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 Darwinadmitted 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."

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 atAustralia's University of Adelaide and works at the South Australian Museum, in a press release.