Fernando Melendez Vazquez, marine biologist and manager of the Science Research Mentoring Program (SRMP) at the Museum, set out to answer that question by helping build the largest evolutionary tree of fishes to date. What he and his collaborators found was unexpected: Endothermy—the ability to generate and maintain an elevated body temperature through metabolic heat—in tunas, billfishes, and great white sharks may have evolved in response to competition with marine mammals, such as whales, which re-entered the oceans some 50 million years ago. By combining fossil evidence, evolutionary models, and genomic data from more than 1,000 marine vertebrates, this research reveals the first evidence of an evolutionary “arms race” between fishes and cetaceans (whales, dolphins, and porpoises). Even more striking, the same genes appear to be actively evolving across endothermic marine animals, suggesting shared biological solutions to life in cold oceans. From ancient seas to modern DNA, this SciCafe reframes endothermy not as a biological oddity, but as evolution in action—shaped by competition, collaboration, and deep time.