Scientists have discovered the earliest footprint of an animal on Earth, dating back 541 million years.
An international research team discovered the fossil tracks in China dating back to the Ediacaran Period, just before the Cambrian Explosion when life on Earth increased rapidly.
In research published in Science Advances, researchers analysed trackways and burrows in the Dengying Formation near the Yangtze Gorges.
The trackways are the earliest discovered indication of when animals evolved appendages.
Professor Shuhai Xiao, a geobiologist at Virginia Tech University, told The Guardian: "Animals use their appendages to move around, to build their homes, to fight, to feed, and sometimes to help mate.
"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."
He explained that the tracks, which are only a few millimetres in width, had to be spotted by tilting rock slabs at different angles.
"The key challenge is to get the lighting right so that the fossils stand out against the background, because the fossils have very low relief," said Professor Xiao.
Animals with bilaterally paired appengages are assumed to have appeared during the Cambrian Explosion, but now their ancestry may be traceable to even further back in history.
An international team of scientists, including researchers from the Nanjing Institute of Geology and Palaeontology, and Virginia Tech in the United States, conducted the study.
They analysed the trackways in rock, usually created when an animal's weight causes a depression in the ground which is later filled in by a different sediment.
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The trackways are irregular, the scientists found, with two rows of imprints that suggest they were created by a bilaterian animal whose appendages raised it above the ground.
No body fossils for these animals have been found yet, however, and the scientists believe such remnants may not have been preserved.