humpback whale's tail sticking out of water
Enlarge / Whales and their kin evolved from land-dwelling mammals, a transition that entailed major physiological and morphological changes—which geneticists have begun to parse.

The ancestors of all four-legged creatures took their first steps on dry land around 400 million years ago. A descendant of the early landlubbers waded back into the water. The magnificent whales, dolphins, and porpoises that glide through the oceans today were created by the back-to- the sea creatures.

In the course of about 10 million years, going back to being aquatic was a drastic move that changed the animals inside and out. The members of this group lost a lot of their hair. For decades, their bizarre body plans perplexed paleontologists, who speculated they might have arisen from a variety of animals.

The cetaceans are the most peculiar and abnormal of mammals.

The Artiodactyla branch of the evolutionary line that spawned cows, pigs, and camels was confirmed by genetics in the late 1990s. Fossils from India and Pakistan identified the closest ancient relatives of cetaceans as deer-like creatures.

Their body plans are just the beginning of their weird behavior. They had to make changes to their bodies to survive in the sea. cetaceans aren't easy to study in the lab and many of the changes aren't obvious in fossils. Genetics brought them to light.

With an increasing availability of cetacean genomes, geneticists can now look for the changes that preceded the back to water transition. Scientists think that many of the ones they see are related to the adaptations that allow cetaceans to dive and thrive in the deep blue sea.


Diving into the depths

The first cetaceans lost more than their legs when they returned to the water. There are a lot of genes in a book of genetic letters that make up a genome. They are like a garbled or fragmented sentence and do not have a fullProtein.

It could happen in two different ways. Animals that lost a certain genes gained a survival edge. Michael Hiller is a researcher at the Senckenberg Research Institute in Germany. The animals would not be worse off if the water didn't have any purpose in it.

Hiller and colleagues compared the genomes of four cetaceans with those of 55 other mammals. The genes became nonfunctional when cetaceans ancestors adapted to the sea. Hiller says they were able to guess why those genes stopped working.

cetaceans no longer have a particular gene involved in making saliva It makes sense that spitting when your mouth is full of water is bad.

Four genes involved in the synthesis and response to melatonin were lost. If whales shut off their brains for hours at a time, they wouldn't be able to breathe. The cetaceans sleep in one hemisphere at a time. Hiller says that melatonin is probably not needed if you don't have regular sleep.

Genetics seem to have changed as a result of the long periods of time that whales have to hold their breath to dive and hunt. Scuba divers know that diving deep can cause small bubbles of nitrogen to form in the blood and seed. Two genes that help kick off blood clotting are no longer functioning in cetaceans. whales and dolphins are able to seal up injuries because the rest of the clotting machinery is intact.


Hiller was surprised by another lost gene that codes for a repair mechanism. He believes that the change has to do with deep dives. When cetaceans come up for a breath, oxygen suddenly floods their bloodstreams, and as a result, they can break DNA. This type of damage is normally repaired by the missing enzyme, but it does so sloppily and often leaves a trail of destruction. The otheridases are more accurate. Hiller thinks that mu was too sloppy for the cetacean lifestyle, unable to handle the amount of oxygen produced by the constant diving. If the repair job was left to the more accurate ones that cetaceans possess, the chances of the oxygen damage being repaired correctly would increase.

The genetic losses in other aquatic mammals are similar to those in whales and dolphins. Both cetaceans and manatees have genes that are normally used to degrade lung tissue. Both groups of animals may have been able to develop elastic lungs because of that deactivation.

Losing isn't all about deep-diving adaptation. The genes that carry instructions for myoglobin are one of the most significant gains. Many divers have a more positive charge on the surface of their myoglobin genes. The myoglobin molecule repels each other. Diving mammals have high concentrations of myoglobin and high concentrations of muscle oxygen because of this.