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It is almost impossible to know how extinct animals behaved; there is no Jurassic Park where we can watch them hunt, mate or avoid predators. But a developing technique is giving researchers a physiological code for deciphering the behavior of extinct species by reconstructing and analyzing the proteins of extinct animals. This molecular necromancy can help them understand traits that are not preserved in fossils.
In the most recent example of this technique in action, scientists led by Sarah Dungan, who completed the work while an undergraduate at the University of Toronto (U of T) in Ontario, have revived visual pigments from some of the earliest cetacean ancestors. The work gave Dungan and her colleagues new insight into how protocetaceans would have lived just after a crucial evolutionary moment: the time roughly 55 to 35 million years ago, when the animals that eventually became whales and dolphins abandoned their terrestrial lifestyles , to return to the sea.
Dungan’s fascination with whale evolution began when he was eight. As a child, she loved spending time in the water and learning about marine biology. Her father told her in passing that the ancestors of modern whales once lived on land. The idea that an animal could transform from living entirely outside water to being unable to live outside it stayed with her. Learning about the evolutionary transition of modern whales—from ocean to land and back again—“completely blew my mind,” she says. “The paper is the end of a story that began when I was very young.”
In 2003, U of T researchers introduced a technique to assemble ancient visual proteins of extinct animals. They have applied the technique to the animal kingdom, learning more about how extinct species saw the world. But studying extinct cetaceans is particularly interesting because the land-ocean transition transformed the animals’ visual realms.
In this study, the researchers compared rhodopsin, the visual pigment responsible for low-light vision, from animals that had blocked the land-ocean transition. They focused on the first cetacean, which lived 35 million years ago and probably swam using powerful muscles in its tail, and the first bipomorph (one of a group of animals that includes cetaceans and hippos) that lived 55 million years ago.
Scientists have yet to find the fossils of the two extinct species. For that matter, they can’t even say exactly what species they are. But Dungan’s technique can infer ancient protein sequences even without this information. The approach follows the evolutionary breadcrumbs left in the proteins of modern animals to understand what the ancient forms would have looked like, even without the bones of the species themselves. By comparing the putative proteins of the first whipomorph and the first cetacean, scientists can pick up the subtle differences in their vision. These differences in vision may reflect differences in animal behavior.
“There’s so much you can learn from 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 her team built a model to predict variants in ancient animals. They produced the visual pigments in the laboratory by genetically modifying cultured mammalian cells and tested the light to which they were most sensitive. The scientists found that compared to the ancient whipomorph, the extinct cetacean was probably more sensitive to blue wavelengths of light. Blue light penetrates deeper into the water than red, so modern deep-sea creatures, 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 adapted quickly to darkness. The eyes of modern cetaceans quickly adapt to dim light, helping them navigate between the bright surface where they breathe and the dark depths where they feed. That discovery is “what really sealed the deal,” Dungan says.
Based on their findings, the scientists believe that early cetaceans likely dived in the ocean’s twilight zone, between 200 and 1,000 meters. Vision was vital during dives. Ancient cetaceans could not echolocate like dolphins, so they relied more on sight.
The finding is surprising, says Lorian Schweikert, a neuroecologist at the University of North Carolina Wilmington, who was not involved in the study. She thought the first cetaceans would have stayed close to the surface. “We started at the bottom, now we’re here,” she jokes, alluding to Drake’s hit 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 changes in the environment.
The most important takeaway from Dungan and her colleagues’ work, Schweikert says, is that it further clarifies the order in which cetacean extreme diving behavior evolved. The rhodopsin study builds on earlier work that painted a similar picture. In a previous study, researchers reconstructed ancient myoglobin and showed that early cetaceans “charged” their muscles with oxygen while holding their breath – further evidence that they were capable divers. Another study, this time on ancient penguins, showed that when the birds made their own transition to marine life, their hemoglobin evolved mechanisms to manage oxygen more efficiently.
Dungan and her colleagues are now directing their molecular Ouija board to resurrect rhodopsin from the earliest mammals, bats and archosaurs. This will help them understand how nocturnal, burrowing and flying evolved.
The approach is “just really fun,” Schweikert says. “You’re trying to look into the past to understand how these animals evolved. I like that we can look at the vision to solve some of these problems.”
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