yes, 20th does include most of the world population. Humans started out within 20 degrees of the equator for the vast majority of human history, and eventually fanned out, but it takes longer than a couple thousand years to make up for the loss in D3 synthesis.

edit: it could be 25th or 30th, i have a hard time finding a single paper amongst thousands, either way it includes 90% of the population of the US, regardless of what parallel.