At https://archaeologynewsnetwork.blogspot.co.uk/2016/06/earliest-footprint… … footprints in the dry region of Eritrea have been dated to 800,000 years ago. Humans living at that time, assumed to be Homo erectus, left behind footprints in what had been sand or silt on the side of a lake – or possibly on the dried up bed of a lake. The footprints are very similar to those of modern people which says a lot about Homo erectus – how different were they really? They appear to have been left behind together with the hoof prints of extinct Pleistocene antelopes. Were they both in flight from something – or were they in pursuit of the animals? You have to read the full article to find that out but suffice to say the sand hardened over time (or possibly even immediately) in order for them to be preserved in situ so to speak.