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.
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 explosion – or if ancestors of Cambrian and modern animal groups can be traced further back in time.
Many organisms from that period have defied efforts to classify them. Their strange bodies – often resembling shapeless sacs or thin, quilted pillows – have no obvious counterparts among living species. These Ediacaran organisms lived in close association with mats of microbes that smothered the seafloor – a type of ecosystem that did not survive the advent of grazing bilaterians.
In spring 2023, Gaorong Li, a PhD student at Yunnan Key Laboratory for Palaeobiology, made a discovery that helps to clarify this fuzzy gap between the weird Ediacaran world and the recognisable, complex animal-dominated Cambrian period. Exploring Ediacaran rocks in the Chinese region of Eastern Yunnan, we found a bizarre worm that lived tethered to the seafloor by an anchoring disc, and which could turn its strange proboscis inside out to collect food.
We nicknamed it the “bugle worm”. Previously, it had been described based only on the disc anchoring it to the seafloor and named Cycliomedusa. In 2024, we went back into the field and pieced together this new fossil community. We found some fossilised organisms characteristic of both the Ediacaran and Cambrian periods. But surprisingly, we also found some that had previously only been known from the time of the Cambrian explosion. Most striking of all, we found the oldest evidence for the group to which we humans belong: the deuterostomes. These now-extinct animals are related to living starfish and acorn worms – the closest invertebrate relatives to humans. This shows our own evolutionary story has its roots in the Ediacaran period.
The Conversation