What Did Ancient Whales See?

Scientists are examining evolutionary breadcrumbs to reconstruct the vision of whales that lived as long as 55 million years ago.

close up of whale
Getty

This article was originally published by Hakai Magazine.

It’s nearly impossible to know how extinct animals behaved; there’s no Jurassic Park where we can watch them hunt or mate or evade predators. But a developing technique is giving researchers a physiological cipher to decrypt the behavior of extinct species by reconstructing and analyzing extinct animals’ proteins. This molecular necromancy can help them understand traits that don’t preserve in the fossil record.

In the most recent example of this technique in action, scientists led by Sarah Dungan, who completed the work as a graduate student at the University of Toronto (U of T) in Ontario, have revived the visual pigments from some of cetaceans’ earliest ancestors. It’s given Dungan and her colleagues a new look into how proto-cetaceans would have lived in the immediate aftermath of a crucial evolutionary juncture: 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 she was 8. As a kid, she loved 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 notion that an animal could transform from living entirely out of water to not being able to live outside of it stuck with her. Learning about the evolutionary transition that modern whales took—from ocean to land and back again—“totally blew me away,” she says. “The paper is the end of a story that started when I was really young.”

In 2003, researchers at U of T pioneered a technique to recreate extinct animals’ ancient visual proteins. They’ve applied the technique across the animal kingdom, learning more about how extinct species saw the world. But studying extinct cetaceans is especially interesting because the land-to-ocean transition transformed the animals’ visual realms.

In the study, the researchers compared rhodopsin, the visual pigment responsible for dim-light vision, in the animals that bookended the land-to-ocean transition. They focused on common ancestors of Cetacea and Whippomorpha (the group of animals that includes cetaceans and hippos), representing a time period of about 55 to 35 million years ago.

Scientists haven’t yet recovered genetic material from the fossils for these two extinct species. For that matter, 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 the evolutionary bread crumbs left in modern animals’ proteins to infer what the ancient forms would have looked like, even without the DNA of the species themselves. By comparing the presumed proteins of Whippomorpha and Cetacea during the land-to-ocean transition, the scientists can glean the subtle differences in their vision. These differences in vision could reflect differences in the animals’ behaviors.

“There’s only so much you can learn from fossil evidence,” Dungan says. “But the eye is a window between the organism and its environment.”

Using the known rhodopsin structures from modern cetaceans, Dungan and her team constructed an evolutionary tree that they used alongside likelihood models to help predict the ancient animals’ variants. They manufactured these visual pigments in the lab in mammalian cells and tested the light they are most sensitive to. The scientists found that compared with ancient Whippomorpha, extinct Cetacea were likely more sensitive to blue wavelengths of light. Blue light penetrates deeper into water than red, so modern deep-sea denizens, including fishes and cetaceans, have blue-sensitive vision. The finding suggests that the extinct Cetacea were comfortable in the deep sea.

The scientists also found that the ancient Cetacea’s version of rhodopsin seems to have adapted particularly quickly to the dark. Today, many modern cetaceans’ eyes quickly adjust to dim light, helping them move between the bright surface where they breathe and the dark depths where they feed. This finding is “what really sealed the deal,” Dungan says.

Based on their findings, the scientists think early Cetacea probably dove to the ocean’s twilight zone, between 200 and 1,000 meters. Eyesight was vital during dives. Ancient Cetacea couldn’t echolocate like dolphins, so they relied more heavily on vision.

The finding is surprising, says Lorian Schweikert, a neuroecologist at the University of North Carolina at Wilmington who wasn’t involved in the study. She thought that the first Cetacea would have stayed near the surface. “Started from the bottom, now we’re here,” she jokes, alluding to Drake’s hit song.

Schweikert says that studying eye physiology is a reliable way to infer an animal’s ecology because visual proteins don’t change much over time. The rare changes almost always correlate with environmental shifts.

The most important conclusion of Dungan and her colleagues’ work, Schweikert says, is that it further clarifies the order in which cetaceans’ 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 transitional ancestors of cetaceans supercharged their muscles’ oxygen supply while they held their breath—further evidence 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 evolved mechanisms to more efficiently manage oxygen.

Dungan and her colleagues are now channeling their molecular Ouija board to resurrect rhodopsin from the earliest mammals, bats, and archosaurs. This will help them understand how nocturnality, burrowing, and flight evolved.

The approach is “just really fun,” Schweikert says. “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.”