Almost all our DNA—99.9 percent of the three billion “letters,” or nucleotides, that make up the human genome—is the same from person to person. But interwoven in that last 0.1 percent are telltale differences. A comparison among, say, East Africans and Native Americans can yield vital clues to human ancestry and to the inexorable progression of colonizations from continent to continent. Until recent years, DNA passed down only from fathers to sons or from mothers to their children has served as the equivalent of fossilized footprints for geneticists. The newest research lets scientists adjust their focus, widening the field of view beyond a few isolated stretches of DNA to inspect hundreds of thousands of nucleotides scattered throughout the whole genome.

One study, co-authored by Harpending, showed that the rate of change of DNA, and thus the pace of evolution, has accelerated over the past 40,000 years.

According to the tree diagrams that document genetic lineages, some gene variants show “deep ancestry”—they are much older than they should be if humans evolved from a single homogeneous group no more than 200,000 years ago; a hint of possible interbreeding. In one study that drew attention in 2006, Bruce T. Lahn of the University of Chicago and his colleagues reported that a version of the Microcephalin gene, which is involved in regulating brain size, contains a haplotype that may have been passed on during an encounter with Neandertals 40,000 years ago.