How Whales Evolved From Land To Water, Gene By Gene
Fifty million years ago, the ancient ancestors of whales and dolphins roamed the land on four legs. But over time, these aquatic mammals have evolved to live fully in the ocean—their genetic makeup changing along the way. Now, a group of scientists have investigated the changes in 85 different genes that were lost in this land-to-sea transition. The results were published in the journal Science Advances this week. Mark Springer, evolutionary biologist and co-author on the paper, discusses the genetic trade-offs that cetaceans have evolved, including an inability to produce saliva and melatonin, and the benefits they provide for a deep-diving, aquatic lifestyle.
Mark Springer is a professor of biology at the University of California, Riverside in Riverside, California.
IRA FLATOW: This is Science Friday. I’m Ira Flatow. Did you know that whales and dolphins and other cetaceans don’t make saliva? That’s what I said. Well, you know, it makes sense, right, if you’re surrounded by water? But they’ve also lost genes involved in blood clotting. Hm. Imagine all the drastic changes that their wolf-like, land-dwelling ancestor had to go through to become the streamlined ocean animals that they are today.
A team of researchers was interested in figuring out how this evolution happened on a genetic level. They mapped out 85 different genes that were lost in this aquatic transition. And their results were published in the journal Science Advances.
To walk us through the genetic steps whales and dolphins had to go through to make it into the water, meet Mark Springer, one of the authors on that study. He’s also a professor of biology at University of California at Riverside. Welcome to Science Friday.
MARK SPRINGER: Thank you. It is a pleasure to be here.
IRA FLATOW: It’s our pleasure to have you. Usually when you’re studying animals from millions of years ago, you look for fossils. But in your study, you looked at molecular fossils. What does that mean?
MARK SPRINGER: So in the genome, we have many different genes. And as you mentioned, the gene that’s expressed in saliva, it’s one of the genes in cetaceans that’s no longer needed. But even though a gene is no longer needed, there are remnants of that gene that are still in the genome. It’s just a dead gene or a fossil gene, if you will.
And it has mutations that have been fixed in that gene and make it inactive. So it’s a broken gene. It can’t do its job. And we were interested in looking for different genes that are broken, that formerly were functional and would have coded for different proteins. And so there are now alignments that are available for many different mammals.
And the alignment that we worked with is an alignment of genome sequences for more than 60 different mammals, including four different cetaceans. And we screened all of the protein coding genes for genes that have these inactivating mutations. And we were looking for genes where the inactivating mutations are shared by all of the different cetaceans. And we ended up with this list of about 85.
IRA FLATOW: 85– these are genes that changed from when they were– let me go backwards from now. What are the closest living relatives, or maybe not living relatives, on land that are close to the cetaceans?
MARK SPRINGER: So for a long time, it was not clear what the closest relative of cetaceans was. But in the last 30 years, we have learned a tremendous amount based on genes and also based on the fossil record. And what came from a study of different genes is that the closest living relative of cetaceans is hippopotamuses. And we think that they diverged from each other about 54 or so million years ago.
And living cetaceans, there’s two main groups of them. There are the baleen whales. And then there are the toothed whales. And they have a common ancestor that they shared with each other, probably about 37 million years ago. So there’s this transitional period from when whales diverged from hippos, until we have the last common ancestor of the toothed whales and the baleen whales. And we were interested in the changes, the genes that were lost on that particular branch in the evolutionary history leading to whales.
IRA FLATOW: OK, so give us an idea of what the steps were. What happened to allow the these land animals to become ocean-dwelling animals?
MARK SPRINGER: So we can learn about the different steps based on the fossil record and also from the genes that we find in the genome. So there are changes that occur with the skin. So the skin is much thicker, and the hair has been lost. Almost all hair has been lost in cetaceans. And that probably just gets in the way and causes additional friction.
You were mentioning saliva. Well, when you’re looking at a lot of these cetaceans, they’re just swallowing food whole anyway. And so they’ve lost a lot of taste receptors. And pretty much every organ system– if you’re looking at the kidneys, if you’re looking at the lungs, if you’re looking at various sensory systems, the eyes, if you’re looking at olfaction or the sense of smell– all of these systems are reorganized.
And what’s great about cetaceans is that this is one of the most remarkable macroevolutionary transitions that in the history of vertebrates. And we have access, not only to a wonderful fossil record now over the last 30 years, but also these genomes. And when we sort of take the genomic fossils and then the fossils that we find in rocks and put it all together, we can kind of piece together and learn about some of these steps.
So if you look at cetaceans, one of the things that they do is dive. And sometimes they stay down a very long time. There’s a beaked whale that has the longest recorded dive of more than two hours. And it was at a depth of almost 3,000 meters. So diving, it’s a very difficult thing to do. Mammals have lungs. They’re not like fish with gills.
And so one of the challenges that cetaceans have is, when they are diving and they’re down for a long time, they only have a limited amount of oxygen. And so one of the things that they do is that they reduce the amount of blood flow to the extremities of the body. And that’s something that we call peripheral vasoconstriction. So the blood vessels leading to the periphery of the body are constricted.
Well, a consequence of that is, it’s more likely that blood clots will form. And so a couple of the genes that we found were inactivated. On this common ancestral branch leading to whales are genes that are involved with blood clot formation. So these genes don’t work anymore. And it makes it less likely that cetaceans will get blood clots during diving.
IRA FLATOW: That’s interesting. Whales have flippers that appear very different than human hands, but the bones and the structures are still in there, right? What about whales that allowed them to move in the water? Did they have to lose genes or gain genes or change the genes that allowed them to rejigger their hands or their limbs into flippers?
MARK SPRINGER: There are a lot of changes in the sequence in the expression of some of the different genes. So some of the hox genes that are involved with the patterning of limbs, the sequence of expression is different. So we’re not finding so much of the gene loss associated with the transition from a fully terrestrial animal with long limbs to a whale, where you have flippers and then you’ve essentially lost the hindlimbs entirely. That seems to be mostly a case of changing the expression and the timing of expression of different genes.
IRA FLATOW: You found the gene that there was a loss of a gene for melatonin.
MARK SPRINGER: Yes.
IRA FLATOW: Why is that important?
MARK SPRINGER: Well, melatonin is commonly known as the sleep hormone. And sleep is very challenging for fully aquatic marine mammals. We don’t have gills. Mammals need to resurface regularly to breathe. So it’s hard to sleep if you need to resurface regularly to breathe.
And ocean waters can also be very cold. And if you fall asleep, you lose body heat. So in a thermally challenging environment, it’s useful to do not have to fall asleep. And so whales have this unique way of sleeping. And they put only one side of the brain to sleep at a time.
And so it’s something that we call unihemispheric sleep. And so they’re alternating between the right side of the brain and the left side of the brain that they’re putting to sleep. So this allows them to do the things that they need to do to stay warm and to breathe without kind of compromising their ability to sleep.
But how does an animal sleep with only one side of the brain at a time, unlike us, where we’re sleeping with both sides of the brain? Well, the melatonin, this sleep hormone, it’s produced at night much more than in the day. So it’s kind of turned on at night and then off during the day. And it’s associated with sleep. And melatonin, it’s produced from what’s sometimes called the happy chemical, which is serotonin.
And there are a couple of enzymatic steps to get there. And both of the genes that code for these enzymes are broken in cetaceans. So they don’t work. So cetaceans can’t make melatonin. And we think that this loss of these genes may have been important in facilitating this unihemispheric sleep that is characteristic of cetaceans. And so it was an important change in this transition to a fully aquatic existence that we see in cetaceans.
IRA FLATOW: What is there about this land to water transition that you find so fascinating? Why do you study it? What interests you about it?
MARK SPRINGER: Well, it’s sort of the reverse of what happened hundreds of millions of years ago. So if you go back maybe 380 to 360 million years ago, our vertebrate ancestors were coming out of the water. And so that was one of the most important events in the history of vertebrate evolution. That’s what gave rise to tetrapods. And now in the Cenozoic, we have a group, whales, that have returned to the water.
So it’s fascinating because it’s a big macroevolutionary transition. And it involves all of the different organ systems of the body. And everything needs to be refashioned. So it’s not just a matter of changing hair color or this or that. It’s a sort of whole scale rearrangement of the entire body plan. So that’s why I think it’s so interesting. And also, because we have this sort of complementary approach, where we can take fossils and we can also learn from genes and try and piece together everything that has happened.
IRA FLATOW: Thank you for taking the time to join with us, Mark. Mark Springer, one of the authors on a study published in Science Advances, he’s also a professor of biology at the University of California at Riverside.