How were human genetics shaped by early migrations from Africa
Early anatomically modern human evolved in Africa, possibly east Africa, at least 200,000 years ago. However, recent finds suggest this might be older. Population bottlenecks, or a reduction in population size, resulted in a relatively less genetically diverse population that would eventually influence all human populations today. Migration out of Africa ensured that the genetic makeup began to be less diverse as humans migrated to other regions and continents.
Population Bottleneck
Between 80,000-70,000 years ago, a population bottleneck has been proposed for modern humans. There is evidence of major climatic change at around this time, perhaps triggered by a super volcanic eruption activity in Lake Toba, Indonesia, that led to rapid global cooling. Some have suggested this led to a diminished modern human population, resulting in a more limited gene pool for modern humans. Others have suggested that the bottleneck may have been triggered much earlier, at about 100,000 years ago. In effect, what results do show is that the genetic diversity is relatively limited for many modern human populations. Where there is greater diversity is found in Africa itself, where different modern African populations show far greater genetic diversity than the rest of modern human populations in all other regions of the globe. Human in general are much less genetically diverse than many other types of animals. Overall, this lack of diversity has suggested a limited number of humans may have been responsible for the variety of populations today. The relatively greater genetic diversity in Africa suggests humans were evolving like many other species in Africa, creating new populations over time. Climatic effects or crises may have then triggered a large-scale migration out of Africa that subsequently led to the more narrow genetic diversity found today in other regions.
Migration From Africa
Scientists do not completely agree on all the genetic and archaeological data, but there may have been two main migrations from Africa (Figure 1). The first may have been relatively small and it may have occurred more than 140,000 years ago. This migration could have resulted in population spreading to West and East Asia, as far as Indonesia. However, the second main migration, between 80,000-50,000 years ago, seems to have been far more significant. In fact, nearly 98% of human genetics found in many global populations appear to trace their genetics to this second migration. In effect, this might have meant that the first migration may not have been successful or it resulted in a more limited population in different regions outside of Africa. The second migration, which could have consisted of sub-migration over the span of time, seemed to spread far and more rapidly across the globe.
Cro-Magnon are generally considered the first anatomically modern humans in Europe at about 50,000 years ago. Already nearly 1 million years ago, the earliest Homo species had migrated to Europe, likely via Eurasia. As modern humans expanded in Europe and Asia, they increasingly came across Neanderthals. At this point, humans were most likely competing with Neanderthals for resources and food. At times, the two species were similar enough to breed, which has resulted in some genetic makeup of modern humans have Neanderthals DNA (some populations range in having 1% to 4% influence in DNA from Neanderthals), but this seems to have been relatively limited. By 20,000 years ago, Neanderthals had disappeared.
In east Asia, east Siberia, Korea, and Japan may have been reached by 35,000 years ago by modern humans. This population that migrated to this region also led to a sub-population that became the first colonizers of the Americas. It is not clear when, but between 40,000-16,500 years ago, modern human migrated over the Beringia land bridge, likely using the ice sheets that formed in the late glacial maximum period when ice sheets and glaciers covered many parts of North America. Recent genetic work has shown that human populations in the Amazon are genetically similar to Australoid populations, or populations that also migrated to east Asia and Australia. In effect, human migrations to the Americas likely traveled along the coastal regions between North and South America.
Genetic Diversity
Genetically, Y-chromosomal Adam, or the genetic ancestor for all living men today (i.e., although this does not mean he was genetic Adam was the only man around), seems to derive between 300-200 thousand years ago, suggesting that the ancestor to all males derived during the early evolution of modern humans. Central to Western Africa seem to be the regions where this early human ancestor influenced subsequent populations. For genetic Eve, or the most recent common matrilineal ancestor Mitochondrial DNA for all humans, is harder to specify, where the dates can range between more than 120,000 years ago. There were likely many female genetic lines but Mitochondrial Eve likely became the only female line. All of these early dates suggest greater human diversity and variation were originating in Africa, long before migrations began to significantly shape subsequent human populations in other parts of the Earth.
As stated, for most human populations, the vast majority of it can be traced to a large wave of migrations that occurred between 80-50 thousand years ago. Overall, humans are about 99.5% similar to each other, suggesting that relatively few populations have influenced the vast majority of our genetic makeup. In Africa, using linguistic and genetic evidence, there were likely about 14 main population groups. These groups show a wider range of genetic variability than many other populations throughout the Earth, which suggests that humans spent a lot of time in Africa before they successfully spread and colonized the remaining parts of Earth (Figure 2). Modern African populations in eastern Africa, between Namibia and South Africa, are likely the most similar to other human populations, which suggests that it was this region that modern humans began the large, second wave migration(s) that eventually colonized the rest of the planet.
Divergence in genetics began to accelerate again at about 40,000 years ago, by which time European and Asian populations began to diverge as human populations migrated into different parts of Eurasia. In southeast Asia, Denisova hominin, an early hominid that shares an ancestor with Neanderthals, may have also bred with early modern humans, similar to Neanderthals in Europe and other parts of Asia. This could explain why up to 4-6% of southeast Asian populations' genetic makeup shows variation from other populations. Additionally, sub-Sahara African populations not only show large variation, but their genetic makeup lacks any genetic input from Neanderthals, suggesting Neanderthals never migrated to areas south of the Sahara.
Summary
Genetic variation among modern humans is relatively limited to most species found on Earth. Populations within Africa were often isolated, developing more diverse genetic variation over time. Once populations left Africa, that diversity began to diminish. Despite physical traits that show variety among humans today in wide regions, the fact is most of those populations show very similar genetic makeup with some input from much older Hominid populations and some early modern humans who migrated before the great migration that occurred between 80-50 thousand years ago. There seems to have been a great split between migratory human populations by about 40,000 years ago, where part of the populations migrated towards Europe and the other part into east Asia and eventually North and South America.