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Human evolution and the origins of human diversity: key points
- Humans shared a last common ancestor with the chimpanzee and the bonobo about 5-6 million years ago.
- The hominin clade has included many species, some of which will have coexisted. Few are in direct ancestral relationship to Homo sapiens.
- Humans are distinguished from the other great apes by specific physical (bipedalism, brain size, hairlessness) and cultural (tool making, language, society) traits.
- Modern humans evolved in Africa and spread across the world, adapting locally to the selective pressures of the climates, food sources and pathogens that they encountered.
- Human diversity is best explained by continuous rather than discontinuous variation.
Routes of human migration
Homo sapiens evolved from Homo erectus in East Africa between 200,000 and 160,000 years ago, but remained largely restricted to that continent for many millennia until about 70,000 to 60,000 years ago, when a relatively small group of those people departed from eastern Africa across the mouth of the Red Sea. They became the founder population for all modern humans living outside Africa, but carried only a small proportion of African genetic diversity. Some of these people migrated around the tropical beaches of Southern and Southeastern Asia, and there is archaeological evidence for human presence in Australia by around 40,000 years ago. Another group appears to have headed northward through the Arabian peninsula to the Levant and Mesopotamia, thence spreading eastwards across the steppes of Central Asia to populate Siberia and, for those who travelled south of the Central Asian massif, India and eventually China. Westward migration of modern humans into Europe appears to have come from two sources, primarily from central Asia via a route north of the Black Sea but also via a Mediterranean route from the Levant. The Americas were populated relatively recently by humans, probably no earlier than 15,000 years ago. The most commonly accepted model is that of one or more migrations by relatively small groups of Siberian hunters over the land mass called Beringia, joining present-day Siberia and Alaska, which existed from about 20,000 to 8,000 years ago because of the fall in sea level during the last glacial period. The North American glaciers began to melt about 15,000 years ago, opening the way for a southward migration that reached the tip of South America 1000 years later. The low genetic diversity of native Americans has caused some investigators to propose an effective population size for the migrating group of no more than 70 individuals. The requirement for advanced technologies to enable humans to cross oceans meant that the last part of the world to be populated by Homo sapiens was Oceania. Current theories of the peopling of Oceania suggest a southward and eastward migration of the Taiwanese Austronesian culture, beginning around 5,500 years ago and reaching the last significant landmass, New Zealand, around 900 years ago. The routes and dates shown are composites of published mitochondrial DNA and Y chromosome data. Numbers represent the time (thousands of years ago, KYA) when settlement was achieved.