A Cambrian underwater seascape showing trilobites dominating the seafloor © Katrina Kenny

Early animals may have evolved much quicker than previously thought

New research by The Natural History Museum and the University of New England in Australia has revealed that a pivotal early evolutionary explosion was far shorter than many experts had thought.

The Cambrian explosion, an important point in the history of life on Earth, took place more than 500 million years ago and was a time of rapid expansion of different forms of life on Earth. It was during this era that most of the major animal groups start to appear in the fossil record.

Dr Greg Edgecombe, Merit Researcher at the Natural History Museum, has been using the diversification of trilobites to try and understand precisely how long the Cambrian explosion went on for. His work suggests that this burst of evolution may have only occurred for around 20 million years, a very brief moment in the grand scheme of Earth's history.

Commenting on the paper, Greg says 'There was a short burst of accelerated evolution at the start, then it flat-lined for the rest of the Cambrian. It means that rather than there being a long and protracted evolutionary explosion throughout the period, it was more of a quick spurt at the start, during which the major animal body plans came into being.’ To understand the Cambrian explosion Greg focused his research on trilobites, an extinct marine animal and one of the earliest-known groups of arthropods. The evolutionary history of the trilobites has previously been relatively unknown. At one point during the Cambrian Period, trilobites suddenly went from being soft-bodied to developing hard shells and were rapidly found in shallow seas all around the world.

Greg and his colleagues looked at the different branches of the trilobite evolutionary tree that appear during the Cambrian, and used methods originally developed for analysing DNA sequence data to track the rate at which anatomical features of these animals changed shape over time. This allowed the team to figure out how quickly animals were evolving during this period, while also giving them clues as to when the last common ancestor to all the trilobites lived.

The team discovered that the common ancestor of all trilobites likely lived 10 to 20 million years before the sudden divergence seen in the fossil record. After trilobites begin to become common as fossils their rate of evolution throughout the majority of the Cambrian was surprisingly stable.

The results finally settle an evolutionary quandary that plagued even Darwin himself, as it questions his idea that evolution occurred gradually over long time periods. This new research shows that fast rates of evolution can, and indeed did, happen during the start of the Cambrian.

The paper ‘Trilobite evolutionary rates constrain the duration of the Cambrian explosion’ is published in the Proceedings of the National Academy of Sciences (USA) on 18th February 2019.

Ends

Notes for editors

Interview: Dr Greg Edgecombe is happy to be contacted for comment.

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