The analogy to human family structures is imperfect, as all analogies in population genetics are, but it may help: USR1 is not the mother of other Native American lineages.

She is more like a very distant great-aunt — someone who shares the same great-grandparent as all the others but who belongs to a branch of the family that went in a different direction from the others very early on.

The branching that created the Ancient Beringian lineage appears, based on the demographic modeling in the 2018 paper, to have occurred at a specific and significant point in time: approximately 20,000 years ago.

This date — rough, uncertain, carrying the standard uncertainty of population genetic estimates — placed the divergence of the Ancient Beringian lineage well before the arrival of the founding population in the Americas in the conventional sense, and approximately at the period that other lines of evidence associate with the Beringian standstill.