Cambrian Explosion Precursors, or Drops in the Bucket?
Two recent presentations, one in person and one in print, tried to fill in the gap of fossils that led to the explosion of diversity in the Cambrian, known as the Cambrian Explosion (see 04/23/2006 entry).
- Darwin’s Dilemma Solved? Dr. J. William Schopf (UCLA), renowned discoverer of Precambrian microfossils, triumphantly announced the solution to “Darwin’s Dilemma” at the Jet Propulsion Laboratory in a slide presentation to scientists 06/14/2006. He showed examples of cyanobacteria, stromatolites (see 03/22/2006) and carbon isotope anomalies. All these, however, showed evidence for microbes, not the complex body plans that burst on the scene in the Cambrian. Moreover, he showed fossil cyanobacteria dated as 3.5 billion and 1.5 billion years old alongside modern species that looked identical; he even speculated that their biochemistry was probably the same.
- Bilateral Split: Fossil embryos with “polar lobes” characteristic of bilaterians (animals with two-sided symmetry) were reported in Science this week from Precambrian deposits in China.1 Though simple microscopic bundles of 3-5 cells, the authors claim, “these fossils imply that lobe formation is an ancient evolutionary device, and that the general strategy of precocious blastomere specification still used in most bilaterian groups was extant at least 40 million years before the Cambrian.” Katherine Unger wrote in the same issue2 that “Some scientists are reluctant to give the fossils their unequivocal endorsement.” No adult forms were found; some speculate they could represent an extinct lineage of early bilaterians.
1Jun-Yuan Chen et al., “Phosphatized Polar Lobe-Forming Embryos from the Precambrian of Southwest China,” Science 312, 16 June 2006: 1644-1646.
2Katherine Unger, “Fossil Embryos Hint at Early Start for Complex Development,” Science 312, 16 June 2006: 1587.
The Schopf talk was egregious for claiming Darwin has been vindicated. For one thing, he gave no clue how microbes exploded into a plethora of complex body plans in the Cambrian. For another, he guaranteed he cannot see evidence against evolution! One of his criteria was, “Does it fit with evolution?” This means that no matter what is found, if it doesn’t fit with evolution, it must be mistaken. Darwin in, Darwin out.
Bilaterian embryos are too little, too late. These are just drops in the bucket, in a bucket too large for Darwin to fill. Read the Paper View article from 04/23/2006 for a detailed account of the problem.