The earliest ancestors of anatomically modern Homo sapiens emerged in a region south of the Zambezi River in Botswana, Africa, according to a new analysis of modern human’s mitochondrial genomes (mtDNA or mitogenome) from the L0 lineage, the oldest known mtDNA lineage on Earth.

“mtDNA acts like a time capsule of our ancestral mothers, accumulating changes slowly over generations,” said Professor Vanessa Hayes, a researcher at the Garvan Institute of Medical Research, the University of Sydney, and the University of Pretoria.

“Comparing mitogenome from different individuals provides information on how closely they are related.”

In the study, Professor Hayes and colleagues collected blood samples to establish a comprehensive catalogue of mitogenomes from the L0 lineage.

“We merged 198 new, rare mitogenomes to the current database of modern human’s earliest known population, the L0 lineage,” said University of Pretoria’s Professor Riana Bornman.

“This allowed us to refine the evolutionary tree of our earliest ancestral branches better than ever before,” added Dr. Eva Chan, from the Garvan Institute of Medical Research.

By combining the L0 lineage timeline with the linguistic, cultural and geographic distributions of different sub-lineages, the scientists revealed that the first anatomically modern Homo sapiens maternal lineage emerged 200,000 years ago in a homeland south of the Greater Zambezi River Basin, which includes the entire expanse of northern Botswana into Namibia to the west and Zimbabwe to the east.

The study authors also analyzed geological, archeological and fossil evidence and found that this region once held Africa’s largest ever lake system, Lake Makgadikgadi.

“Prior to modern human emergence, the lake had begun to drain due to shifts in underlying tectonic plates,” said Dr. Andy Moore, a geologist at Rhodes University.

“This would have created, a vast wetland, which is known to be one of the most productive ecosystems for sustaining life.”

The ancient wetland ecosystem provided a stable ecological environment for modern humans’ first ancestors to thrive for 70,000 years.

“We observed significant genetic divergence in the modern humans’ earliest maternal sub-lineages, that indicates our ancestors migrated out of the homeland between 130,000 and 110,000 years ago,” Professor Hayes said.

“The first migrants ventured northeast, followed by a second wave of migrants who travelled southwest. A third population remained in the homeland until today.”

“In contrast to the northeasterly migrants, the southwesterly explorers appear to flourish, experiencing steady population growth.”

“The success of this migration was most likely a result of adaptation to marine foraging, which is further supported by extensive archaeological evidence along the southern tip of Africa,” the researchers noted.

To investigate what may have driven these early human migrations, the team reconstructed Southern Africa’s climate history for the past 250,000 years.

“Our simulations suggest that the slow wobble of Earth’s axis changes summer solar radiation in the Southern Hemisphere, leading to periodic shifts in rainfall across southern Africa,” said Professor Axel Timmermann, Director of the IBS Center for Climate Physics at Pusan National University.

“These shifts in climate would have opened green, vegetated corridors, first 130,000 years ago to the northeast, and then around 110,000 years ago to the southwest, allowing our earliest ancestors to migrate away from the homeland for the first time.”

“These first migrants left behind a homeland population. Eventually adapting to the drying lands, maternal descendants of the homeland population can be found in the greater Kalahari region today,” Professor Hayes said.

The research is described in a paper in the journal Nature.

_____

Eva K.F. Chan et al. Human origins in a southern African palaeo-wetland and first migrations. Nature, published online October 28, 2019; doi: 10.1038/s41586-019-1714-1