Population Bottlenecks and Volcanic Winter
The "Weak Garden of Eden" model for the origin and dispersal of modern humans posits a spread around 100,000 years ago followed by population bottlenecks. Then, around 50,000 years ago, a dramatic growth occurred in genetically isolated, small populations. In a 1998 article, Stanley Ambrose proposed an alternative hypothesisa volcanic winter scenarioto explain recent human differentiation. The bottleneck was caused by a volcanic winter resulting from the super-eruption of Toba in Sumatra. If Ambrose's hypothesis is correct, modern human variations differentiated abruptly through founder effect, genetic drift, and adaptation to local environments after around 70,000 years ago.
Ambrose points out that the Out of Africa dispersal date of around 100,000 years ago fits the generally warm, humid last interglacial period, 130 -74,000 years ago. An impressive body of paleontological evidence shows an Afro-Arabian biotic community expanded northward during this period. Several such multi-species dispersals out of Africa have occurred during previous interglacial phases. He considers the variants of the Replacement model to be more accurate and realistic than the Multiregional models.
The number of DNA mutations within a population increases temporally. When a population has passed through a bottleneck, the mutation distribution evidences the bottleneck. DNA studies have identified a significant bottleneck (or bottlenecks) during the last glacial period.
The Multiple Dispersals model proposes a population bottleneck occurred when cold, dry climates isolated populations in Africa. Additional bottlenecks occurred through physical bottlenecks such as the Sinai Peninsula. The first dispersal of anatomically modern humans, to the Levant around 100,000 year ago, is evidenced by early modern human skeletons in the Near East. According to Ambrose, this first dispersal apparently failed to permanently establish modern humans outside of Africa. Genetic evidence shows that non-African populations can be divided into southern Australasian and northern Eurasian populations that divided 50-75,000 years ago.
In contrast, Ambrose's model proposes a scenario of a globally synchronous bottleneck. If bottlenecks were caused by the cold climate, duration was approximately 10,000 years with release 60,000 years ago. If the eruption of Toba alone caused the bottleneck, then release may have followed within a few decades of the volcanic winter 71,000 years ago, or the bottleneck could have lasted 1000 years, during the coldest portion of the Ice Age following the Toba eruption. In the bottleneck scenarios, more individuals survived in the African tropical refugia, resulting in the greatest genetic diversity survival in Africa.
Ambrose concludes that bottlenecks occurred among genetically isolated human populations because of a six-year long volcanic winter and subsequent hyper-cold millennium after the cataclysmic super-eruption of Toba. This volcanic winter played a role in recent human differentiation. The resultant combination of founder effects and genetic drift may account for low human genetic diversity as well as population differences associated with so-called races. The bottleneck hypothesis offers an explanation for why humans exhibit so little genetic variation, yet superficially appear diverse. It also affords an explanation for the apparent recent coalescence of mtDNA and African origins.
Ambrose, Stanley H. 1998. Late Pleistocene human population bottlenecks, volcanic winter, and differentiation of modern humans. Journal of Human Evolution 35:115-118.