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Who we are and how we got here

By David Reich

Ancient DNA and the new science of the human past

  • Introduction
    • DNA consists of twin chains of molecules called nucleotides made from the chemicals adenine (A), cytosine (C), guanine (G) and thymine (T). Each chain is about three billion chemical blocks in length
    • In contrast, genes are fragments of these chains, generally around a thousand nucleotides long.
    • Random variations in these inherited sequences are called mutations.
    • If you compare two people's mutations, the more differences you find between their genes, the further they are away from sharing a common ancestor.
    • All humans today are descended from a single female ancestor, known as Mitochondrial Eve, and she lived in Africa no more than 200,000 years ago
    • If the old multi-regional theory held true, then any shared ancestor we had would be close to an incredible two million years old and part of the despersal of Homo erectus globally around 1.8 million years ago
    • If Mitochondrial Eve is seen as our shared ancestor, then modern humans must have evolved in Africa and only spread across the world 50,000 years ago
  • The ancestors of modern non-African humans interbred with both Neanderthals and Denisovans
    • Neaderthals descended from Homo erectus
    • Modern non-African share common mutations with Neanderthals, interbred around 54000 to 49000 years ago
    • non-African genomes are between 1.5-2.1 percent Neanderthal
    • Denisovans was discoverd using a strange fingerbone found at Denisova Cave in Siberia, 400 mutational differences from modern humans found. Modern humans have only around 200 mutational differences from Neanderthals
    • Denisovians more closer to Neanderthals than either species was to modern humans.
    • Sequence showed Denisovans are actually closer to New Guineans than to other modern populations; 3 to 6 percent of New Guinean ancestory is Denisovan.
    • Neanderthals were first discovered in the Neander Valley in Germany - "tal" being german for "valley"
  • Migrations patterns thousands of years ago informed the ancestry and languages of modern Europeans
    • Otzi the Iceman, 5300 year old naturally mummified corpse who was unearthed in the Alps in 1991
  • Indian ancestry is dual in nature, which is still reflected in its languages and caste system
    • Indo-Aryan in the north
    • Dravidian in the south
    • 2007 DNA study showed that all Indians are a mix of two ancient populations, one called Ancestral North Indians (ANI), who were related to Western Eurasians, and the other Ancestral South Indians (ASI), who aren't related to any populations outside the India.
  • Native Americans are descended from two separate migrations, but their exact history is not yet clear
  • Modern East Asians are descended from migrations that began in the Chinese agricultural heartland
  • Africa has seen just as much population movement as the rest of the world
  • Population mixing is sometimes driven by gender inequality among social groups
  • DNA demonstrates that differences exist between populations, but they don't justify racist generalizations