Sooner or later, genetic mutations would have occurred that allowed Homo sapiens’ immune systems to mount an effective response to Neanderthal pathogens. Because people with these gene variants were more likely to survive, over time they spread widely through the population. But Homo sapiens took a much quicker route to immunity—by interbreeding. Reproducing with another closely related species is an unintentional kind of biohacking: it immediately endows a species with gene variants that are already adapted to the new environment.
