Last updated November 15, 2019 at 3:31 pm
Australian scientists have scoured human DNA to discover modern human’s hometown, and constructed the story of our spread from Africa.
Why This Matters: You’ve always got a link to your hometown. The question is where is ours?
A vast inland oasis in present-day northern Botswana was the hometown of all modern humans, according to a genetic analysis of modern-day Africans.
Hundreds of thousands of years ago modern humans began spreading throughout and beyond Africa, but the original site where they first lived has been one of controversy. Competing theories abound, and some experts argue there is no single site. However, Australian researchers believe they have tracked down the place all humans can call home.
And while it is now an area of desolate salt flats, 200,000 years ago life there would have been very different, say the researchers.
Genetic analysis reveals our hometown
All people belong to a particular haplogroup depending on the sequence of their mitochondrial DNA. Researchers then construct a genetic tree of these haplogroups, which can track where different populations first cropped up and how people dispersed around the world.
The most ancient haplogroup in modern people is L0 (L-zero). To find out where it first arose, the researchers examined the mitochondrial DNA from around 200 Khoe-Sān people, an indigenous foraging community whose DNA has been rarely studied. Those results were then compared with around 1000 other mitochondrial genomes from around Africa. From this, they could then refine the earliest branches of the human evolutionary tree.
Because this lineage is today found only in people in southern Africa, the researchers say it’s likely that people carrying the L0 lineage lived in southern Africa and formed the ancestral population for all living humans,
“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 and geological records, the team was then able to reconstruct a vivid picture of an ancient human homeland.
The story of humanity’s rise
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, began partially drying and forming smaller lakes. This created a vast fertile region called the Makgadikgadi–Okavango wetlands, which would have been similar to today’s UNESCO heritage site the Okavango delta, also in Botswana.
“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.
We are all Khoe-San
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.
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.
Ultimately, ancient genomes, rather than modern ones, could help to fill in the gaps, he says.
Not all experts convinced
The results have also raised skepticism among some international researchers.
“I’m persuaded that southern Africa was an important area for human evolution,” population geneticist Aylwyn Scally from England’s University of Cambridge told the journal Science. He questioned whether studies of living people’s DNA can reveal the precise location of ancestors. “It would be astonishing if all our genetic ancestry at this time arose in one small homeland.”
Sarah Tishkoff of the University of Pennsylvania also finds the results problematic. She argues had the researchers traced both the mothers mitochondrial DNA lineage, and the evolution of the Y chromosome inherited from fathers, the results may have come out differently.
However, Hayes responded that mitochondrial DNA was an accurate tool for tracking human lineages, as it doesn’t get shuffled in early foetal development the way some other genes do. “It acts like a time capsule for our ancestral mothers.”
Additionally, most of the Khoisan speakers’ Y chromosome data has disappeared as men mixed with other groups, she told Science.
Tishkoff and others remain unconvinced, also suggesting that ancient populations could have moved vast distances across Africa, and only moved into the area in the millenia since the rise of the L0 lineage.
While it might seem comforting to think of humans as having a single hometown, it’s not the only possibility. While the scientific debate continues, one thing can be agreed upon – the emergence of humans around 200,000 years ago sparked off an ecological revolution that will likely never be seen on Earth again.
Additional reporting by Ben Lewis