Academic journal article Perspectives on Science and Christian Faith

The Fossil Record of the Cambrian "Explosion": Resolving the Tree of Life

Academic journal article Perspectives on Science and Christian Faith

The Fossil Record of the Cambrian "Explosion": Resolving the Tree of Life

Article excerpt

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 snails, crabs, or sea urchins) 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 Institute for Creation Research 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 [sic] 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? (2)

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. (3)

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. … 
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