Patterns of hominin dispersal
The overall pattern
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Hominins originated in Africa. All the earliest hominin fossils are African, and the deepest genetic divisions in living humans are within African populations — both lines of evidence point the same way.
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From there, hominins dispersed out of Africa in more than one episode, separated by roughly 1.5 million years.
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The first dispersal was by Homo erectus, from around 1.9–1.8 million years ago. It reached western Asia and, over time, as far as eastern and southeast Asia. These populations persisted for a very long time.
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The second dispersal was by Homo sapiens. Our species arose in Africa by around 300,000 years ago, and the major dispersal beyond Africa occurred from roughly 70,000–60,000 years ago, with evidence of earlier, smaller movements that appear to have left little genetic trace in living populations.
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This means dispersal was not a single event. Describing it as one migration is the commonest error in this topic.
The later sequence
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After leaving Africa, Homo sapiens reached the remaining habitable world in a rough sequence:
- Australia and New Guinea, from roughly 65,000–50,000 years ago. This is significant because it required crossing open water even at the lowest sea levels — the first firm evidence of deliberate sea travel.
- Europe, from roughly 45,000 years ago.
- The Americas, from roughly 20,000–15,000 years ago, entering via Beringia — the land bridge exposed between Siberia and Alaska when sea level was low during glaciation.
- The remote Pacific, much later, by deliberate voyaging.
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New Zealand was among the last major landmasses settled by people, reached by Polynesian voyagers in roughly the 13th century CE. This required sustained open-ocean navigation over thousands of kilometres — the final stage of a dispersal that began in Africa, and one accomplished by sophisticated navigational knowledge rather than drift.
What made dispersal possible
- Dispersal was limited by climate, not by distance. Moving into cold, high-latitude environments required solving problems the body alone could not:
- Fire — warmth, and cooking to extract more energy from available food.
- Clothing — insulation replacing lost body hair, and tailored clothing much more effective than draped hides.
- Shelter — constructed, allowing occupation where no caves existed.
- Tools — access to new food sources, and hunting of unfamiliar prey.
- Watercraft — required to reach Australia and, later, the Pacific.
- These are cultural solutions. Dispersal is therefore a clear case of culture removing a biological constraint, and it is why this topic sits alongside the cultural trends.
What drove dispersal
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Several factors, and questions usually supply evidence for one or more:
- Climate change. Glacial cycles repeatedly opened and closed routes. Low sea level exposed land bridges such as Beringia; wetter phases made the Sahara passable, and arid phases closed it.
- Following prey, as animal populations shifted with climate.
- Population pressure, pushing groups into neighbouring unoccupied land.
- Resource availability — coastlines in particular offered predictable food and a route that required no new adaptations to follow.
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Note that dispersal need not be deliberate long-distance migration. A population expanding its range by a few kilometres a generation covers continental distances over thousands of years. Most dispersal is better understood as range expansion than as a journey.
Interbreeding with other hominins
- Dispersing Homo sapiens populations encountered other hominin species already present in Eurasia, and genetic evidence shows they interbred with them.
- Living people outside Africa carry a small percentage of DNA from these other populations, and the proportions differ between regions — evidence of separate encounters at different times and places.
- This matters conceptually, and it is worth stating: it means the picture is not a simple replacement of one species by another, and it complicates the biological species concept, since these populations were distinct enough to be named separate species yet produced fertile offspring.
The genetic signature of dispersal
- Genetic diversity in living human populations decreases with distance from Africa, following the routes people took.
- The explanation is a serial founder effect: each new population was founded by a subgroup of the previous one, carrying only a sample of its alleles. Repeat this many times along a route, and diversity declines steadily with each step.
- This is a strong argument, because it is a quantitative pattern predicted by the dispersal model and not easily explained otherwise.
Selective advantage
- Dispersal itself is favoured where a population is at or near the local carrying capacity: individuals moving into unoccupied land face less competition for food and space, so they obtain more resources and leave more offspring.
- The cost is real — unfamiliar territory, unknown resources, novel pathogens and no local knowledge — so dispersal is favoured only when local competition is severe enough, or the new area rich enough, to outweigh it.
- Reaching new environments then created new selection pressures, producing the regional biological variation seen in living humans — for example in skin pigmentation, which reflects the trade-off between protection from UV damage and the UV needed for vitamin D synthesis at different latitudes.
Worked Example
Worked Example
Researchers compare genetic diversity in living human populations along a route from East Africa through Asia to the Americas.
- Genetic diversity is highest in African populations and decreases steadily with distance travelled along the route.
- The deepest genetic divisions between living populations are within Africa.
- Populations in the Americas have the lowest diversity.
- Fossil and archaeological dates along the route become progressively younger in the same direction.
- Populations outside Africa carry a small proportion of DNA from other hominin species; African populations carry very little.
Explain what this evidence shows about hominin dispersal.
Answer:
Origin in Africa.
Two independent observations point to an African origin.
- Genetic diversity is highest in Africa. A population that has existed longest has had the most time to accumulate mutations, so it holds the most variation. Higher African diversity indicates African populations have the longest continuous history.
- The deepest genetic divisions are within Africa. The deepest divisions separate the earliest-diverging lineages. If those lie within Africa, the root of the tree is African, and all non-African populations descend from a subset of African variation.
The decline in diversity: a serial founder effect.
Diversity decreasing steadily with distance is explained by repeated founder effects.
- Each new population was founded by a small subgroup of the previous population.
- Those founders carried only a sample of the parent population's alleles, so some alleles were left behind — this is the founder effect, and it reduces diversity in the new population.
- Repeating this many times along a route makes diversity decline at each step, producing a steady decrease with distance.
The Americas show the lowest diversity because they lie at the end of the longest route, so they have been through the most founder events. Entry was also via a narrow corridor at Beringia, which is likely to have imposed a particularly severe bottleneck.
This is strong evidence, because the quantitative pattern — a steady decline tracking distance along a specific route — is predicted by the dispersal model and is not easily explained otherwise.
The archaeological dates confirm the direction.
Genetic data alone give the branching order but not absolute timing. Fossil and archaeological dates becoming progressively younger along the same route show that people arrived later the further along it they were.
This matters because it is an independent confirmation: the genetics and the dating have entirely different sources of error, so their agreement makes the conclusion far stronger than either alone.
The other-hominin DNA.
Non-African populations carry DNA from other hominin species; African populations carry very little. This indicates that dispersing Homo sapiens encountered and interbred with other hominins already living in Eurasia, after leaving Africa. Populations that remained in Africa had no such encounters, which is why the signal is largely absent there.
It also shows the dispersal was not simple replacement — the incoming and resident populations were distinct enough to be named separate species yet produced fertile offspring, which complicates the biological species concept.
Cautions.
- Living populations are evidence about the present. Populations have moved, mixed and gone extinct since, so present-day genetics is an imperfect record of past distributions. Ancient DNA from remains is better where it can be recovered, but it preserves poorly, especially in warm climates.
- Diversity is affected by more than dispersal. Later bottlenecks, expansions and admixture also alter it, so the pattern reflects a population's whole history, not only its distance from Africa.
- Dates are provisional. They shift as sites are found and dating improves.