Nature
Study reveals the two animals that have reached the point of no return: they will never be able to set foot on land again
A Proceedings of the Royal Society B study of 5,600 mammals found that two species will never return to dry land.
Scientists believe that the first animals to migrate to dry land first crawled out of the sea more than 350 million years ago. Those first intrepid adventurers gradually became accustomed to life on land, evolving limbs and respiratory systems perfectly suited to their new habitat.
Those animals went on to become the tetrapods, a broad category of vertebrates with four limbs and distinct digits, including mammals. Most of those mammals have remained land animals but, around 250 million years ago, some turned back and returned to the seas.
Could sea mammals return to the land?
That aquatic return has left some researchers wondering whether another reversal could be possible. Namely, could sea mammals return to the land one day
The journal Proceedings of the Royal Society B included a study of 5,600 mammal species to discuss whether it is possible. The notion of ‘irreversible’ evolution was first forwarded in the 19th century by Belgian palaeontologist Louis Dollo and his findings have been supported by the Royal Society study.
Researchers found that whales and dolphins are very unlikely to ever be able to return to the land, having now evolved so distinctly for the water.
In the course of the study the mammals in question were split into four categories: fully terrestrial; aquatic adaptations but land-mobile; limited land locomotion; and fully aquatic. Whales and dolphins were classed by researchers in the ‘fully aquatic’ group, those which are deemed completely adapted for their aquatic environment. Researchers believe that dolphins and whales have now passed an evolutionary threshold, after which they will never be able to revert to life on the land.
For example, they have a vastly increased body mass to retain heat and a carnivorous diet to support that physique. However it would be impossible to replicate that sort of diet and maintain their heightened metabolism on dry land.
Lead study author Bruna Farina explained: “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.” It looks like the chance of an fully aquatic animals, such as whales and dolphins, returning to dry land are non-existent.
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