Deciphering the evolutionary origin of the exceptional slow pace-of-life of marine endotherms

All organisms face a certain risk of dying before reproducing. Given that reproducing earlier can also ensure that offspring start breeding sooner, there is strong pressure on individuals to reproduce as early as possible. Why, then, some organisms mature late, defer reproduction and age slowly? Major evolutionary transitions in life history are believed to result from the invasion of niches altering extrinsic mortality and imposing new adaptive challenges. However, limited research on these transitions has hindered understanding their role in the evolution of extremely slow-lived strategies. We reveal here that the invasion of marine environments by birds and mammals triggered profound adaptive shifts towards extremely slower life histories, particularly in pelagic lineages. Such life history convergences were facilitated by the slow-paced nature of their non-marine ancestors, and were associated with adaptations for enhanced energy acquisition and storage, facilitating a long reproductive lifespan at the expense of extended development. Ancestral traits and lifestyle changes might thus have been essential in shaping the evolution of exceptionally slow life histories.