The first mammals to return to the sea more than 35 million years ago looked into the depths.
The visual systems of whales and dolphins, collectively known as cetaceans, evolved from a common ancestor with strong underwater vision, according to a new study.
Both whales and hippos are thought to have evolved from four-legged mammals about 50 million years ago. Although both are aquatic, only one of these branches can dive into the depths of the ocean.
The findings are based on a mammalian eye protein known as rhodopsin, which is particularly sensitive to weak blue light, like that found in the deep ocean.
By analyzing the genes for this protein in modern whales and some related mammals, the researchers were able to predict the genetic sequence of the ancestors that first allowed underwater diving.
When expressed in cells grown in vitro, this signature sequence was able to “revive” a long-lost chromosomal protein.
Compared to terrestrial mammals, this protein is more sensitive to lower light levels. It also responds quickly to changes in light intensity.
And if such a sensitive protein was present in the first aquatic cetaceans, the researchers believe that this creature could have searched for food at depths of 200 meters or more (about 650 feet) when the light begins to fade in the ocean.
The authors of the study conclude: “These inherited shifts in rhodopsin function collectively suggest that some of the earliest aquatic whales fully submerged into the mesopelagic zone. In addition, our reconstructions show that this behavior arose before the divergence of odontocetes and right whales.”
Instead, all cetaceans seem to have a common ancestor that can see at depth, even those now fishing in shallow water.
Previous studies of ancient whale fossils have shown that the first water whale had a dolphin-like body with a combination of swimming tail and hind limbs.
However, the current study is one of the first to look at how this creature’s eyes function in search of underwater food.
The study was published in a journal. Proceedings of National Academy of the science.
Source: Science Alert.