Scientists are suggesting that these trails - dubbed the Shibantan trackways - could very well be the oldest footprints on Earth. All previously identified animals with paired appendages were from the Cambrian period (530-540 million years ago), a time that witnessed an outburst of different and more complex forms of life. "The footprints are organized in two parallel rows, as expected if they were made by animals with paired appendages," Xiao writes. "Also they are organized in repeated groups, as expected if the animal had multiple paired appendages." The identity of what made the tracks, Xiao said, "is difficult to determine in the absence of body remains at the end of the trackways."