According to new research, the visual systems of modern whales, dolphins and seals – collectively known as cetaceans – 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 about 50 million years ago. While 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 going out to 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 originally allowed for 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 was present in the first aquatic cetacean, the researchers believe that this creature could forage at depths of 200 meters or more (about 650 feet), where light begins to fade in the ocean. “Altogether, these ancestral 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 baleen whales. Instead, it seems that all cetaceans had an ancestor that could see deep, even those that now hunt in shallow water. Then, explains evolutionary biologist Belinda Chang, “later species evolved all the different foraging specialties that we see in modern whales and dolphins today.” Previous studies of the fossil remains of ancient whales have suggested that the first aquatic cetacean had a dolphin-like body with a combination of tails and vestigial hind limbs for swimming. The current study, however, is one of the first to investigate how this creature’s eyes might have worked in its search for underwater food. Even more impressive, the authors did it without a natural fossil. “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 the condition tends to be bad,” says evolutionary biologist Sarah Dungan from the University of Toronto. “If you’re interested in how genes and DNA evolve, you rely on mathematical modeling and a robust sample of genes from living organisms to complement what we understand from the fossil record.” The study was published in the Proceedings of the National Academy of Sciences.
