The first mammals to return to the sea more than 35 million years ago had eyes for the deep.
According to new research, the visual systems of modern whales, dolphins and porpoises – collectively known as cetaceans – all descended from a common ancestor with powerful underwater vision.
Both whales and hippos are believed to have evolved from four-legged land mammals about 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 skill developed is still a big mystery, but the new findings suggest that the transition happened shortly after entering the sea.
The findings are based on a protein in the mammalian eye known as rhodopsin, which is particularly sensitive to dim, blue light like that found in the deep ocean.
By analyzing the genes behind this protein for living whales and some related mammals, the researchers were able to predict the ancestral gene sequence that first allowed deep underwater diving.
When expressed in lab-grown cells, this signature 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.
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 (about 650 feet), where light begins to fade in the ocean.
“Taken together, these heritable changes in rhodopsin function suggest that some of the first fully aquatic cetaceans could dive into the mesopelagic zone,” the study authors conclude.
“Furthermore, our reconstructions show that this behavior arose before the divergence of toothed whales and cetaceans.”
Instead, it seems that all cetaceans had an ancestor that could see in the depths, even those that now hunt in shallow water.
Then, explains evolutionary biologist Belinda Chang, “later species developed all the diverse foraging specializations we see in modern whales and dolphins today.”
Previous studies of fossilized remains of ancient whales suggested that the first aquatic cetaceans had a dolphin-like body with a combination of tail flukes and rudimentary hind limbs for swimming.
However, the current study is one of the first to investigate how this creature’s eyes might have worked in its search for underwater food.
Even more impressive is that the authors did this without a physical fossil.
“Fossils are the gold standard for understanding evolutionary biology. But despite what Jurassic Park would have you believe, extracting DNA from fossils is rare because the condition is usually 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 strong sampling of genes from living organisms to complement what we understand from the fossil record.”
The research was published in the Proceedings of the National Academy of Sciences.
Add Comment