Image: Ttrilobite of Cambrian Period.
During the Cambrian Period of geological time (543-248 mya) there was an explosion of life forms. Three dozen animal phyla (kinds) appeared each with its own distinctive body plan. All of this happened in a relatively short period of geologic time (c.10 million years). Darwin was aware that the main divisions of the animal kingdom suddenly appeared. This convinced him that all existing species of the same group had descended from a single progenitor. (Darwin, Great Books of the Western World, Encyclopedia Britannica, Inc., 1952, pp. 163)
But during Darwin's time, they had not found Precambrian life forms and this presented a problem for his theory of gradual evolution. He stated that if his theory was correct, countless living creatures had to have lived in Precambrian times. Since 1950, Precambrian single celled life forms have been found dating to c. 3.5 billion years ago. They left structures called Stromatolites which are colonial structures formed by photosynthesizing cyanobacteria and other microbes. About 1.4 billion years ago we find eukaryote cells, that is, more complex single cells similar to the amoeba and paramecium.
Ediacara multicellular fossil
On the eve of the Cambrian explosion we do find multicellular organisms (Ediacara). Stephen Gould points out that Ediacara animals were soft-bodies and a worldwide fauna. If they are ancestors of Cambrian creatures, why can't we find soft forms leading up to Cambrian hard body form? (Stephen J. Gould, Wonderful Life, W.W. Norton & Co., NY, 1989, p.57-60)
What did cause the Cambrian proliferation of life forms?
Darwin suggested violent environmental changes may have speeded up evolution. Granted that environmental changes occurred prior to the Cambrian period especially an increase of the oxygen level, in the atmosphere and oceans, something else was needed -- more genetic information, but where did it come from?
Biologists have looked for a master control plan (genetic tool kit) in ancient life forms such a jelly fish and sponges but to no avail. The jelly fish body, for example, is symmetrical, it mouth is also its anus and its nervous system is a decentralized web. (Carl Zimmer, Evolution, Harper Collins, Pub., 2002, p. 122)
Here science can only theorize. It is presumed that an animal type existed prior to this explosion which contained all the genetic information necessary for the diversity of life forms. "Exactly what kind of body that common ancestor had is difficult to say. But paleontologists shouldn't be surprised to unearth some some inch long creature that lived not long before the Cambrian explosion with a wormlike body; a mouth, a gut and an anus; muscles and a heart; nervous system organized around a nerve cord and a light-sensing organ; and, finally, some kind of outgrowths on its body -- if not actual legs or antennae, then perhaps, appendages around the mouth to help it eat. (Carl Zimmer, p. 122)
Even if such a fossil could be found, still it does not tell us how this creature came into being, except that genetic information became encoded into earth's life somehow. And so science must continue to search on.
Oh, the depth of the riches and wisdom
and knowledge of God!
How unsearchable are his judgments
and how inscrutable his ways!