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What If Humans Were Cold-blooded Creatures?

VO: Noah Baum WRITTEN BY: Caitlin Johnson
Human beings are warm-blooded mammals. It's a fact of basic science. But, what if they weren't? How would our lives be different if we were actually cold-blooded? In this video, Unveiled discovers how our bodies would change for a cold-blooded lifestyle, the dangers we'd face but also the surprising benefits we'd suddenly enjoy. Scientifically speaking, it's about more than just an advanced race of hyper-intelligent lizard people... What do you think would happen if humans were cold-blooded?

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What if Humans Were Cold-Blooded Creatures?

Humans can live almost anywhere on the planet. We’re capable of adapting to the harsh conditions of the arctic circle, the high altitudes atop the Andes, and even in the midst of tropical jungles. In fact, few species are spread as far and wide as humankind, with us even aiming to venture to the stars and colonise alien worlds. But could evolution have taken us down a totally different path?

This is Unveiled, and today we’re answering the extraordinary question; What if humans were cold-blooded creatures?

First off, what do we actually mean by “warm” and “cold-blooded”? Neither are the best nor most specific of terms. Biologists would rather speak of “endothermic” and “ectothermic” creatures, referring to animals that either produce their own body heat, or don’t. As well as that, we have “homeothermic” or “poikilothermic” creatures, meaning an animal either maintains its own internal body temperature, or it relies on its environment to regulate it.

All told, as humans, we’re endothermic homeotherms, because we produce our own body heat and maintain it ourselves, through our metabolism. Most other mammals and birds also do this, but cold-blooded animals – or “ectothermic poikilotherms” – are incapable of regulating their own temperature. That’s why you see lizards in the desert bathing in the baking sunlight, because they need to warm up their bodies before their muscles and brains can work.

Interestingly, though, even these aren’t the only options that the natural world can throw up. Palaeontologists now believe that some dinosaurs were actually “mesothermic”, meaning that they survived via a combination of creating and maintaining their own body heat and utilising their environment. But for this video, let’s just focus on the basic, non-dino categories.

If humans had evolved to be cold-blooded instead of warm-blooded, or even if we all miraculously turned cold-blooded overnight, one of the first big changes we’d all notice is we’d be eating less food. A lot less food. Even the so-called ‘king of reptiles’, the crocodile, can actually go more than twelve months without eating anything at all. They’re painted as ruthless killers of the deep, but typical crocs only average about fifty meals a year – or just one a week.

If humans ate so infrequently – rather than tucking into three square meals a day plus snacks – our food bills would definitely be much cheaper, it’d also be much easier to feed everybody around the world, and we’d only need a tiny fraction of the land we use now for agriculture. Theoretically, nobody would ever have to go hungry.

Of course, you still could eat three meals a day, if you wanted to. But, with the much slower metabolism of a cold-blooded creature, the calories wouldn’t have anywhere to go, and the added food would translate into fat. In lots of warm-blooded animals, fat is pretty important and essential – keeping mammals like seals and whales warm in freezing oceans, for example. But it could prove deadly very quickly were we cold-blooded, causing us to overheat and die. Luckily, as a cold-blooded being, you should never feel hungry enough for this to ever actually happen… but over-indulgence would definitely be off the menu.

Assuming that you don’t eat too much, though, it’d actually be harder to get sick as a cold-blooded animal. The constant heat of an endothermic homeotherm (as we currently are) provides a perfect incubator for germs, which is why it’s so easy for us to get sick from just being near other sick people. But, if we were cold-blooded, our bodies wouldn’t be able to cultivate these germs as easily, staving off potential infections.

There is a deadly caveat, though, because our new-found cold-bloodedness wouldn’t make it impossible to get sick; and if you did fall ill, the consequences could be dire. You may not spread your disease, but if you got just a little too cold – say your heating breaks, or the wind picks up when you’re waiting for a bus – your lowered body temperature could damage your immune system. If an ectotherm is too cold for too long, their body could stop functioning, leaving them wholly unable to fight off disease and infection.

As warm-blooded creatures, we can live anywhere with relative ease, usually by just adding or removing layers of clothing. We can even survive the likes of Antarctica with the right equipment. But, if we were cold-blooded, the state of our environment becomes a matter of life or death every single day. Where most warm-blooded creatures maintain a body temperature of around 37 degrees Celsius, a cold-blooded creature is exactly the same temperature as its environment. So, if humans were suddenly cold-blooded, we’d be much better off living in jungles and deserts, along the equator and in the tropics, never straying too far north or south.

If we did branch out to a colder region we’d need to build complex habitats, designed to maintain a constant, reliable, 24/7 temperature. Going outside for just a couple of minutes could result in death, so human settlements would have to be high-tech places – with our fundamental need for tolerable temperatures at their heart.

Strangely, though travelling to places like Canada, Russia or Scandinavia would be a lot harder, travelling into space might actually be easier if we were a species of advanced ectotherms. This is because it’d theoretically be easier to go into torpor if we were ectothermic, which is a state of total inactivity some animals can enter by slowing their metabolism to a stop.

Putting humans into deep sleep is a staple sci-fi idea, but it’d be all the easier if we could just do this naturally, without the need for complex technologies that we haven’t yet invented. Send some deep space astronauts on a shuttle to Mars and (if they’re cold-blooded) they could nap through the whole journey, meaning they needn’t take as much food and water with them, and they needn’t deal with the loneliness. Though, of course, actually living on Mars would be just as tricky for any ectotherm, since the Red Planet’s average surface temperature is minus-60 degrees Celsius.

There’s one final thing that’s pretty important, though: brain function. The human brain uses about 20% of our current energy levels, which is 20% of our daily caloric intake. Clearly, if we only ate one meal a week, our human brains wouldn’t receive the same power to reach the same levels… It’s why the world isn’t really run by a race of advanced lizard people. If there actually were lizard people, they’d be evolutionarily obliged to spend most of their days sunbathing just to get their brains and bodies to work – and they’d have to stay stuck on the equator, to even begin to match the potential of a warm-blooded rival. So, even with space travel, the problems would again outweigh the plus points. Yes, we might stand more of a chance of actually getting to other planets, but would a cold-blooded crew be clever enough to work out what to do next? Probably not.

With this in mind, if humans had always been cold-blooded, evolution wouldn’t have happened in even remotely the same way. We certainly wouldn’t be such a social species, and our hunter/gatherer instincts wouldn’t have developed as they did. Instead, we’d all have sat around waiting for our next meal to arrive, expending all of our limited energy on trying to catch it in one strike – just like real-world reptiles do. It’s why you see crocodiles floating ominously in the water for hours on end, biding their time, for absolute efficiency.

Even if we did have some intelligence, we wouldn’t have needed to co-operate to do things like build, farm, and generally invent. The only time we’d ever really need to have met other people would’ve been for procreation. And, even then, reptiles and fish aren’t exactly known for their parental instincts, instead tending to leave their offspring to fend for themselves, or else ditch them before they’ve even hatched. And, as well as all of that, we probably wouldn’t even be bipedal. Instead, we’d crawl around, spreading our bodies to expose as much of our skin to the sun as possible, in order to increase our brain function.

True, nobody would ever go hungry because of our extremely low metabolisms. But, we also probably wouldn’t care if anybody went hungry in the first place, because an ectothermic race of humans would essentially all be lazy, antisocial lizards. And, that’s what would happen if humans were cold-blooded creatures.

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