The sudden appearance of a range of modern animals about half a billion years ago, during evolution’s “big bang”, has intrigued and puzzled generations of biologists from Charles Darwin onwards.
A new study by Greg Edgecombe from London’s Natural History Museum, Julien Soubrier from the University of Adelaide, and I, and published today in Current Biology, 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 compound interest “the most powerful force in the universe”, 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 Cambrian explosion.
Within a geologically brief interval of perhaps 20 million years, virtually every modern animal phylum made its fossil debut, including arthropods (represented today by insects, crustaceans and arachnids), molluscs (clams, snails, octopus and squid) and chordates (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’s big bang
Evolution’s 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 Charles Darwin feel uneasy: 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 “read” 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 accommodating supercomputer, 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’s big bang? Many game-changing adaptations 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 “arms races” 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 “worth” of evolutionary change.
That’s a huge amount of evolution: for instance, whales and bats both diverged from a tiny shrew-like common ancestor over perhaps as few as 65 million years.
This helps resolves Darwin’s 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.
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