It may surprise many people that cetaceans descended from ancestors that lived on land. This is clear from many of their characteristics. Cetaceans breathe air, nurse their young with milk, and their flippers are surrounded by bones that distinguish five “toes.” Before birth, the small hind limbs in their fetuses disappear. If you look at whales and hippos, which are close relatives, you will notice that they both have similar characteristics. Both whales and hippos evolved from four-legged, ungulate (ungulate) ancestors that roamed the earth about 50 million years ago. Today’s ungulates are animals such as hippos, giraffes, deer, pigs and cows. The cetacean ancestors returned to the sea and, over 8 million years, evolved into sea-dwelling creatures. Basilosaurs, which were these huge prehistoric whales, were originally thought to be dinosaurs, before scientists realized they were mammals. Basilosaurus was longer than modern whales, with shorter hind legs and front flippers. They had nostrils between the tip of the snout and the forehead of Basilosaurus, as well as the same ear bones found in modern whales. Basilosaurus was halfway between the prehistoric ungulate ancestors and the modern whale. No one knows why some prehistoric ungulates returned to the sea. Some believe they liked to feed on plants near large bodies of water because they could hide in the water whenever a predator approached. As they spent more time there, their bodies adapted to moving in water, with their front legs morphing into flippers, replacing their fur, and their bodies becoming smoother to move through water more easily. Their tails become longer, stronger and larger, allowing them to move through the sea. Meanwhile, their hind legs shrank in size. The nostrils moved upwards, enabling them to breathe without having to lift their heads up as they swam. As their diet changed, some of them became ballers, losing their teeth. One of the biggest questions scientists have wrestled with is understanding how these animals adapted to see in the deep. Recent evidence suggests that rhodopsin, or “viral purple,” a protein found in a mammal’s eye, is responsible for this evolutionary leap. Rhodopsin is a light-sensitive receptor protein that is important for visual phototransduction, the process by which light is converted into electrical signals in the striatal cells, cone cells, and light-sensitive ganglion cells of the retina of the eye. Rhodopsin is very sensitive to light, enabling clear vision in low light conditions. The researchers, Sarah Z Dungan and Belinda SW Chang, published a paper, “Ancient whale rhodopsin rewires dim-light vision in a major evolutionary transition: Implications for ancestral diving behavior,” which shows exactly why whales were in position to develop the power of vision in the deep. Dungan and Chang reconstructed the ancient sequence in which whales learned breath-hold diving for the purpose of underwater foraging. This has always been very difficult to do, but they believed that by looking at their co-evolving sensory systems, they could gain some insight into what this sequence actually looks like. They also used the protection resurrection to see how the activation of light occurred during the transition from the land to the water world. Whales have rhodopsin that is more sensitive to light in dim conditions than rhodopsin in land mammals. The authors found that decay rates of this light-activated rhodopsin increased in prehistoric cetaceans, suggesting that dark adaptation happened (relatively) very quickly. Overall, they found that the whales’ prehistoric ancestors were able to dive about 650 feet or more, in what is known as the midplague zone, where light begins to fade. This was before the split between toothed whales and whales. So we can say that cetaceans had the same ancestor, which started with breathing diving and evolved into today’s modern form. At a later stage, they evolved all the features that made it easier for them to forage in the sea. In the past, researchers believed that prehistoric cetaceans were dolphin-like, with tails and hind limbs. However, prior to this work, scientists had not progressed very far in determining how they evolved the ability to see deep sea. Most impressively, Dungan and Chang achieved these results without having to examine fossils. This may shock some people, but the reality is that prehistoric fossils are so old that it is extremely unlikely that a researcher would get any DNA from them. So researchers must rely on computer models and gene samples from existing organisms to map the evolutionary path of prehistoric creatures.
