The first mammals to return to the sea, more than 35 million years ago, had eyes for the deep.
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 about 50 million years ago. While both have aquatic lifestyles, only one of these branches can dive deep into the ocean.
When and why that skill evolved is still very much a mystery, but the new findings suggest the transition happened shortly after taking to the sea.
The findings are based on a protein in the mammalian eye known as rhodopsin, which is particularly sensitive to dim, 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 gene sequence that first allowed for deep underwater dives.
When expressed in lab-grown cells, this signature sequence was able to 'resurrect' a long-lost pigment protein.
Compared to land mammals, this protein appears much more sensitive to low light levels. It also responds rapidly to changes in light intensity.
If such a sensitive protein existed in the first aquatic cetacean, researchers think 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 shifts in rhodopsin function suggest that some of the first fully aquatic cetaceans could dive into the mesopelagic zone," the authors of the study conclude.
"Moreover, our reconstructions indicate that this behavior arose before the divergence of toothed and baleen whales."
Instead, it seems all cetaceans shared an ancestor that could see in the deep, even those that now hunt in shallow waters.
Then, explains evolutionary biologist Belinda Chang, "later species evolved all the diverse foraging specializations we see in modern whales and dolphins today."
Previous studies on 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 vestigial hind limbs for swimming.
The current study, however, is one of the first to investigate how this creature's eyes might have worked in its search for underwater food.
Even more impressive, the authors did so without a physical fossil.
"The fossil record is the gold standard for understanding evolutionary biology. But despite what Jurassic Park would have you believe, extracting DNA from fossil specimens is rare because the condition tends to be poor," says evolutionary biologist Sarah Dungan from the University of Toronto.
"If you're interested in how genes and DNA are evolving, you rely on mathematical modeling and a strong sample of genes from living organisms to complement what we understand from the fossil record."
The study was published in the Proceedings of the National Academy of Sciences.
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