The first mammals to return to the sea, more than 35 million years ago, had keen eyes for the deep.
According to recent research, 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 mammals 50 million years ago. Although both enjoy aquatic lifestyles, only one branch may dive deep into the ocean.
The time and why it evolved is still a mystery, but new observations suggest that the transition occurred shortly after the trip to the sea.
The findings are based on a protein found in the mammalian eye known as rhodopsin, which is particularly sensitive to dim, blue light like that found in the deep ocean.
Researchers were able to reconstruct the ancestral gene sequence that first permitted deep underwater dives by studying the genes behind this protein in living whales and other similar mammals.
When expressed in lab-grown cells, this signature sequence was able to'resurrect' a long-lost pigment protein.
This protein appears to be much more sensitive to low light conditions than other mammals. It also responds rapidly to changes in light intensity.
Researchers believe that if such a sensitive protein existed in the first aquatic cetacean, this organism might have foraged for food at depths of 200 meters or more (around 650 feet) where light is starting to fade in the ocean.
"These ancestral shifts in rhodopsin function suggest that some of the first fully aquatic cetaceans might find their way into the mesopelagic zone," according to the authors of the study.
"Our reconstructions demonstrate that this behavior evolved before the divergence of toothed and baleen whales."
Instead, it appears that all cetaceans shared an ancestor who could see in the deep, even those who now hunt in shallow waters.
"Later species evolved all the different foraging specializations we see today in modern whales and dolphins," says evolutionary biologist Belinda Chang.
Previous investigations on ancient whale fossils have suggested that the first aquatic cetacean had a dolphin-like body with a swimming vestigial hind limbs.
The present investigation, however, is one of the first to investigate how this creature's eyes might have acted in its search for underwater food.
Even more remarkable, the authors did so without a physical fossil.
"The fossil record is the gold standard for understanding evolutionary biology. Nevertheless, extracting DNA from fossil specimens is rare, because the condition tends to be poor," according to University of Toronto evolutionary biologist Sarah Dungan.
"You rely on mathematical modeling and a robust sample of living organism genes to complement what we know from the fossil record"
The study was published in the Proceedings of the National Academy of Sciences.