Our planet’s origins and its ancient history has fascinated human beings since the dawn of civilization. According to Science Alert, over 35 million years ago, the ancestor of modern cetaceans, i.e. fully aquatic mammals such as whales, dolphins and porpoises, returned to the sea, aided by its unique eyesight that allowed it to see in the deep.
Return to the Sea
It may surprise many people that cetaceans evolved from ancestors that lived on the land. This is clear from many of their features. Cetaceans breathe air, they nurse their young with milk, and their flippers encase bones that mark out five “fingers”. Before birth, the little back limbs in their embryos disappear.
If you look at whales and hippos, who are close relatives, you will notice that the two have similar features. Both whales and hippos evolved from four-legged, hoofed (ungulate) even-toed ancestors that roamed the earth some 50 million years ago. Today’s ungulates are animals such as hippos, giraffes, deer, pigs and cows.
The ancestors of cetaceans returned to the sea and, over 8 million years, evolved into creatures that lived in the sea.
The Basilosaurus, who were these massive prehistoric whales, were initially thought to be dinosaurs, before scientists realised they were mammals. The Basilosaurus was longer than modern whales, with smaller hind legs and front flippers. They had nostrils between the snout’s tip and Basilosaurus’ forehead, as well as the same ear bones we find on modern whales. The Basilosaurus was halfway in the transition from prehistoric ungulate ancestors and the modern whale.
Nobody knows why some prehistoric ungulates returned to the sea. Some believe that they liked to feed on plants near large water bodies because they could hide in the water whenever a predator approached. As they spent more time there, their bodies adapted to moving in the water, with their front legs morphing into flippers, blubber replacing their fur, and their bodies becoming smoother to move through the water more easily. Their tails become longer, stronger and bigger, allowing them to move through the sea. Meanwhile, their back legs were reduced in size. The nostrils moved upwards, enabling them to breathe without having to move their heads up as they swam. As their diet changed, some of them became baleen, losing their teeth.
One of the biggest questions that scientists have wrestled with is figuring out how these animals adapted to seeing in the deep. Recent evidence suggests that rhodopsin, or “virual 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 electric signals the rod cells, cone cells and photosensitive ganglion cells of the retina of the eye. Rhodopsin is very sensitive to light, making it possible to see clearly in low-light conditions.
Researchers, Sarah Z Dungan and Belinda S. W. Chang, published a paper, “Ancient whale rhodopsin reconstructs dim-light vision over a major evolutionary transition: Implications for ancestral diving behavior”, that shows just why whales were able to evolve the power of sight in the deep.
Dungan and Chang reconstructed the ancient sequence in which whales learnt breath-hold diving for the purposes of underwater foraging. This has always been very challenging to do, but they believed that by looking at their coevolving sensory systems, they could gain some insight as to what this sequence actually looks like. They also used protection resurrection to see how light-activation occurred during the transition from terrestrial to aquatic world.
Whales have rhodopsin’s that are more sensitive to light in dim conditions than rhodopsin’s in in land mammals. The authors found that the decay rates of these light-activated rhodopsin rose in prehistoric cetaceans, suggesting that adaptation to the dark occured (relatively) very quickly. Overall, they found that prehistoric ancestors of whales were able to dive some 650 feet or more, into what’s known as the mesopelagic zone, where light starts to dim. This occured before the split between toothed and baleen whales.
So, we can that cetaceans had the same ancesto, who started off with breath-hold diving a they 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 cetacean were dolphin-like, with tails flukes as well as vestigial hind limbs. However, before this work, scientists had not gone far in determining how they evolved the capacity of deep sea sight. What is more impressive is that Dungan and Chang achieved these results without having to examine any fossils. This may shock some people, but the reality is that prehistoric fossils are so old that it’s extremely unlikley that a researcher can get any DNA from it. So researchers have to rely on computer models, and samples of genes from existing organisms, to map out the evolutionary path of prehistoric creatures.