Shaking up the Tree of Life.

18 NOVEMBER 2016 • VOL 354 ISSUE 6314 817 I n 2010, a comparison between the genomes of a Neandertal and people today settled what anthropologists and geneticists had debated for decades: Our ancestors had indeed mated with their archaic cousins, producing hybrid children. They, in turn, had mated with other modern humans, leaving their distant descendants—us—with a permanent Neandertal legacy. Not long afterward, DNA from another archaic human population, the Denisovans, also showed up in the modern human genome, telling a similar story (Science, 23 December 2011, p. 1629). For researchers and the public alike, this evidence of interbreeding among distinct human populations—so different some still argue there was more than one species— created a shock wave. Suddenly hybridization “just captured our imagination,” says Michael Arnold, an evolutionary biologist at the University of Georgia in Athens. “That genomic information overturned the assumption that everyone had.” The techniques that revealed the Neandertal and Denisovan legacy in our own genome are now making it possible to peer into the genomic histories of many organisms to check for interbreeding. The result: “Almost every genome study where people use sensitive techniques for detecting hybridization, we find [it]—we are finding hybridization events where no one expected them,” says Loren Reiseberg, an evolutionary biologist at the University of British Columbia in Vancouver, Canada. By Elizabeth Pennisi FEATURES Two captive Heliconius species mate, illustrating the rampant hybridization among plants and animals in nature.