In 2010, extraordinary circumstances allowed geneticists to reconstruct the first full genome of an ancient human: the DNA came from a hairball, frozen 4,000 years in Greenland soil. Since then, methods have improved so much in cost and efficiency that individual papers now report genomic data from hundreds of dead people (here, here, here). Ancient DNA (aDNA) has now been published from well over 2,000 human ancestors, stretching as far back as 430,000 years ago.

But around 70 percent of