That's hundreds of millions of years before dinosaurs started roaming Earth, about 245 million years ago.
The scientists in China have discovered the fossilized animalfootprints of the ancient times. They observed the trackways, which were not regular, and after analyzing their characteristics, they reached the conclusion that these were formed by bilaterian animals with paired appendages.
For comparison, non-bilateral animals include sponges, corals, jellyfish, and anemones.
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.
The researchers don't yet know exactly what animal left these tracks, and unfortunately we may never know.
The scientists weren't able to locate the body fossils of the animals that made these traces.
Xiao's team found the footprints while tilting rock slabs at different angles. Precisely what the creature looked like is a mystery, though: nothing this old with legs has been discovered to date.
Scientists from the Chinese Academy of Sciences' Nanjing Institute of Geology and Paleontology and Virginia Tech have teamed together to find the prints and analyze them.
"Ediacaran trace fossils provide key paleontological evidence for the evolution of early animals and their behaviors", researchers write in their study.
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.
The Ediacaran Period happened before the Cambrian Explosion, which is when life on Earth began to increase and diversified. Today, these creatures are spread throughout the Earth and are among the most diverse forms of animal life on the planet, notes the report.
The trackways appear to be connected to burrows, suggesting that the animals may have periodically dug into sediments, perhaps to mine oxygen and food.
He also said that arthropods and annelids or their ancestors are possible.
"Although the exact identity of the trace maker of the Shibantan trackways is hard to determine in the absence of body remains at the end of the trackways, we suggest that the trace maker was probably a bilaterian animal with paired appendages", the authors reported.
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