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Measured by virtually any criterion one might propose, studies of Precambrian life have burst forth since the mid-1960s to culminate in recent years in discovery of the oldest fossils known, petrified cellular microbes nearly 3,500 million years old, more than three-quarters the age of the Earth. Precambrian paleobiology is thriving---the vast majority of all scientists who have ever investigated the early fossil record are alive and working today; new discoveries are being made at an ever quickening clip....After more than a century of trial and error, of search and final discovery, those of us who wonder about life's early history can be thankful that what was once "inexplicable" to Darwin is no longer so to us. --J. William Schopf
The Cambrian explosion refers to the quality of the fossil record during the first 30 million years of the Cambrian Period (roughly 570 to 500 million years ago). During that 30-million-year period, "mollusks, starfish, arthropods, worms, and chordates (including vertebrates)"* evolved. There were sponges, bryozoans, hydrozoans, brachiopods, and a few species of stalked echinoderms.* As Richard Dawkins notes: "It's as though they were just planted there, without any evolutionary history" (Dawkins 1996: 229). Dawkins doesn't claim to know why there is so little pre-Cambrian fossil evidence but he suspects "it might be that many of these animals had only soft parts to their bodies: no shells or bones to fossilize" (Dawkins 1996: 230).
The oldest fossilized bacteria date from about 3.5 billion years ago. Two billion years later algae—organisms with cells, a nucleus, and chromosomes—appeared. Marine invertebrates with hard shells and skeletons of chitin or lime are more conducive to fossil preservation than soft-bodied creatures. Perhaps adding to the conditions that were conducive to preserving fossils during the Cambrian Period was the fact that most landmasses on the planet at that time were in the Tropics or the southern hemisphere.
Before the Cambrian Period, life on earth had emerged but large animals had not evolved. One explanation for this is that respiration was not possible as there was not much oxygen content in the atmosphere and the oceans until the Cambrian. Another explanation is that the earth was a frozen snowball until about the Cambrian Period and that sudden melting brought about a "climate shock" that triggered the evolution of multi-cellular animals. There are other proposed explanations, as well.*
For some reason, creationists think the Cambrian explosion is evidence that counts against evolution but supports their hypothesis that an invisible magical being created species individually. As Jerry Coyne notes:
many animals and plants do not show up as fossils until well after the Cambrian explosion: bony fishes and land plants first appeared around 440 million years ago, reptiles around 350 million years ago, mammals around 250 million years ago, flowering plants around 210 million years ago, and human ancestors around 5 million years ago. The staggered appearance of groups that become very different over the next 500 million years gives no support to the notion of instantaneously created species that thereafter remain largely unchanged. If this record does reflect the exertions of an intelligent designer, he was apparently dissatisfied with nearly all of his creations, repeatedly destroying them and creating a new set of species that just happened to resemble descendants of those that he had destroyed.
The fact is, the fossil record is imperfect. As Richard Dawkins notes: "if we arrange all our available fossils in chronological order, they do not form a smooth sequence of scarcely perceptible change" (Dawkins 1996: 229). Eldredge and Gould proposed the theory of punctuated equilibrium on the belief that some of the gaps in the fossil record represent what actually happened: some gaps are due to evolution occurring in sudden bursts followed by periods where no evolutionary change took place in given lineages. But even Eldredge and Gould recognized that some gaps many be due to imperfections in the record.
Creationists love to quote Gould and Dawkins out of context. Christie Syftestad of Roseville writes that "Stephen Jay Gould himself admitted that fossil evidence completely contradicts natural selection" (letter to the editor, Sacramento Bee, Dec. 19, 2005). Not quite. In his final published work, The Structure of Evolutionary Theory, Gould wrote that an important reason for writing the book was to present
a tight brief for substantial reformulation in the structure of evolutionary theory, with all threads of revision conceptually united into an argument of different thrust and form, but still sufficiently continuous with its original Darwinian base to remain within the same intellectual lineage and logic (2002: p. 33).
Furthermore, unlike creationists and ID advocates, Gould was a scientist. The lack of fossil evidence from pre-Cambrian times did not mean it was time to give up science and bring in a god or some other magical being to wave his wand and create species individually just as it says in the Bible! No, Gould notes proudly that paleontologists have been scouring the appropriate sediments on earth for whatever evidence they might find of pre-Cambrian life:
For example, the earth's first prominent assemblage of animals, named the Ediacara fauna for the Australian locality of its first discovery but now known from all continents, lived from about 600 million years ago right up to the explosion, with perhaps a few forms surviving beyond. These large creatures (up to a meter in length in one case, though most specimens occupy the range of centimeters to decimeters) tend to be highly flattened in form, composed of numerous sections that seem to be "quilted" together (certainly not segmented in any metameric way), and appear to possess no body openings. Although some researchers have sought the origin of a few bilaterian phyla within this fauna, the comparisons seem farfetched and many paleontologists regard the Ediacaran animals as an early expression of pre-bilaterian possibilities of diploblast design (with modern cindarians and a few other groups surviving as a remnant of this fuller diversity), while other experts have regarded them as an entirely separate (and failed) experiment in multicellular life or even as a group of marine lichen! (Gould 2002: 1157).
The creationists and ID advocates are anti-scientific propagandists. They assume that no amount of scientific investigation will ever produce more data of relevance to understanding the processes of evolution that have taken place over the past several billion years on this planet. In short, their only interest in science is to find areas where scientists see problems to be investigated and declare that the problems can't be solved except by appealing to a magical being who can do anything that's needed to make the data fit with somebody's understanding of the Bible.
Some of these pious frauds go so far as to claim that evolution is false and not scientific. That's the position of Frank Sherwin of the Institute for Creation Research, an outfit of religious zealots who see their Christian mission as proving the "scientific bankruptcy of evolution." According to Sherwin (in an article written by his equally pious sister, Elisabeth), the Cambrian explosion is one of the "four irrefutable arguments" against evolution. Frank believes that a true scientist believes in the Bible and appeals to a magical invisible being to do his science for him. His sister says she accepts intelligent design rather than evolution because she's "irritated by the arrogance of evolutionists who claim to have all the answers" (Sherwin 2005). If either Sherwin sibling actually read anything by Dawkins or Gould or an other evolutionary scientist they would have to lie their way to heaven if they tried to maintain that evolutionists are the ones who claim to have all the answers. The ID and creationist attackers of evolution only appear when evolutionists don't have the answers. And the pious creationists appear mainly to declare that it is hopeless to search any further, that it's time to give up and recognize that only a miracle can solve the problem. Then, in a glorious non sequitur they declare, as Frank Sherwin does, that "the world view of a person who thinks they came from bacteria is likely to be substantially different from the world view of someone who thinks they were created in [some god's] image" (Sherwin 2005). That's the moral of their story, it seems.
This is the talk of a propagandist, not a scientist. (What you gonna believe? that you're ephemeral slime or an immortal spirit?) It is the talk of a man who has no true interest in the magnificent and magical universe around him. It is the talk of a man who has an imaginary friend that allows him to stop thinking and dogmatically declare that he has all the answers so there is no need to investigate this wonderful world of living things any further. There are many straws such a man might grasp, but the Cambrian explosion shouldn't be one of them.
Sherwin, Elisabeth. (2005). "The best thing about origins debate is getting people to think," Davis Enterprise, Dec. 19.