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Evolutionary biologists resolve 'Darwin's dilemma'

Darwin was troubled by how rapidly fossils appeared, 540 million years ago, but a group of Australian scientists has calculated that early evolution was only four to five times as fast as later evolution: 'perfectly consistent with Darwin's theory.'

Marine life during the Cambrian explosion (~520 million years ago). Since life didn't colonize land for millions of years, all early creatures lived in water.

Katrina Kenny & Nobumichi Tamura/University of Adelaide

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When creationists argue against evolution, they'll often mention "Darwin's Dilemma," the head-scratching paleontological reality that the fossil record appears to have begun all at once, about half a billion years ago. The "Cambrian explosion," as it's called, refers to the rapid appearance of most modern animal groups between 540 and 520 million years ago, during the Cambrian Era.

When writing his "Origin of Species," Charles Darwin admitted that he couldn't explain why the fossil record began so abruptly. "The several difficulties here discussed, namely ... the sudden manner in which several whole groups of species first appear in our European formations;—the almost entire absence, as at present known, of formations rich in fossils beneath the Cambrian strata,—are all undoubtedly of the most serious nature." While acknowledging that these constituted serious holes in his theory, he trusted that later scientists would resolve them.

One group of scientists now say they've solved a big chunk of the puzzle: they've made the first-ever estimates of the rates of evolution during the Cambrian explosion – and it wasn't as fast as it sounds. While the word "explosion" implies a split-second transformation, evolution in the Cambrian was only 4 to 5 times faster than in later years, they say. That's like the difference between flying in a puddle-jumper versus a commercial jet. It's faster, to be sure, but it's not the difference between walking and the speed of light.

"Quite rapid," says lead author Michael Lee, "but perfectly consistent with Darwin's theory of evolution."

In the 150 years since Darwin published his revolutionary theory, evolutionary biologists have tackled several of the different mysteries of the "Cambrian explosion."


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