Environment Institute researchers Mike Lee and Julien Soubrier, with Greg Edgecombe have resolved a mystery that has puzzled scientists for centuries, Darwin's Dilemma. They have published their findings in, Mike explains below in an article he produced for .
Evolution鈥檚 鈥榖ig bang鈥 explained (and it鈥檚 slower than predicted)
[caption id="attachment_5502" align="alignleft" width="300"] Meet your flatmate. A living arthropod (centipede Cormocephalus) crawls over two 515-million-year-old relatives (Estaingia trilobites) which lived during the Cambrian explosion. All are found on what is now Kangaroo Island, southern Australia. Michael Lee[/caption]
The sudden appearance of a range of modern animals about half a billion years ago, during evolution鈥檚 鈥渂ig bang鈥, has intrigued and puzzled generations of biologists from Charles Darwin onwards.
A new study by from London鈥檚 Natural History Museum, from the 成人大片, and I, and published today in , suggests that the evolution of all these animals during the lower Cambrian can be explained by only a relatively minor increase in evolutionary rates - sustained over 20-30 million years.
Minor changes acting cumulatively over substantial periods generate huge differences: Albert Einstein termed 鈥渢he most powerful force in the universe鈥, and his observation also applies to evolutionary change.
The near-simultaneous appearance of a plethora of advanced animals around 530 million years ago is known as the .
Within a geologically brief interval of perhaps 20 million years, virtually every modern animal made its fossil debut, including (represented today by insects, crustaceans and arachnids), (clams, snails, octopus and squid) and (sea-squirts and vertebrates).
Alongside these familiar forms were a range of much more bizarre creatures, such as Opabinia (with five eyes and a stalked jaw), Diania (which looked like a walking cactus), and Anomalocaris (which looked like the head of a lobster grafted onto the body of a squid).
Measuring evolution鈥檚 big bang
Evolution鈥檚 big bang has fascinated and perplexed scientists for hundreds of years. Some scientists have speculated (with little evidence) that animals evolved at light speed during this time, hundreds or thousands of times faster than they are evolving today.
It certainly made : he thought incremental evolution through natural selection could not easily explain such an abrupt pattern.
Rather, he predicted that modern animal groups would appear in a staggered fashion, preceded by a range of precursors. Such reservations have predictably been exploited by opponents of evolution.
But it has been notoriously difficult to measure evolutionary rates during this pivotal interval of earth history, for good reason. = Such inferences require an exceptional fossil record.
To observe and date changes in ancestral and descendant species, we need complete animals preserved in rocks that span a substantial and continuous interval of time. Unfortunately, the Cambrian fossil record is far too patchy: good fossils are thinly scattered across time and space, making it impossible to directly 鈥渞ead鈥 evolutionary rates from the rocks.
Branches on the evolutionary tree
Our team used a novel approach to measure evolutionary rates during this pivotal moment in earth history, using living animals. Just as astronomers can infer much about the origin of the universe from the nature and movements of the galaxies today, we can learn much about the Cambrian explosion (the origin of animals) from the anatomy, genes, and current evolutionary trajectory of living animals.
We focused on arthropods, the dominant group of animals ever since the Cambrian, today making up more than 80% of animal life.
We reconstructed a detailed evolutionary tree of arthropods and inferred how much change had occurred on every branch of this tree, purely by looking at living animals. Then, using fossils, DNA and an , we worked out how fast each branch was evolving.
It turns out that - on average - Cambrian animals were evolving about five times faster than modern animals. This is fast, but nowhere near the speediest estimates that had been previously thrown about.
More importantly, such rates are totally consistent with Darwinian evolution by natural selection: for instance, mammals that colonise novel island habitats evolve a few times faster than their mainland relatives.
What fuelled the explosion?
What could have driven prolonged rapid (but not impossibly fast) evolution during evolution鈥檚 big bang? Many first appeared during the Cambrian explosion, such as vision, predation, burrowing, and active swimming.
These innovations would also have opened up totally new niches that animals would have raced to exploit, and would have triggered rapid evolutionary 鈥渁rms races鈥 between predators and prey.
The magnitude of these innovations meant their evolutionary effects could have reverberated for substantial time. While the Cambrian explosion occurs over a time interval that is considered short on geological scales, it still occupied a considerable period.
The inferred five-fold increase in evolutionary rates, acting over perhaps 20 million years, would generate 100 million years 鈥渨orth鈥 of evolutionary change.
That鈥檚 a huge amount of evolution: for instance, whales and bats both diverged from over perhaps as few as 65 million years.
This helps resolves Darwin鈥檚 dilemma: moderately elevated evolutionary rates - sustained over 20 million years in the early Cambrian - could easily explain the relatively sudden appearance of a range of highly divergent modern animals.
The study discussed in this article was funded by Australian Research Council grants to Mike Lee.
This article was originally published at .
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