Around 400 million years ago, the ancestor of all four-legged friends took his first steps on the mainland. About 350 million years later, a descendant of these early landlubbers did a U-turn and waded back into the water. Over time, the back-to-sea creatures evolved into animals very different from their land-trotting cousins: they became the magnificent whales, dolphins, and porpoises that glide the oceans today.
The return to aquatic animals was a drastic step that would change animals both internally and externally within about 10 million years—an evolutionary wink. Members of this group, now called whales, dropped their hind legs for powerful leeches and almost all lost their hair. Hers for decades Bizarre body plans baffled paleontologistswho speculated that they may have evolved from creatures as diverse as marine reptiles, seals, marsupials such as kangaroos, and even a now-extinct group of wolf-like carnivores.
“Whales are, by and large, the most peculiar and devious of mammals,” wrote one scientist in 1945.
Then, in the late 1990s, genetic data confirmed that whales were part of the same Line of evolution that produced cows, pigs, and camels– a branch called Artiodactyla. Fossils from what is now India and Pakistan later completed this family tree and identified the closest ancient relatives of whales as small, wading deer-like creatures.
But their body plans are just the beginning of the whales’ insanity. To survive in the sea, they also had to make internal changes, changing their blood, saliva, lungs, and skin. Many of these changes are not evident in fossils, and whales are not easy to study in the laboratory. Instead, once again, it was genetics that brought them to light.
With increasing availability of whale genomes, geneticists can now look for the molecular changes that accompanied the transition back to the water. While it’s impossible to be sure about the influence of any particular mutation, scientists suspect that many of the mutations they see correspond to adaptations that allow whales to dive and thrive in the deep blue sea.
Plunge into the deep
The first whales lost much more than just legs when they returned to the water: Entire genes became dysfunctional. In the vast book of genetic letters that make up a genome, these defunct genes are among the easiest changes to spot. They stand out like a garbled or fragmented sentence and no longer encode a complete protein.
Such a loss can happen in two ways. Perhaps having a particular gene was somehow detrimental to whales, giving animals that lost it a survival advantage. Or it could be a “use it or lose it” situation, says genomics scientist Michael Hiller of the Senckenberg Research Institute in Frankfurt, Germany. If the gene had no purpose in the water, it would randomly accumulate mutations and the animals would be no worse off if it stopped working.
Hiller and colleagues delved into the transition back to water by comparing the genomes of four whales — dolphin, orca, sperm whale and minke whale — with those of 55 land mammals, plus a manatee, a walrus and the Weddell seal. Some 85 genes became dysfunctional When the whales’ ancestors adapted to the sea, the team got in touch scientific advances in 2019. In many cases, Hiller says, they were able to guess why those genes stopped working.
For example, whales no longer have a certain gene –SLC4A9—involved in salivation. That makes sense: what’s the point of spitting if your mouth is already full of water?
Whales also lost four genes involved in synthesizing and responding to melatonin, a hormone that regulates sleep. The ancestors of whales probably discovered pretty quickly that they couldn’t surface to breathe if they turned their brains off for hours. Modern whales sleep one brain hemisphere at a time while the other brain hemisphere stays alert. “Anyone who doesn’t sleep as regularly as we know them probably doesn’t need melatonin,” says Hiller.
The long periods during which whales must hold their breath to dive and hunt also appear to have driven genetic changes. As divers know, deep diving means small pockets of nitrogen can form in the blood and clumps of semen — something that was likely harmful to early whales. Coincidentally, two genes (F12 and KLKB1), which normally help trigger blood clotting, are no longer functional in whales, presumably reducing this risk. The rest of the clotting machinery remains intact, so whales and dolphins can still seal injuries.
Another lost gene – and this one surprised Hiller – encodes an enzyme that repairs damaged DNA. He believes this change also has to do with deep dives. When whales surface to breathe, oxygen suddenly floods their bloodstream, and as a result, reactive oxygen molecules that can break apart DNA. The missing enzyme—DNA polymerase mu—usually repairs this type of damage, but it does so sloppily and often leaves behind mutations. Other enzymes are more accurate. Perhaps, Hiller thinks, mu was just too sloppy for the whales’ lifestyle, unable to handle the amount of reactive oxygen molecules produced by the constant diving and resurfacing. Omitting the imprecise enzyme and leaving the repair to more accurate enzymes, which whales also possess, may have increased the likelihood that oxygen damage was repaired correctly.
Whales are not the only mammals to have returned to the water, and genetic losses in other aquatic mammals often resemble those in whales and dolphins. For example, both whales and manatees have disables a gene called MMP12, which normally breaks down the stretchy lung protein called elastin. Perhaps this deactivation helped both groups of animals evolve highly elastic lungs, allowing them to quickly exhale and inhale about 90 percent of their lung volume when they surfaced.
Deep customizations it’s not just about losses. A notable win lies in the gene that contains instructions for myoglobin, a protein that supplies muscles with oxygen. Scientists have studied myoglobin genes in diving animals from tiny water shrews to giant whales discovered a pattern: In many divers, the surface of the protein is more positively charged. This would cause the myoglobin molecules to repel each other like two north magnets. Researchers theorize that this allows diving mammals to maintain high levels of myoglobin without the proteins fusing together, and thus high levels of muscle oxygen when they dive.
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