The oldest known footprints on Earth, left by an ancient creepy-crawly more than 500 million years ago, have been discovered in China.
Previously, scientists had discovered footprints as old as 530-540m years, but none predating the Cambrian period, which also began at this time and marked an explosion in the diversity and complexity of life on Earth.
"The footprints are organised in two parallel rows, as expected if they were made by animals with paired appendages".
The trackways were found in the Yangtze Gorges area of South China, and date to the Ediacaran Period 541 million to 635 million years ago.
"The irregular arrangement of tracks in the. trackways may be taken as evidence that the movement of their trace maker's appendages was poorly coordinated and is distinct from the highly coordinated metachronal (wave-like) rhythm typical of modern arthropods", the Chinese and American team led by Dr Shuhai Xiao, from Virginia Tech in the U.S., wrote in the journal Science Advances.
The presence of paired appendages (a primitive version of legs and arms) in the anatomy of this prehistoric creature is mirrored in the way the fossil footprints are laid out, Xiao explains.
The research by a Chinese team appears in Science Advances journal.
The trackways are somewhat irregular, consisting of two rows of imprints that are arranged in series or repeated groups, according to the study.
This is a group of animals characterised by having paired appendages - in this case, perhaps, paired legs.
"Animals use their appendages to move around, to build their homes, to fight, to feed, and sometimes to help mate", one of the researchers, geobiologist Shuhai Xiao from Virginia Tech University, told The Guardian.
No body fossils for these animals have been found yet, however, and the scientists believe such remnants may not have been preserved.
Trackways and burrows excavated in situ from the Ediacaran Dengying Formation.
Still, due to the proximity of the track marks to fossilised burrows discovered nearby, the researchers hypothesise the creature may have exhibited "complex behaviour", such as periodically digging into sediments to mine oxygen and food among its riverbed habitat. The fossils date back to almost 3.5 billion years ago and are strong evidence of the earliest life that existed on Earth.
While science has previously recorded that bilaterian animals, such as arthropods and annelid worms, first emerged during the "Cambrian Explosion" (541 to 510 million years ago), this finding proves that these creatures actually evolved earlier, confirming the suspicion shared by some researchers.