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It is almost impossible to know how extinct animals behaved; there is no Jurassic Park where we can see them hunting, mating or evading predators. But a developing technique is giving researchers a physiological cipher to decipher the behavior of extinct species by reconstructing and analyzing proteins from extinct animals. This molecular necromancy can help them understand traits not preserved in the fossil record.
In the most recent example of this technique in action, scientists led by Sarah Dungan, who completed the work while a graduate student at the University of Toronto (U of T) in Ontario, have revived the visual pigments of some of the earliest cetacean ancestors. . The work has given Dungan and his colleagues a new look at how protocetaceans would have lived immediately after a crucial evolutionary juncture: the time roughly 55 to 35 million years ago when the animals that eventually became in whales and dolphins left their land. lifestyles to return to the sea.
Dungan’s fascination with the evolution of whales began when he was eight years old. As a child, she loved spending time in the water and learning about marine biology. His father told him in passing that the ancestors of modern whales once lived on earth. The idea that an animal could transform from living completely out of water to not being able to live out of it stuck with her. Learning about the evolutionary transition modern whales made, from ocean to land and back again, “totally blew my mind,” he says. “The diary is the end of a story that began when I was very young.”
In 2003, U of T researchers pioneered a technique to assemble the ancient visual proteins of extinct animals. They have applied the technique to the entire animal kingdom, learning more about how extinct species saw the world. But studying extinct cetaceans is particularly interesting because the transition from land to ocean transformed the animals’ visual fields.
In this study, the researchers compared rhodopsin, the visual pigment responsible for dim-light vision, in animals that completed the land-to-ocean transition. They focused on the first cetacean, which lived 35 million years ago and probably swam using powerful tail muscles, and the first whippomorph (one of a group of animals that includes cetaceans and hippos), which live 55 million years ago.
Scientists have yet to discover the fossils of the two extinct species. Because of this, they can’t even say precisely what species they are. But Dungan’s technique can infer ancient protein sequences even without this information. The approach follows evolutionary clues left in the proteins of modern animals to figure out what the ancient forms would have looked like, even without the bones of the species. By comparing the presumed proteins of the first whippomorph and the first cetacean, scientists can tease out the subtle differences in their vision. These differences in vision could reflect differences in animal behavior.
“You can only learn so much from the fossil evidence,” says Dungan. “But the eye is a window between the organism and its environment.”
Using an evolutionary tree and the known structures of rhodopsin from modern cetaceans, Dungan and his team built a model to predict variants in ancient animals. They made the visual pigments in the lab by genetically modifying cultured mammalian cells and tested the light to which they are most sensitive. The scientists found that compared to the ancient whippomorph, the extinct cetacean was probably more sensitive to blue wavelengths of light. Blue light penetrates deeper into water than red light, so modern inhabitants of the deep sea, including fish and cetaceans, have blue-sensitive vision. The find suggests the extinct cetacean was comfortable in the deep sea.
The scientists also found that the ancient cetaceans’ version of rhodopsin quickly adapts to the dark. The eyes of modern cetaceans quickly adjust to dim light, helping them move between the bright surface where they breathe and the dark depths where they feed. That finding is “what really sealed the deal,” says Dungan.
Based on their findings, the scientists think that the first cetaceans probably sank in the twilight zone of the ocean, between 200 and 1,000 meters. Sight was vital during the dives. Ancient cetaceans could not echolocate like dolphins, so they relied more on vision.
The finding is surprising, says Lorian Schweikert, a neuroecologist at the University of North Carolina Wilmington who was not involved in the study. He thought that the first cetaceans would have stayed close to the surface. “It started from the bottom, now we’re here,” he quips, alluding to the success of Drake’s song.
Schweikert says studying eye physiology is a reliable way to infer an animal’s ecology because visual proteins don’t change much over time. Rare changes almost always correlate with environmental changes.
The most important takeaway from Dungan and his colleagues’ work, says Schweikert, is that it further clarifies the order in which cetacean extreme diving behaviors evolved. The rhodopsin research builds on previous work that painted a similar picture. In a previous study, researchers reconstructed ancient myoglobin and showed that early cetaceans “supercharged” their muscles’ oxygen supply while holding their breath, further proof that they were capable divers. Another study, this time on ancient penguins, showed that when the birds had their own transition to marine life, their hemoglobin developed mechanisms to handle oxygen more efficiently.
Dungan and his colleagues are now channeling their molecular Ouija board to resurrect rhodopsin from early mammals, bats and archosaurs. This will help them understand how nocturnal, burial and flight evolved.
The approach is “a lot of fun,” says Schweikert. “You’re trying to look into the past to understand how these animals evolved. I love that we can look at vision to solve some of these problems.”