Over time, this group — spread across a large area and subject to the ordinary processes of isolation and genetic drift — begins to differentiate.

One branch separates and continues along a distinct trajectory.

The remaining population continues to differentiate, eventually producing the various lineages that we associate with North and South American Indigenous populations.

Under the model proposed in the 2018 paper, the lineage leading to USR1 was one of the early branches — perhaps the earliest well-documented one.

The fact that it diverged before other sampled populations does not make it the ancestor of those populations.

It makes it a sister lineage — a branch that shares a common ancestor with the other branches but that represents a distinct trajectory from the point of divergence.