Population Y was proposed as a distinct ancestral component that contributed to some South American populations and that was most genetically similar, among modern and ancient populations, to Australasians — specifically to populations from Australia, New Guinea, and the Andaman Islands.

The paper was careful about what it was and was not claiming.

It was not claiming that modern Australians had sailed to South America.

It was claiming that the statistical signal in certain South American genomes was most similar to the signal expected from a contribution of Australasian-like ancestry — the kind of ancestry that, in the deep human past, characterized the populations that spread out of Africa early and populated the southern route through Asia toward Australia and Melanesia.

The paper proposed several models that could explain the signal.