Understanding the Cambrian Explosion Part 2 Definition


The term Cambrian explosion derives its name from a unique fossil record where we find a sudden appearance of animals during the Cambrian period of geologic time (541-485.4 million years ago).   Two of the best sites for this early fossil evidence are found in Chengjiang, China and the Burgess Shale Formation, located in the Canadian Rockies of British Columbia.  Both of these sites are highly regarded for their ability to preserve ancient soft body parts.

It is during the Cambrian period where we find at least 19 and as many as 35 (40 total) phyla having made their appearance on earth.  Phylum is the basic body plans or better said a grouping of organisms that share basic body architecture.  Of the 7 biological classifications we find phylum the 2nd highest level just below kingdom.  To understand the classifications I will use humans as an example.  I will begin with the lowest level and work upwards.

  1. Species = sapiens.
  2. Genus = homo
  3. Family = hominids
  4. Order = primates
  5. Class = mammals
  6. Phylum = chordates
  7. Kingdom = animals.

The class of phyla forms the highest biological category in the animal kingdom, with each phylum exhibiting a unique structural body plan.  Some common body plans are cnidarians (corals and jellyfish), mollusks (squids and shellfish), arthropods (insects and trilobites), echinoderms (sea star and sea urchin), and chordates, which includes all vertebrates (with backbone or spinal cord) including human beings.

The geological time period of the Cambrian explosion has been dated at 530 million years ago and lasted around 5 million years (some say 2 million).  Geologically speaking, 5 million years represents 0.11 percent of the Earth’s history.[1]  To understand how small this window of geological time is just picture it as one minute in a 24 hour day.  To imagine this time period in linear fashion, you can picture 4 inches on a 100 yard football field.

What makes this period of time unique is this sudden appearance of this vast array of phyla with what appears to be no clear morphological antecedents (prior body plans) in earlier strata.[2]   In fact some authorities have even estimated that all animal phyla came into existence during the Cambrian period.  This sudden appearance of phyla comes out of nowhere and then is followed by stasis.  Scientists have had a difficult time understanding how these major body plans exhibit such a considerable morphological isolation from one another and then the fossil record seems to come to a halt.

The problems posed by the Cambrian explosion are:

  1. The sudden appearance of most animal body plans.
  2. The sudden appearance of vastly different forms of life.
  3. The sudden appearance of information.
  4. The persistence of morphological (body plans) isolation.  This means that major differences remain constant and are not bridged by intermediaries.  Examples are cone jellies and vertebrate fishes that remain unchanged through geological time and remained separate over time.
  5. Top down and not bottom up pattern of appearance. Top down refers to the higher life forms (phyla) appear before the family, genus, and species or lower life forms.  This is very perplexing to many scientists!

How did all these unique body plans suddenly come into existence without precursors?  Why was this time period followed by stasis?  What scientific explanation best fits the fossil evidence as it exists?  Can natural selection explain the sudden appearance of all these unique body plans?  Can punctuated equilibrium explain this anomaly in the fossil records?

Go to part 3 here

[1] John Angus Campbell and Stephen Meyer, eds., Darwinism, Design, and Public Education (Michigan State University Press, 2003) pg. 326

[2] Ibid.

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