It is almost impossible to know how extinct animals behaved. There is no Jurassic Park where we can watch them hunt or mate or avoid predators. But a developing technique is giving researchers a physiological cipher for deciphering the behavior of extinct species by reconstructing and analyzing proteins from extinct animals. This molecular necromancy can help them understand features not preserved in the fossil record.
In the latest 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 visual pigments from some of the earliest cetacean ancestors . The work gave Dungan and her colleagues a new look at how protoceteans would have lived just after a critical evolutionary juncture: the time about 55 to 35 million years ago, when the animals that eventually became whales and dolphins left their land-based habitats. lifestyle to return to the sea.
Dungan’s fascination with whale evolution began when she was eight. As a child, she enjoyed spending time in the water and learning about marine biology. Her dad told her in passing that the ancestors of modern whales once lived on land. The idea that an animal could go from living completely out of water to not being able to live out of it stuck with her. Learning about the evolutionary transition that modern whales went through – from ocean to land and back again – “completely blew my mind,” he says. “The newspaper is the end of a story that began when I was very young.”
In 2003, researchers at U of T pioneered a technique to assemble ancient visual proteins of extinct animals. They applied the technique across the animal kingdom, learning more about how extinct species saw the world. But the study of extinct cetaceans is particularly interesting because the land-to-ocean transition transformed the animals’ visual realms.
In this study, researchers compared rhodopsin, the visual pigment responsible for dim-light vision, from animals that made the land-to-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 whippomorph (one of a group of animals that includes cetaceans and hippos), which lived 55 million years ago.
Scientists have yet to discover the fossils for the two extinct species. For that matter, they can’t even tell exactly what kind it is. 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 figure out what the ancient forms would have looked like, even without the bones of the species itself. By comparing the putative proteins of the first flagellum and the first cetacean, scientists can find the subtle differences in their vision. These differences in vision could reflect differences in animal behavior.
“There’s only so much you can learn from fossils,” 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 variations in ancient animals. They made the optical 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 likely more sensitive to blue wavelengths of light. Blue light penetrates deeper into water than red, so modern deep-sea inhabitants, including fish and cetaceans, have blue-sensitive vision. The find suggests the extinct cetacean was comfortable in the deep sea.
The scientists also discovered that the ancient cetaceans’ version of rhodopsin quickly adapts to the dark. The eyes of modern cetaceans adapt quickly to dim light, helping them navigate 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 believe that early cetaceans probably dove in the ocean’s twilight zone, between 200 and 1,000 meters. Vision was vital during dives. Ancient cetaceans could not vocalize 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. He thought the first cetaceans would have stayed close to the surface. “I started from the bottom now we’re here,” she jokes, referring 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 are almost always associated with environmental changes.
The most important takeaway from Dungan and her colleagues’ work, Schweikert says, is that it further clarifies the order in which cetacean extreme diving behaviors evolved. The rhodopsin research builds on earlier 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 evidence that they were skilled divers. Another study, this time on ancient penguins, showed that when the birds made their transition to marine life, their hemoglobin evolved mechanisms to handle oxygen more efficiently.
Dungan and her colleagues are now channeling their molecular Ouija board to resurrect rhodopsin from early mammals, bats and archosaurs. This will help them understand how nightlife, hit and run, evolved.
The approach is “just really fun,” says Schweikert. “Try to look into the past to understand how these animals evolved. I love that we can see the vision to solve some of these problems.”
