Early human migrations in southern Africa

Comparing the DNA profiles of living people who are indigenous to different parts of the world has achieved a lot as regards tracing the migrations of their ancestors and amalgamations between and separations from different genetic groups along the way. Most such analyses have centred on alleles in DNA from mitochondria (maternal) and Y chromosomes (paternal), and depend on the assumption that rates of mutation (specifically those that have neither negative nor positive outcomes) in both remain constant over tens of thousand years and genetic intermixing through reproduction. Both provide plausible hypotheses of where migrations began, the approximate route that they took and the timing of both departures from and arrival at different locations en route. Most studies have focused on the ‘Out of Africa’ migration, which began, according to the latest data, around 80 ka ago. Arrival times at various locations differ considerably, from around 60 ka for the indigenous populations of Australia and New Guinea, roughly 40 ka for Europe and ~12 ka for the Americas. Yet an often overlooked factor is that not all migrating groups have descendants that are alive today. For instance, remains of anatomically modern humans (AMH)have been found in sediments in the Levant as old as 177 ka (see: Earliest departure of modern humans from Africa, January 2018), and between 170 to 210 ka in southern Greece (See: Out of Africa: The earliest modern human to leave). Neither have yielded ancient DNA, yet nor are their arrival times compatible with the ‘route mapping’ provided by genetic studies of living people. Such groups became extinct and left no traceable descendants, and there were probably many more awaiting discovery. Maybe these mysteries will be penetrated by DNA from the ancient bones, should that prove possible.

The recorded history of AMH within Africa began around 286 to 315 ka in Morocco (see: Origin of anatomically modern humans, June 2017) and their evolutionary development may have spanned much of the continent, judging by previously discovered fossils in Ethiopia and South Africa that are older than 200 ka. Again, ancient DNA has not been extracted from the oldest fossils; nor is that likely to be possible because the double helix breaks down quickly in hot and humid climates. Genetic data from living Africans are growing quickly. An additional 198 African mtDNA genomes reported recently have pushed up the total available for analysis, the bulk of them being from eastern and southern Africa (Chan, E.K.F. and 11 others 2019. Human origins in a southern African palaeo-wetland and first migrations. Nature, v. 575, p. 185-189; DOI: 10.1038/s41586-019-1714-1). The study focuses on data from the KhoeSan ethnic group, restricted to areas south of the Zambezi River, who speak a language with distinctive  click consonants. Some KhoeSan still practice a hunter-gatherer lifestyle. Previous genetic studies showed the KhoeSan to differ markedly from other inhabitants of southern Africa, and they are widely regarded as having inhabited the area for far longer than any other groups. A sign of this emerges from their mtDNA in a genetic lineage signified as L0. Comparing KhoeSan mtDNA with the wider genetic database allowed the researchers to plot a ‘family tree’. Measures of the degree of difference between samples push back the origin of L0 and the KhoeSan themselves to roughly 200 ka.

The Okavango Delta today during the wet season (Credit: Wikimedia Commons)

It turns out that the LO lineage has several variants, whose geographic distributions allow the approximate place of origin for the lineage and directions of later migration from it to be mapped. It seems that LO was originally indigenous to the modern Okavango Delta and Makgadikgadi salt flats of Botswana. People carrying the original (L0k) variant are estimated to have remained in the broad area for about 70 thousand years. During that time it was all lush, low-lying wetland around a huge, now vanished lake. The hydrology of the area was dramatically split by regional tectonic activity at around 60 ka. The lake simply evaporated to form the salt pan of the Makgadikgadi, leaving only the seasonal Okavango Delta as a destination for flood water. People carrying Lok stayed in the original homeland whereas other shifted. Migration routes to the northeast and towards the southwest and south are crudely mapped by the distribution of the other L0 variants among modern populations. They followed ‘green corridors’ between 130 and 110 ka, the collapse of the ecosystem leaving a small group of the founding population isolated from its descendants.

The paper claims that the former Botswana wetlands were the cradle of the first modern humans. Perhaps in southern Africa, but other, older AMH remains found far off and perhaps undiscovered elsewhere are more likely. But that can only be reconciled with the KhoeSan study by ancient DNA from fossils. Criticism of the sweeping claims in the paper has already been voiced, on these grounds and the study’s lack of data on paternal DNA or whole genomes from the sampled population.

See also: Gibbons, A. 2019. Experts question study claiming to pinpoint birthplace of all humans. Science (online); DOI: 10.1126/science.aba0155