By Alan Gibson
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Who we are and how we got here

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Issue 436

We are all profoundly mixed up genetically, and our ancestors were always moving. These are just two of the discoveries that David Reich presents in this exciting book about the ancient DNA revolution.

Reich starts by explaining how rapidly analysis into ancient DNA has developed. Since 2001, when the human genome was sequenced for the first time, research has ballooned as costs have diminished and automation has mushroomed.

He outlines the process whereby researchers seek out the mutations within segments of an individual’s genome and compare them to those gathered from thousands of others. Working on the logic that the higher the density of differences separating two genomes on any segment, the longer it has been since they shared a common ancestor, geneticists have been able to discover a history that archaeologists, despite much valuable research and analysis, have only ever hypothesised.

Reich, from his own laboratory at Harvard, has been a leading figure in many of the revelations. Along with a host of other geneticists, he and his colleagues have begun to track the movement of humans over almost 200,000 years. They have revealed ancient humans’ relationships with other ancient populations, such as the Neanderthals and Denisovans — a subspecies of people who once roamed regions between Siberia and South East Asia — and discovered an increasing number of other unique examples.

Among the most important findings are: that the people who live in a particular place today almost never exclusively descend from the people who lived in the same place far in the past; that in the relatively recent past, human populations were just as different from each other as they are today; and that populations have never simply developed from one place but have moved back and forth across whole continents, interacting with other subspecies along the way to produce a complex ancestry within all of us.

The findings also put paid to the idea that present-day humans living in many parts of Africa and Eurasia have a common ancestor in Homo erectus, a species that made crude stone tools and had a brain about two-thirds the size of ours, and who came out of Africa around 1.8 million years ago. But researchers have been unable to find any DNA sequences among people that go back that far. On the contrary, the sequences they have found reveal ancestry going back ten times more recently, showing that humans today largely descend from a much later expansion from Africa.

Many of these findings are nectar for anti-racists. Not only do they dispel any notion that humans can be grouped biologically into “primeval” groups and races. They show that humans have always migrated, driven not just by environmental changes but by the affect this has had on the human genome, and particularly the mutations that led to the development of language and conceptual thought.

One warning, however. To get to the bones of this book takes concentration. Reich weaves through the analysis to come up with fabulous findings, but you have to follow closely or you’ll get lost within the three billion nucleotides that make up your own genome.

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