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.
The oldest known footprints on Earth, left by an ancient creepy-crawly more than 500 million years ago, have been discovered in China.
Previously, it was believed animals with pairs of legs capable of leaving such footprints first appeared during the "Cambrian Explosion" about 541 to 510 million years ago.
The characteristics of the trackways indicated that they were produced by bilaterian animals with paired appendages that raised the animal body above the water-sediment interface.
Researchers on the study came from the Nanjing Institute of Geology and Palaeontology of the Chinese Academy of Sciences, and Virginia Tech in the United States.
Although it's not clear what animal left these ancient tracks behind - since only the trace fossils (evidence that an animal has been there) were discovered, and not the fossils themselves - the footprints date back 551 million to 541 million years ago, to the Ediacaran Period.
However, this creature - which provides the earliest evidence of an animal with legs - would have existed around 10 million years before then. They published their findings Wednesday in the journal Science Advances.
This remarkable discovery is hailed in a study, published yesterday in the journal Science Advances by a research team from Virginia Tech University in the USA and the Nanjing Institute of Geology and Palaeontology (NIGP) of the Chinese Academy of Sciences.
"The trackways are somewhat irregular, consisting of two rows of imprints that are arranged in series or repeated groups", explained notes from the Chinese Academy of Sciences.
An worldwide 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. They are one of the most diverse animal groups in existence today.
"Animals use their appendages to move around, to build their homes, to fight, to feed, and sometimes to help mate", Xiao told The Guardian.
The scientists weren't able to locate the body fossils of the animals that made these traces. They hoped that the sunlight would reveal subtle marks left by ancient organisms on the Shibantan trackways.
Trackways and burrows excavated in situ from the Ediacaran Dengying Formation. This means that the mystery animal might have periodically dug into the ocean floor's sediments and microbial matts, possibly to mine for oxygen and food, the researchers said.