I wondered if this latest report could be the result of a fluke, people clinging to a tree trunk across the Pacific to wash up at (modern) California. It's thought that a lot of species in the Americas originally rafted that way across the Atlantic from Africa and Europe; why not humans? Why not the Pacific? (I should check trade winds and currents, but I'm just speculating, not authoring a paper.) And if the accident didn't transport a breeding pair, yep, dead-end, just a smear of DNA on the soil.

Solitary human males tend to leave a lot of DNA smears on the ground.

I think you're probing a conceptual blind spot most of us have. We think of the first people in a place as the founders of a population we can easily trace forward in time, as we've been able to do with the more recent human discoveries (Clovis, etc.).

But sometimes Og just wanders away from the cave blind-drunk on fermented berries, and just keeps walking. Or torrential rains wash the tree to which you're clinging down the river and out to sea. Shit happens, and most of the time there's little trace of it.

That's the fate of most of humanity, just a smear of DNA in the soil.