How did modern humans come to populate the whole planet, and what does the evidence say about where we started?
Explain the Out of Africa hypothesis for the spread of modern humans, compare it with the multiregional hypothesis, and evaluate the supporting evidence
A focused answer to the WACE Year 12 Human Biology Unit 4 dot point on the spread of modern humans. The Out of Africa hypothesis and the migration of Homo sapiens, the contrasting multiregional hypothesis, and the fossil, genetic and mitochondrial DNA evidence.
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What this dot point is asking
WACE wants you to explain how modern humans came to be spread across the globe, compare the two main hypotheses, and weigh the evidence. This brings together the hominin trends, the evidence for evolution and the population genetics you have already studied.
The Out of Africa hypothesis
The Out of Africa hypothesis (also called the replacement model) proposes that anatomically modern Homo sapiens first evolved in Africa, then spread out across the rest of the world in one or more waves of migration. As they spread, they replaced the earlier hominin populations they met, such as Homo erectus in Asia and the Neanderthals in Europe, rather than evolving from them. Under this model, all living humans share a recent common origin in Africa.
The multiregional hypothesis
The multiregional hypothesis offers a different explanation. It proposes that modern humans evolved more or less simultaneously in several different regions, from the local populations of Homo erectus that had spread out of Africa much earlier. Under this model, gene flow between regions kept all the populations as a single evolving species, so there is no single recent African origin for all modern humans.
Comparing the two hypotheses
The key difference is origin and replacement. Out of Africa says one recent African origin with later replacement of other hominins. Multiregional says several regional origins from earlier hominins, linked by gene flow. Out of Africa predicts that the greatest genetic diversity, and the oldest modern human fossils, should be found in Africa, while multiregional predicts continuity between earlier and modern populations in each region.
The evidence
- Fossil evidence: the oldest fossils of anatomically modern Homo sapiens are found in Africa, with younger modern-human fossils appearing progressively later as you move away from Africa, consistent with a spreading migration.
- Genetic diversity: modern African populations have the greatest genetic diversity, which fits an African origin where humans have lived longest, with reduced diversity in populations descended from the smaller groups that migrated out (a founder effect).
- Mitochondrial DNA: mitochondrial DNA is inherited only from the mother and accumulates mutations at a roughly steady rate, acting as a molecular clock. Tracing it back points to a recent common maternal ancestor in Africa.
Some interbreeding did occur, since modern non-African humans carry a small amount of Neanderthal DNA, so the replacement was not absolutely complete, but the overall pattern strongly supports the African origin.
How this maps to the exam
Expect questions that ask you to describe the Out of Africa hypothesis, compare it with the multiregional hypothesis, or evaluate which is better supported using fossil, genetic and mitochondrial DNA evidence. The strongest answers state that independent lines of evidence agree in pointing to a recent African origin.