How hominins spread across the world

Out of Africa, the first time: Homo erectus

• All the earliest hominin fossils are African; the oldest hominin fossils outside Africa are Homo erectus, at Dmanisi in Georgia, about 1.8 million years ago, followed by sites in Java and China.
• Homo erectus had long legs and a body built for walking long distances, a larger brain, and Acheulean tools, and probably used fire later in its history, so it could follow game into new habitats.
• A diet with more meat gave it a food source available in many environments, unlike the narrow plant diets of earlier hominins.
• Its descendants in Asia lasted until about 110 000 years ago, and small island hominins such as Homo floresiensis on Flores may descend from it.

Where did Homo sapiens come from? Two models

• The Out of Africa (replacement) model says Homo sapiens evolved in Africa only, then spread out across the world and replaced the older hominins (such as Neanderthals) with little or no interbreeding.
• The multiregional model says Homo erectus populations in Africa, Europe and Asia each evolved into modern humans, held together as one species by gene flow between regions.
• The oldest Homo sapiens fossils are African (Jebel Irhoud in Morocco, about 300 000 years ago), and modern humans appear much later elsewhere, which supports an African origin.
• DNA now shows that some interbreeding did happen, so most scientists accept a modified version: a recent African origin with a small amount of mixing with the hominins already outside Africa (sometimes called the assimilation model).

The spread of Homo sapiens

• Small groups left Africa earlier, reaching the Levant by about 180 000 years ago, but the main dispersal that gave rise to all living non-Africans happened about 70 000 to 50 000 years ago.
• People moved along the coast of southern Asia and reached Australia, by boat across open sea, by at least 50 000 years ago; Europe was reached by about 45 000 years ago.
• The Americas were reached across the Bering land bridge from Siberia, by at least 15 000 years ago and possibly earlier.
• Remote Oceania, the islands out of sight of each other, was the last region reached: Aotearoa was settled only about 750 years ago.

Try it: when did it happen?

Evidence from mtDNA and the Y chromosome

• Mitochondrial DNA (mtDNA) is passed only from mother to child, does not recombine, has many copies per cell and mutates fairly fast, so its changes record the maternal line over tens of thousands of years.
• The Y chromosome is passed only from father to son, and most of it does not recombine, so it records the paternal line in the same way.
• All living people's mtDNA traces back to one woman (nicknamed mitochondrial Eve) who lived in Africa roughly 150 000 to 200 000 years ago, and the deepest Y chromosome lineages are also African.
• African populations hold the most diversity in both, and all non-African lineages form one branch inside the African tree, which fits a small group leaving Africa and founding everyone else.
• She was not the only woman alive; other women's mtDNA lines simply died out when they had no daughters, by chance.

Try it: mtDNA or the Y chromosome?

The serial founder effect

• Each time a small group split off and moved on, it carried only part of its parent population's alleles: a founder effect.
• Repeated step after step across the world, this means genetic diversity falls the further a population is from East Africa, measured along likely walking routes.
• Studies of many populations find exactly this pattern, with the lowest diversity in the Americas and the Pacific, the last places settled.
• The same pattern would not appear if modern humans had evolved separately in each region, so it is strong evidence for an African origin.

Interbreeding with Neanderthals and Denisovans

• Neanderthals lived in Europe and western Asia from about 400 000 to 40 000 years ago; Denisovans are known mostly from DNA, first read from a finger bone in Denisova Cave in Siberia in 2010, and lived across Asia.
• Comparing their genomes with ours shows that people of non-African descent carry about 1 to 2% Neanderthal DNA, so Homo sapiens interbred with Neanderthals soon after leaving Africa, probably in the Middle East.
• People in Papua New Guinea and Aboriginal Australians carry about 4 to 6% Denisovan DNA, and smaller amounts are found across East Asia and among Pacific peoples, including Māori.
• Some inherited alleles were selected because they helped: a Denisovan version of the EPAS1 gene helps Tibetans cope with low oxygen at high altitude, and some immune genes came from Neanderthals.
• A bone from Denisova Cave belonged to a girl with a Neanderthal mother and a Denisovan father, so these groups also met and bred with each other.

Settling the Pacific and Aotearoa

• Near Oceania (New Guinea, the Bismarck Archipelago and the Solomons), where islands are in sight of each other, was settled more than 40 000 years ago.
• About 3000 years ago, people of the Lapita culture, known from their stamped pottery and speaking Austronesian languages whose roots trace back to Taiwan, sailed into Remote Oceania and reached Fiji, Tonga and Sāmoa.
• After a gap of about 1800 years, voyagers settled the Society Islands and then, in a rapid burst, the far points of East Polynesia: Hawaiʻi, Rapa Nui and Aotearoa.
• They sailed double-hulled waka, navigating by stars, swells and birds, and carried kūmara, taro, hue, kiore and kurī.
• The evidence includes radiocarbon dates on short-lived seeds and on kiore-gnawed seeds and bones, the DNA of kiore (which could only travel with people), human mtDNA, pottery and language.

What Achieved, Merit and Excellence look like

• Achieved describes the evidence; Merit explains what it shows about where people came from; Excellence combines several lines of evidence and judges how far each supports the conclusion.