Scientists say modern humans arose in northern Botswana

The ancestral home of anatomically modern humans can be traced back 200,000 years to northern Botswana, according to a new study that challenges the idea that they emerged in East Africa’s Great Rift Valley.

Today the region is dominated by dry salt pans and desert, but it was once home to the enormous Lake Makgadikgadi, which broke up to form a lush wetland that supported a wide variety of life.

“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 this homeland was,” said University of Pretoria researcher Vanessa Hayes, who is one of the co-authors of the research, published on Monday in the peer-reviewed journal Nature.

Hayes, who is also affiliated with the Garvan Institute of Medical Research in Australia, is part of a multidisciplinary team of international scientists that merged new and rare mitochondrial DNA (mtDNA) from 198 South Africans and Namibians of Khoe San descent with the current database of modern humans’ oldest known lineage, to trace their maternal line back in time to a “homeland” in the heart of Southern Africa.

MtDNA is genetic material that is passed from a mother to her children, and changes so little from one generation to the next that a modern-day person’s mtDNA is in all likelihood identical to their maternal ancestor dozens of generations ago.

When rare mutations did occur in the past, a woman’s descendants were likely to live near her, and their unique mtDNA “signature” would move with them if they migrated.

The team used geological, archaeological and fossil evidence to show that the region to which they had traced the mtDNA lineage back 200,000 years ago was at that stage home to a huge wetland, twice the size of Lake Victoria, and surrounded by an extremely dry, and inhospitable landscape.

The mtDNA data indicates that there were no splits in the lineage until 130,000 years ago, when climate change increased the region’s rainfall and opened up a new green “corridor” that enabled modern humans to migrate to the northeast.

A second green corridor opened up to the southwest about 110,000 years ago. These dispersals paved the way for modern humans to later migrate out of Africa across the world.

University of WisconsinMadison anthropologist John Hawks, who was not involved in the study, said scientists face an immense challenge trying to piece together the puzzle of early modern human evolution in Africa, and the evidence is not yet good enough to say exactly how or where it happened.

“MtDNA can only tell one small part of the story, because it is a single locus that reflects only a single line of ancestry,” he said.

Data obtained from nuclear genome sequencing conflict with the mtDNA story, he said. For example, the ancestry of Khoe San-speaking people show evidence of differentiation from other Africans as early as 300,000 years ago. Thus some scientists favour the idea of multiple regions in Africa giving rise to modern humans, rather than a single place.

Hayes said mtDNA is the most reliable genetic tool currently available for timeline predictions, but conceded it cannot yield information about ancestral lines that became extinct.

“Our convergence time is absolutely in line with fossil records. We are all fully aware of the limitations of all our disciplines when predicting the past,” she said.