Newly-Discovered Fossil Suggests Complex Animals Evolved Much Earlier Than Previously Thought

X-ray microtomography image of trace fossil in sediment. Luke Parry - University of Bristol

Scientists have identified markings in a new set of trace fossils that suggest complex life existed earlier than previously thought. It is an important discovery that could completely change what we thought we knew about the evolutionary timeline.

The fossils were found in sediment in the Corumbá region of western Brazil, and are over half-a-billion years old. The study is published in the journal Nature Ecology and Evolution.


"This is an especially exciting find due to the age of the rocks – these fossils are found in rock layers which actually pre-date the oldest fossils of complex animals – at least that is what all current fossil records would suggest," explained Dr Russell Garwood, from Manchester's School of Earth and Environmental Sciences, in a statement.

Trace fossils are fossils that show signs of life, rather than being the remains of life themselves. Think footprints, tracks, and feces instead of bone fragments and fingernails. In this particular case, the rocks have imprints left by a creature burrowing through the sediment hundreds of millions of years ago. It is, scientists think, the earliest sign of an organism with muscle control and the ability to actively move. 

The scientists believe the burrows were formed by "nematoid-like organisms", which look a lot like modern-day roundworms. They might not sound all that impressive, but they are considered complex life forms and, like humans, come under a group of animals called bilaterians. Bilaterians have one layer of symmetary, whereas simple creatures, including jellyfish, have multiple. Archeological evidence tends to be quite a bit younger, with the first fossils appearing in the Cambrian period (545 million to 495 million years ago). 

"Our new fossils show that complex animals with muscle control were around approximately 550 million years ago, and they may have been overlooked previously because they are so tiny," said Luke Parry, lead author from the University of Bristol.


They really are tiny. From the size of the burrows, scientists have worked out that these ancient creatures measured just 40 to 300 micrometers – about the width of a strand of human hair. They would have moved around the sediment, wiggling and leaving behind burrows as they go.

The thing that is really fascinating is their age. The burrows were fossilized during a period known as the Ediacaran-Cambrian transition (around 541 million years ago), an important turning point in evolutionary history. To give that some context: The first dinosaurs appeared around 230 million years ago during the Mesozoic Era, and modern Homo sapiens have only been around for the past 200,000 or so years. 

"The evolutionary events during the Ediacaran-Cambrian transition are unparalleled in Earth history," said Garwood. "That's because current fossil records suggest that many animal groups alive today appeared in a really short time interval."

"Our discovery highlights an unexplored window for tracking animal evolution in deep time," Parry added.


  • tag
  • fossil,

  • burrows,

  • Ediacaran-Cambrian,

  • nematoid,

  • ancient organism,

  • bilaterians