A new study by an international team of researchers, led by scientists from the University of Bristol, has revealed the origins and evolution of animal body plans.

Animals evolved from unicellular ancestors, diversifying into thirty or forty distinct anatomical designs. When and how these designs emerged has been the focus of debate, both on the speed of evolutionary change, and the mechanisms by which fundamental evolutionary change occurs.

Did animal body plans emerge over eons of gradual evolutionary change, as Darwin suggested, or did these designs emerge in an explosive diversification episode during the Cambrian Period, about half a billion years ago?

The further back in time you go, the patchier our understanding of life on Earth gets. That’s because fossils from those early years are extremely hard to come by and interpret, for a number of reasons. Now, British scientists have used a different method known as a molecular clock to plot out a rough timeline of all life on Earth, tracing the first organisms back to about 4.5 billion years ago.

When did animals originate? In research published in the journal Palaeontology, we show that this question is answered by Cambrian period fossils of a frond-like sea creature called Stromatoveris psygmoglena.

The Ediacaran Period lasted from 635 to 542m years ago. This era is key to understanding animal origins because it occurred just before the “Cambrian explosion” of 541m years ago, when many of the animal groups living today first appeared in the fossil record.

Yet when large fossils from the Ediacaran Period were first identified during the 20th century they included unique frond-like forms, which were not quite like any living animal. This prompted one of the greatest debates still raging in evolution. What exactly were these enigmatic fossils, often called the Ediacaran biota?