The new Nature paper argues that the ancestral homeland of modern humans today was in northern Botswana, based on mitochondrial DNA. The study (Chan et al. 2019) dates changes in the L0 lineage and finds that AMHs are descended from a small community of Khoisan hunter-gatherers who lived 200,000 years ago in vast wetlands encompassing Botswana’s Okavango Delta. The L0 lineage split 130,000 years ago when some of the founder population moved north-east along a green vegetated route. A second wave of migration headed south-west about 20,000 years later as rainfall also increased vegetation in that direction.

Abstract
Anatomically modern humans originated in Africa around 200 thousand years ago (ka)1,2,3,4. Although some of the oldest skeletal remains suggest an eastern African origin2, southern Africa is home to contemporary populations that represent the earliest branch of human genetic phylogeny5,6. Here we generate, to our knowledge, the largest resource for the poorly represented and deepest-rooting maternal L0 mitochondrial DNA branch (198 new mitogenomes for a total of 1,217 mitogenomes) from contemporary southern Africans and show the geographical isolation of L0d1’2, L0k and L0g KhoeSan descendants south of the Zambezi river in Africa. By establishing mitogenomic timelines, frequencies and dispersals, we show that the L0 lineage emerged within the residual Makgadikgadi–Okavango palaeo-wetland of southern Africa7, approximately 200 ka (95% confidence interval, 240–165 ka). Genetic divergence points to a sustained 70,000-year-long existence of the L0 lineage before an out-of-homeland northeast–southwest dispersal between 130 and 110 ka. Palaeo-climate proxy and model data suggest that increased humidity opened green corridors, first to the northeast then to the southwest. Subsequent drying of the homeland corresponds to a sustained effective population size (L0k), whereas wet–dry cycles and probable adaptation to marine foraging allowed the southwestern migrants to achieve population growth (L0d1’2), as supported by extensive south-coastal archaeological evidence8,9,10. Taken together, we propose a southern African origin of anatomically modern humans with sustained homeland occupation before the first migrations of people that appear to have been driven by regional climate changes.


Fig. 3: Reconstructed and simulated climatic conditions during the out-of-homeland migration.

During the out-of-homeland period (130–110 ka), our model simulation supports humid conditions to the northeast that facilitated the first dispersals, concurring with L0f coalescence (around 125 ka) (Fig. 3d). By contrast, the region southwest of the homeland experienced an approximately 15-kyr-long megadrought before an orbital shift created the favourable humid conditions that led to the dispersal of the L0d1’2 lineage (around 113 ka) (Fig. 3f), which is also supported by palaeo-data (Fig. 3c). This is also around the time the northeast L0a and southwest L0g migrants last share a common ancestor (around 117 ka). During the last glacial period (approximately 100–11 ka), we observe a reduction in the amplitude of the changes in orbital-scale hydroclimate and overall drying within the homeland (Fig. 3b), whereas the southwest coastal hydroclimate was dominated by precessional variability and showed relatively agreeable environmental conditions (Fig. 3c, f). Notably, periods of deceleration and acceleration in the estimates of the effective population size of the L0d1’2 lineage coincide with regional changes in hydroclimate, further linking climate, population size and evolution.

We propose that the Makgadikgadi–Okavango palaeo-wetland was the possible homeland of AMHs. Although one cannot exclude the possibility of a polycentric origin34, this deltaic–lacustrine ecosystem would have provided an ideal geographical locality for the evolution and 70-kyr-long sustained existence of the deepest-branching maternal founder population of AMHs. Increased humid conditions, supported by palaeo-lake system reconstructions35, between 130 and 110 ka would have opened green corridors for successful northeast–southwest migrations, supporting a pull scenario. Drying within the homeland following the out-of-homeland period, supported by hydroclimate data (110–100 ka) and a model simulation (100–80 ka), would have created a push scenario, in which a reduced carrying capacity of the land would have increased pressure to seek out climatically more favourable regions. We propose that the southwest migrants maintained a successful coastal forager existence, while the northeast migrants—similar to the later-branching population of L1’6—gave rise to ancestral pastoral and farming populations. A recent publication36 provides further mitochondrial evidence to support the northeast out-of-homeland migration route and expansion into eastern Africa around 70–60 ka. Revealing a southern African homeland for the emergence and extended subsistence of the L0 lineage, we propose that an out-of-homeland migration event, which was probably driven by astronomically induced regional shifts in hydroclimate, shaped the present-day ethnic and genetic diversity of modern humans.

https://www.nature.com/articles/s41586-019-1714-1