In 2010 a comparison between a Neandertal genome and genomes from people today turned up evidence of ancient liaisons, a discovery that belied the common idea that animal species can't hybridize or, if they do, will produce infertile offspring—think mules. Such reproductive isolation is part of the classic definition of a species. This discovery brought credence to other work in plants, Darwin's finches in the Galápagos Islands, tropical butterflies, mosquitoes, and a few other animals showing that hybridization was not just common, but also important in shaping evolution. 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. As more examples are discovered, researchers are questioning the definition of species and rethinking whether the tree of life is really a "net" of life.