Who We Are and How We Got Here: Ancient DNA and the New Science of the Human Past

by David Reich

Going back deeper in time, a person’s genome gets scattered into more and more ancestral stretches of DNA spread over ever-larger numbers of ancestors. Tracing back fifty thousand years in the past, our genome is scattered into more than one hundred thousand ancestral stretches of DNA, greater than the number of people who lived in any population at that time, so we inherit DNA from nearly everyone in our ancestral population who had a substantial number of offspring at times that remote in the past. There is a limit, though, to the information that comparison of genome sequences provides about deep time. At each place in the genome, if we trace back our lineages far enough into the past, we reach a point where everyone descends from the same ancestor, beyond which it becomes impossible to obtain any information about deeper time from comparison of the DNA sequences of people living today. From this perspective, the common ancestor at each point in the genome is like a black hole in astrophysics, from which no information about deeper time can escape. For mitochondrial DNA this black hole occurs around 160,000 years ago, the date of “Mitochondrial Eve.” For the great majority of the rest of the genome the black hole occurs between five million and one million years ago, and thus the rest of the genome can provide information about far deeper time than is accessible through analysis of mitochondrial DNA.22 Beyond this, everything goes dark.