Enlarge / Whales and their relatives evolved from land mammals, a transition that led to major physiological and morphological changes, which geneticists have begun to analyze.
Hayes Baxley/National Geographic for Disney+
About 400 million years ago, the ancestor of all four-limbed creatures took their first steps on dry land. Fast-forward about 350 million years, and a descendant of those early earth-dwellers has flip-flopped: he's gone back into the water. Over time, the creatures returned to the sea gave rise to animals very different from their terrestrial counterparts: they became the magnificent whales, dolphins and porpoises that glide through the oceans today.
Returning to aquatic life was a drastic decision that would change animals inside and out, in the space of about 10 million years - a blink of an eye in evolutionary terms . Members of this group, now called cetaceans, gave up their hind limbs for powerful flukes and lost almost all of their hair. For decades, their bizarre body plans have puzzled paleontologists, who have speculated they may have originated from creatures as varied as marine reptiles, seals, marsupials like kangaroos and even a now-extinct group of carnivores. resembling wolves.
"Cetaceans are on the whole the most peculiar and aberrant mammals", wrote a scientist in 1945.
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. Later, fossils from modern India and Pakistan fleshed out this family tree, identifying the earliest cetacean relatives as small deer-like creatures.
But their body plans are just the start of cetacean weirdness. To survive in the sea, they also had to make internal modifications, altering their blood, saliva, lungs and skin. Many of these changes are not evident in fossils, and cetaceans are not easily studied in the laboratory. Instead, it was once again genetics that brought them to light.
With the increasing availability of cetacean genomes, geneticists can now search for the molecular changes that have accompanied the return to the water. While it's impossible to be certain how influential any particular mutation is, scientists suspect that many of the ones they see correspond to adaptations that allow cetaceans to dive and thrive in the ocean. a deep blue.
Dive into the depths
The first cetaceans lost more than legs when they returned to the water: entire genes became non-functional. In the vast book of genetic letters that make up a genome, these missing genes are among the easiest changes to detect. They come off as a scrambled or fragmented sentence, and no longer encode a complete protein.
Such a loss can occur in two ways. Perhaps having a particular gene was somehow detrimental to cetaceans, so animals that lost it gained 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 use in water, it would randomly accumulate mutations and animals wouldn't be worse off when it stopped working.
Hiller and his colleagues delved into the transition to water by comparing the genomes of four cetaceans (dolphin, killer whale, sperm whale, and minke whale) with those of 55 land mammals plus a manatee, walrus, and harbor seal. Weddell. . Some 85 genes became nonfunctional when cetacean ancestors adapted to the sea, the team reported in Science Advances in 2019. In many cases, says Hiller, they were able to guess why those genes are gone.
For example, cetaceans no longer have a particular gene—SLC4A9—involved in making saliva. Makes sense: what's the point of spitting when your mouth is already full of water?
Cetaceans have also lost four genes involved in the synthesis and response to melatonin, a hormone that regulates sleep. The ancestors of whales probably discovered fairly quickly that they couldn't surface to breathe if they turned off their brains for hours on end. Modern cetaceans sleep one night...
Enlarge / Whales and their relatives evolved from land mammals, a transition that led to major physiological and morphological changes, which geneticists have begun to analyze.
Hayes Baxley/National Geographic for Disney+
About 400 million years ago, the ancestor of all four-limbed creatures took their first steps on dry land. Fast-forward about 350 million years, and a descendant of those early earth-dwellers has flip-flopped: he's gone back into the water. Over time, the creatures returned to the sea gave rise to animals very different from their terrestrial counterparts: they became the magnificent whales, dolphins and porpoises that glide through the oceans today.
Returning to aquatic life was a drastic decision that would change animals inside and out, in the space of about 10 million years - a blink of an eye in evolutionary terms . Members of this group, now called cetaceans, gave up their hind limbs for powerful flukes and lost almost all of their hair. For decades, their bizarre body plans have puzzled paleontologists, who have speculated they may have originated from creatures as varied as marine reptiles, seals, marsupials like kangaroos and even a now-extinct group of carnivores. resembling wolves.
"Cetaceans are on the whole the most peculiar and aberrant mammals", wrote a scientist in 1945.
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. Later, fossils from modern India and Pakistan fleshed out this family tree, identifying the earliest cetacean relatives as small deer-like creatures.
But their body plans are just the start of cetacean weirdness. To survive in the sea, they also had to make internal modifications, altering their blood, saliva, lungs and skin. Many of these changes are not evident in fossils, and cetaceans are not easily studied in the laboratory. Instead, it was once again genetics that brought them to light.
With the increasing availability of cetacean genomes, geneticists can now search for the molecular changes that have accompanied the return to the water. While it's impossible to be certain how influential any particular mutation is, scientists suspect that many of the ones they see correspond to adaptations that allow cetaceans to dive and thrive in the ocean. a deep blue.
Dive into the depths
The first cetaceans lost more than legs when they returned to the water: entire genes became non-functional. In the vast book of genetic letters that make up a genome, these missing genes are among the easiest changes to detect. They come off as a scrambled or fragmented sentence, and no longer encode a complete protein.
Such a loss can occur in two ways. Perhaps having a particular gene was somehow detrimental to cetaceans, so animals that lost it gained 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 use in water, it would randomly accumulate mutations and animals wouldn't be worse off when it stopped working.
Hiller and his colleagues delved into the transition to water by comparing the genomes of four cetaceans (dolphin, killer whale, sperm whale, and minke whale) with those of 55 land mammals plus a manatee, walrus, and harbor seal. Weddell. . Some 85 genes became nonfunctional when cetacean ancestors adapted to the sea, the team reported in Science Advances in 2019. In many cases, says Hiller, they were able to guess why those genes are gone.
For example, cetaceans no longer have a particular gene—SLC4A9—involved in making saliva. Makes sense: what's the point of spitting when your mouth is already full of water?
Cetaceans have also lost four genes involved in the synthesis and response to melatonin, a hormone that regulates sleep. The ancestors of whales probably discovered fairly quickly that they couldn't surface to breathe if they turned off their brains for hours on end. Modern cetaceans sleep one night...
Enlarge / Whales and their relatives evolved from land mammals, a transition that led to major physiological and morphological changes, which geneticists have begun to analyze.
Hayes Baxley/National Geographic for Disney+
