Who We Are and How We Got Here

[...] After a grenade explosion in a room, could the exact position of each object prior to the explosion be reconstructed by piecing together the scattered remains and studying the shrapnel in the wall? Could languages long extinct be recalled by unsealing a cave still reverberating with the echoes of words spoken there thousands of years ago? Today, ancient DNA is enabling this kind of detailed reconstruction of deep relationships among ancient human populations.

Spurred by the revolutionary technology of plant and animal domestication, which could support much higher population densities than hunting and gathering, the farmers of the Near East began migrating and mixing with their neighbours. But instead of one group displacing all the others and pushing them to extinction, as had occurred in some of the previous spreads of hunter-gatherers in Europe, in the Near East all the expanding groups contributed to later populations. The farmers in present-day Turkey expanded into Europe. The farmers in present-day Israel and Jordan expanded into East Africa, and their genetic legacy is greatest in present-day Ethiopia. Farmers related to those in present-day Iran expanded into India as well as the steppe north of the Black and Caspian seas. They mixed with local populations there and established new economies based on herding that allowed the agricultural revolution to spread into parts of the world inhospitable to domesticated crops. The different food-producing populations also mixed with one another, a process that was accelerated by technological developments in the Bronze Age after around five thousand years ago. This meant that the high genetic substructure that had previously characterised West Eurasia collapsed into the present-day very low level of genetic differentiation by the Bronze Age. It is an extraordinary example of how technology – in this case, domestication – contributed to homogenisation, not just culturally but genetically. It shows that what is happening with the Industrial Revolution and the information revolution in our own time is not unique in the history of our species.

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Surprisingly, the ancient DNA revolution, through its discovery of the pervasiveness of ghost populations and their mixture, is fuelling a critique of race that has been raised by scholars in the past, but was never prominent because of a lack of support from hard scientific facts.³⁷ By demonstrating that the genetic fault lines in West Eurasia between ten thousand and four thousand years ago were entirely different from today’s, the ancient DNA revolution has shown that today’s classifications do not reflect fundamental ‘pure’ units of biology. Instead, today’s divisions are recent phenomena, with their origin in repeating mixtures and migrations. The findings of the ancient DNA revolution suggest that the mixtures will continue. Mixture is fundamental to who we are, and we need to embrace it, not deny that it occurred.

Some of the most striking examples of the complexity of human population structure in Africa are the patterns of natural selection on the continent. People of West African ancestry today have a high rate of sickle cell disease, conferred by a mutation that changes the blood protein haemoglobin, the molecule that more than any other is responsible for ferrying oxygen around the body. This mutation has risen to substantial frequency under the pressure of natural selection in several places in Africa: in far West Africa (e.g., Senegal), in west-central Africa (e.g., Nigeria), and in central Africa (whence the mutation spread to eastern Africa and southern Africa via the migrations associated with the Bantu expansion). The reason this mutation has risen to such a high frequency in each of these populations is that if a person carries one copy of the mutation from either of his or her parents, it protects against the infectious disease malaria. Malaria is so dangerous that the protection provided to the approximately 20 percent of the population who carry one copy of the sickle cell mutation is balanced in evolutionary terms with the cost that the approximately 1 percent of the population has to pay in carrying two copies of the mutation and suffering from sickle cell disease, which kills in childhood without treatment. Strikingly, the mutation has arisen independently in each of three locations in Africa, which we know from the fact that the sequences on which it resides are all different. From a naive perspective this seems surprising, as one would think that a mutation like this would be so advantageous to the people who carry it that once it arose it would spread around the vast malaria zone of Africa propelled by a tailwind of natural selection if there was even a small rate of interbreeding among neigh-bors.⁴²

A similar pattern is seen for the mutations in the lactase gene that confer an ability to digest cow’s milk into adulthood. The genetic basis for lactase persistence is completely different in North Africans and in the Fulani of West Africa than it is in the Masai of Sudan and Kenya, who carry different mutations, albeit in the same gene.⁴³