The Khoisan people currently reside in southern Africa and maintain a hunter-gatherer lifestyle.
Their numbers have been in decline ever since the “Bantu Expansion” around 3,000 years ago, when a population of subsistence farmers from West Africa made an eastward and southward expansion.
Despite their dwindling numbers, they maintain the greatest genetic diversity of all human populations.
They also have the most ancient Y-chromosome and mitochondrial DNA lineages (two types of molecular clocks) of any human population on Earth.
High genetic diversity is normally associated with large population groups, so what’s the deal?
There are two possibilities: Khoisan people had either a large ancestral population, or lots of recent interbreeding. Stephan Schuster, Hie Lim Kim, and their colleagues used high-powered genomic testing to shed some light on the mystery.
Their new study analyzed the entire genome of five Khoisan individuals and compared their results to genomic information from other major human populations.
They uncovered three major clues:
First: there was very little genetic evidence of interbreeding with other populations.
Second: the Khoisan group appears to have been the first to split off from the rest of the other early human populations, around 150,000 years ago.
They are distinct not only from Asians and Europeans, but also from other Africans. This explains why they have such ancient DNA.
Third: the population size of all human groups in the study declined dramatically between 30,000-120,000 years ago, but the Khoisan population declined the least.
Because of that slower decline, Khoisans have been the most populous ethnic group throughout much of the last 120,000 years of human history.
The team suggests that the same climate changes that negatively affected most human populations lead to more favorable conditions in the areas that the Khoisan population inhabited.
Most of Africa became much drier. But the Southern Khoisan region likely grew wetter. It's a lot easier to be a hunter-gatherer when things are growing and animals are thriving.