The first mammals to return to the ocean 35 million years ago had deep-set eyes. According to new research, the visual systems of modern whales, dolphins and porpoises – known collectively as cetaceans – all derive from a common ancestor that possessed powerful underwater vision.
Both whales and hippos are believed to have evolved from four-legged land mammals about 50 million years ago. Although both have an aquatic lifestyle, only one branch can dive deep into the ocean.
When and why this skill evolved is still a mystery, but the new findings suggest that the transition occurred soon after the move to the ocean.
The findings are based on a protein in the mammalian eye called rhodopsin, which is particularly sensitive to the dim, blue light found in the deep sea.
By analyzing the genes behind this protein for living whales and some related mammals, the researchers were able to predict the ancestral gene sequence that allowed the first underwater diving.
When expressed in lab-grown cells, this signature sequence was able to ‘resurrect’ long-lost pigment proteins. Compared to terrestrial mammals, this protein appears to be more sensitive to low light levels. It also responds quickly to changes in light intensity.
If such a sensitive protein had been present in the first aquatic cetacean, the researchers believe the 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 first fully aquatic cetaceans could have dived into the mesopelagic zone,” the study authors concluded.
“Moreover, our recreation shows that this conduct emerged before the difference of toothed and baleen whales.” Instead, it seems that all cetaceans shared an ancestor that could see deep, even those that now hunt in deeper waters.
Then, explains evolutionary biologist Belinda Chang, “later species developed all the diverse foraging skills we see today in modern whales and dolphins.”
Previous studies of the fossil remains of ancient whales have suggested that the first aquatic cetaceans had a dolphin-like body with a tail fluke and a combination of hind limbs for swimming.
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The current study, however, is one of the first to investigate how the creature’s eyes may have functioned in its search for food underwater.
Even more impressive, the authors did so without a body fossil. “The fossil record is the gold standard for understanding evolutionary biology,” says Sarah Dungan, an evolutionary biologist at the University of Toronto. gets worse.”
“Assuming you’re keen on how qualities and DNA are advancing, you depend on numerical demonstrating and strong examples of qualities in living beings to supplement what we comprehend from the fossil record.”
