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the cambrian 'explosion'

Wells' second 'question' centres on what's often been called the Cambrian 'explosion' - the seemingly rapid appearance in the fossil record of a wide range of different organisms. ('Rapid' = over a period of 10-20 million years or so.)

DARWIN'S TREE OF LIFE. Why don't textbooks discuss the "Cambrian explosion," in which all major animal groups appear together in the fossil record fully formed instead of branching from a common ancestor -- thus contradicting the evolutionary tree of life? 

The simple answer is, they don't. (Which should be the end of the story, but this one keeps on coming up in various forms & forums.) What does Wells mean by 'major animal groups'? Taxonomic classes are major groupings. But many of them - amphibians, reptiles, birds & mammals - don't appear in the Cambrian at all, but much later in the piece. Nor is it correct to describe the Cambrian animals as 'fully formed' representatives of their class or phylum. They're ancient. Many of them look quite different from modern representatives of those groups (read Wonderful Life or The Crucible of Creation to get a feel for what they were like). And while they might show some of the key features of their taxon, they often don't possess them all.

What's more, many scientists regard the term 'explosion' as something of a misnomer. It does tend to imply that there was a rapid burst of extensive adaptive radiation during the Cambrian. But data from both the fossil record and molecular biology suggest that the origins of many of the Cambrian groups lie much further back in time. And the picture we are getting from these data does indeed support the idea of an evolutionary tree of life.

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