While GWAS is an invaluable tool for discovering the genomic basis of traits that are variable within a species, it cannot discover the genomic basis of traits that are shared by all members of a species. This limitation exists because GWAS relies on being able to discover genetic features that differ between individuals with and without a trait of interest. In humans, though, having an internal skeleton and a body temperature that is basically constant even when external temperatures change are both species-defining features, making it impossible by definition to compare the genomes of humans with and without these traits. For discovering the genomic basis of such traits, comparative genomics—comparison of genomes across not individuals but species that do, as opposed to do not, have a given trait—is an essential tool. The 1999 article by Stephen O’Brien and colleagues, “The Promise of Comparative Genomics in Mammals,” was among the earliest commentaries on the power of a comparative approach for discerning the genomic basis of mammalian traits that vary across species.