The spread of modern humans: WACE Year 12 Human Biology Unit 4
“Explain the Out of Africa hypothesis for the spread of modern humans, compare it with the multiregional hypothesis, and evaluate the supporting evidence”
The Out of Africa hypothesis states that modern humans (Homo sapiens) evolved in Africa and then migrated out to populate the rest of the world, replacing earlier hominins such as Homo erectus and Neanderthals. The competing multiregional hypothesis says modern humans evolved in several regions from earlier hominins. Fossil, genetic and mitochondrial DNA evidence strongly supports the Out of Africa model.
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 Out of Africa hypothesis is the model that modern humans (Homo sapiens) evolved in Africa and then migrated out to colonise the rest of the world, replacing earlier hominin populations. It is also called the replacement model because the migrating modern humans replaced, rather than descended from, the hominins already living in those regions.
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.
Several independent lines of evidence support the Out of Africa model: the oldest fossils of anatomically modern humans are found in Africa; genetic diversity is greatest in African populations, as expected if humans originated there and only a subset migrated out; and analysis of mitochondrial DNA points to a recent common ancestor in Africa. The agreement of fossil and molecular evidence is what makes the case strong.
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.
Why African populations are the most diverse
If modern humans originated in Africa and only a relatively small group migrated out to populate the rest of the world, then that migrating group carried only a sample of the African gene pool (a founder effect). African populations, having lived in Africa the longest, would therefore retain the greatest genetic diversity, while populations further along the migration route would show progressively less. This is exactly what the genetic data show, which supports the Out of Africa hypothesis over the multiregional model. A marker wants you to connect the observation (greatest diversity in Africa) to the prediction of the hypothesis (an African origin with a founder effect during migration).
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.
A common error is mixing up the two migrations out of Africa. Homo erectus left Africa much earlier and spread across Asia and Europe; the Out of Africa hypothesis is about the later migration of anatomically modern Homo sapiens, who replaced those earlier populations. Be clear which species and which migration you are discussing. Also avoid saying replacement was total; modern non-African humans carry a small amount of Neanderthal DNA, so limited interbreeding occurred.
Exam-style questions
Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
WACE 2022-style6 marksCompare the Out of Africa hypothesis with the multiregional hypothesis for the origin and spread of modern humans, and evaluate one type of evidence that supports the Out of Africa hypothesis.Show worked answer →
A 6 mark response needs both hypotheses contrasted plus an evaluated piece of evidence.
- Out of Africa hypothesis
- Modern Homo sapiens evolved once in Africa and then migrated outward, replacing earlier hominin populations such as Homo erectus elsewhere with little or no interbreeding.
- Multiregional hypothesis
- Modern humans evolved in parallel from earlier Homo populations in several regions at once, with gene flow between regions maintaining a single species.
- Key contrast
- Out of Africa proposes a single recent African origin and replacement; multiregional proposes multiple regional origins with continuity.
- Evaluating mitochondrial DNA evidence
- Mitochondrial DNA, inherited maternally, shows the greatest genetic diversity in African populations and traces back to a common ancestor in Africa. This supports Out of Africa because the oldest, most diverse lineages are African, consistent with a single African origin. A strength is that mtDNA mutates at a fairly steady rate, allowing a molecular clock estimate.
Markers reward the single-origin-replacement versus multiple-origins-continuity contrast and a clearly evaluated line of evidence.
WACE 2023-style4 marksExplain how fossil evidence and genetic evidence together support the conclusion that modern humans originated in Africa.Show worked answer →
A 4 mark answer needs both evidence types linked to an African origin.
Fossil evidence. The oldest fossils of anatomically modern Homo sapiens have been found in Africa, with younger modern human fossils appearing progressively in other continents, consistent with a spread out of Africa.
Genetic evidence. African populations show the greatest genetic diversity, which indicates they are the oldest populations, because diversity accumulates over time. Populations outside Africa are a subset of African diversity, as expected if small groups migrated out.
Markers reward oldest modern fossils in Africa and greatest genetic diversity in Africa both pointing to an African origin.