Human Populations: Origins and Evolution

Abstract

The origins of modern human diversity have long been debated within a framework set by two hypotheses: ‘Out‐of‐Africa’ versus multiregional evolution. Humans are genetically closer to each other than members of all other primate species, most of our genetic diversity is accounted for by individual differences within populations, and only a small fraction of the species' genetic variance represents differences among populations or geographic groups. All these findings are hard to reconcile with the notion of parallel evolution in different continents, implicit in the multiregional model; the alternative hypotheses are now better described as either complete replacement of archaic human populations, or as partial assimilation into anatomically modern populations. Critical information about human demographic history has emerged from analyses of genomic diversity, clearly supporting an African origin of our species, followed by dispersal of rather small groups of people in the other continents. However, ancient deoxyribonucleic acid from fossil specimens seems to suggest low, but significant, levels of hybridisation between anatomically archaic and modern humans in the course of the latter's expansion from Africa, although alternative explanations of the data cannot be ruled out.

Key Concepts:

  • Genetic differences between humans are smaller than in any other primate species.

  • Genetic differences between human populations account for a minor fraction of the species' diversity.

  • Most human polymorphic alleles are present in all continents.

  • Each population contains an ample subset of human allelic variants.

  • African populations show the highest genome diversity worldwide, and alleles in other continents can largely be regarded as subsets of African diversity.

  • Differences between populations are structured in the geographical space, showing evidence of repeated founder effects as humans expanded to colonise the whole planet.

  • Models of genetic replacement, partial or total, of archaic human forms by anatomically modern humans account for current diversity better than any alternative models.

  • Non‐African people consistently show an excess of DNA similarity with the Neandertal genome, which is often (but not necessarily) interpreted as a consequence of archaic hybridisation between Neandertals and anatomically modern people dispersing from Africa.

Keywords: genomic diversity; DNA diversity; out‐of‐Africa; multiregional evolution; coalescence; neandertal; admixture

Figure 1.

Four main models of human evolution (top) and the relative contribution of African genes to the gene pool of non‐African populations implied by each of them (bottom). In each of the four panels corresponding to the different models, the past is at the root and the present is at the tips of the tree. The assimilation and replacement models do not rule out the possibility of genetic exchanges between different continental populations (represented by the thin horizontal arrows for the model of multiregional evolution). Reproduced with permission from Stoneking . © Nature Publishing Group.

Figure 2.

A comparison of the model assuming hybridisation between modern humans dispersing from Africa and Neandertals (top) and of the model of ancient population structure in Africa (bottom). The colours of the dots suggest levels of genotype similarity. In both (a) and (d) (corresponding to perhaps 800 000 years ago) the Neandertals' ancestors move from Northern Africa into Europe, but in (d) African populations are genetically structured, with a North–South difference. In (b) and (e) (corresponding to perhaps 60 000 years ago) anatomically modern people move from Africa into the Near East; however, in (b) they admix with Neandertals (as indicated by the red arrow) whereas in (e) they do not (as indicated by the diagonal red line). In (c) and (f) (corresponding to any time between 29 000 years ago and the present) Neandertals have gone extinct, but in both cases Europeans are genetically closer to them than Africans, either because they have incorporated some Neandertal genes (darker edge of the dots in (c)), or because they share a set of recent ancestors (similar colours of the dots in (e)).

Figure 3.

A highly schematic view of the origins of current genome diversity, under a replacement model of human evolution. This scheme is also valid, with modest modifications, under a model of limited assimilation of anatomically archaic populations. Dates along the black lines are years before present, dots of different colours represent different genotypes, the distribution of which roughly corresponds to archaeological evidence on human occupation of different regions. Dots of new colours appear in the maps in the course of time (e.g., red and violet in Africa at 70 000 BP, burgundy in India at 10 000 BP), representing the effect of mutation. As only part of the African alleles (yellow, orange and light green dots) are carried into Eurasia by dispersing Africans from 60 000 years bp, diversity in modern Eurasian populations is largely a subset of African diversity. Moreover note how the expansion in Eurasia generates broad gradients of genotype frequencies. Reproduced with permission from Barbujani et al..

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Further Reading

Balding D, Bishop M and Cannings C (2007) Handbook of Statistical Genetics, 3rd edn. New York: Wiley.

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Stringer C (2012) Lone Survivors: How We Came to Be the Only Humans on Earth. New York: Henry Holt.

Tattersall I (2012) Masters of the Planet: The Search for Our Human Origins. New York: Palgrave MacMillan.

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Barbujani, Guido, and Colonna, Vincenza(Oct 2013) Human Populations: Origins and Evolution. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001794.pub3]