The Cambrian Explosion – Biology’s Big Bang?
The Cambrian Explosion relates to an abrupt appearance of a wide range of organisms, mainly invertebrates, with hard (fossilizable) parts in Cambrian strata which mainstream scientists date from about 540 million years ago. They were complex, well-developed organisms with many types of differentiated cells, and it is widely conceded that evolution of these organisms from unicellular precursors within such a short period of time is highly doubtful.
Now, it can be granted that organisms without skeletons will leave few if any fossils, so it should not be too surprising if one evolving line were to appear suddenly in the fossil record. It is surprising, however -- at least within an evolutionary framework -- that such a wide variety of fossilizable forms should appear at more or less the same time. The number of radically different body plans which appear in such a very short period of geological time (about 13 million years) is greater than at any other. Accounting for the abrupt and sudden appearance of these organisms is one of the leading challenges in evolutionary biology.
The Cambrian Explosion – Is it curtains for Darwinism?
There are two principle options which are invoked in an effort to account for the sudden diversity: (1) the different forms arose at the start of the Cambrian from a single common stock from which the various forms radiated, or (2) the divergence happened much earlier. If they diverged at the beginning of the Cambrian then it does some way to explaining the absence of preceding fossils but then it remains inexplicable how the wide divergence of characteristics occurred so suddenly, i.e. that the various phyla appear so different as to be unrelated. If, on the other hand, they diverged considerably before the Cambrian era (so as to allow for the divergence of various forms, then it is extraordinary that all the various lines should reach a fossilizable stage at much the same time.
One example of a Cambrian-appearing organism is the sponge. There are four classes of sponges, all of which appear abruptly in the Cambrian. Despite a relatively unspecialized structure, the classes are distinct and difficult to relate to one another. There is no sign of intermediary fossils and there is certainly no consensus as to how they could potentially have evolved from a common ancestor. Despite their primitive form, the sponges are quite separate from the rest of the animal kingdom.
Similarly, as far as the arthropods are concerned, the different subphyla of trilobites, horseshoe crabs and crustaceans arise in the Cambrian. Furthermore, the crustaceans are exceedingly diverse. All four major classes of the crustaceans and many lower taxa are found in the Cambrian; but, again, despite this multitude of fossils, no trace can be found of any transitional forms which would link the different groups to a common ancestor.
One attempt to account for the Cambrian explosion involves the proposal that there was a substantial increase in oxygen about this time which stimulated rapid evolutionary progress -- but such a suggestion ignores the sheer improbability of biological macromolecules, whether oxygen is plentiful or not. The enigma is compounded because, not only do different phyla appear suddenly, but also -- within most of the phyla -- very distinct classes arise, again at more or less the same time.
The Cambrian Explosion – Summary
In summary, what is generally proposed, is the extraordinary coincidence that these diverse types of organism with their radically different skeletons all reached fossilizable stage within a relatively short period of time. The Cambrian explosion raises the kinds of questions that occur repeatedly regarding the fossil record. First is that major new types of organism appear suddenly and abruptly. Second, many different lines, exhibiting the same sort of significant development, arise about the same time. There exists such a radical diversity that it becomes implausible that they shared a recent common ancestor. It also seems unrealistic that the same sort of advance could have arisen independently in several lines, especially simultaneously.