For comparison, non-bilateral animals include sponges, corals, jellyfish, and anemones.
Today, a crucial piece of the puzzle emerges as scientists unearthed the oldest known fossil footprints in history, revealing that bilaterian animals existed millions of years earlier than initially thought. "Unless the animal died and was preserved next to its footprints", said Xiao, "it is hard to say who made the footprints". Researchers say that these prints were made by creatures with appendages and this discovery is proof that there were indeed animals with limbs in the Ediacaran period.
Xiao's team found the footprints while tilting rock slabs at different angles.
Still, this discovery means that paleontologists will have to revise their vision of how life developed in Earth's primordial oceans.
Researchers from the Nanjing Institute of Geology and Palaeontology of the Chinese Academy of Sciences in collaboration with the Virginia Polytechnic Institute and State University examined trackways and burrows in sediment dating back to the Ediacaran Period.
Dozen of fossils were discovered in an Ediacaran Period Dengying Formation limestone strata, which dates from between 551m and 541m years ago, in the Three Gorges area of Yi Chang city.
The fossil reportedly consists of two rows of imprints that represent the earliest known record of an animal that has legs.
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.
"Previously identified footprints are between 540 and 530 million years old".
Near the ancient footprints, the team found fossilized burrows, which suggests that the animal might have been periodically tunneling into sediments and microbial mats, either in search of food or perhaps to mine for oxygen.
The trackways are the earliest discovered indication of when animals evolved appendages. Other research has suggested that the evolutionary roots of bilaterians should go back further than that, but fossils had never turned up until now.