The genetic distance between two species captures key information about both their shared evolutionary history and their distinguishing characteristics. These features have long been of particular interest to students of human evolution, where the search for the changes that ‘make humans unique’ necessitates an understanding of how unique, exactly, we are. At the genetic level, part of that answer now comes from comparisons between modern humans and archaic hominins. But for most of the history of molecular anthropology, it necessarily involved comparisons against other extant primates, especially members of the genus Pan (chimpanzees and bonobos), our closest living relatives.

In 1975, Mary-Claire King and Allan Wilson published a masterful analysis of the genetic distance between humans and chimpanzees, producing an estimate of average nucleotide dissimilarity (approximately 1.1%) that is remarkably close to the estimate produced by the chimpanzee genome project thirty years later (around 1.23%, excluding indels and structural variants). However, unlike the genome project — and nearly all efforts to study genetic variation, differentiation and divergence today — King and Wilson derived their estimate without a single piece of nucleotide sequencing data.