Footprints
"Footprints uncovered in Kenya show that as early as 1.5 million years ago an ancestral species, almost certainly Homo erectus, had already evolved the feet and walking gait of modern humans."
This is truly fascinating, particularly with the accompanying photograph. I love little glimpses like this into the distant past, and a footprint is such a vivid and relatable image (and a ready-made metaphor of course.) 1.5 million years ago someone who was not yet fully human but with a foot essentially the same as ours walked upright, and we can see their footprint. For some reason this really touched me.
The only earlier prints are apparently more than twice as ancient, and much more apelike.