If you’re going to commit to something, commit all the way. Sound advice for everyone — but something the planet’s aquatic mammals know full well. That’s because a new study published in the Proceedings of the Royal Society B has found that once mammals turn to a fully aquatic life, there’s no going back, evolutionarily speaking.
“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,” lead study author Bruno Farina told Live Science in an interview. Farina is a doctoral student at the University of Fribourg in Switzerland.
According to current evolutionary theory, the first fish began hauling themselves out of the water over 400 million years ago. These animals are the ancestors of all tetrapods (vertebrates with four limbs and digits). One example is a tiktaalik, a transitional crocodile-like species that sported both lungs and gills. Scientists first discovered a tiktaalik fossil on Ellesmere Island in 2004.
About 150 million years later, some tetrapods ventured back to the briny blue and became the fully aquatic mammals we know and love today.
But is a return to land impossible for these animals? Or have they just not gotten around to it yet? That’s what scientists involved with the study set out to discover.
You can’t go home again
One of science’s first paleontologists was a Belgian named Louis Dollo. Dollo posited that evolution was, in fact, a one-way street — an idea that came to be known as “Dollo’s Law” — and it’s been argued about ever since.
“Just look at the world around you, right here on the ocean floor. Such wonderful things surround you. What more is you lookin’ for?” A musical crab to this orca, probably. Photo: Shutterstock
To make their contribution to the debate, Farina and his co-authors divided 5,600 mammal species into four distinct groups covering fully terrestrial species, fully aquatic species, and everything in between. Then they created a model that really drills down on which aquatic traits, if any, have been lost over the years.
“One of the main points of our work was to include the entire gradient of adaptations from fully terrestrial to fully aquatic forms,” Farina shared with Live Science.
The results? The same adaptations that make a fully aquatic life possible (like, for instance, increased mass) make it that much harder to climb back out onto land. Meanwhile, semi-aquatic adaptations can and do sometimes get lost in the evolutionary shuffle.
In other words, according to this study, at least, Dollo was both right and wrong. There’s a threshold when it comes to aquatic adaptations. Stay on one side, and you can re-adapt to land. Pass it, and you’re in too deep (literally) to back out. It’s a concept a certain musical crustacean would approve of.
So that’s a bummer, because dolphins, whales, and their relatives are pretty cool, and it would be sweet to hang out with them on land in 100 million years or so. But who’s to say humans (or some version of us) won’t be swimming around down there with them as the millennia roll on?
