An Ancient Creature That Could See In The Dark Is Hiding In The Eyes Of Whales

The first mammals to return to the sea, more than 35 million years ago, had eyes for the depths.

According to new research, the visual systems of modern whales, dolphins and porpoises – collectively known as cetaceans – all derive from a common ancestor with powerful underwater vision.

Both whales and hippos are thought to have evolved from a four-legged land mammal around 50 million years ago. Although both have an aquatic lifestyle, only one of these branches can dive deep into the ocean.

When and why this skill evolved is still a mystery, but the new findings suggest the transition happened shortly after setting sail.

The findings are based on a mammalian eye protein known as rhodopsin, which is particularly sensitive to faint blue light like that found in the deep ocean.

By analyzing the genes behind this protein for living whales and some related mammals, researchers were able to predict the ancestral genetic sequence that first enabled deep underwater diving.

When expressed in lab-grown cells, this signature sequence was able to “resurrect” a long-lost pigment protein.

Compared to terrestrial mammals, this protein seems much more sensitive to low levels of light. It also reacts quickly to changes in light intensity.

If such a sensitive protein existed in the first aquatic cetacean, researchers believe this creature could have foraged for food at depths of 200 meters or more (about 650 feet) where light begins to fade in the ocean.

“Taken together, these ancestral changes in rhodopsin function suggest that some of the earliest fully aquatic cetaceans may dive into the mesopelagic zone,” the study authors conclude.

“Furthermore, our reconstructions indicate that this behavior arose before the divergence of toothed and baleen whales.”

Instead, it seems that all cetaceans shared an ancestor that could see into the depths, even those that now hunt in shallow waters.

Then, explains evolutionary biologist Belinda Chang, “later species evolved all of the various foraging specializations that we see in modern whales and dolphins today.”

Previous studies of the fossilized remains of ancient whales have suggested that the first aquatic cetacean had a dolphin-like body with a combination of tail flukes and residual hind limbs for swimming.

The current study, however, is one of the first to investigate how this creature’s eyes might have functioned in its underwater foraging.

Even more impressive, the authors did it without a physical fossil.

“The fossil record is the gold standard for understanding evolutionary biology. But despite what Jurassic Park would have you believe, DNA extraction from fossil specimens is rare because the condition tends to bad,” says evolutionary biologist Sarah Dungan of the University of Toronto.

“If you’re interested in the evolution of genes and DNA, you rely on mathematical modeling and a solid sample of genes from living organisms to supplement what we understand from the fossil record.”

The study was published in the Proceedings of the National Academy of Sciences.

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