Scientists from China and the US have found what they say are the earliest animal footprints ever discovered. The tracks and burrows, dating back half a billion years to the Ediacaran Period, were made by some of the earliest bilaterian animals, and reveal that more complex lifeforms arose earlier than previously thought.

Multicellular life got its start in the Early Ediacaran Period about 635 million years ago, perhaps buoyed by the melting of "Snowball Earth." But fossils from the rest of the Ediacaran are relatively rare, so we don't know much about what forms life took until the "explosion" during the Cambrian Period, beginning about 541 million years ago.

That's largely because Ediacaran life hadn't yet evolved the kinds of hard bones and shells that fossilize easily, so scientists usually have to rely on trace fossils instead – burrows, tracks and other secondary evidence of their existence.

For the new study, researchers from Virginia Tech and the Nanjing Institute of Geology and Palaeontology (NIGP) looked at trackways and burrows excavated from the Ediacaran Dengying Formation in the Yangtze Gorges area in South China, which are between 551 and 541 million years old. Similar burrows around the same age left by tiny worm-like creature have previously been found in Brazil, but the Chinese fossils suggest the presence of more complex organisms.

The Chinese tracks show two rows of imprints arranged in repeating groups, indicating the animals that left them would have been bilaterian – they had pairs of legs. The tracks were found next to burrows, suggesting that the creatures were tunnelers, likely in search of food or oxygen.

Bilaterians are one of the most common body types in the world, currently and throughout history, but previous fossil evidence for them only goes back as far as the Cambrian. Other research has suggested that the evolutionary roots of bilaterians should go back further than that, but fossils had never turned up until now. This new find is the first direct evidence of animals with appendages during the Late Ediacaran Period.

The researchers don't yet know exactly what animal left these tracks, and unfortunately we may never know. Whatever they were, they might not have had the right kinds of bodies to leave fossil remains.

The research was published in the journal Science Advances.