When cetaceans (whales, dolphins, and porpoises) transitioned from life on land to life in the sea about 50 million years ago, 85 genes became inactivated in these species, according to a new study. While some of these gene losses were likely neutral, others equipped cetaceans with "superpowers" for surviving in the open ocean, facilitating deep dives and paving the way for a unique new sleeping style. Although previous studies have provided insights into the genomic changes that drove transformations as the ancestors of modern cetaceans transitioned from a terrestrial to a fully-aquatic lifestyle, these genomic changes remain incompletely understood. Matthias Huelsmann et al. searched for gene-inactivating mutations from 19,769 genes in 62 mammal species, screening for those that were turned off after cetaceans split from the ancestors of modern hippopotamuses but before toothed whales split from baleen whales. Among the inactivated genes identified in cetaceans, the researchers found a gene involved in saliva secretion, which became unnecessary since the watery surroundings sufficiently lubricate food and dilute salivary digestive enzymes, as well as two genes that promote blood clot formation but do not affect wound sealing, without which whales gain protection from blood clots while diving. The cetaceans also lost lung-related genes, allowing their lungs to temporarily collapse as they descend into the deep--an occurrence that would be alarming in humans, but which makes whales better divers by reducing their buoyancy and protecting them from injuries caused by rapid pressure changes when they resurface. Further, Huelsmann and colleagues found that cetaceans lost all genes required for synthesizing melatonin, which may have led them to evolve a type of sleep in which one brain hemisphere stays awake so they can continue to resurface and generate heat as needed.
