Evolution Basics: The Cambrian Diversification and Assembling Animal Body Plans, Part 1

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September 12, 2013 Tags: Genetics, History of Life

Today's entry was written by Dennis Venema. You can read more about what we believe here.

Evolution Basics: The Cambrian Diversification and Assembling Animal Body Plans, Part 1
"Mobius" Modern hanging mobile art sculpture by Julie Frith

Note: This series of posts is intended as a basic introduction to the science of evolution for non-specialists. You can see the introduction to this series here. In this post we introduce the Cambrian diversification and discuss how the step-by-step origin of modern-day “body plans” makes testable predictions about patterns in the fossil record.

The Cambrian “Explosion” 

In the last post in this series, we discussed the evidence for two ancient endosymbiosis events (leading to mitochondria and chloroplasts) that had a profound impact on the subsequent evolution of eukaryotic diversity. A second event that would profoundly shape the future of animal life on earth was the dramatic diversification of animal groups during the Cambrian, a period stretching from 542 million years ago until 488 million years ago.

What is especially intriguing about the Cambrian period is that it represents the first fossil record for many animal groups that bear a discernible resemblance to animals that still exist today. “Discernible” of course does not mean that what we see in the Cambrian is familiar – Cambrian fauna are markedly different from present-day organisms – but rather the first appearance of traits in the fossil record that we recognize as characteristic of groups of related organisms we observe today. In other words, certain traits we observe in the Cambrian are familiar, though the combination of traits we observe in Cambrian animals is often different from those seen in modern groups. Nonetheless, there is an interest in determining when the groups we see in the present first arose – the first “arthropods,” or “vertebrates,” for example.

If you’re wondering if this introduces something of a “present-day bias” to studying the fossil record, you’re correct – effectively, scientists are using the characteristics of present day organisms to attempt to place extinct organisms into groups of relatedness. Before we see how this plays out in studies of the Cambrian, however, we’ll have to explore a few deeper concepts about phylogenies than we have examined thus far.

Evolution and taxonomy

Biologists have been trying to do taxonomy – i.e. group organisms into logical categories – since the time of Linnaeus in the 1700s. Given the explanatory power of evolutionary theory and its current place as a foundational theory in biology, this practice now attempts to group organisms by their evolutionary relatedness. In this approach, the most logical classifications are said to be monophyletic – a technical term that simply means consisting of a common ancestral population and all of its descendant species. An easy way to recognize a monophyletic group is to imagine a phylogeny as a mobile – a monophyletic group can be “snipped off” the mobile with only one cut. Any other type of grouping would require two or more cuts to be made. For example, for the following phylogeny, a grouping of A, B and C is monophyletic, but a grouping of B, C, and D is not:


One cut (the blue line) removes a “monophyletic” group from a phylogeny. The science of classifying organisms (taxonomy) is an attempt to organize species into monophyletic groups.


The grouping of B, C, and D is not monophyletic, since it requires at least two “cuts” to remove it from its phylogeny. The correct taxonomic group that encompasses (B, C and D) also must include A if it is to be monophyletic.

Using monophyletic groups as the basis for taxonomy comes with challenges, however. One challenge arises when we apply the natural tendency for using combinations of traits found in present-day organisms as the basis for classifying all organisms throughout evolutionary history. Let’s work through an example to see what the issues are.

Will the first “real” arthropod please stand up?

Arthropods are a highly diverse and successful group of organisms that include present-day insects, crustaceans and arachnids (i.e. spiders and scorpions), among others. The evidence also points to arthropods being a monophyletic group. All living arthropods have a suite of defining characteristics such as a hard external skeleton (exoskeleton), specialized body segments, and specialized appendages. While these characteristics are useful for defining modern arthropods, these criteria become less useful as we travel back through the evolutionary history of arthropods. The reason is simple – from an evolutionary point of view, one would not expect these different traits to arise as a unit in one fell swoop. Rather, one would expect that these traits would arise over time in the lineage leading to modern day arthropods. If so, and if populations were diverging away from the arthropod lineage to form species as these traits were being acquired, we would expect to find species in the fossil record that do not have the full suite of “arthropod” characteristics, but only some:

For example, based on the above phylogeny we might expect to find two groups of “arthropod-like” organisms in the fossil record:  species that have only (1) of the three traits (specialized appendages only), as well as a second group (2) with specialized appendages and segments. If such species (or groups of species) existed, it would simultaneously provide information on how the characteristic suite of arthropod features was acquired over time, and blur the distinction between arthropods and other forms of life. Indeed, these species would represent “transitional forms” in the sense that they have intermediate sets of characteristic features that indicate the steps the arthropod lineage took to achieve the “modern” suite of characteristics.

In other words, the taxonomic group “arthropods” is somewhat of an arbitrary classification, since we are choosing to “cut off” a monophyletic group at a particular location on the phylogeny when it would also be just as appropriate to cut if off at a different location further back in time and include more species (or later in its history, and include less).

While understanding the evolutionary history of a monophyletic group does not lend itself to black-and-white, either/or types of taxonomic classification, it is very useful for determining how complex body plans arose in a step-by-step fashion. In our next post, we’ll explore this idea further by examining some Cambrian animals in more detail.

For further reading

Budd, G.E. (2008). The earliest fossil record of the animals and its significance Phil. Trans. R. Soc. B  363, doi: 10.1098/rstb.2007.2232

Budd, G.E. and Telford, M.J. (2009). The origin and evolution of arthropods. Nature 457, 812-817 doi:10.1038/nature07890

 


Dennis Venema is professor of biology at Trinity Western University in Langley, British Columbia. He holds a B.Sc. (with Honors) from the University of British Columbia (1996), and received his Ph.D. from the University of British Columbia in 2003. His research is focused on the genetics of pattern formation and signaling using the common fruit fly Drosophila melanogaster as a model organism. Dennis is a gifted thinker and writer on matters of science and faith, but also an award-winning biology teacher—he won the 2008 College Biology Teaching Award from the National Association of Biology Teachers. He and his family enjoy numerous outdoor activities that the Canadian Pacific coast region has to offer. Dennis writes regularly for the BioLogos Forum about the biological evidence for evolution.

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glsi - #82732

September 28th 2013

Although not from the Cambrian, I found this short video rather humorous and not unlike the diagrams that Dr. Venema has drawn above.

 http://fr.news.yahoo.com/video/un-poisson-fossile-découvert-en-193716127.html

Never mind that it’s in French, just watch the little video which illuminates the state of evolutionary “science”.  Just the other day it seemed so certain we were all descended from a cartilaginoid shark. Whoops, then somebody found a bony fish even older than the shark!  No problem!  Just redraw the diagram bypassing the shark and meet your new great, great, great granddaddy! 


Dennis Venema - #82744

September 30th 2013

glsi, that video, judging from the diagrams, is very misleading - which is not uncommon for popular news videos discussing evolution, alas.

If you look at the last diagram in the post above, what the folks in the video are erroneously claiming is that the stem groups (1 and 2 above) are ancestors of the crown group - no so. I deal with this in a lot more detail in the next post that will run Thursday this week.


glsi - #82798

October 6th 2013

Dennis,

That’s another way of saying that your diagrams don’t match another biologist’s diagrams.  I really doubt that the “popular news” drew the relationships between the stem groups and the crown groups.  It’s that these things are highly subjective and the lines are easily redrawn based on new discoveries or simply the perspective or opinion of the one claiming to know.  To watch these things change over time is to discover for oneself that evolutionary science is truly just a theory.


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