The oldest known "footprints" left by an animal have been uncovered in southern China.
The identity of the creature that made the 546-million-year-old tracks is still unknown, but they come from the period when the earliest animals are thought to have evolved.
The fossil consists of two rows of imprints that represent the earliest known record of an animal with legs.
The research by a Chinese team appears in Science Advances journal.
Team-members are unclear whether the creature had two legs or several. But they say the tracks probably belong to a bilaterian.
This is a group of animals characterised by having paired appendages - in this case, perhaps, paired legs. They are one of the most diverse animal groups in existence today.
These legs raised the animal's body above the sediment it was moving across.
The trackways were found in the Yangtze Gorges area of South China. The rocks they come from are dated to between 551 million and 541 million years old.
"Previously identified footprints are between 540 and 530 million years old. The new fossils are probably up to 10 million years older," the study's co-author Zhe Chen, from the Chinese Academy of Sciences, told AFP.
He added: "At least three living groups of animals have paired appendages (represented by arthropods, such as bumblebees; annelids, such as bristle worms; and tetrapods, such as humans).
"Arthropods and annelids, or their ancestors, are possibilities."
The animal appears to have paused from time to time, since the trackways seem to be connected to burrows that may have been dug into the sediment, perhaps to obtain food.