And then they found a human skull.
The Punín-1 skull came out of sediment that, geologically, appeared to be Pleistocene in age. The same layers that contained the mastodon bones contained, apparently, a human skull. If the skull was really as old as the sediment suggested, it would be extraordinarily significant: evidence of human presence in South America during a period far earlier than the existing scientific consensus about when people had arrived in the Americas.
But the skull had other features that attracted attention. Its prominent brow ridges — the projecting bone above the eye sockets — were notable for what appeared to be an archaic character. The skull looked, to some of the scientists who examined it, like something that might not be fully modern Homo sapiens. It prompted comparisons to skulls from other parts of the world in ways that generated two distinct hypotheses.