Out of Africa: How Homo sapiens Spread Across the World

“Out of Africa” is the name for the spread of modern humans, Homo sapiens, from Africa across the rest of the world. The main dispersal, from which nearly all living non-Africans descend, happened roughly 70,000 to 50,000 years ago, when a relatively small population crossed into south-west Asia and then spread rapidly as far as Australia and Europe. It is one of the most important events in the human story, explaining why people everywhere are so closely related and why Africa holds the greatest human genetic diversity.

Background: an African species

Homo sapiens evolved in Africa. The oldest fossils generally assigned to our species come from Jebel Irhoud in Morocco, dated to about 300,000 years ago, and from Omo Kibish in Ethiopia, more than 230,000 years old. For most of its history our species lived only in Africa, alongside other kinds of humans elsewhere: Neanderthals in Europe and western Asia, Denisovans in parts of Asia, and late Homo erectus and other groups in South-East Asia.

Humans had left Africa before. Homo erectus and its relatives reached Georgia, China and Java from around 1.8 million years ago. The question of how our own species came to replace these older populations was one of the great debates of 20th-century science.

The debate: one origin or many?

In the 1980s two models competed. The multiregional hypothesis, associated with Milford Wolpoff and Alan Thorne, argued that modern humans evolved gradually in several regions from local Homo erectus populations, linked by gene flow. The recent African origin model, championed by Chris Stringer of the Natural History Museum in London and others, argued that modern humans arose in Africa and later replaced archaic populations elsewhere.

In 1987 Rebecca Cann, Mark Stoneking and Allan Wilson published a study of mitochondrial DNA, which passes only from mother to child. They found that all living people’s mitochondrial lineages trace back to a single ancestral lineage in Africa roughly 200,000 years ago, popularly nicknamed “Mitochondrial Eve”. The deepest branches were all African. This strongly supported a recent African origin.

Later studies of the Y chromosome and of whole genomes confirmed the pattern. Genetic diversity declines with distance from Africa, as expected if small groups repeatedly split off and moved on, each carrying only part of the diversity of the group they left. The genomes of all non-Africans share a common ancestry from a single population that left Africa within the last 100,000 years.

Earlier exits

Fossils show that Homo sapiens left Africa several times before the main dispersal.

• Misliya Cave, Israel: an upper jaw dated to roughly 177,000 to 194,000 years ago.
• Apidima Cave, Greece: a partial skull reported in 2019 as Homo sapiens of about 210,000 years ago, though this identification is debated.
• Skhul and Qafzeh, Israel: burials of early modern humans from about 120,000 to 90,000 years ago.
• Al Wusta, Saudi Arabia: a finger bone of about 85,000 years ago, from a time when Arabia had lakes and grasslands.

These groups seem to have died out or been absorbed, leaving little or no detectable ancestry in living people. In the Levant, Neanderthals reoccupied the region after the Skhul and Qafzeh people. Researchers sometimes call these “failed” dispersals, though they may have resulted in early mixing with Neanderthals.

The main dispersal

Genetic dating, though imprecise, places the ancestors of today’s non-Africans leaving Africa roughly 70,000 to 50,000 years ago. The founding population was relatively small, perhaps a few thousand people, which created a genetic bottleneck that is still visible in non-African genomes.

Shortly after leaving, these people met and interbred with Neanderthals somewhere in south-west Asia. Recent genetic studies place the main period of mixing at roughly 50,000 to 45,000 years ago. As a result, all living non-Africans carry roughly 1 to 2 percent Neanderthal DNA (see Neanderthal interbreeding). Further east, the migrants met Denisovans, and people in New Guinea and Aboriginal Australians today carry several percent Denisovan DNA.

Why the dispersal succeeded this time is debated. Possible factors include climate shifts that opened green corridors across Arabia, growing population sizes in Africa, and new technologies and social networks, such as more efficient hunting weapons and long-distance exchange of materials.

Routes out of Africa

Two main routes are proposed, and both may have been used.

The northern route ran from north-east Africa along the Nile, through Sinai and into the Levant. It requires no sea crossing but passes through desert that was only passable in wetter periods.

The southern route crossed the Bab-el-Mandeb strait between the Horn of Africa and Yemen. During cold phases of the Ice Age sea levels were lower, narrowing the strait to perhaps a few kilometres, though it probably never closed completely. From there people may have followed the coasts of Arabia, Iran and India, living partly on shellfish and fish, in what is sometimes called the coastal or “southern dispersal”.

Spreading across the world

Once in Asia, modern humans spread remarkably quickly.

• South-East Asia: teeth from Lida Ajer cave in Sumatra have been dated to about 73,000 to 63,000 years ago, and remains from Tam Pa Ling in Laos are also very early, although some of these dates are debated.
• Australia: reaching Sahul, the ice-age continent joining Australia and New Guinea, required several sea crossings. Humans had arrived by at least 50,000 years ago and possibly by 65,000 years ago (see peopling of Australia).
• Europe: Homo sapiens may have briefly occupied Grotte Mandrin in southern France around 54,000 years ago. By about 45,000 years ago they were present at Bacho Kiro in Bulgaria and Ranis in Germany.
• Northern Asia: a thigh bone from Ust’-Ishim in western Siberia, about 45,000 years old, gave one of the oldest modern human genomes yet sequenced.
• The Americas: people reached the Americas much later, from Siberia across Beringia, by at least about 16,000 years ago and perhaps several thousand years earlier.

Within a few thousand years of their arrival in each region, the older human populations, including Neanderthals in Europe, disappeared. Whether modern humans outcompeted them, absorbed them or simply arrived as they were already declining is still debated.

How scientists piece the story together

The Out of Africa story rests on several independent kinds of evidence, each with its own strengths and weaknesses.

Fossils show where and when people with modern anatomy lived, but they are rare and often fragmentary, and assigning a single tooth or jaw to a species can be contentious.

Archaeology records stone tools, hearths, shell beads and ochre. Tools can show that people were present even where no bones survive, but similar tools were sometimes made by different kinds of humans, so the maker is not always certain.

Dating methods vary with age. Radiocarbon dating works only back to about 50,000 years ago, close to its limit for this period. Older sites rely on luminescence dating, which measures when sand grains were last exposed to light, and on uranium-series dating of cave deposits. Disagreements over dates lie behind many of the debates about early arrivals.

Genetics compares the DNA of living people to reconstruct population splits, bottlenecks and mixing. Since 2010, ancient DNA has allowed scientists to sequence genomes from people who lived tens of thousands of years ago, including Neanderthals and Denisovans. This field, pioneered by Svante Pääbo, showed directly that early modern humans in Europe sometimes had Neanderthal ancestors only a few generations back, as at Peștera cu Oase in Romania, about 40,000 years ago.

Because each method has gaps, the strongest conclusions are those on which fossils, archaeology and genetics agree. The broad outline of an African origin and a main dispersal within the last 100,000 years is one of them.

Myths and facts

• Myth: humans left Africa in a single journey. The dispersal was a slow spread of populations over many generations, with groups moving perhaps only a few kilometres in a lifetime and some later moving back into Africa.
• Myth: Africans are “less evolved” or closer to the ancestral population. All living people have the same amount of evolutionary history. African populations simply contain more genetic diversity because humans lived there far longer.
• Myth: one “Eve” was the only woman alive. Mitochondrial Eve was one of many women alive at the time; she is simply the one whose mitochondrial line happens to survive in everyone today.
• Fact: the Out of Africa model has been refined, not overturned. Ancient DNA shows that modern humans interbred with archaic groups, so the replacement was not total, but the core of the model remains widely accepted.

By the numbers

Timeline

• c. 1.8 million years ago: Homo erectus and relatives spread from Africa into Eurasia.
• c. 300,000 years ago: Early Homo sapiens at Jebel Irhoud, Morocco.
• c. 194,000-177,000 years ago: Homo sapiens at Misliya Cave, Israel.
• c. 120,000-90,000 years ago: Early modern humans buried at Skhul and Qafzeh.
• c. 70,000-50,000 years ago: The main dispersal of Homo sapiens out of Africa.
• c. 65,000-50,000 years ago: Humans reach Australia.
• c. 50,000-45,000 years ago: Main period of interbreeding with Neanderthals.
• c. 45,000 years ago: Modern humans established in Europe and Siberia.
• 1987: The “Mitochondrial Eve” study supports a recent African origin.
• 2010: The first Neanderthal genome confirms interbreeding with modern humans.

Why it matters

The Out of Africa dispersal explains why all living people are closely related and why human differences are so recent and shallow. It shows that every population outside Africa shares African ancestors, and it frames many later questions, from the extinction of the Neanderthals to the first sea voyages and the settlement of every continent except Antarctica.