A set of footprints discovered in an ancient seafloor in the Yangtze Gorges in China reveals what creature on Earth is first to develop feet.

An international team of scientists found that the fossil of an animal appendage dates back to the Ediacaran period, sometime between 541 and 635 million years ago. Scientists said the footprints may belong to a primitive ancestor of modern-day insects or worms, which is the oldest ever discovered throughout history.

Professor Shuhai Xiao, senior author of the study and geobiologist at Virginia Tech University, said their findings allow them to understand what species were first to evolve with legs.

"It is important to know when the first appendages appeared, and in what animals, because this can tell us when and how animals began to change to the Earth in a particular way," Xiao said.

Animals use their appendages or outgrowths to move around, make their homes, feed, and find mates.

The study is published June 6 in the journal Science Advances.

Footprint Analysis

Xiao's team found the footprints while tilting rock slabs at different angles. They hoped that the sunlight would reveal subtle marks left by ancient organisms on the Shibantan trackways.

The fossil measures only a few centimeters away, although the researchers did not know how the species looked like. Further analysis also showed that that the footprints have two rows of imprints, suggesting that the creature lived along the riverbed.

The marks lead to burrows, which indicate that the organism scoured food, microbes, and oxygen into the sediments. However, Xiao said they are uncertain if the creature belonged to the arthropod family or whether it has many or two legs.

"Although the exact identity of the trace maker of the Shibantan trackways is difficult to determine in the absence of body remains at the end of the trackways, we suggest that the trace maker was probably a bilaterian animal with paired appendages," the authors reported.

Oldest Fossil Discovered So Far

In 2017, scientists at UCLA and the University of Wisconsin-Madison reported that the microscopic fossils found in a piece of rock in Western Australia are the oldest so far. The fossils date back to nearly 3.5 billion years ago and are strong evidence of the earliest life that existed on Earth.

The study was published in the Proceedings of the National Academy of Sciences. There are 11 microbial specimens identified from the fossils, including extinct ones that belong to the Archaea group.