A vast inland oasis in present-day northern Botswana was once home to the founder population of all modern humans, according to a genetic analysis of modern-day Africans.
The analysis, published in the journal Nature, used mitochondrial sequences, which are passed from generation to generation through the maternal lineage.
All people belong to a particular haplogroup, as determined by sequences in their mitochondrial genome. Constructing a genetic tree of these haplogroups can tell scientists where different populations first cropped up, and how people dispersed around the world.
The most ancient haplogroup in modern people is L0 (L-zero). The highest frequency of L0 haplotypes is in southern Africa in the Khoe-Sān people, indigenous foraging communities who speak language with “click” sounds.
Vanessa Hayes, from Australia’s Garvan Institute of Medical Research, and colleagues homed in on this ancient lineage, pooling more than a thousand L0 mitochondrial genomes from southern Africans to work out where the lineage first arose.
“It is a unique region of the world where these pockets still live today in genetic isolation,” says Hayes.
By combining genetic data with climate modelling, the team was able to reconstruct a vivid picture of an ancient human homeland.
According to the analysis, the L0 lineage was born approximately 200,000 years ago, around the same time that a massive prehistoric palaeo-lake twice the size of Africa’s largest lake, Lake Victoria, partially dried into a vast fertile region called the Makgadikgadi–Okavango wetlands.
“Modern humans appear to have thrived there for 70,000 years,” says Hayes. By her reckoning, there was little to draw them – or other large animals such as giraffes, lions and zebras – away from the oasis and into the surrounding arid landscape.
That changed around 130,000 years ago. At that time, a region to the northeast of the wetlands became more humid. This opened up a green corridor of vegetation that supported migrations away from the homeland.
A lineage that split off at this time is now only present in people north of the Zambesi River.
When a 15,000-year-long megadrought southwest of the homeland broke, favourably humid conditions developed there, too. People started wandering along that green corridor about 113,000 years ago and continued to disperse along the southern coast of Africa.
The lineages that split off from the ancestral homeland population at this time are only located south of the Zambesi River.
Drying of the homeland from 100,000-80,000 years ago would have reduced the carrying capacity and perhaps pushed additional early humans to venture out from the Makgadikgadi–Okavango wetlands.
A later-branching population in eastern Africa – known as L1’6 – went on to split into the populations that migrated out of Africa around 70,000 years ago and are ancestral to all non-Africans alive today.
Nevertheless, Hayes and her team suggest the homeland they have identified could be the location where the ancestor of all anatomically modern humans first evolved.
“I believe we were all Khoe-Sān at one stage,” she says.
But the new narrative doesn’t fit all of the evidence, according to human origins researcher Richard (Bert) Roberts, from the University of Wollongong in Australia, who wasn’t involved in the study.
For instance, he says, stone tools found on the southern coast of South Africa date to 167,000 years ago, which does not fit the new analysis.
Another find that would need to be explained is the 315,000-year-old skull from Jebel Irhoud in Morocco in the north of Africa, the oldest known member of our species.
“[It] begs the question of whether the full history of [anatomically modern human] occupation of the region is captured by this new dataset,” says Roberts, who also questions the conclusions that can be drawn from the mitochondrial genome alone.
“Several recent DNA studies have shown that rather different histories can be reconstructed using whole genomes instead of mitogenome,” he says.