Natures wonders: whale series genetically explored (Introduction)

by David Turell , Sunday, February 26, 2023, 16:51 (634 days ago) @ David Turell

New explanations:

https://knowablemagazine.org/article/living-world/2022/evolution-whales-land-to-sea

"Going back to being aquatic was a drastic move that would change the animals inside and out, in the space of about 10 million years — an eyeblink in evolutionary terms. Members of this group, now called cetaceans, dropped their hind limbs for powerful flukes and lost nearly all their hair. For decades, their bizarre body plans perplexed paleontologists. (my bold)

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"Then, in the late 1990s, genetic data confirmed that whales were part of the same evolutionary line that spawned cows, pigs and camels — a branch called Artiodactyla. Fossils from modern-day India and Pakistan later fleshed out that family tree, identifying the closest ancient relatives of cetaceans as small, wading deer-like creatures.

"But their body plans are just the start of cetaceans’ weirdness. To survive in the sea, they also had to make internal modifications, altering their blood, saliva, lungs and skin. Many of those changes aren’t obvious in fossils, and cetaceans aren’t easily studied in the lab. Instead it was, once again, genetics that brought them to light.

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"The first cetaceans lost a lot more than legs when they went back to the water: Entire genes became nonfunctional. In the vast of book of genetic letters that make up a genome, these defunct genes are among the easiest changes to detect. They stand out like a garbled or fragmented sentence, and no longer encode a full protein.

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"Some 85 genes became nonfunctional when cetaceans’ ancestors adapted to the sea, the team reported in Science Advances in 2019. In many cases, Hiller says, they could guess why those genes became defunct.

"For example, cetaceans no longer possess a particular gene — SLC4A9 — involved in making saliva. That makes sense: What good is spit when your mouth is already full of water?

"Cetaceans also lost four genes involved in the synthesis of and response to melatonin, a hormone that regulates sleep. The ancestors of whales probably discovered pretty quickly that they couldn’t surface to breathe if they shut off their brains for hours at a time. Modern cetaceans sleep one brain hemisphere at a time, with the other hemisphere staying alert. “If you don’t have the regular sleep as we know it anymore, then you probably do not need melatonin,” says Hiller.

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"Cetaceans aren’t the only mammals that returned to the water, and the genetic losses in other aquatic mammals often parallel those in whales and dolphins. For example, both cetaceans and manatees have deactivated a gene called MMP12, which normally degrades the stretchy lung protein called elastin. Maybe that deactivation helped both groups of animals develop highly elastic lungs, allowing them to quickly exhale and inhale some 90 percent of their lungs’ volume when they surface.

"Deep-diving adaptations aren’t all about loss, though. One conspicuous gain is in the gene that carries instructions for myoglobin, a protein that supplies oxygen to muscles. Scientists have examined myoglobin genes in diving animals from tiny water shrews all the way up to giant whales, and discovered a pattern: In many divers, the surface of the protein has a more positive charge. That would make the myoglobin molecules repel each other like two north magnets. This, researchers suspect, allows diving mammals to maintain high concentrations of myoglobin without the proteins glomming together, and thus high concentrations of muscle oxygen when they dive.

"The migration of the nasal opening is one of many morphological changes that accompanied the transition from land to water. The earliest cetaceans (beginning at left) had nasal bones (gray) and nasal openings (black) near the tip of their snouts, like many land-dwelling mammals. As cetaceans transitioned to life underwater, their bones retracted and nasal openings migrated to the top of the head, becoming blowholes.

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"The researchers reported in 2016 that they found hundreds of genes that showed just this pattern in members of these three different aquatic groups. Genes under such dialed-up evolutionary pressure included ones that code for proteins in the skin, and a gene encoding the liquid surfactant that coats the inside of the lungs. It’s difficult to know exactly how those genetic changes altered the animals’ physiology for the better, but protection from germs is Clark’s best guess.

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"Scientists know they are only just beginning to plumb the genetic depths of cetacean evolution. Now, with dozens of cetacean genomes available to study, and with new analytic techniques under development, they are poised to further probe the aquatic transition, along with other exciting moments in cetacean evolutionary history. Dolphins alone offer a wealth of questions: How did they diversify into so many types? They make up nearly half of cetacean species today. How did they and other toothed whales pick up the skill of echolocation, navigating the ocean via sound? And how did dolphin brains get so large, with a brain-to-body-size ratio to rival that of great apes?"

Comment: once again we see the adverse complexities that faced mammals returning to water. The nine stages in whales in only ten million years suggests a special driving force, perhaps a designing God? Many minor discoveries skipped for brevity.