Researchers agree that the first multicelluar animals on Earth were circular in shape - much like the jellyfish and sea anemones of today - and it was only at a later stage in evolution that body shapes with bilateral symmetry (a right and a left side) began to emerge.

It was a crucially important development in history because features such as legs, fins and wings can only develop on bilateral organisms. But precisely when these creatures came on to the scene is something of a mystery.

The assumption is that most of them arose during a dramatic diversification of animal life - dubbed the Cambrian explosion - some 540-500 million years ago.

The ancestors of nearly all major modern animal groups, or phyla, can be found in the fossil record from this period.

Early emergence

However, evidence has been growing that they might have emerged much earlier. And a new study published in the journal Science would appear to confirm this by identifying a group of modern flatworms called the Acoela as the living descendants of an early lineage from this pre-Cambrian time.

The scientists were attracted to the Acoela because of their uniqueness. Unlike other flatworms they have no digestive tract of any sort.

To find out whether they should be reclassified, the team undertook an analysis of a specific gene common to the Acoela - the "18S rDNA" gene - and many other animals. This gene mutates at a constant rate and can be used to measure the _genetic distance_ between organisms.

In other words, the more variation that exists between organisms, the more distantly they are related - and the greater the time since they diverged from their common ancestor.

Fossil search

According to the results of the study, the Acoela were the first group of bilaterals to split off from the radial (circular) organisms, well before the other flatworms arose in the midst of the Cambrian explosion.

"It may well be that the origin of bilateral organisms occurred a bit earlier on, [before the Cambrian explosion,] from simple animals whose fossils have not been recovered," says Jaume Baguņā of the University of Barcelona.

"This may mean that we need to look more carefully at pre-Cambrian rocks or sediments to search for primitive bilateral animals."

The scientists suggest that the Acoela should now be classified in their own animal group.