Some recent work [Daniel Y.C. Wang, Sudhir Kumar, and S. Blair Hedges Proceedings of the Royal Society of London (Series B), January 22, 1999 issue] using so-called "molecular clocks" has placed the time of divergence of animals, plants and fungi at around 1500 million years ago. The origin of nematode worms is placed at around 1200 million years ago, and, the sponges and coelenterates, somewhere between 1500 and 1200 million years ago.
If the results of his team's genetic study are correct, Hedges says the scientific question must change from "How did all these species evolve so suddenly early in the Cambrian period?" to "Why don't we see any fossils of these species long before the Cambrian period?"
We think this is a good question to ask.
Although there are suggested explanations, (generally having to do with not having hard body parts that can be preserved), the posited explanations certainly sound a lot like arm-waving. Some scientists have theorized that the ancestors of creatures that suddenly and mysteriously appeared in the Cambrian era may have been soft-bodied creatures that could not have been preserved in the fossil record. These scientists must reckon with the data emerging from China: Any suggestion that Precambrian forms were soft-bodied creatures that therefore could not be preserved is emasculated by the recent discovery of an elegantly preserved soft-tissued fauna in the Early Cambrian Chengjiang site in China, and by the persistent absence of those forms, or any other life forms, in the earlier rocks with similar lithologies also available at the Chengjiang site. Professor Chen, who reported on these finds recently, insisted that the biological “explosion” exhibited at Chengjiang is not an illusory artifact of an incomplete fossil record, but an accurate preservation of the true history of life on earth. Accordingly, he suggests biologists should construct theories of life’s origin in light of this paleontological evidence, not in spite of it.
How are evolutionists dealing with these data? Some argue against the molecular clock. For instance, when Levinton gave his paper [at the 1996 GSA meetings in New Orleans] he stated that the molecular clock can be best compared to a sun dial in the shade, which isn’t very encouraging for his method, but he and his colleagues still believed that it yielded data sufficient to test the theory of the rapid evolution of life at the base of the Cambrian....
Howeverr, from their molecular clock data they concluded that the initial divergence of metazoan life forms occurred about 1.2 billion years ago (+/- 50 to 250 million years) . The base of the Cambrian is currently dated at about 543 million years ago , so their conclusions require a half billion years of metazoan history before the Cambrian. [As mentioned above, this number appears now to be closer to a billion years.] They also concluded that the beginning of Metazoan phyla was not an explosion, but was somewhat spread out during that (billion years) time.
A couple of days later these papers were discussed in a 'Hot topics discussion' during the noon hour. Four scientists gave brief presentations on the new ideas about the Cambrian explosion, followed by audience questions and comments. Many questions dealt with technicalities of their research method, but two questions stand out. A little background is necessary before dealing with these questions. The proposal that complex metazoan animals, ancestral to such things as molluscs, trilobites, vertebrates, sea urchins, corals, and many others, existed for a half billion years before the Cambrian implies that they lived all that time without leaving a fossil record. This pretty much requires that before the Cambrian they existed as soft worm- or larvae-like forms, with the general genetic make-up of the Cambrian groups but without their skeletonized morphology.
Now the questions. The first of the two questions was - why are trace fossils (fossil tracks, trails, and burrows) so rare before the base of the Cambrian, if these animals existed for that half billion years? An internationally recognized expert on trace fossils stood up, presumably to answer the question. However, he talked about other things and the very important question never was answered. At the end of the discussion another scientist stood up and commented on the implication that all the skeletonized phyla developed skeletons at about the same time in the Cambrian. He asked - why are all these types of animals living for so long and then all making skeletons all at once? He then asked, with some vigor - 'Why are you avoiding the real question?' After a pause, one member of the original presenters answered 'because it’s really hard (a hard question)'. He went on to say that they hoped answers would come from further study of developmental biology.
These two questions were apparently not asked by people who doubted the evolution theory, but by evolutionary scientists willing to ask the hard questions that need to be addressed as they try to test between different hypotheses. The fact remains that the Cambrian explosion is one of the big challenges to naturalistic theories that looms larger with every passing day. For additional information on this topic see the paper The Trilobite: Enigma of Complexity elsewhere on this site.
2010 Arthur V. Chadwick, Ph.D.