Embracing change and challenges fosters growth, but there’s no turning back once we cross certain thresholds.
Evolution understands this too well— and now, a recent study published in the Proceedings of the Royal Society B found that when a mammal becomes fully aquatic, it becomes nearly impossible to return to terrestrial landscapes.
Is evolution reversible?
Around 350 to 400 million years ago, the first fish ventured onto land, developing primitive limbs that enabled them to move about. Over time, their descendants evolved into the tetrapods we know today.
While this transition from water to land occurred only once, the reverse transition from land to water has happened repeatedly, prompting researchers to question whether aquatic mammals could ever revert to a terrestrial lifestyle.
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.
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