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Around 50 million years ago, a hippo-like animal wandered into the water. And over time, through the wonders of evolution, we ended up with whales and dolphins— the group of animals biologists refer to as cetaceans.

Though they're really different from their land-dwelling cousins, cetaceans have kept a lot of their ancestor's traits. Like, they still have multi-chambered stomachs, even though they don't need them to ferment plant material. They've also gained several new features.

But, perhaps what's most intriguing are the things they've lost. We often think of adaptation as shaping or adding things, but often, genes that disappear or are shut off are just as important in the process — if not more so. And that's why examining what whales have lost has helped scientists gain a better understanding of what it takes to live the life aquatic.

I'm not talking about obvious stuff, like their lack of hind legs or general hairlessness. I'm talking about things you'd really think they'd miss. Like melatonin, for instance.

That's the hormone that gets us and most other animals ready for snooze time, among other things. But whales and dolphins don't produce it. So they don't go into that wonderful, deep, restorative kind of sleep most animals desperately need to recharge.

You'd think that'd be a bad thing. Like if it were possible to not sleep, animals would do that. But they can get the rest they need by sleeping unihemispherically instead.

Essentially, they put one half of their brain to sleep at a time. And that's much safer for them, since if they did totally conk out, they'd sink and drown. There are also other potential upsides to ditching melatonin.

That's because it helps lower core body temperature by increasing heat loss from the skin. And that, too, could be deadly in the ocean, since water wicks away heat much faster than air. But losing melatonin also means that cetaceans don't have a circadian rhythm.

Their bodies don't tell time like ours do. They don't feel energized in the morning, or tired at night. In fact, we're not really sure how they decide when to sleep, though it may have more to do with food availability than anything else.

And you know how much human babies sleep? Dolphin babies never stop. And that means their parents don't rest either.

And I thought my life was exhausting. Though, I guess, there's less for a baby dolphin to run into in the middle of the ocean. So maybe they're less likely to hurt themselves?

Speaking of injuries, cetaceans have also lost some seemingly important genes in that department. Specifically, they don't have the ability to form clots within blood vessels. These clots, called thrombi, are thought to be really important because they plug small holes—so, you know, your blood vessels don't spring leaks.

But it turns out they're more dangerous than helpful when you dive for a living. That's because, when whales and dolphins descend to the depths in search of food, their heart rate and blood flow slows down. Also, thanks to the increased pressure, their veins and arteries become narrower, and they often end up with bubbles of nitrogen in their blood.

All of which would normally increase the risk of forming a clot that isn't needed. And such clots can end up cutting off blood flow and therefore oxygen to vital tissues. So for cetaceans, it's probably safer to never form thrombi at all.

You might think that would put them at a slight disadvantage when they do need to prevent blood loss. But, fortunately, they've hung onto and even improved upon other genes that help them repair wounds. And while we're on the subject of diving, the behavior might also explain why cetaceans and many other marine mammals have lost the function of a gene called paraoxonase 1 — or PON1, for short.

This enzyme is found in all mammalian landlubbers, including us. And that may be because it plays a role in protecting against heart disease. It basically chops up certain fats if they've had an electron stolen by some other molecule, a process known as oxidation.

And that's important because these oxidated fats rile up the immune system and promote the fatty buildups along blood vessel walls that can lead to heart disease. So you might think whales and dolphins would need this enzyme. But apparently, they don't, and there are probably two main reasons for that.

One, their diets have changed dramatically—from plants to marine animals. And that's shifted the balance of the fats they consume towards ones which are more resistant to oxidation. And two, they've ramped up other ways of dealing with oxidation.

This is where diving comes in. You see, marine mammals likely deal with a lot more oxidation than their non-aquatic counterparts. That's because, when preparing to dive, they suck in huge amounts of oxygen, which is then slowly used up as they move about in the depths.

So, their tissues go through repeated cycles of oxygen overloading and deprivation as they repeatedly dive and resurface. And that ultimately creates a lot of oxidizing molecules. These would wreak havoc, if it weren't for the army of antioxidants that cetaceans have.

And, apparently, those other antioxidants have made PON1 kind of extraneous. Unfortunately PON1 also breaks down the toxic pesticide by-products called organophosphates. In fact, it's the only known defense mammals have against these powerful neurotoxins.

And that may mean cetaceans are defenseless, which sucks, since these toxins keep leaching from our farmlands and ending up in the waters where they live. So, the loss of this gene has become a bit of an issue— but not for the reasons you'd have thought. Overall, it's kind of amazing what they've been able to lose without terrible consequences.

The transition from land to sea was truly an incredible evolutionary feat, and it couldn't have happened without ditching a few things. But those losses gave them the chance to thrive in their new environment, and hopefully they will continue to do so, even with the new challenges they face today. Thanks for watching this episode of SciShow!

If you want to learn more about whales, you might want to check out our episode on why they don't deafen themselves with those super loud calls they make. They can communicate with one another from over tens thousand kilometers away— can you even imagine how loud you'd have to yell to do that?! And if you just want to keep learning about science in general, be sure to click that subscribe button and ring the notification bell! [♪ OUTRO].