Fifty-eight footprints, of two or three hominids walking north about 2.6 million years ago in what is now Tanzania, have been preserved by a lucky sequence of natural events. Fine volcanic ash from a neigbouring eruption was on the ground when light rain fell and formed with the ash a material like wet plaster. This retained a cast of the passing hominids' footprints, which then dried to become hard. A subsequent layer of volcanic ash sealed them and preserved them. They were discovered in 1976.
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