The first mammals to return to the sea, more than 35 million years ago, had eyes specially adapted for the depths.

According to new research, the visual systems of whales and dolphins all derive from a common ancestor with powerful underwater vision.

Both whales and hippos are believed to have evolved from a four-legged land mammal around 50 million years ago. Although both have an aquatic lifestyle, only one of these branches can dive deep into the ocean.

When and why this ability evolved is still a big mystery, but the new findings suggest that the transition happened shortly after they took to the sea.

Mammalian eyes and their secrets

The findings are based on a protein in the mammalian eye known as rhodopsin, which is particularly sensitive to dim, blue light such as that found in the deep ocean.

By analyzing the genes underlying this protein in living whales and some related mammals, the researchers were able to predict the ancestral genetic sequence that first allowed deep diving.

When expressed in cells grown in the lab, this sequence was able to “resurrect” a long-lost pigment protein.

Compared to land mammals, this protein appears to be much more sensitive to low light levels. It also responds quickly to changes in light intensity.

A common ancestor

If such a sensitive protein existed in the first aquatic cetacean, the researchers believe that this creature could have searched for food at depths of 200 meters or more, where light begins to fade in the ocean.

Instead, it appears that all cetaceans shared a deep-seeing ancestor, even those that now hunt in shallow water.

Previous studies of the fossilized remains of ancient whales suggested that the first aquatic cetacean had a dolphin-like body.

However, the current study is one of the first to investigate how this creature’s eyes might have worked in underwater foraging.

Fossil specimens are very rare

Even more impressive is that the authors did this without a physical fossil, they write ScienceAlert.

“The fossil record is the gold standard for understanding evolutionary biology. But despite what Jurassic Park would have you believe, extracting DNA from fossil specimens is rare because their condition tends to be poor,” says evolutionary biologist Sarah Dungan of the University of Toronto.

“If you’re interested in how genes and DNA evolve, you rely on mathematical modeling and a powerful sample of genes from living organisms to complement what we understand from the fossil record.”

The study was published in Proceedings of the National Academy of Sciences.

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