555-Million-Year-Old Fossils Show Earliest Evidence of Animal Handedness

Fossils of the worm-like creature Spriggina floundersi show the earliest known evidence that animals favored one side of the body over the other. Scott Evans at the American Museum of Natural History and colleagues examined 100 specimens collected in South Australia.

The animals lived 555 million years ago during the Ediacaran Period in a shallow ocean and moved by wriggling left or right across the seafloor.

Twice as many of the clearly bent specimens curve leftward in the rock, which means the living animals bent rightward because the fossils preserve mirror-image impressions formed when storms buried them in sand. Evans said around 50 specimens display clear bends and that the pattern matches handedness observed in animals today.

Some specimens show multiple bends in both directions, indicating the creatures could turn either way.

The study was published in Scientific Reports. Evans noted that the findings demonstrate foundational traits such as bilateral symmetry and handedness evolved in the Ediacaran before the Cambrian diversification of animal life. Russell Bicknell at Flinders University said the presence of functional asymmetry this deep in the fossil record provides information on how such behaviors emerged over time.