Scientists have found that once the mammal is entirely aquatic, it has crossed a threshold that makes a return to terrestrial environments almost impossible.

The likelihood of marine mammals such as orcas, bottlenose dolphins, and whales constantly evolving to be able to return to land is nearly nonexistent, according to a study. Researchers found these adaptations have enabled the animals to survive in water and crossed an evolutionary threshold, marking the end of their journey for terrestrial existence.

Between 350 and 400 million years before the beginning of time, The first fish swam out from the ocean and onto the land. These vertebrates with awkward postures had the beginnings of limbs, which allowed them to move around, and later generations changed into the tetrapods we see in the present.

Tetrapods are vertebrates sporting four legs and distinct digits, including reptiles, amphibians, and mammals. While most mammals were on the land, a few (starting about 250 million years ago) went back to the ocean with adaptations that enabled them to make the most of the habitats they found.

The transition to land occurred only once. However, the return to the water was repeated — prompting researchers to wonder if marine mammals can be adapted to terrestrial life and, If not, why.

In the study released on July 12, 2012, in Proceedings of the Royal Society B, Researchers examined more than 5600 mammal species to explore the issue. They discovered that the shift from water to land is most likely “irreversible.”

The concept that evolution cannot be reversed was first proposed through the work of Belgian paleontologist Louis Dollo in the 19th century. The principle, also known by the name Dollo’s Law, says that once a particular characteristic is lost over time, it is doubtful that it will be seen again in the next generation.

To test this concept for mammals, the researchers divided the thousands of animals into four categories: entirely terrestrial species, those with some adaptations to aquatic life but remain in motion on land, those with restricted mobility on land, and marine groups similar to whales..A tiktaalik illustration is an ancestor of tetrapods, believed to have begun to transition from land to sea around 375 millennia ago. (Image credit Mark Garlick/Getty Images)

Researchers developed the model to study species’ evolution with branches that indicate common ancestral roots. By comparing these species’ traits, they constructed models that predicted the chance of developing particular characteristics.

“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,” the lead researcher Bruna Farina, a Ph.D. student studying at the University of Fribourg, in Switzerland said to Live Science.

The research team discovered a line of separation between semiaquatic and fully aquatic animals, and once this threshold is crossed, the aquatic adaptations become irreversible. The transition to marine environments was caused by various modifications, such as increased body mass, which allowed them to keep warm in colder climates, and a carnivore diet to help support their increased metabolisms. Bruna told Live Science that these changes could make it more difficult for them to compete with terrestrial species.

“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 explained. Thus, the chances of aquatic animals such as dolphins and whales returning to land are almost nonexistent.

“While Dollo’s law makes a regular appearance in macroevolutionary studies of this kind, the authors have been able to debunk the myth that sea-to-land transitions are not entirely improbable,” Virag Sharma is an expert of comparative genomes at the University of Limerick who was not involved in the research said to Live Science.

He also said that the study focuses on mammals only. Therefore, future research could examine if the same irreversibilities are present in other lineages of tetrapods.