Slightly Accelerated Evolution enough to Explain 'Darwin's Dilemma'

Researchers from the University of Adelaide in Australia and the Natural History Museum in London claim to have resolved an evolutionary dilemma that haunted Charles Darwin

"To the question of why we do not find rich fossiliferous deposits belonging to these...periods prior to the Cambrian system, I can give no satisfactory answer." Those words, written by Darwin in his 1859 work The Origin of Species, surmise what has come to be known as "Darwin's Dilemma," the sudden appearance of a plethora of modern animals groups in the fossil record during the early Cambrian period.

Publishing their work in the journal Current Biology, the University of Adelaide-led research team presents the first estimates for the rate of evolution during the so-called "Cambrian explosion" when most modern animal groups appeared between 540 and 530 million years ago.

"The abrupt appearance of dozens of animal groups during this time is arguably the most important evolutionary event after the origin of life," said lead author Michael Lee of the University of Adelaide's School of Earth and Environmental Sciences and the South Australian Museum.

"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," he said. "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."

Lee and his colleagues' explanation for the dilemma is that the rates of both anatomical and genetic evolution during the Cambrian explosion were five times faster than they are today, an estimation that Lee calls "quite rapid, but perfectly consistent with Darwin's theory of evolution."

To reach their conclusion the researchers quantified the anatomical and genetic differences between living animals and then established a timeframe over which those differences started to accumulate. By analyzing the fossil record and employing intricate mathematical models, the researchers found what they claim to be sufficient evidence that just moderately accelerated rates of evolution present results consistent with the seemingly sudden appearance of many groups of advanced animals in the fossil records during the Cambrian explosion.

The research was focuses on arthropods like insects, crustaceans and arachnids, which are the most diverse animal group both in the Cambrian period and today.

"It was during this Cambrian period that many of the most familiar traits associated with this group of animals evolved, like a hard exoskeleton, jointed legs, and compound (multi-faceted) eyes that are shared by all arthropods," said study co-author Greg Edgecombe of the Natural History Museum in London. "We even find the first appearance in the fossil record of the antenna that insects, millipedes and lobsters all have, and the earliest biting jaws."