Over half a billion years ago, an early creature scuttled across a patch of seafloor that would eventually become China. Preserved for millennia in the rock, these fossil imprints are thought to be the earliest footprints ever discovered.
But there’s a catch, since scientist didn’t think that legs evolved until millions of years later.
The trace fossils have been discovered in rocks dating to between 551 and 541 million years old, during a period known as the late Ediacaran. This is of real significance because it was previously believed that complex life that we might recognize – including the evolution of appendages such as legs – did not really kick-start until the period that followed, known as the Cambrian.
During the Ediacaran plants were yet to colonize land and the oceans were dominated by simple, soft multicellular organisms that resembled worms, fronds, and weird sedentary bags. It was thought that it was only during the Cambrian explosion, which occurred approximately 541 million years ago, that life suddenly became more complex and the oceans around the planet were abruptly teaming full of strange arthropods, trilobites, and even early mollusks.
Yet the fossils found in the Yangtze Gorges area of South China are set to question this order of events. The tracks seem to show that a small bilaterian animal – meaning that it is bilaterally symmetrical like you, me, and crabs – scampered across a microbial mat on a sea floor, seemingly going from burrow to burrow.
The trackways, described in Science Advances, show two rows of imprints that are arranged in repeated units, not unlike a crustacean or insect. As this was a time before hard shells had evolved, it is impossible to say what the animal that left the marks all that time ago looked like, as its soft body has not been preserved.
“Animals use their appendages to move around, to build their homes, to fight, to feed, and sometimes to help mate,” Professor Shuhai Xiao, senior author of the research, told The Guardian. “It is important to know when the first appendages appeared, and in what animals, because this can tell us when and how animals began to change the Earth in a particular way.”
The "particular way" in which Professor Xiao is describing relates to the churning up of the sediment as such animals would if they were wandering along the floor. You might not think that such little animals could make much of a difference, but even tiny creatures today are thought to be influencing the major ocean currents. These fossils suggest that perhaps animals were altering the nutrient cycle much earlier than anyone previously thought.