Scientists in China have discovered fossils of what they believe to be the oldest footprints seen so far. About 551 years ago some animal walked through the mud next to what is now the Yangtze river and left its tiny footprints in the soft ground. By chance, these prints were preserved and fossilised so that we can see them today. The scientists can distinguish the pattern made by the animal from other sorts of fossilised structures. The tracks clearly show two rows of impressions left in the mud. Each row consists of groups of little indentations. This resembles the pattern that a small animal with rows of paired legs. It is distinct from other patterns which could be linked to burrows or holes.

^{Image from Zhe Chen et al, Sci. Adv. 4 eaao6691 (2018) (CC BY-NC)}

Why is this find so important?

Scientists generally believe that the life on Earth grew rapidly during the Cambrian explosion. This was a period in Earth's history in which the number of different species increased over a short period of time. The Cambrian explosion is dated by most experts to have happened between 540 to 510 million years ago. During the Cambrian explosion, complex life forms evolved. It was generally thought that before this event only simple life forms existed on Earth.

The new find dates more than 10 million years before the Cambrian explosion. It was not expected to find any indication of a complex life form for this time. What is more the sediment in which the footprints were found have been dated quite precisely. This means there is little margin for error and the footprints are with great certainty really older than the Cambrian explosion.

Unfortunately, the researchers cannot identify exactly what type of animal was responsible for the footprints. This would only be possible if the animal had died near the footprints and would have also been preserved in the same way. So the scientists have to guess what the animal might have been.

There are three types of species that have paired appendages such as the animal would have had. The first group are arthropods. Arthropods include all types of insects such as bees or flies, all types of spiders but also crabs. The second type of animal with paired legs are the annelids. This group contains all many worms and leeches. The third group are the tetrapods which include humans but also dinosaurs. Tetrapods have developed only very much after the Cambrian explosion. This means that the footprints must have come from either an early arthropod or annelid.