A friend today mentioned that I should explore the Biologos Series on Earth's Cambrian Explosion period. Since I always like to start with Wikipedia I first went there to the article, the Cambrian Explosion, to skim through it. Apparently it was in this primordial period some 500 million years ago that life quickly transitioned from life in the sea to life on land after preceding a lengthy period of "unconformity" where both land-and-sea became prepared for sustainable life by geologic upheaval. Add to this early period the necessary ingredients for life like the formation of oxygen produced from massive microbial sea mats,
an oxygenating process necessary for early photosynthesis, the creation of land-bound nutrients by massive ice seas originating from an earlier Snowballed Earth with its resulting glaciation periods, and an increase in the concentration of calcium in the Cambrian seawater, and you get life!
--- From Wikipedia Article ---
The "Cambrian explosion" can be viewed as two waves of metazoan expansion into empty niches: first, a coevolutionary rise in diversity as animals explored niches on the Ediacaran sea floor, followed by a second expansion in the early Cambrian as they became established in the water column.[4]
The rate of diversification seen in the Cambrian phase of the explosion is unparalleled among marine animals: it affected all metazoan clades of which Cambrian fossils have been found. Later radiations, such as those of fish in the Silurian and Devonian periods, involved fewer taxa, mainly with very similar body plans....[12]
Although the recovery from the Permian-Triassic extinction started with about as few animal species as the Cambrian explosion, the recovery produced far fewer significantly new types of animals.[120]
Ecosystem engineering
Evolving organisms inevitably change the environment they evolve in. The Devonian colonization of land had planet-wide consequences for sediment cycling and ocean nutrients, and was likely linked to the Devonian mass extinction. A similar process may have occurred on smaller scales in the oceans, with, for example, the sponges filtering particles from the water and depositing them in the mud in a more digestible form; or burrowing organisms making previously unavailable resources available for other organisms.[118]
Discredited hypotheses
As our understanding of the events of the Cambrian becomes clearer, data has accumulated to make some hypotheses look improbable. Causes that have been proposed, but are now discounted[citation needed] include the evolution of herbivory, vast changes in the speed of tectonic plate movement or of the cyclic changes in the Earth's orbital motion, or the operation of different evolutionary mechanisms from those that are seen in the rest of the Phanerozoic eon.
Complexity threshold
The explosion may not have been a significant evolutionary event. It may represent a threshold being crossed: for example a threshold in genetic complexity that allowed a vast range of morphological forms to be employed.[119]
... Hence, an evolutionary complexity threshold was reached which triggered a massive and rapid diversification of life which opened up an exceptionally wide range of previously unavailable ecological niches. When these were all occupied, limited space existed for such wide-ranging diversifications to occur again, because strong competition existed in all niches and incumbents usually had the advantage....
... There were two similar explosions in the evolution of land plants: after a cryptic history beginning about 450 million years ago, land plants underwent a uniquely rapid adaptive radiation during the Devonian period, about 400 million years ago.[12] Furthermore, Angiosperms (flowering plants) originated and rapidly diversified during the Cretaceous period.
Other Helpful Wikipedia Links:
Now all this is very interesting to me as a non-specialist but it also makes me wish that I had taken classes on evolution when I was at university. I did take the obligatory biology class and attempted a short run at a geology class, but at the last, my interest was more in mathematics, chemistry, and physics. And just for fun, my electives found me 3 years deep in Attic Greek and its literature, with a short course on Roman History, and another on Sumerian/Ugaritic History (pre-Mosaic, Pentateuchal eras). But like most students I was always short on time-and-money and had to keep moving towards my major.
And so, like many of you, I have a basic, rudimentary knowledge of the Earth's development, and will, from time-to-time, watch the NOVA, History, and Discovery TV Channels showing the early drift of the continents; the evolutionary development of biotic environments becoming separated from itself within the drift of those land masses; the formation of mountains from uplifted sea coasts; and even the odd microcellular genome stuff. Still, it is nothing rigorous like you would get by studying it in class with other interested students marked by a capable professor who has taught it for years and years.
Nonetheless, my friend went on to tell me excitedly of the Cambrian Explosion, which having been out to the Canadian Rockies, and having purposely visited the Royal Tyrrell Museum of Palaeontology, I could recall the Burgess Shale area high atop the distant mountains overlooking the Red River Valley stretching some thousand miles over the vast Canadian Prairies of Saskatchewan, Manitoba, North Dakota and Minnesota. It was there, on those riparian heights - which were originally a part of an ancient sea bed - that one could look at a dazzling 20-million year span where primordial life sprang into being. From it came a violent explosion of multi-cellular life-forms which have been especially preserved in this area as "soft fossils" uniquely so because of the geologic conditions that allowed their preservation.
Moreover, apparently there is not a little controversy over the discovery of the Cambrian period's rapid development of life which reached a tipping point, or threshold, for complex life to not only form, but explode! Begging the question, "How could such a pronounced and rapid change take place when evolution supposedly takes hundreds of millions of years and not tens of millions of years to occur?" However, the fact remains is that it did as evidenced by the fossil record. And perhaps even on a shorter timescale than that (some have theorized anywhere between 3 to 10 million years) making it all the more remarkable. But then I remembered Conway's "Game of Life" laid out with very simple initial conditions which moved rapidly from single, multi-blocked sets, to large, diverse, complex sets over a very short span of time, showing that this remarkable evolutionary period is quite possible after all.
So to help explore this evolutionary riddle, Biologos has created some space for an introduction to the Cambrian period, along with a lengthy series on evolution itself by Dennis Veneman (I last counted somewhere around 28 articles to date). Here are the links:
Moreover, they have also provided a six-part series on the problem this Cambrian period apparently poses by its rapid diversification which I have provided in whole below. Even so, here are the links:
The Cambrian “Explosion”, Transitional Forms, and the Tree of Life, Dec 3, 2010
The Cambrian “Explosion”, Part 2, Dec 11, 2010
The Cambrian “Explosion”, Part 3, Dec 17, 2010
The Cambrian “Explosion”, Part 4, Feb 8, 2011
The Cambrian “Explosion”, Part 5, Mar 4, 2011
The Cambrian “Explosion”, Part 6, Mar 11, 2011
Final Thoughts on the Text of Genesis
For myself, I find the history of evolution as remarkable as its own legacy apparently forebodes on the subject. I love controversies like this and would rather seek for better questions than for better answers. I should also remark that my view of the Genesis creation record is one that is non-literal and grounded in seeing it as an ancient narrative (I personally don't like the word "myth") written in rebuke to the more ancient Sumerian, Egyptian, and Babylonian creation accounts. But this has been discussed elsewhere.
More simply said, I don't attempt to force an ancient view of cosmogony and the genesis of life into the passages of Genesis like many theistic evolutionists will do. I am content in my own type of "evolutionary creation" that, though similar in thought to its older form, does not need to force it into the biblical record itself to find its relevancy. I wouldn't have expected that of the biblical writers in their oral legends, and nor would God. Even so, today's sciences would befuddle and baffle the ancients back then, and if hearing of it would probably burn all heretics at the stake immediately and without hesitation. And I can likewise imagine 4000 years from now the same being said of us by those futurists who may look upon our 21st century postmodern generations in bafflement and bewilderment as well. Thinking our societies' arcane, primitive, and quite backwards. And so it goes... we do the best we can with what we have and understand as the Lord gives us knowledge through creation's ancient accounts.
Lastly, I did find an helpful article written by an theistic evolutionist that likes to look at Genesis as a pattern describing modern evolutionary science. Though I don't find the same sympathies with the biblical text as he does, even so I found his reasoning helpful enough for readers to peruse and come away with perhaps a better understanding of the Cambrian Explosion and the difficulties it presents to the modern mind. Here's the link:
R.E. Slater
September 19, 2013
http://biologos.org/questions/cambrian-explosion
In a Nutshell
The “Cambrian Explosion” refers to the appearance in the fossil record of most major animal body plans about 543 million years ago. The new fossils appear in an interval of 20 million years or less. On evolutionary time scales, 20 million years is a rapid burst that appears to be inconsistent with the gradual pace of evolutionary change. However, rapid changes like this appear at other times in the fossil record, often following times of major extinction. The Cambrian Explosion does present a number of interesting and important research questions. It does not, however, challenge the fundamental correctness of the central thesis of evolution.
In Detail
The term “Cambrian Explosion” refers to the appearance and rapid diversification of most major living animal body plans (phyla) in the fossil record within an interval of perhaps 20 million years or less, a relatively short period in evolutionary history. This time is known as the Early Cambrian, and began around 543 million years ago. This time interval is recorded by some spectacular fossil deposits that include superbly preserved fossils of these early animals. Two famous examples are the Burgess Shale in Canada, and the Chengjiang in China.1 Despite the claims of some, the Cambrian was not the beginning of multicellular animal life; the latter has a fossil record that extends back at least 30 million years earlier.2
The Cambrian Explosion is often posed as a challenge for evolution because the sudden burst of change in the fossil record appears to be inconsistent with the more typical gradual pace of evolutionary change. However, although different in certain ways, there are other times of very rapid evolutionary change recorded in the fossil record -- often following times of major extinction. The Cambrian Explosion does present a number of challenging and important questions because it represents the time during which the main branches of the animal tree of life became established. It does not create a challenge to the fundamental correctness of the central thesis of evolution, the descent of all living species from a common ancestor. This important period in the history of life extended over millions of years, plenty of time for the evolution of these new body plans (phyla) to occur. Furthermore, the fossil record provides numerous examples of organisms that appear transitional between living phyla and their common ancestors. The ongoing research about the Cambrian period is an exciting opportunity to advance our understanding of how evolutionary processes work, and the environmental factors shaping them.
The major animal body plans that appeared in the Cambrian Explosion did not include the appearance of modern animal groups such as: starfish, crabs, insects, fish, lizards, birds and mammals. These animal groups all appeared at various times much later in the fossil record.3 The forms that appeared in the Cambrian Explosion were more primitive than these later groups, and many of them were soft-bodied organisms. However, they did include the basic features that define the major branches of the tree of life to which later life forms belong. For example, vertebrates are part of the Chordata group. The chordates are characterized by a nerve cord, gill pouches and a support rod called the notochord. In the Cambrian fauna, we first see fossils of soft-bodied creatures with these characteristics. However, the living groups of vertebrates appeared much later. It is also important to realize that many of the Cambrian organisms, although likely near the base of major branches of the tree of life, did not possess all of the defining characteristics of modern animal body plans. These defining characteristics appeared progressively over a much longer period of time.4
Interpretations of the “Cambrian Explosion”
Not all scientists accept the idea that the Cambrian Explosion represents an unusually rapid evolutionary transition. The fossil record is notoriously incomplete, particularly for small and soft-bodied forms. Some researchers argue that the apparent rapid diversification of body plans is an artifact of an increase in the rate of fossilization, due in part to the evolution of skeletons, which fossilize more effectively.5 Many of the early Cambrian animals possessed some type of hard mineralized structures (spines, spicules, plates, etc.). In many cases these, often very tiny, mineralized structures are all that are found as fossils. There were major changes in marine environments and chemistry from the late Precambrian into the Cambrian, and these also may have impacted the rise of mineralized skeletons among previously soft-bodied organisms. 6
Most scientists are persuaded that something significant happened at the dawn of the Cambrian era and view the Cambrian Explosion as an area of exciting and productive research. For example, scientists are now gaining a better understanding of what existed before the Cambrian Explosion as a result of new fossil discoveries. Recent discoveries are filling in the fossil record for the Precambrian fauna with soft-bodied organisms like those in the Ediacaran Assemblages found around the world.7 Late Precambrian fossil discoveries also now include representatives of sponges, cnidarians (the group that includes modern jellyfish, corals and anemones), mollusks and various wormlike groups. Some of the new fossil discoveries, in fact, appear to be more primitive precursors of the later Cambrian body plans. The discovery of such precursors shows that the Cambrian organisms did not appear from thin air.8 Further discoveries will no doubt reveal more clearly the relationship of Precambrian organisms with the creatures found in the Burgess Shale and Chengjiang deposits.9
Genomic studies provide further insights into the origins of the Cambrian Explosion. Although the genetic divergence of organisms would have preceded the recognition of new body plans in the fossil record, accumulating genomic data is broadly consistent with the fossil record.10 Both point to the rise of the bilateria (bilaterally symmetric invertebrate animals) in the latest Precambrian Ediacaran, and their ecological explosion in diversity in the Cambrian.
Unanswered Questions
The sudden change of the Cambrian Era was, in relative terms, not too sudden for the process of evolution. The changes during the Cambrian Era did not occur over decades, centuries, or even thousands of years; they occurred over millions of years—plenty of time for evolutionary change. However, for millions of years beforehand, body plans of animals had remained relatively constant. Not until this time period did a significant change occur. The remaining questions are: What triggered the Cambrian Explosion? And why did so much change occur at this time? Several different theories address the origin of the Cambrian Explosion, proposing that dramatic environmental changes must have opened up new niches for natural selection to operate upon. These proposals include the runaway glaciation theory,11 which proposes that glaciers briefly covered much of the earth, and the resultant loss of habitat created bottlenecks where evolution could act more rapidly. Another theory suggests that a change in atmospheric oxygen led to this sudden burst in evolutionary changes.12 Yet another proposal is that major changes in the seafloor, from algae mat-covered surfaces in the late Precambrian to soft muddy bottoms later in the Cambrian, had dramatic evolutionary and ecological impacts.13
The Cambrian Era Fossils, Providing Answers
While the causes of the Cambrian Explosion remain a topic of open and exciting debate, the continued fossil discoveries from the Cambrian and Precambrian Eras are bringing more clarity to the evolutionary puzzle. These fossils provide valuable insight, particularly for envisioning the common ancestors of diverse groups. For instance, both vertebrates (fish) and echinoderms (sea urchins, starfish) are part of the group called deuterostomes. Without fossil evidence, it is hard to envision what a common ancestor would look like for these very different creatures. The Cambrian fossils are filling in the picture.14
Further Reading
DVDs
See part 2 of the PBS series on Evolution, which can be rented, for example, through Netflix or can be purchased through PBS.
Books
Conway Morris, Simon. The Crucible of Creation. Oxford: Oxford University Press, 1998.
Falk, Darrel R. “The Fossil Record.” In Coming to Peace with Science: Bridging the Worlds between Faith and Biology. Downers Grove, IL: InterVarsity Press, 2004.
David Campbell and Keith Miller, “The ‘Cambrian Explosion’: A Challenge to Evolutionary theory?” In Keith Miller (ed.), Perspectives on an Evolving Creation (Grand Rapids, MI: Wm. b. Eerdmans Pub. Co., 2003), 182-204.
Online
Derek Briggs, Douglas Erwin, and Frederick Collier, The Fossils of the Burgess Shale (Washington: Smithsonian Institution Press, 1994). Junyuan Chen and Guiqing Zhou, “Biology of the Chengjiang Fauna,” in Junyuan Chen, Yen-nien Cheng, and H.V. Iten (eds.), The Cambrian Explosion and the Fossil Record, Bulletin of the National Museum of Natural Science no. 10 (Taichung, Taiwan, China, 1997), 11-105.
David Campbell and Keith Miller, “The ‘Cambrian Explosion’: A Challenge to Evolutionary theory?” in Keith Miller (ed.), Perspectives on an Evolving Creation (Grand Rapids, MI: Wm. b. Eerdmans Pub. Co., 2003), 182-204.
Darrel R. Falk, Coming to Peace with Science: Bridging the Worlds between Faith and Biology (Downers Grove, IL: InterVarsity Press, 2004), 95.
Graham Budd and Soren Jensen, “A Critical Reappraisal of the Fossil Record of the Bilaterian Phyla,” Biological Reviews 75 (2000): 253-295.
Darrel R. Falk, Coming to Peace with Science: Bridging the Worlds between Faith and Biology (Downers Grove, IL: InterVarsity Press, 2004), 94
Simon Conway Morris, The Cambrian Explosion, course, September 16, 2007, from The Faraday Institute of Science and Religion, MP3, Download Video, (accessed December 18, 2008); and S.T. Brennan, T.K. Lowenstein, and J. Horita, 2004, “Seawater chemistry and the advent of biocalcification,” Geology 32 (2004): 473-476.
M.A. Fedonkin, “Vendian faunas and the early evolution of metazoa,” In, J.H. Lipps and P.W. Signor (eds.), Origin and Early Evolution of the Metazoa (New York: Plenum Press, 1992), p.87-129. G.M. Narbbonne, M. Laflamme, C. Greentree, and P. Trusler, “Reconstructing a lost world: Ediacaran rangeomorphs from Spaniard’s Bay, Newfoundland,” Journal of Paleontology 83, no. 4 (2009): 503-523.
David Campbell and Keith Miller, “The ‘Cambrian Explosion’: A Challenge to Evolutionary theory?”
-
Kevin J. Peterson et al., “The Ediacaran Emergence of bilaterians: Congruence between the genetic and the geological fossil records,” Philosophical Transactions of the Royal Society B 363 (2008), 1435–43.
P.F. Hoffman and D.P. Schrag, “The snowball Earth hypothesis: testing the limits of global change,” Terra Nova 14 (2002): 129-155.
Simon Conway Morris, The Cambrian Explosion; and D.A. Fike, J.P. Grotzinger, L.M. Pratt, and R.E. Summons, “Oxidation of the Ediacaran ocean,” Nature 444 (2006): 744-747.
David Bottjer, James Hagadorn, and Stephen Dornbos, “The Cambrian Substrate Revolution,” GSA Today 10, no. 9 (2000): 1-7.
Shu, D-G., et al., “Primitive deuterstomes from the Chengjiang Lagerstatte ( Lower Cambrian, China),” Nature 414 (2001): 419-424.
* * * * * * * *
The Cambrian "Explosion," Transitional Forms, and the Tree of Life
www.BioLogos.org
BY KEITH B. MILLER, DEPARTMENT OF GEOLOGY,
KANSAS STATE UNIVERSITY
http://biologos.org/uploads/projects/miller_white_paper.pdf
The
BioLogos Foundation • www.BioLogos.org/projects/scholar-essays
This
essay is an update and extension of Campbell, D., and K.B. Miller, 2003,
"The Cambrian Explosion: A challenge to evolutionary theory?" in
Miller, K.B. (ed.), Perspectives on an Evolving Creation: Grand Rapids, MI: Wm.
B. Eerdmans Publ. Co., p.182-204
Introduction: What’s all the fuss?
The
most fundamental claim of biological evolution is that all living organisms
represent the outer tips of a diversifying, upward-branching tree of life. The
"tree of life" is an extremely powerful metaphor that captures the
essence of evolution. Like the branches of a tree, as we trace individual lines
of descent (lineages) back into the past (down the tree) they converge with
other lineages toward their common ancestors. Similarly, these ancient lineages
themselves converge with others back in time. Thus, all organisms, both living
and extinct, are ultimately connected by an unbroken chain of descent with
modification to a common ancestral trunk among single-celled organisms in the
distant past.
This
tree metaphor applies as much to the emergence of the first representatives of
the major groups of living invertebrates (such as annelids, snails, or
arthropods) as it does to the first appearance and diversification of
dinosaurs, birds, or mammals. This early diversification of invertebrates
apparently occurred around the time of the Precambrian/Cambrian boundary over a
time interval of a few tens of millions of years. This period of rapid
evolutionary diversification has been called the "Cambrian Explosion."
The
Cambrian explosion has been the focus of extensive scientific study,
discussion, and debate for decades, and is increasingly receiving attention in
the popular media. It has also received considerable recent attention by
evolution critics as posing challenges to evolution. These critics argue that
the expected transitions between major invertebrate groups (phyla) are absent,
and that the suddenness of their appearance in the fossil record demonstrates
that evolutionary explanations are not viable.
What
are some of the arguments of the evolution critics? John Morris of the ICR
writes:
"If
evolution is correct, the first life was quite simple, evolving more complexity
over time. Yet the Cambrian Explosion of Life has revealed life's complexity
from the start, giving evolution a black eye. The vast array of complex life
that appears in the lowest (or oldest) stratigraphic layer of rock, with no
apparent ancestors, goes hard against evolutionary dogma. Evolution's desperate
attempt to fill this gap with more simple ancestral fossils has added more
injury. .... Think of the magnitude of this problem from an evolutionary
perspective. Many and varied forms of complex multi-celled life suddenly sprang
into existence without any trace of less complex predecessors. There are
numerous single-celled forms at lower stratigraphic levels, but these offer
scant help in solving the mystery. Not one basic type or phyla of marine
invertebrate is supported by an ancestral line between single-celled life and
the participants in the Cambrian Explosion, nor are the basic phyla related to
one another. How did evolution ever get started?"1
Intelligent
Design advocate Stephen Meyer and others have written:
"To
say that the fauna of the Cambrian period appeared in a geologically sudden
manner also implies the absence of clear transitional intermediates connecting
the complex Cambrian animals with those simpler living forms found in lower
strata. Indeed, in almost all cases, the body plans and structures present in
Cambrian period animals have no clear morphological antecedents in earlier
strata.2
And:
"A
third feature of the Cambrian explosion (as well as the subsequent fossil
record) bears mentioning. The major body plans that arise in the Cambrian
period exhibit considerable morphological isolation from one another (or
"disparity") and then subsequent "stasis." Though all
Cambrian and subsequent animals fall clearly within one of a limited number of
basic body plans, each of these body plans exhibits clear morphological
differences (and thus disparity) from the others. The animal body plans (as
represented in the fossil record) do not grade imperceptibly one into another,
either at a specific time in geological history or over the course of
geological history. Instead, the body plans of the animals characterizing the
separate phyla maintain their distinctive morphological and organizational
features and thus their isolation from one another, over time."3
Are
these critiques warranted? To what extent is the Cambrian explosion really
problematic for the evolutionary picture of an unbroken tree of life extending
back to the earliest life on Earth?
Geologic Time Scales: How big was the bang?
The
relative rapidity of the diversification of invertebrates during the Cambrian
explosion is set against the backdrop of the Earth’s geologic and biologic
history. Geologic time is unfamiliar to most people, and its shear vastness is
difficult to grasp.
Two
lines of evidence impact our understanding of the duration of the animal
diversification that led to the appearance of the major groups of living
invertebrates. The first is the dating of critical strata within the geological
timeline such as the Precambrian-Cambrian boundary and various important
fossil-bearing horizons. The second is the time of appearance of the first
widely recognized fossil representatives of the major living groups (phyla) of
invertebrate animals. The latter is in considerable flux as new fossil
discoveries are made.
Originally,
the base of the Cambrian had been set at the earliest appearance of organisms
with mineralized skeletons - particularly trilobites. However, a diverse
collection of tiny mineralized plates, tubes and scales was discovered to lie
below the earliest trilobites.4 This
interval of "small shelly fossils" was designated the Tommotian.
Because of the presence of even earlier tiny mineralized tubes and simple
burrows, there was no internationally accepted definition for the boundary
until 1994. At that time, the base of the Cambrian was placed at the first
appearance of a particular collection of small fossil burrows characterized by
Treptichnus pedum.
Until
the early 1990s the age of the Precambrian-Cambrian boundary was not tightly
constrained, and was estimated to be about 575 million years ago. However, in
1993 new radiometric dates from close to the accepted Precambrian-Cambrian
boundary revealed that it was significantly younger -- about 544 million years.5 A more precise date of 542 ± 0.3
million years has recently been formally accepted by the International
Commission on Stratigraphy. The basis for this date was the discovery that a
sharp worldwide fall (or negative spike) in the abundance of the isotope
carbon-13 was coincident with the Cambrian boundary as previously defined. In
Oman, this isotopic marker also coincides with a volcanic ash layer that
yielded the 542 million year date using uranium/lead radiometric methods.6 This horizon also marks the last
occurrence of several fossils characteristic of the underling late Precambrian
Ediacaran Period.7
Such extinction events
are commonly used to subdivide the geologic time scale.
The
earliest diverse fossil invertebrate communities of the Cambrian are
represented by the Chengjiang, in China. These deposits are dated at 525-520
million years. The famous Burgess Shale is considerably younger, dating at
about 505 million years, and the end of the Cambrian Period is set at 490
million years. The Cambrian Period thus lasted for 52 million years. To put
this in perspective, the time elapsed since the extinction of the dinosaurs at
the end of the Cretaceous has been 65 million years. The Cambrian was a very
long period of time.
If
the Cambrian explosion is understood to comprise the time from the base of the
Cambrian to the Chengjiang fossil beds, then this period of diversification in
animal body plans appears to have lasted about 20 million years. However, not
all living animal phyla with a fossil record first appear within this time
window. The colonial skeleton-bearing bryozoans, for example, are not known
from the fossil record until the end of the Cambrian around 491 million years
ago.8 More significantly, several living
invertebrate phyla have a fossil record that extends into the late
Neoproterozoic before the Cambrian. Sponges have been recognized as early as
580 million years, cnidarians (the group including jellyfish and anemones) are
present among the Ediacaran animals at around 555 million years, and the stem
groups (see discussion below) for some other phyla were also likely part of the
Ediacaran communities.9
Defining
the Cambrian explosion is not as straightforward as it might seem. Although
there was clearly a major burst of evolutionary innovation and diversification
in the first 20 million years or so of the Cambrian, this was preceded by an
extended period of about 40 million years during which metazoans (multicellular
animals) arose and attained critical levels of anatomical complexity. The
Ediacaran saw the appearance of organisms with the fundamental features that
would characterize the later Cambrian organisms (such as three tissue layers,
and bilaterally symmetric bodies with a mouth and anus), as well as the first
representatives of modern phyla. The base of the Cambrian is not marked by a
sharp dramatic appearance of living phyla without Precambrian roots. It is a
subjectively defined point in a continuum. The Cambrian "explosion"
appears to have had a "long fuse."
Classifying Animals: What’s in a name?
The
procedure of classifying organisms is called taxonomy, and the general name for
individual groups is "taxa." Significantly, the first question that
needs to be addressed is -- What is a phylum? A phylum is often identified as a
group of organisms sharing a basic "body plan," a group united by a
common organization of the body. However, phyla can be understood
fundamentally, like all other taxonomic categories, as groupings of taxa that
are more closely related to each other than to any other group.
The
most widely accepted method for grouping organisms today is called cladistics.
In cladistics all taxonomic groups are monophyletic, that is all of the members
of the group are descended from a common ancestor that is the founding member
of that taxon. A branch of the tree of life whose members all share the same
ancestor is called a "clade" - thus the term cladistics.
Closely-related taxa that do not share the same common ancestor are called
"sister" taxa. These sister taxa commonly resemble each other more
than the descendant relatives resemble the ancestors of their clade. As a
result, placing these organisms into their correct monophyletic groups can be
very difficult. Thus, organisms within a given phylum may bear close
similarities to those from another closely-related sister phylum. In fact, the
assignment of a given organism or fossil specimen to a phylum can be just as
problematic as assignments to lower-ranked taxa such as classes, orders,
families, etc.10
Further
complicating the assignment of fossil organisms to phyla is that the anatomical
characteristics that are used to define living phyla did not appear
simultaneously, but were added over time. This has resulted in the distinction
between "crown groups" and "stem groups" (see below) in the
scientific literature. A crown group is composed of all the living organisms
assigned to that phylum, plus all the extinct organisms that were descended
from the common ancestor of those living organisms. The stem group is composed
of organisms more closely related to one living phylum than to another, but
that do not possess all of the distinguishing characters of the crown group. It
turns out that the organisms appearing in the early Cambrian are, with few
exceptions, not crown groups but stem groups. That is, the complete suite of
characters defining the living phyla had not yet appeared. Many crown groups
actually do not appear in the fossil record until well after the Cambrian.11
The
existence of stem groups provides a way to understand how the basic body plan
of a living invertebrate could have been built up in steps. The major
invertebrate groups are often portrayed by evolution critics as possessing
anatomies that are both irreducible in organization and separated from other
groups by unbridgeable gaps. No transitions could exist even in principle. This
view is illustrated by the following comment by John Morris.
"Let's
suppose you want to find the forefathers of the clams, a prominent resident of
the Cambrian Explosion, for instance. As you follow the fossil clues into ever
"older" strata, what do you find? You find clams. The first or lowest
occurrence of clams is abrupt or sudden. There are no ancestors that are not
clams. An evolutionary lineage is impossible to discern, for clams have always
been clams. Fossil clams are quite abundant, found all over the world in rocks
of every age, and clams live today. Great variety among them abounds, but they
are still clams. Variety does not speak to ancestry. The same is true of all
animals found in the Cambrian Explosion. How can evolutionary scientists use
the fossils as evidence of a common descent of all life?"12
The
phylum Mollusca, to which clams belong, actually illustrates well how modern
body plans could evolve from earlier stem groups. There is a well-documented
series of transitional forms that extends from pre-mollusks (stem mollusks)
through primitive early mollusks to the first unambiguous clams. The animals in
this group gradually acquired the whole set of characteristics we now use to
define "clam". The earliest known mollusk-like organism is Kimberella
from the late Neoproterozoic Ediacaran. It is a primitive organism that appears
to lack several features characteristic of modern mollusks and is thus a
considered a stem mollusk. The first likely "crown group" mollusks
appear in the earliest Cambrian as part of the "small shelly fauna."
While recognizable as mollusks, many of these fossils belong either to sister
groups or to stem groups of living classes. The earliest fossil bivalves ("clams")
are linked through a series of transitional forms to two of these extinct
groups - the rostroconchs and the cap-shaped helcionelloids. The hinged valves
of clams appear to have evolved by the lateral compression of cap-shaped shells
and then the thinning and loss of shell material along the hinge line.13 The characters that we use to
identify "clams" did not appear as a complete package, but were
acquired over time.
Some
critics of evolution make much of the "top-down" versus the
"bottom-up" pattern of appearance of higher taxa. That is,
phylum-level diversity reaches its peak in the fossil record before class-level
diversity, and the class-level diversity before that of orders, etc. These
critics interpret this apparent "top-down" pattern as contrary to
expectations from evolutionary theory. For example, Stephen Meyer and others
have argued:
"Instead
of showing a gradual bottom-up origin of the basic body plans, where
smaller-scale diversification or speciation precedes the advent of large-scale
morphological disparity, disparity precedes diversity. Indeed, the fossil
record shows a "top-down" pattern in which morphological disparity
between many separate body plans emerges suddenly and prior to the occurrence
of species-level (or higher) diversification on those basic themes."14
However,
this pattern is an artifact, being generated by the way in which species are
assigned to higher taxa. The classification system is hierarchical with species
being grouped into ever larger and more inclusive categories. When this
classification hierarchy is applied to a diversifying evolutionary tree, a
"top-down" pattern will automatically result. Consider species
belonging to a single evolving line of descent given genus -level status. This
genus is then grouped with other closely related lines of descent into a
family. The common ancestors of these genera are by definition included within
that family. Those ancestors must logically be older than any of the other
species within the family. Thus the family level taxon would appear in the
fossil record before most of the genera included within it. Another way of
looking at this is the fact that the first appearance of any higher taxon will
be the same as the first appearance of the oldest lower taxon within the group.
For example, a phylum must be as old as the oldest class it contains. Most
phyla contain multiple classes, which in turn include multiple orders, and so
forth. Thus, each higher taxon will appear as early as the first of the
included lower taxa.
Additionally,
higher taxonomic levels typically reflect more general aspects of the body
plan. Thus, a poorly preserved specimen may be confidently assigned to a
particular phylum, but not to any one class. Similarly, a primitive fossil
might have the distinctive features of a particular phylum, but not be clearly
assignable to any particular class because it is a transitional form -- that
is, a stem group or a sister group to a living class of organisms. Both of
these factors would promote the earlier recognition of higher taxonomic
categories than lower ones. The "top- down" pattern of taxa
appearance is therefore entirely consistent with a branching tree of life.
There
is one last bias in our reconstruction of the past that is generated by the
process of assigning organisms to particular phyla. Because phyla are defined
by particular anatomical character traits, they cannot be recognized in the
fossil record until after those specific characters evolve. However, the splitting
of the branch of the tree of life to which a phylum belongs may have occurred
many millions of years previous to the evolution of those characters. The
actual first appearance of a phylum thus occurs after significant anatomical
evolution has occurred along that particular branch of the tree. Branching
points in the tree of life will always be older than the named taxa.15
The Fossil Record: Is there enough evidence?
There
are two opposite errors which need to be countered about the fossil record: 1)
that it is so incomplete as to be of no value in interpreting patterns and
trends in the history of life, and 2) that it is so good that we should expect
a relatively complete record of the details of evolutionary transitions within
all or most lineages.
What
then is the quality of the fossil record? It can be confidently stated that
only a very small fraction of the species that once lived on Earth have been
preserved in the rock record and subsequently discovered and described by
science.16
There
is an entire field of scientific research referred to as "taphonomy"
-- literally, "the study of death." Taphonomic research includes
investigating those processes active from the time of death of an organism
until its final burial by sediment. These processes include decomposition,
scavenging, mechanical destruction, transportation, and chemical dissolution
and alteration. The ways in which the remains of organisms are subsequently
mechanically and chemically altered after burial are also examined -- including
the various processes of fossilization. Burial and "fossilization" of
an organism's remains in no way guarantees its ultimate preservation as a
fossil. Processes such as dissolution and recrystallization can remove all
record of fossils from the rock. What we collect as fossils are thus the
"lucky" organisms that have avoided the wide spectrum of destructive
pre- and post-depositional processes arrayed against them.
Soft-bodied
organisms, and organisms with non-mineralized skeletons have very little chance
of preservation under most environmental conditions. Until the Cambrian nearly
all organisms were soft-bodied, and even today the majority of species in
marine communities are soft-bodied. The discovery of new soft-bodied fossil
localities is always met with great enthusiasm. These localities typically turn
up new species with unusual morphologies, and new higher taxa can be erected on
the basis of a few specimens! Such localities are also erratically and widely
spaced geographically and in geologic time.
Even
those organisms with preservable hard parts are unlikely to be preserved under
"normal" conditions. Studies of the fate of clam shells in shallow
coastal waters reveal that shells are rapidly destroyed by scavenging, boring,
chemical dissolution and breakage. Occasional burial during major storm events
is one process that favors the incorporation of shells into the sedimentary
record, and their ultimate preservation as fossils. Getting terrestrial
vertebrate material into the fossil record is even more difficult. The
terrestrial environment is a very destructive one: with decomposition and
scavenging together with physical and chemical destruction by weathering.
The
potential for fossil preservation varies dramatically from environment to
environment. Preservation is enhanced under conditions that limit destructive
physical and biological processes. Thus marine and fresh water environments
with low oxygen levels, high salinities, or relatively high rates of sediment
deposition favor preservation. Similarly, in some environments biochemical
conditions can favor the early mineralization of skeletons and even soft
tissues by a variety of compounds (eg. carbonate, silica, pyrite, and
phosphate). The likelihood of preservation is thus highly variable. As a
result, the fossil record is biased toward sampling the biota of certain types
of environments, and against sampling the biota of others.
In
addition to these preservational biases, the erosion, deformation and
metamorphism of originally fossiliferous sedimentary rock have eliminated
significant portions of the fossil record over geologic time. Furthermore, much
of the fossil-bearing sedimentary record is hidden in the subsurface, or
located in poorly accessible or little studied geographic areas. For these
reasons, of those once-living species actually preserved in the fossil record,
only a small portion have been discovered and described by science. However,
there is also the promise of continued new and important discovery.
The
forces arrayed against fossil preservation also guarantee that the earliest
fossils known for a given animal group will always date to some time after that
group first evolved. The fossil record always provides only minimum ages for
the first appearance of organisms.
Because
of the biases of the fossil record, the most abundant and geographically
widespread species of hardpart-bearing organisms would tend to be best
represented. Also, short-lived species that belonged to rapidly evolving lines
of descent are less likely to be preserved than long-lived stable species.
Because evolutionary change is probably most rapid within small isolated
populations, a detailed species-by -species record of such evolutionary
transitions is unlikely to be preserved. Furthermore, capturing such
evolutionary events in the fossil record requires the fortuitous sampling of
the particular geographic locality where the changes occurred.
Using
the model of a branching tree of life, the expectation is for the preservation
of isolated branches on an originally very bushy evolutionary tree. A few of
these branches (lines of descent) would be fairly complete, while most are
reconstructed with only very fragmentary evidence. As a result, the large-scale
patterns of evolutionary history can generally be better discerned than the
population-by-population or species-by-species transitions. Evolutionary trends
over longer periods of time and across greater anatomical transitions can be
followed by reconstructing the sequences in which anatomical features were
acquired within an evolving branch of the tree of life.
Before the "Explosion": What went bang?
A
very important question is what organisms existed before the Cambrian
"explosion." Were there Precambrian precursors, or did the Cambrian
explosion really happen in a biological vacuum? Many critics of evolution claim
that the Precambrian is devoid of fossils that could represent body plans
ancestral to those of the Cambrian invertebrates.
The
words of Darwin are often cited as evidence of the seriousness of the problem
for evolution.
"There
is another and allied difficulty, which is much more serious, I allude to the
manner in which species belonging to several of the main divisions of the
animal kingdom suddenly appear in the lowest known fossiliferous rocks. Most of
the arguments which have convinced me that all the existing species of the same
group are descended from a single progenitor, apply with equal force to the
earliest known species."17
When
Darwin published his model of descent with modification by means of natural
selection, knowledge of the fossil record was in its infancy. In particular,
the Precambrian and Early Cambrian fossil record was virtually unknown. Even
the fossils of the now famous Burgess Shale and similar units were as yet
undiscovered. After more than a century of paleontological work, the situation
has changed dramatically. In keeping with evolutionary expectations, fossils
are now known from the late Precambrian and early Cambrian that record several
dramatic transitions in the history of life.
The
presence of Late Precambrian animals was recognized in the 1950s and became
widely publicized by the early 1970s. These are the famous Ediacaran fossils
named for fossil-rich beds in the Ediacaran Hills of South Australia and now
recognized at sites throughout the world. These organisms are typically
preserved as impressions in sandstones and siltstones. Associated with these
fossils are trails and simple burrows of organisms that show a limited increase
in complexity and diversity toward the Cambrian.
The
record of life actually extends far beyond the Ediacaran fossils (~565-545 My)
into the deep geologic past. Fossils of algae, protists, and bacteria are
present throughout much of the Precambrian. The earliest convincing fossils of
bacteria are recognized in rocks 3.5 billion years old, and chemical signatures
point to the presence of life even earlier. Finely layered mounds (called
stromatolites) produced by the activity of mat-building bacteria and algae
appear at about this time and become relatively abundant by around 2.7 billion
years ago. Evidence of eukaryotic algae, possessing membrane-bounded nuclei and
internal organelles, dates to about 1500 million years ago, or earlier if chemical
evidence is accepted. Multicellularity had appeared by 1000 million years ago
in the form of diverse and relatively advanced seaweeds. The earliest fossils
of metazoans (multi-celled animals) may be represented by simple disk-shaped
fossils found in rocks 610-600 million years old.18
The
earliest unambiguous indication of the rise of metazoan life is preserved in
the spectacular phosphorite deposits of the Doushantuo Formation of China
dating to at least 580 million years ago. Phosphate can preserve organisms and
tissues in such great detail that individual cells can often be recognized.
Where environmental conditions are ideal for this type of preservation,
extraordinary fossil deposits may result. In the case of the Doushantuo,
phosphatization has preserved not only a variety of algal remains, but also the
cellular tissues of sponges and millimeter-sized tubes that might represent
stem cnidarians.19
However, even more
spectacular is the preservation of metazoan eggs and early embryos. These
embryos are of uncertain affinities but may represent cnidarians or even
bilaterians (animals with bilateral symmetry).20
The
Ediacaran fossils provide the next window into the rise of metazoans. These
fossil-bearing units span from about 575 million years to the base of the
Cambrian, and are found in south Australia, Namibia, the White Sea coast of
Russia, and Newfoundland. The enigmatic soft-bodied organisms were preserved as
impressions, or molds, on the surfaces of sandstone and siltstone layers. These
sediment layers accumulated in shallow-marine environments where the seafloor
was covered by firm microbial algal mats. The microbial mats covering the
seafloor appear to have been important in determining the lifestyles of the
Ediacaran organisms, as well as their unique mode of preservation.21
Most
soft-bodied impressions of the Ediacaran (or Vendian) can roughly be placed
into three general groups -- disks, fronds, and flat-bodied,
bilaterally-symmetric forms. The biological affinity of these fossils is very
difficult to determine and highly debated.22 Disks
are the earliest appearing, and most common, Ediacaran fossils. They have often
been identified as medusoids ("jellyfish") but many appear to have
been attached to the bottom, and none bear clear structures that would place
them in a living group. Some do clearly possess tentacles around their margins
suggesting a stem or sister group relationship to the cnidarians. Some
sack-shaped fossils might even be stem anthozoans (the cnidarian group that
includes anemones and corals).23
A
few disk-shaped fossils may be related to other living phyla. One such form
appears to be a sponge that might be assignable to the modern class of
hexactinellids.24
Another is a small disk
that has a raised center with five radial grooves that has been interpreted as
a stem echinoderm (the phylum that includes modern starfish and sea urchins)
that lacked the characteristic porous calcareous plates and other diagnostic
features of true echinoderms.25
The
frond-shaped forms include organisms that were attached to the bottom by a
stalk, and others that appear to have been free lying. These fossils have also
been assigned by some workers to a group of modern cnidarians (the "sea
pens") or to ctenophores. However, like the disks, the fronds are fairly
diverse and some may be unrelated to living phyla.26 Others, although likely not able to
be placed into a living cnidarian group, may be stem cnidarians, or even stem
anthozoans. The discovery of better preserved fronds in the Cambrian that
closely resemble some of the Ediacaran fossils would seem to support this
interpretation.27
The
bilaterally-symmetric forms of the Ediacaran are the most diverse and most
enigmatic fossils of the late Precambrian. Some of these fossils may represent
early experiments on the pathway to the living phyla.28 For example, Dickinsonia and the
similar Yorgia are fairly large flat highly-segmented forms that some workers
have interpreted as annelids or stem annelids, while others have seen
resemblances to other worm phyla or even chordates. These organisms do appear
to have been able to move about the bottom as seen by associated crawling and
resting traces. Even if not members of a living phylum, these organisms appear
to at least be mobile bilateral metazoans (or bilaterians). Another bilateral
form that has been the subject of much recent attention is Kimberella. This 555
million year old fossil has been interpreted as a stem mollusk.29 Scratch marks found associated with
Kimberella indicate that it had some form of feeding structure (though probably
not a true mollusk radula) that enabled it to graze the abundant algal mats.
Other bilateral fossils have been interpreted to bear similarities to
arthropods, although these interpretations are disputed.
An
important, but less attention-getting, component of the Ediacaran fossil record
is the presence of trace fossils such as trails, burrows and feeding traces.
Except in the few cases mentioned above, there are no body fossils preserved of
the organisms that made these traces. These traces tend to be small unbranched
sediment-filled burrows that run horizontally along the sediment surface or
under the microbial algal mats. Somewhat more complex burrows appear toward the
base of the Cambrian including irregularly branching burrows and shallow
vertical burrows.30
These traces are
important because they point to the existence of small worm-like organisms that
were probably feeding on and in the algal mats that covered extensive areas of
the seafloor. The biological identity of these organisms is unknown, although
they were clearly bilateria.
There
is one more set of fossils that are known from the late Ediacaran (550-543
million years) that reveal yet another aspect of the metazoan diversity before
the Cambrian. These fossils include tiny calcified or phosphatized tubes, cones
and goblet-shaped structures that record the presence of animals capable of
producing mineralized skeletons. They are commonly embedded within algal
buildups that formed reef-like structures, and may be quite abundant.31 These algal-metazoan reefs
foreshadow the later algal reefs of the Cambrian. The very peculiar cm-sized
goblet-shaped Namacalathus (found as calcified fossils) lived attached to the
algal mounds by stalks. Although the preserved shape of these fossils is consistent
with that of cnidarians, their biology is uncertain. The cone-in-cone
structures of Cloudina, and the more tubular Sinotubulites could have been
produced by various types of worms such as serpulids. However, as with the
trace fossils, the identity of the actual tube formers remains unknown. A
significant observation of the Cloudina fossils is that many of them are
perforated by borings. These borings provide the first clear evidence of
predation before the Cambrian.
It
is clear from the above discussion of the latest Precambrian, that the Cambrian
explosion did not occur in a biological vacuum. Although many of the fossil
specimens are enigmatic and difficult to classify, they nonetheless show
significant biological diversity. Furthermore, at least a few living phyla had
already appeared by the beginning of the Cambrian, and other forms likely
represented stem groups related to later-evolving phyla.
Ground Zero: What were the Cambrian animals like?
One
of the most important features of the Cambrian "explosion" was the
very rapid diversification of organisms with shells, plates, and various other
types of hard parts. A wide variety of soft-bodied organisms are also known
from the Cambrian. Although some fossils can be assigned to living phyla, there
are also specimens that appear to represent stem groups or intermediates
between modern phyla, as well as specimens of unknown relationship.
Representatives of several living classes and other lower taxonomic categories
also appear in the Cambrian. A few deposits with exceptionally good
preservation of fossils, such as the Burgess Shale in Canada, contribute to the
wide range of taxa known from the Cambrian. Such deposits with exceptional
preservation are known as Konservat-Lagerstätten (from the German
"conservation deposits"). Similar deposits have since been found
around the world in the Early to Middle Cambrian, notably the Early Cambrian
Chengjiang fauna of China. Additionally, trace fossils become much more varied,
complex, and abundant in the Cambrian, suggesting a newly widened range of
animal activity.
Some
of the very first fossils to appear near the base of the Cambrian are tiny
skeletal plates, spines, tubes, and cap-shaped shells that have been called the
"small shelly fossils."32 Among
these are the spicules of different groups of sponges, and the shells of the
earliest known "crown group" mollusks and brachiopods. However, the
biological identities of many of these tiny skeletal elements were completely unknown
until fairly recently. Well-preserved complete fossils in the Chengjiang, and
other fossil lagerstätten around the world, have revealed that some of these
small shelly fossils were actually the spines and "armoring" of
larger metazoans. More detailed analysis of other fossils has revealed that
they may represent the stem groups of living phyla, rather than evolutionary
dead ends.
The
discovery of complete specimens from later in the early Cambrian has revealed
that a variety of scales, plates and spines found among the small shelly
fossils actually fit together and overlapped to cover the bodies of slug-like
organisms.33
These organisms are the
halkieriids and wiwaxiids. The halkieriids bore conical mollusk-like shells as
well as calcareous structures similar to the chitinous bristles typical of
polychaete annelid worms. The slightly younger Wiwaxia was covered in
scale-like and spine-like structures even closer to those of the polychaetes,
and also possessed a radula diagnostic of mollusks. These various unusual
organisms bear resemblances to both mollusks and polychaete annelid worms,
which are closely related phyla. Thus these organisms would appear to be
positioned somewhere on the evolutionary tree near the branching point of the
mollusks with the annelids.
Other
cap-shaped fossils from the earliest Cambrian are the helcionelloids. These are
interpreted as monoplacophoran-like crown group mollusks. As discussed earlier
in the section on "Classifying Animals", there is good fossil
evidence of the transition from these primitive cap-shaped helcionelloids to
the first bivalves. There are also likely fossil transitions from helcionelloids
to the first gastropods.
Another
important group of organisms represented by small plates in the early Cambrian
are the lobopods. Lobopodians, until very recently an enigmatic group of
strange fossils, were "caterpillar-like" organisms with fleshy lobed
limbs and mineralized plates or spines running along their backs. They are
similar to the living Onychophora, or velvet worms, but are considered a
distinct group.34
The oldest known
lobopodian bears certain similarities to a distinctive group of worms called
the palaeoscolecid priapulids that also bore small plates or tubercles along
their bodies.35
Lobopods may have been
derived from these worms that also have an early Cambrian fossil record. Furthermore,
the lobopods have become recognized as the critical link in reconstructing the
assembly of the arthropod body plan. They have anatomical features in common
with the arthropods, particularly with peculiar Cambrian stem arthropods such
as Opabinia and Anomalocaris that are preserved in the younger Chengjiang and
Burgess fossil beds. These later organisms possessed lobopod limbs but also had
gill flaps along their bodies and jointed feeding appendages. Intermediates
between lobopodians and the early stem group arthropods have also been
discovered that possessed gills.36 Of
even greater interest is the evidence available from the extraordinary
preservation of muscle tissue in a few of these transitional organisms. These
specimens suggest a progression of steps in the transformation of internal
anatomy from lobopodians to true arthropods.37
The
tommotiids, a group of roughly conical-shaped shells composed of calcium
phosphate, have until recently been one of the most enigmatic of the small
shelly fossils. However, new discoveries of articulated specimens have shown
that pairs of symmetrical skeletal elements fit together to form an open cone
that was attached to the seafloor at the base. An opening at the base indicates
the presence of a muscular attachment structure likely similar to the pedicle
of brachiopods. The paired shells also have features similar to the tiny
paterinids, crown group brachipods with calcium phosphate shells that also
appear in the early Cambrian.38 These
fossils therefore appear to represent stem brachiopods that were themselves
derived from armored tubular filter feeders attached to the seafloor.
Following
the appearance of the small shelly fossils, the diverse metazoan fossil
communities of the Chengjiang in China are dated at around 525-520 million
years, 20 million years after the beginning of the Cambrian. The exceptional
preservation in these fossil beds is similar to that of the Burgess Shale
deposits that are dated around 515-505 million years. These extraordinary
fossil sites give us our best views into the composition of marine biological communities
from this time, preserving both soft-bodied organisms and those with
mineralized skeletons.39
These beds contain
abundant and diverse sponges and cnidarians, as well as priapulid worms,
annelid worms, lobopods, stem mollusks such as Wiwaxia, and brachiopods.
However, probably the most dramatic characteristic of the Chengjiang and
Burgess type deposits is the abundance and diversity of arthropods.
Arthropods
comprise 50% or more of all of the fossil specimens collected from these beds.
These fossils include stem arthropods such as the anomalocarids, trilobites
which came to dominate the Paleozoic, and some species that appear to be
crustaceans and chelicerates. However, most of the fossils belong to primitive
stem groups that likely represent evolutionary experimentations after the
appearance of true arthropods but before the rise of most living arthropod
groups. In the Burgess Shale one such primitive species (Marrella) alone
comprises a third of all fossil specimens. These fossils show unusual
arrangements, and types, of appendages.
The
chordates (that include vertebrates), hemichordates (that include the living
"acorn worms"), and echinoderms (that include the living starfish and
echinoids) are all deuterostomes and have the same pattern of early embryo
development. Although the modern representatives of these phyla appear
extremely different, they are actually closely-related branches on the tree of
life, and are understood to have evolved from a common ancestor. Some rare, but
very significant, specimens in the Chengjiang seem to be stem chordates and
stem echinoderms, as well as specimens that have been interpreted as organisms
close to the common ancestors of chordates and echinoderms. These rather simple
Cambrian organisms possess the anatomical characteristics that would be
expected in organisms that had acquired some but not all of the distinctive
features of chordates or echinoderms.
A very primitive stem group of deuterostomes,
called ventulicolians, has recently been described that might represent the
anatomy of organisms near the base of the deuterostome evolutionary branch that
were ancestral to both the
chordates and echinoderms. These soft-bodied organisms possessed segmentation
and oval structures interpreted as gill slits, and a terminal mouth.
Significantly, another group of primitive deuterostomes, called vetulocystids,
bears similarities to the ventulicolians as well as to some of the bizarre
early echinoderms.40
These organisms were
likely anchored to the sediment and possessed an echinoderm-like mouth and
respiratory openings.41
They may in fact
represent organisms ancestral to the first echinoderms that were characterized
by peculiar globular and asymmetrical shapes.
The
most primitive group of chordates are the urochordates, or tunicates, that have
a sack-like adult body that filters seawater through pharyngeal slits. In their
tadpole-like larval form, they possess stiff notochords (a structure diagnostic
of chordates) that is lost in the adult form. A likely tunicate has been
described from the Chengjiang.42 Another
group of primitive chordates are the cephalochordates (represented today by the
lancelets) that possess a notochord as adults, pharyngeal slits, and muscles
arranged in parallel bundles. Some fossils have been interpreted as stem
cephalochordates.43
Lastly, and of
particular interest, is a fossil that may be a stem vertebrate.44 Haikouichthys, in addition to a
notochord, gill pouches and muscle bundles, also appears to have had some
structures characteristic of vertebrates. These vertebrate features include a
cavity surrounding the heart, a dorsal fin, and cartilage around the head and
as a series of elements along the notochord. The Chengjiang thus includes
fossil specimens that occupy several significant transitional positions from
primitive deuterostomes, to stem echinoderms and stem chordates.
The
fossils of the Cambrian explosion were indeed diverse and included organisms
that can be assigned to a number of living phyla. As we have seen, these fossil
organisms were also largely representative of stem groups that possessed some,
but not all, of the diagnostic features that define the major groups of living
organisms. The body plans of phyla were assembled piecemeal. Furthermore,
important transitional steps between living phyla and their common ancestors
are also preserved. These include: the rise mollusks from their common ancestor
with the annelids, the evolution of arthropods from lobopods, the likely
evolution of brachiopods from tommotids, and the rise of chordates and
echinoderms from early deuterostomes. While the picture is far from complete,
the spectacular fossil discoveries from the early and middle Cambrian strongly
support the conclusion that the major branches of the animal tree of life are
joined to a common metazoan trunk.
Possible Causes of the Cambrian Radiation: What lit the
fuse?
Numerous
hypotheses exist for the geologically rapid diversification of invertebrates in
the Cambrian, proposing various key evolutionary innovations or environmental
triggers. Critical levels of ecological or behavioral complexity may also have
stimulated diversification. At the molecular level, organisms may have reached
a key threshold of genetic organization or evolved a key gene.
A
number of important environmental changes occurred in the late Precambrian and
in the early Cambrian that likely had important consequences for the early
evolution of metazoans. Near the end of the Precambrian there were several
episodes of nearly global glaciation in which sea ice and continental
glaciation extended to the equatorial regions. The last of these "snowball
Earth" episodes was about 635 million years ago.45 This time just precedes the earliest
fossil evidence of metazoans. The major changes in ocean temperature and
chemistry associated with the transition from a snowball Earth to a greenhouse
world would likely have had profound effects on life. In particular, isotopic
data indicates that the oceans became increasingly oxygenated after the end of
the last of the global glaciations.46 Higher
oxygen levels would have been critical for aerobic respiration and the evolution
of larger body sizes.
The
advent of mineralized hard parts was an important part of the Cambrian
"explosion." The ability of organisms to secrete hard parts had
important consequences for both metazoan evolution, and for the preservability
of these organisms in the fossil record. Much of the rapid increase in fossil
diversity during the early Cambrian is among organisms with resistant hard
parts. Changes in seawater chemistry may have played an important role in
permitting or stimulating mineral precipitation by marine organisms. With the
right concentrations of certain ions, normal physiological processes, such as
respiration or photosynthesis, may cause precipitation. Such biomineralization
could then be modified through natural selection. In addition, hard parts
represent a handy way to store useful ions, or remove toxic ones. Carbonate and
phosphate ions, present in most skeletons, are also good buffers against pH
changes. Recent work on seawater chemistry during the latest Proterozoic and
early Cambrian has indicated a major change in calcium ion concentrations
between 544 and 515 million years.47 This
time interval coincides with the onset of widespread biomineralization in the
fossil record.
The
rise of hard parts would likely have had important behavioral consequences.
Hard skeletons provide firm attachments for muscles, enabling various
activities and motions not otherwise possible, and skeletons would have helped
to support larger and more complex bodies. Hard parts also would provide a
protective armor against predators, and evidence for predation is found almost
as early as the first skeletal elements appear in the fossil record.
Predator-prey interactions seem particularly effective at producing an
evolutionary escalation, with the prey evolving defenses and the predator
evolving ways to overcome them. Animals with mineralized armor would promote
selection for harder jaws and claws in the predators. The more effective
predators would in turn increase selective pressure for more resistant
skeletons in the prey.
Changes
in animal behavior can also change the physical environment. A major
environmental change in the early Cambrian came as a result of increased
complexity and intensity of bioturbation (burrowing, digging, or other moving
and mixing of the sediment by organisms). Burrowing can be a response to
escaping predation or seeking out food resources. These evolving behaviors also
disrupted the existing seafloor habitat. For much of the Precambrian and into
the early Cambrian, microbial and algal mats largely covered the seafloor.
These mats provided a stable base for sessile animals and kept mud out of the
water, making it easy for filter feeders to obtain relatively high amounts of
food and low amounts of sediment. The advent of algal grazers, extensive
burrowing and other bioturbation disrupted these mats. This created problems
for animals adapted to the old seafloor pattern, but provided a new habitat of
muddy seafloors.48
Additionally, the
constant burrowing unearthed buried nutrients, making them accessible to
animals at the surface of the sediment.
Available
food resources and ecological roles were also altered with the appearance of
planktonic or swimming metazoans in the early Cambrian. Prior to the Early
Cambrian, there is no evidence for macroscopic zooplankton or swimming animals.
However, in the Cambrian several actively swimming, plankton-feeding animals
appeared. At the same time, many kinds of planktonic algae became extinct and
the surviving forms were much smaller. Evolution of swimming and
plankton-feeding ability leads to the diversification of plankton feeders, but
it also affects the bottom-dwelling organisms.49 Both the fecal material and the carcasses of these
animals would have fallen to the bottom, moving large quantities of nutrients
from the water column, where they were previously inaccessible to animals, to
the sea floor. Even today, most of the nutrients in the deep sea come from
these sources.
This
brief survey of possible factors in the Cambrian explosion illustrates how
ocean chemistry, environment, ecology and animal behavior are complexly
intertwined. Complex positive and negative feedbacks make it very difficult to
tease out which factor was most critical to the rapid diversification of metazoan
life at the end of the Precambrian and early Cambrian. However, evidence from
multiple sources strongly suggests that several significant changes in the world’s
ocean environment conspired to light the fuse of evolutionary innovation.
Conclusions
Given
our current and continually growing knowledge of the deep past, it is
increasingly clear that the rise of multicellular animals is not an
impenetrable mystery. While there is much that is not known, and will never be
known, there is also much that has been discovered, and much excitement for
what will yet be learned. The animals of the Cambrian did not appear in all
their modern complexity out of a void, but rather provide pointers to their
common ancestry. Despite the claims of evolution skeptics, the fossil record
provides multiple examples of organisms displaying transitional anatomies. The
anatomical characters that define the body plans of the major living animal
phyla, can be seen to have been acquired piecemeal during the early evolution
of the metazoa. Just as with all other taxonomic groups (e.g. classes, orders,
families, genera, species), the divisions between phyla break down as we move
closer to their times of origin from common ancestors. The tree of life
continues to stand tall.
Notes
1.
Morris, J.D., 2008, The Burgess shale and complex life, Acts & Facts 37
(10): 13
2.
Meyer, S.C., M. Ross, P. Nelson, & P. Chien. 2003. The Cambrian explosion:
biology's big bang. Pp. 323-402 in J. A. Campbell & S. C. Meyer,
eds., Darwinism, Design and Public Education: Michigan State University Press,
Lansing, p. 326.
3.
Meyer, S.C., M. Ross, P. Nelson, & P. Chien. 2003. The Cambrian explosion:
biology's big bang. Pp. 323-402 in J. A. Campbell & S. C. Meyer,
eds., Darwinism, Design and Public Education: Michigan State University Press,
Lansing, p. 333.
4.
Rozanov, A.Y., 1984, "The Precambrian-Cambrian boundary in Siberia,"
Episodes 7: 20-24. Rozanov, A.Y., and A.Y. Zhuravlev, 1992, "The Lower
Cambrian fossil record of the Soviet Union," IN J.H. Lipps and P.W. Signor
(eds.), Origin and Early Evolution of the Metazoa: Plenum, New York, p.205-282,
5.
Bowring, S.A,, J.P. Grotzinger, C.E. Isachsen, A.H. Knoll, S.M. Pelechaty, and
P. Kolosov, 1993, "Calibrating rates of Early Cambrian evolution,"
Science 261: 1293-1298.
6.
Gradstein, F.M., J.G.Ogg, A.G. Smith, et. al., 2004. A Geologic Time Scale
2004. Cambridge University Press.
7.
Amthor, J. E.; J.P. Grotzinger,; S. Schröder, S.A. Bowring, J. Ramezani, M.W.
Martin, and A. Matter, 2003, "Extinction of Cloudina and Namacalathus at
the Precambrian-Cambrian boundary in Oman". Geology 31: 431–434.
8.
Landing, E., A. English,and J.D. Keppie, 2010, "Cambrian origin of all
skeletonized metazoan phyla - Discovery of Earth’s oldest bryozoans (Upper
Cambrian, southern Mexico)," Geology 38: 547-550.
9.
See the section "Before the explosion: What went bang?" below for
details.
10.
See the discussion in the chapter "The Nature of Phyla" in Valentine
J.W., 2004, On the Origin of Phyla, Univ. of Chicago Press. Also see Miller,
K.B., 2003, "Common descent, transitional forms, and the fossil
record," IN, K.B. Miller (ed.), Perspectives on an Evolving Crreation, Wm.
B. Eerdmans, Grand Rapids.
11.
Budd, G.E. and S. Jensen, 2000, "A critical reappraisal of the fossil
record of the bilaterian phyla," Biological Reviews 75: 253- 295. Conway
Morris, S., 2000, "The Cambrian ‘explosion’: Slow-fuse or
megatonnage?", Proceedings of the National Academy of Science 97(9):
4426-4429.
12.
Morris, J. 2008. The Burgess Shale and Complex Life. Acts & Facts. 37 (10):
13.
13.
Gubanov, A.P., , A. V. Kouchinsky, and J. S. Peel,1999, "The first
evolutionary-adaptive lineage within fossil molluscs," Lethaia 32:
155-157. Kouchinsky, A.V., 1999, "Shell microstructures of the Early
Cambrian Anabarella and Watsonella as new evidence on the origin of the
Rostroconchia," Lethaia 32: 173-180.
14.
Meyer, S.C., M. Ross, P. Nelson, & P. Chien. 2003. The Cambrian explosion:
biology's big bang. Pp. 323-402 in J. A. Campbell & S. C. Meyer,
eds., Darwinism, Design and Public Education: Michigan State University Press,
Lansing, p. 346.
15.
See the discussion in the chapter "The Nature of Phyla" in Valentine
J.W., 2004, On the Origin of Phyla, Univ. of Chicago Press.
16.
A more expanded discussion of this topic can be found in Miller, K.B., 2003,
"Common descent, transitional forms, and the fossil record," IN, K.B.
Miller (ed.), Perspectives on an Evolving Crreation, Wm. B. Eerdmans, Grand
Rapids.
17.
C. Darwin, 1872, On the Origin of Species by Means of Natural Selection, 6th
ed., p 234-255.
18.
Summaries of the early fossil record of life can be found in Schopf, J.W.
(ed,), 1983, Earth’s Early Biosphere: Its Origin and Evolution, Princeton
University Press; and Knoll, A.H., 2003, Life on a Young Planet: The First
Three Billion Years of Evolution on Earth, Princeton University Press. During
the writing of this essay, a new fossil discovery from Australia has indicated
the presence of possible sponge-grade metazoans in rocks 640-650 million years
ago. See Maloof, A.C., et al., 2010, "Possible animal-body fossils in
pre-Marinoan limestones from South Australia," Nature Geoscience
doi:10.1038/ngeo934.
19.
Li, C-W., J-Y Chen, and T-E Hua, 1998, "Precambrian sponges with cellular
structures," Science 279: 879-882. Xiao, S, X. Yuan, and A.H. Knoll, 2000,
"Eumetazoan fossils in terminal Proterozoic phosphorites?,"
Proceedings of the National Academy of Science 97(25): 13684-13689.
20.
J-Y Chen, et al., 2000, "Precambrian animal diversity: Putative
phosphatized embryos from the Doushanto Formation of China," Proceedings
of the National Academy of Science 97 (9): 4457-4462. Xiao, S., and A.H, Knoll,
2000, "Phosphatized animal embryos from the Neoproterozoic Doushantuo
Formation at Weng’an, Guizhou, south China," Journal of Paleontology 74
(5): 767-788. J-Y Chen et al., 2006, "Phosphatized polar lobe-forming
embryos from the Precambrian of southwest China," Science 312: 163-165.
Xiao, S., J.W. Hagadorn, C. Zhou, and X. Yuan, 2007, "Rare helical
spheroidal fossils from the Doushantuo lagerstatte: Ediacaran animal embryos
come of age?," Geology 35 (2): 115-118.
21.
Seilacher, A., 1999, "Biomat-related lifestyles in the Precambrian,"
Palaios 14: 86-93.
22.
Fedonkin, M.A., 1992, "Vendian faunas and the early evolution of
metazoa," IN, J.H. Lipps and P.W. Signor (eds.), Origin and Early
Evolution of the Metazoa, Plenum Press, New York, p.87-129. Jenkins, R.J.F.,
1992, "Functional and ecological aspects of Ediacaran assemblages,"
IN, J.H. Lipps and P.W. Signor (eds.), Origin and Early Evolution of the
Metazoa, Plenum Press, New York, p.131-176..
23.
Gehling, J.G., 1988, "A cnidarian of actinian-grade from the Ediacaran
Pound Subgroup, South Australia," Alcheringa 12:299- 314.
24.
Gehling, J. G., and Rigby, K., 1996, "Long expected sponges from the
Neoproterozoic Ediacara fauna of South Australia," Journal of Paleontology
70(2): 185-195.
25.
Gehling, J.G., 1987, "Earliest known echinoderm -- a new Ediacaran fossil
from the Pound Subgroup of South Australia," Alcheringa 11:337-345.
26.
Narbbonne, G.M., M. Laflamme, C. Greentree, and P. Trusler, 2009,
"Reconstructing a lost world: Ediacaran rangeomorphs from Spaniard’s Bay,
Newfoundland," Journal of Paleontology 83(4): 503-523.
27.
Conway Morris, S., 1993, "Ediacaran-like fossils in Cambrian Burgess
Shale-type faunas of North America," Palaeontology 36 (3): 593-635.
28.
Dzik, J., 2003, "Anatomical information content in the Ediacaran fossils
and their possible zoological affinities," Integrative and Comparative
Biology 43: 114-126. Fedonkin, M.A., 2003, "The origin of the Metazoa in
light of the Proterozoic fossil record," Paleontological Research 7(1):
9-41.
29.
Fedonkin, M.A., and Waggoner, B.M., 1997, "The later Precambrian fossil
Kimberella is a mollusc-like bilaterian organism," Nature 388:868-871.
30.
Crimes, T.P., 1992, "The record of trace fossils across the
Proterozoic-Cambrian boundary," IN, J.H. Lipps and P.W. Signor (eds.),
Origin and Early Evolution of the Metazoa, Plenum Press, New York, p.177-202.
Zhu, M., 1997, "Precambrian- Cambrian trace fossils from eastern Yunnan,
China: Implications for Cambrian explosion," IN Junyuan Chen, Yen-nien
Cheng, and H.V. Iten (eds.), The Cambrian Explosion and the Fossil
Record, Bulletin of the National Museum of Natural Science No. 10 (Taichung,
Taiwan, China), p. 275-312.
31.
Chen, Z., S. Bengtson, C-M. Zhou, H. Hua, and Z. Yue., 2008, Tube structure and
original composition of Sinotubulities: Shelly fossils from the late
Neoproterozoic in southern Shaanxi, China, Lethaia 41: 37-45. Hofmann, H.J.,
and E.W. Mountjoy, 2001, Namacalathus-Cloudina assemblage in Neoproterozoic
Miette Group (Byng Formation), British Columbia: Canada’s oldest shelly
fossils, Geology 29: 1091-1094. Grotzinger, J.P., W.A. Watters, and A.H. Knoll,
2000, Calcified metazoans in thrombolite-stromatolite reefs of the terminal
Proterozoic Nama Group, Namibia, Paleobiology 26(3): 334-359.
32.
For detailed descriptions of the variety of small shelly fossils see: Rozanov,
A.Y., and A.Y. Zhuravlev, 1992, "The Lower Cambrian fossil record of the
Soviet Union" (p.205-282), and Jiang, Z-W., "The Lower Cambrian
fossil record of China" (p.311 -333), IN J.H. Lipps and P.W. Signor
(eds.), 1992, Origin and Early Evolution of the Metazoa: Plenum, New York,
33.
Dzik, J., 1993, "Early metazoan evolution and the meaning of its fossil
record," Evolutionary Biology 27:339-386. Conway- Morris, S., and J.S.
Peel, 1995, "Articulated halkieriids from the Lower Cambrian of north
Greeland and their role in early protostome evolution," Philosophical
Transactions of the Royal Society London B 347:305-358. See also Caron, J-B.,
A. Scheltema, C. Schander, and D. Rudkin, 2006, "A soft-bodied mollusc
with radula from the Middle Cambrian Burgess Shale," Nature 442: 159-163.
34.
Ramsköld, L., 1992, "Homologies in Cambrian Onychophora," Lethaia 25:
443-460. L. Ramsköld, L., and H. Xianguang, 1991, "New early Cambrian
animal and onychophoran affinities of enigmatic metazoans," Nature 351:
225-228.
35.
Liu, J., D.Shu, J. Han, Z. Zhang, and X. Zhang, 2008, "Origin,
diversification, and relationships of Cambrian lobopods," Gondwana
Research 14:277-283.
36.
Chen, J-Y., L. Ramsköld, and G-G. Zhou, 1994, "Evidence for monophyly and
arthropod affinity of Cambrian giant predators," Nature 264: 1304-1308. G.
E. Budd, G.E., 1996, "The morphology of Opabinia regalis and the
reconstruction of the arthropod stem group," Lethaia 29: 1-14.
37.
G. E. Budd, 1998, "Arthropod body-plan evolution in the Cambrian with an
example from anomalocaridid muscle," Lethaia 31: 197-210.
38.
Skovsted, C. B., G.A. Brock, J.R. Paterson, L.E. Holmer, and G.E. Budd, 2008,
"The scleritome of Eccentrotheca from the Lower Cambrian of South
Australia: lophophorate affinities and implications for tommotiid
phylogeny," Geology 36:171–174. Skovsted, C.B., et al., 2009, "The
scleritome of Paterimitra: an Early Cambrian stem group brachipod from South
Australia," Proceedings of the Royal Society B 276: 1651-1656.
39.
Excellent descriptions of these fossil communities can be found in the
following books: Briggs D., D. Erwin, and F. Collier, 1994, The Fossils of the
Burgess Shale (Washington: Smithsonian Institution Press). Conway Morris, S.,
1998, The Crucible of Creation: The Burgess Shale and the Rise of Animals (New
York: Oxford Univ. Press). Chen J., and G. Zhou, 1997, "Biology of the
Chengjiang Fauna," IN Junyuan Chen, Yen-nien Cheng, and H.V. Iten (eds.), The
Cambrian Explosion and the Fossil Record, Bulletin of the National Museum
of Natural Science No. 10 (Taichung, Taiwan, China), p.11-105.
40.
Shu, D-G., et al., 2001, "Primitive deuterstomes from the Chengjiang
Lagerstatte ( Lower Cambrian, China)," Nature 414: 419 -424.
41.
Shu, D-G., S. Conway Morris, J. Han, Z-F. Zhang, and J-N. Liu, 2002,
"Ancestral echinoderms from the Chengjiang deposits of China," Nature
430: 422-428.
42.
Shu, D-G., L. Chen, J. Han, and X-L. Zhang, 2001, "An early Cambrian
tunicate from China," Nature 411: 472-473.
43.
Chen, J-Y., J. Dzik, G.D. Edgecombe, L. Ramskold, and G-Q. Zhou, 1995, "A
possible early Cambrian chordate," Nature 377: 720-722. Chen, J-Y., D-Y
Huang, and C-W. Li, 1999, "An early Cambrian craniate-like chordate,"
Nature 402: 518-522.
44.
Shu, D-G, et al., 1999, "Lower Cambrian vertebrates from south
China," Nature 402: 42-46. Shu, D-G., et al., 2003, ""Head and
backbone of the early Cambrian vertebrate Haikouichthys," Nature 421:
526-529
45.
Hoffman, P.F. and D.P. Schrag, 2002, "The snowball Earth hypothesis: testing
the limits of global change," Terra Nova 14: 129 -155.
46.
Fike, D.A., J.P. Grotzinger, L.M. Pratt, and R.E. Summons, 2006,
"Oxidation of the Ediacaran ocean," Nature 444: 744-747.
47.
Brennan, S.T., T.K. Lowenstein, and J. Horita, 2004, "Seawater chemistry
and the advent of biocalcification," Geology 32: 473 -476.
48.
Bottjer, D., J. Hagadorn, and S. Dornbos, 2000, "The Cambrian Substrate
Revolution," GSA Today 10 (9): 1-7. Dornbos, S.Q. and D.J. Bottjer, 2000,
"Evolutionary paleoecology of the earliest echinoderms: Helicoplacoids and
the Cambrian substrate revolution," Geology 28: 839-842.
49.
Butterfield, N.J., 2001, "Ecology and evolution of Cambrian
plankton," IN, A.Y. Zhuralev and R. Riding (eds.), The Ecology of the
Cambrian Radiation (New York: Columbia University Press), p. 200-216.
* * * * * * * *
ADDENDUM
* * * * * * * *
To the Reader,
It should be noted that the three views presented above spoke pointedly of Earth's early biologic history from a (i) scientific viewpoint (Wikipedia), a (ii) theistic evolutionary viewpoint, and an (iii) evolutionary creationist viewpoint. I did not purposely intend this arrangement but when I look back upon each section it becomes readily apparent.
As such, each discussion approached the material according to its own philosophical perspective, each making a variety of suggestions as to what some of the irregularities may be within their own given contexts. No less would one expect to find similar anomalies in earlier, and later, evolutionary periods from a past so distant that it is remarkable we even know as much as we do eons-and-eons removed from the primordial events.
And what did each viewpoint demonstrate?
View (i) showed scientific anomalies within the Cambrian period; view (ii) attempted to reconcile known evolutionary knowledge with biblical passages; and view (iii) attempted an interaction with "Young Earth Creationists" (YEC) and "Intelligent Design" (ID) groups.
Moreover, each view's evolutionary description was arranged according to the audience it was written to - whether to scholarly groups, or to Christians trying to figure out evolution's relationship to the Genesis account, or to non-evolutionary Christians who think the whole thing is bunk and work feverishly to disprove it by jot-and-by-tittle, because if they were to accept it, it would change their belief structure (an example of this is Stephen C. Meyer's book, Darwin's Doubt).
Which then answers the question posed in the title of this posting, "Why all the fuss over Earth's remarkable Cambrian Explosion?" For the scholars in group (i) it is a delightful problem to work out by conjecture and by future discoveries. They are not unsettled by the anomalies but work all the harder to discern Earth's ancient past by trying to ask the right questions. For the theistic evolutionists in group (ii) they too are attempting to work within the limitations of their Bible-centered professions, most of whom believing God to have spoken to the ancient writers of Genesis in a prophetic fashion ahead of today's more modern scientific era. While the evolutionary creationists of group (iii) jettison any hope of finding today's science in the Genesis records by flatly accepting evolution as the most proper description of our origins. And then moving their attention to the actual problem at hand which is how to read yesterday's Bible in today's more distant cultures using a variety of hermeneutical approaches.
What are these? To read some biblical passages in a non-literal fashion following the literary cues that lay within the text itself: is the passage a narrative account (some call these historical "myths,' which I purposely shun because of the misleading connotations that this description brings with it however apt that it seems). Is the text a psalm, a poem, a parable? Are the Bible's historical passages a composite mix of narrative accounts each compounded by that era's earlier assemblage of salvific understanding within their given context? Naturally one would then ask how that understanding changed as it progressed from individual perceptions, to family contexts, to tribal and national legacies. There then is the additional problem of how the text was read existentially and phenomenologically by each receiving era as each generation progressed further and further away from the actuating moment. What did a pile of rocks, a lamp, a priest, a house, a road, or an event, mean to the original recipients about God (Abraham, Moses, Joshua, David, the prophets, Jesus, the disciples) as versus what that same pile of rocks et. al. might later mean to a more nuanced society thinking through the larger glens of its salvific heritage. There is also the idea of enculturation to take into account... how did people in their societal contexts change towards their understanding of a biblical event, or a narrative story, from one generation to the next, as their regional contexts changed by world event, time, and distance?
So interpreting the Bible by flatly stating a literal, inerrant reading seems boxed-in and quite naïve as a position that an earnest Christian might make. It flies-in-the-face-of-facts as it were. But the good news is that we get a more interesting Bible that can have a much larger, more timeless meaning to global cultures quite distant from the biblical event. One that forces each succeeding generation towards its own due diligence if God's Word is to be relevant to humanity's needs. As a Brazilian met by slavery and poverty a Liberation Gospel might proceed. As an African immersed in animism and oppression a totalitarian gospel might arise. From America's wealth and secularism an anti-secular, postmodern gospel might proceed. From Europe's blended mix of dissimilar cultures and steady beat towards socialism perhaps a Radical Theology rises forth. But to expect to read the Bible from an ancient Greek mindset as the early Church Fathers once did; or from a Medieval Scholastic viewpoint as the Catholic church did in Aquinas' day; or even as an Enlightened Reformational record as today's Evangelicalism attempts to create, makes the Bible socially irrelevant and quite foreign.
But by using the relevant examples that science and evolution afford shows quite clearly how the Christian faith must adapt its ancient faith and doctrines towards today's newer skepticisms and disbelief. We misunderstand God if we think that His missional role doesn't change from one generation to the next. We make of Him a lesser god than the great God that He is by showing our disbelief by not expanding His Word to meet society's many needs and searching questions. It becomes one of religious pride and hubris instead of humble submission. The Pharisees in Jesus' day didn't like Jesus challenging their doctrines no less than we do today.... However, we must show faith that God knows what He is doing and need not be troubled when all that we thought we knew about God may change. Like the Bereans of the NT, we set about our tasks to discern, and to judge, the voices of today's newest Apostles of Jesus. But when all has been discussed and proved we move forthwith heralding an ancient faith tradition that is willing to change when deemed necessary. Thank you for your consideration.
R.E. Slater
September 20, 2013
Index to past discussions -
Index to past articles on "Science & Religion"
