NEW YORK – The ancestors of all humans alive today likely had a sustained homeland in a region of Africa that is now part of northern Botswana, according to a mitochondrial DNA analysis conducted by an international team of researchers.
Anatomically modern humans arose in Africa about 200,000 years ago, but exactly where this occurred has been unclear. Early skeletal remains have been recovered in eastern Africa, but genetic data has pointed to the emergence of modern humans in southern Africa.
By analyzing more than 1,200 mitogenomes, researchers led by the Garvan Institute of Medical Research's Vanessa Hayes traced a deep branching mitochondrial lineage back to find that this L0 lineage arose in a paleo-wetland about 200,000 years ago before dispersing out of that region some 70,000 years later when a green corridor opened up following a climatic shift.
"We have known for a long time that modern humans originated in Africa and roughly 200,000 years ago, but what we hadn't known until this study was where exactly this homeland was," Hayes said during a press briefing to discuss the new analysis, which appeared in Nature today.
The maternal human phylogenetic tree has two major branches, but the deep-rooted L0 lineage, which includes mostly southern African haplogroups, is the oldest. For the study, Hayes and her colleagues focused on the L0 lineage and sequenced the mitochondrial genomes of 198 contemporary southern African individuals with that haplotype, both Khoesans and non-Khoesans. They folded these new mitogenomes in with 1,019 other, previously published L0 mitogenomes.
The researchers then traced the emergence of the L0 lineage to about 200,000 years ago, about 20,000 and 50,000 years earlier than previous estimates. This lineage remained stable for about 70,000 years, a period for which the researchers did not find any new divergences. "These early humans must have stayed within the homeland region and not left this region," Hayes said.
By adding geographic data on top of the mitochondrial genome data, the researchers found that the L0 lineage likely arose in the Makgadikgadi-Okavango paleo-wetland, a lush green region that was twice the size of modern-day Lake Victoria, south of the Zambezi River, and was surrounded by drier regions.
About 130,000 years ago, though, people did begin to disperse out of this region, the researchers noted, first to the northeast. One of these split-off lineages is still found northeast of the Zambezi River today, but not to the south of it. About 110,000 years ago, though, people also began to move to the southwest, as the researchers observed more lineage splitting occurring there, and these lineages are still found among modern southern African populations but not among populations that live north of the Zambezi River.
Archaeological data also indicates that modern humans were present there around 100,000 and 160,000 years ago, the researchers noted.
Using climatic data and models, they tied these two dispersal events to a time of increased humidity in those outlying areas, which likely opened up green corridors through which people could migrate.
Then between 100,000 and 110,000 years ago, the Makgadikgadi-Okavango region itself became drier, likely also prompting migration from the region, though others remained in what the researchers suggest is a human homeland.
"We used genomic data, we overlaid [it] with multiple other disciplines to really tell the story of our earliest maternal lineage," Hayes said. "We pinpoint a homeland region. We discover that modern humans appear to thrive there for 70,000 years and then we describe the first human explorations."