An analysis of 41 ancient African genomes led by Mary Prendergast and David Reich suggests that the spread of herding and farming into eastern Africa affected human populations in phases, involving multiple movements of - and gene flow among - ancestrally distinct groups. The work reveals how, as these novel food-producing strategies spread in Africa, human populations focused on them "mixed" with human forager populations. The origin and spread of domestic animals across the globe has historically affected the underlying genetic composition of human populations. However, in Africa - where domestic sheep, goats, and cattle of southwest Asian origin were first introduced to northeastern parts of the country about 8,000 years ago - it has been difficult to identify the impact of interactions among migrating food producers and local hunter-gatherers, particularly as foraging persisted. Some hypotheses suggest pastoralism was introduced by migrants from Sudan and/or Ethiopia in a series of small movements that spread to descendants farther south. Others support the local adoption of novel livestock practices in the country, or even two distinct expansions of herders into eastern Africa. Testing these models has been inhibited by several factors to date, the authors say. Here, to better examine the genetic impacts of the spread of herding and farming on foraging communities within Africa, Prendergast, Reich and colleagues sequenced tooth and bone DNA from 41 ancient eastern African individuals that lived approximately 4,000 to 100 years ago. Through analyses of these and other ancient and modern-day African genomes, the authors conclude there were two phases of admixture associated with the spread of pastoralism, one between 6,000 and 5000 years ago in northeastern Africa, and the second about 4,000 years ago between this admixed group and eastern African foragers. Herders entering new environments would interact in diverse ways with indigenous foragers, according to the data, resulting in "varying cultural responses and blurred archaeological boundaries as groups adopted some of each other's cultural practices." Often this meant minimal gene flow between herders and foragers, boundaries maintained even during the Iron Age. This model of herding spread in Africa supports some previous theories while rejecting others, the authors say.
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Journal
Science