Origin: A Genetic History of the Americas

By the beginning of the Upper Paleolithic, H. sapiens populations living
outside of Africa were large enough and sufficiently geographically
dispersed to accumulate genetic variation that distinguished populations in
different regions of the world. The genome of an ancient man whose
remains were found in Tianyuan Cave in northern China showed that by
about 43,000 to 40,000 years ago, you could tell the difference genetically
between people living in western Eurasia and people living in East Asia,
although the differences were subtle and there was still considerable mixing
between them i (1).
GENETIC LEGACIES FROM ARCHAIC HUMANS
Anatomically modern H. sapiens (AMHS), or humans who look
like us, interbred with Neanderthals sometime around 65,000
and 50,000 years ago and with Denisovans sometime around
55,000 and 45,000 years ago (and possibly at other times as
well). These introgression events, as geneticists rather primly
call them, took place in different regions of the world.
Subsequent movement and mixing of people have led to these
traces of ancestry dispersing throughout the globe. Currently,
traces of Denisovan ancestry are found mostly in populations
living in Asia, South Asia, Southeast Asia, and Melanesia.
Traces of Neanderthal ancestry are seen at highest levels in
East Asians, Europeans, and North Africans. There are low
levels of Neanderthal ancestry in East African populations,
possibly because of more recent gene flow from AMHS carrying
these alleles.
Traces of gene flow from early AMHS have been found in the
genomes of Altai Neanderthals. So far, no Neanderthal
mitochondrial lineages have been found in AMHS populations,
which strongly suggests that these mating events were between
Neanderthal males and AMHS females. There’s significant
evidence that our genomes have been slowly purging
themselves of most archaic-derived ancestry generation by
generation, but there are a few exceptions. Some alleles that