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Evolutionary biologists resolve 'Darwin's dilemma'

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The problem of the abrupt beginning to the fossil record was resolved in the 1950s, with the discovery of pre-Cambrian life forms. Why hadn't they left fossils like their Cambrian cousins? Simple: These squishy creatures, similar to slugs, worms, and jellyfish, didn't have bones or shells that could fossilize. With that discovery, scientists had proof that the Cambrian explosion didn't mark the dawn of life – just the beginning of life with hard parts.

But that's its own mystery, and has remained a lingering challenge to evolution: How could so many unrelated creatures all evolve with shells, exoskeletons, carapaces, and other hard parts, all at once?

"The abrupt appearance of dozens of animal groups during this time is arguably the most important evolutionary event after the origin of life," says Dr. Lee, who teaches at Australia's University of Adelaide and works at the South Australian Museum, in a press release.

"These seemingly impossibly fast rates of evolution implied by this Cambrian explosion have long been exploited by opponents of evolution. Darwin himself famously considered that this was at odds with the normal evolutionary processes. However, because of the notorious imperfection of the ancient fossil record, no one has been able to accurately measure rates of evolution during this critical interval, often called evolution's Big Bang."

Until now.

With colleagues from Australia and London's Natural History Museum, Lee measured anatomic and genetic differences between living animals, and used fossils and mathematical models to establish the rate at which those differences accumulated. Their models showed that quick, but not lightning fast, evolution could explain the arrival of the many kinds of animals in the Cambrian explosion.

In the Cambrian, animals evolved visible differences about 4 times faster than in later years, and molecular evolution was even faster – about 5.5 times the rate of subsequent evolution, say the researchers.

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