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.
While the footprints were well-preserved, scientists don't know exactly what animal made the tracks since there were no body fossils found in the site.
"This is considered the earliest animal fossil footprint record", the researchers wrote in the report.
This means that the symmetrical creature appeared before the Cambrian Period, Chen noted.
While bilaterian animals - including arthropods and annelids - were suspected to have first stretched their innovative legs prior to the Cambrian explosion, in what's called the Ediacaran Period, before now there was no evidence for it in the fossil record.
The Chinese and American team led by Dr Shuhai Xiao, from Virginia Tech in the USA, wrote in the journal Science Advances: "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".
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The fossils date to the Ediacaran Period, which was between 635 to 541 million years ago.
As Xiao explains, knowing when the first legged animal appeared on Earth is a crucial detail, considering that the movement of sediments triggered by that first walking creature as it trotted over our planet's surface could have had a major impact on the Earth's geochemical cycles and climate.
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. Carved in limestone, the trackways consist of two rows of imprints arranged in repeated groups.
Further, the footprints left behind by its multiple feet suggest that this sea-dwelling animal had paired appendages that raised its body above the ocean floor.
The study claims that the footprint dates to about ten million years before the explosion that took place in Cambria.
"But unless the animal died (and preserved) next to its footprints, it is hard to say who made the footprints", he said. The trackways also indicate a connection to burrowing, suggesting that whatever animal made the tracks might have had a habit of digging into sediments.
As modern arthropods and annelids served as appropriate analogs for the interpretation of this fossil, the researchers posit the animal in question could be the ancestor of either of the two groups.