The Cambrian “Explosion”, Part 2

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December 11, 2010 Tags: History of Life

Today's entry was written by Keith Miller. Please note the views expressed here are those of the author, not necessarily of The BioLogos Foundation. You can read more about what we believe here.

The Cambrian “Explosion”, Part 2

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.1

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" in the scientific literature (see figure above). 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.2

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?”3

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 (fig.1) 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 (fig. 2) and the cap-shaped helcionelloids (fig. 3). 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.3 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.”4

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.5

Notes

1. 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.
2. 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.
3. Morris, J. 2008. The Burgess Shale and Complex Life. Acts & Facts. 37 (10): 13.
4. 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.
5. 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.


Keith Miller is research assistant professor of geology at Kansas State University in the United States. He is editor of Perspectives on an Evolving Creation (Eerdmans, 2003), an anthology of essays by prominent evangelical Christian scientists who accept theistic evolution. He is also a member of the executive committee of the American Scientific Affiliation, an association of Christians in the sciences, and a board member of Kansas Citizens for Science, a not-for-profit educational organization that promotes a better understanding of science.

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sy - #43442

December 11th 2010

Dr. Miller

Thank you for tis highly informative artcle on cladistics, a field which I have always found difficult. I am curious as to whether you would agree that genetic analysis has the potential to render a good deal of classical taxonomy obsolete. I am sure you dealt with this question before, and I know that in many cases good DNA is not available. Still, would you agree that body plans and other structural elements aside, DNA sequence analysis can always provide the gold standard for classification?


pds - #43636

December 13th 2010

“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.”

This is simply not true.  There is no reason that all the phylum level differences needed to appear in the Cambrian explosion.  That fact is not an artifact of how we classify species. 

We could have seen a pattern of simple body plans turning into more complex body plans slowly over time.  That is what Darwinism would predict.  Instead, we see virtually all the body plans appearing in a 5-10 million year period, and then no new body plans appearing for the next 500 million years.


R Hampton - #43653

December 13th 2010

pds,
I have talked with numerous YEC and ID proponents who use the term to define macroevolution and from their point of view, the raptor/bird is a “new” (post-Cambrian explosion) body plan. How do you define “body plan”?


unapologetic catholic - #43672

December 13th 2010

” then no new body plans appearing for the next 500 million years.”

You mean besides plants, insects, fish, birds and tretrapods, just to name a few?


Robert Byers - #43688

December 14th 2010

There was no explosion. This is just a snapshot of the world before the flood. its just like in the Amazon today. Great diversity from rapid adaptation. Theres nothing impressive about this life. Its the natural way that diversity be great.
Anyways its all wrong. its unreasonable to claim confidence of the years ago these critters were frozen in sediment.
The geology is without foundation.
What if the geology was wrong?
How would conclusion on these fossils be forced to changed?
if greatly then admit its mostly a geological case here and only second biological.
Watch the math.


Paul D. - #43706

December 14th 2010

Robert Byers, the biological and genetic case for evolution is even more compelling than geology.


pds - #43741

December 14th 2010

R Hampton,

Here I am using “body plan” to mean more or less the same thing as “phylum.”

Unap. Cath.,

To clarify, virtually no new phyla appear later.  Do any of the examples that you mention appear after a slow, gradual build up demonstrated by a transitional series of forms in the fossil record?


pds - #43745

December 14th 2010

“The earliest known mollusk-like organism is Kimberella (fig.1) 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 relationship (if any) between Kimberella and later animals is not at all settled.  It is not considered “a stem mollusk” by many.  The general consensus is that the Ediacara creatures are a very different fauna from what comes later.  Some wonder if the Ediacara creatures are even part of the animal or plant kingdoms.


John - #43767

December 14th 2010

pds wrote:

“Here I am using “body plan” to mean more or less the same thing as “phylum.””

Because “body plan” is much fuzzier, and better facilitates the Gish Gallop.

Aren’t literalists who focus on phyla and body plans implicitly conceding the thing that most upsets them—common ancestry for humans and the rest of the great apes?


Argon - #43855

December 15th 2010

Robert Byers - #43688 “Great diversity from rapid adaptation.”

Some proponents of a Global Flood and Noah’s Ark seem to assume rapid diversification and speciation at greater rates than many biologists and yet don’t accept evolution.


R Hampton - #43871

December 15th 2010

pds,
I’m sure you know that complex multicellular life forms are not conducive to fossilization, and that the absence of such fossils prior to 600 mya is the basis for the concept of an explosive evolutionary event during the Cambrian. However, a recent discovery of a rich fossil bed more than 2 billlion years old in Gabon strongly suggests that “body plans” are much, much older than first thought (http://www.physorg.com/news197132216.html)

Even so, of the 36 phyla, Arthropoda itself accounts for most of the known animals species and together with Chordata, represents the vast majority of “common” animals. So it’s the great evolutionary diversity within these two body plans - not the total phylum - that prompt incredulity from IDers. For example, all mammals are members of Chordata, but what upsets many in the ID community is the notion that the whales evolved from four-legged carnivores despite sharing the same “body plan”.


Robert Byers - #44083

December 17th 2010

Argon.
Yes. It was this way. As long as it fits in genesis its okay. Evolution doesn’t. Biology is glorious and options can be beyond present imagination.


Paul D. - #44205

December 18th 2010

Argon:

“Some proponents of a Global Flood and Noah’s Ark seem to assume rapid diversification and speciation at greater rates than many biologists and yet don’t accept evolution.”

Indeed. If speciation has taken place at the rate some Creationists suggest (to allow for a limited animal manifest on the Ark), there ought to be little resemblance between today’s plant and animal life and the life that existed, say, at the time of Christ. However, the flora and fauna of the first century are mostly the same as those of today.


Keith Miller - #50632

February 9th 2011

Responding to Sy:

Cladistics was already displacing traditional Linnaean taxonomy that was based on the concept of ideal types, and “evolutionary taxonomy” that sought to group organisms that shared certain evolutionary adaptations, before genetic analysis was possible.  The power of cladistics is that the grouping of organisms is independent of evolutionary assumptions (and independent of the position of organisms in time) and thus can be used to test evolutionary hypotheses.  It also works for a wide range of different types of data—and is very well suited to the analysis of genomic data.  I don’t see either morphological or genetic data as taking precedence. They each bring something different to the reconstruction of the evolutionary tree.  With time, these two data sources are more and more converging on the same conclusions. 

Keith


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