Since the dramatic paper which I just posted on came out, suggesting a doubling of time that various Siberians have been in the Americas, from 15K y/a to 30K, there have been cautionary responses. First, identifying stone tools can be difficult, especially in caves, as stalactites or other falling rocks landing on stones and breaking them into sharp pieces is not very different from striking a stone on purpose to make a spearpoint on first hit. Sorting out what is intentional and what is accidental is difficult.
But to the larger point, the amount of genetic divergence is not absolutely reliant on groups arriving at widely different times. There is a fully formed theory - with some evidence - that groups lived in different parts of Beringia right from the start, in isolation from each other, coming into the New World by various routes and times. Though they were genetically similar when they expanded from what are now the Anadysky and Chutotka Districts of Russia (East Siberia, sort of Kamchatka on the Risk board), they didn't follow the same route, eat the same stuff, or move at the same time. There were more than a few, but most died out. There were main survivors who were the ancestors of most of the first arrivals, the Amerinds, but a few others who contributed at least a bit to the pool here and there.
They didn't have to arrive 30K years ago to do this. Moving out from the main interactive group in Siberia at 30K, staying in diverse place in Beringia 22K-15K, then moving into the New World then also fits the genetic pattern, even with the weird bits.
Even with a time machine you would need helicopters with a lot of fuel to check all this out - and you'd need to know where to look.
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