Dolphins are forever bound to sea — they've reached a point of no return

Belgian paleontologist Louis Dollo initially proposed that evolution was irreversible in the 19th century. Dollo's law states that once a complex trait is lost in a lineage, it is unlikely to reappear in subsequent generations.

To investigate this idea in mammals, scientists analyzed over 5,600 mammal species, categorizing them into four groups: fully terrestrial, semi-aquatic with some land mobility, limited land locomotion, and fully aquatic species such as whales.

"One of the main points of our work was to include the entire gradient of adaptations from fully terrestrial to fully aquatic forms and to test if these adaptations were irreversible," said lead study author Bruna Farina, a doctoral student at the University of Fribourg in Switzerland, in a Live Science article. 

The study revealed a threshold between semi-aquatic and fully aquatic species. Once this threshold is crossed, the adaptations to an aquatic lifestyle become irreversible. 

Transitioning to aquatic environments necessitated multiple changes, including increased body mass to retain heat in colder surroundings and a carnivorous diet to support their heightened metabolism. These adaptations could hinder competition against terrestrial organisms.

While shifting from fully terrestrial to semi-aquatic in incremental steps is possible, the researchers concluded that an irreversible threshold exists for particular aquatic adaptations. 

"We found that it's possible to go from fully terrestrial to semiaquatic in [small steps], but there's an irreversible threshold for some aquatic adaptations," Farina said. 

Therefore, she explained that the chance of fully aquatic animals, like whales and dolphins, returning to the land is virtually zero. 

Virag Sharma, a researcher in comparative genomics at the University of Limerick not involved in the study, praised the researchers for dispelling the myth that sea-to-land transitions are highly improbable.

Nevertheless, he noted that the study focused solely on mammals, leaving room for future investigations into whether similar irreversibilities apply to other tetrapod lineages.

The complete study was published in the Proceedings of the Royal Society B on July 12 and can be found here.

Study abstract:

Secondary transitions to aquatic environments are common among vertebrates, and aquatic lineages display several adaptations to this realm, some of which might make these transitions irreversible. At the same time, discussions about secondary transitions often focus only on the marine realm, comparing fully terrestrial with fully aquatic species. This, however, captures only a fraction of land-to-water transitions, and freshwater and semi-aquatic groups are often neglected in macroevolutionary studies. Here, we use phylogenetic comparative methods to unravel the evolution of different levels of aquatic adaptations across all extant mammals, testing if aquatic adaptations are irreversible and if they are related to relative body mass changes. We found irreversible adaptations consistent with Dollo's Law in lineages that rely strongly on aquatic environments, while weaker adaptations in semi-aquatic lineages, which still allow efficient terrestrial movement, are reversible. In lineages transitioning to aquatic realms, including semi-aquatic ones, we found a consistent trend towards an increased relative body mass and a significant association with a more carnivorous diet. We interpret these patterns as the result of thermoregulation constraints associated with the high thermal conductivity of water leading to body mass increase consistently with Bergmann's rule and to a prevalence of more nutritious diets.