Animal life is extraordinarily diverse and complex, having colonised almost all environments on Earth – from hostile hydrothermal vents in the deep sea to the skies across our continents.

But the planet was not always teeming with complex animal life. For the first 3.7 billion years after it originated, life was small, simple and largely confined to the oceans. This microbe-dominated world was a tumultuous place, with several major swings in its climate.

But all this appears to have changed about 538 million years ago (mya) during the Cambrian period. This critical juncture in the history of life saw animals bursting on to the scene in an event known as the “Cambrian explosion”.

All sorts of animals easily recognisable as groups alive today appeared in the fossil record, from echinoderms (starfish, sea cucumbers, urchins) and arthropods (spiders, crustaceans, insects) to various types of worm. This seemingly abrupt appearance of animals in a geological “blink of an eye” has puzzled scientists from Charles Darwin onwards.

Many of these new lifeforms belonged to a group of animals called Bilateria, so-named for their symmetrical left and right sides. This group now contains all animals with brains and complex musculature.

However, a longstanding question for palaeontologists has been whether this astonishing diversification event happened all at once during the Cambrian........

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