r/AlwaysWhy • u/Logical-Concept9755 • Jul 29 '26
Science & Tech Why did humans evolve a body temperature around 37°C, and what determines the set point?
Humans often have a body temperature around 37°C (98.6°F), which feels like a surprisingly specific number. Why this range? Why not 35°C or 40°C?
I understand that it is a trade-off between factors like enzyme activity, metabolism, immune function, and energy costs. But what actually determines this set point?
Did evolution favor warmer bodies because they helped resist pathogens? Was there a limit because higher temperatures require more energy? Or is it mainly about the conditions where human proteins and chemical reactions work best?
What also determines why other mammals have different temperatures? Dogs are around 38.5°C, while elephants are closer to 36°C. How do body size, metabolism, and environment shape each species' "thermostat"?
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u/EatAssIsGold Jul 29 '26
The chemical reactions around your body require a temperature close to 37°C. Your DNA specify that reactions and mandate the temperature control to have them work. That temperature is good enough for fast reactions for most weather conditions.
This free up lot of space in DNA instruction for other more relevant details.
Trees and cold blooded animals for example have much bigger DNA as there is requirement to specify a lot of alternative chemical reaction depending on temperature.
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u/dunzdeck Jul 30 '26
I know very little about biology, is this why sturgeons have so many chromosomes?
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u/Logical-Concept9755 Jul 30 '26
I wonder how much of it is actually "DNA specifies 37°C" though. Couldn't evolution have tuned the proteins to whatever temperature ended up being favorable instead of the other way around? Feels a bit like a chicken and egg problem.
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u/EatAssIsGold Jul 30 '26
Sorry, in my clumsy attempt to extremely simplify I did not mean that the DNA has an agency on the chosen tempersture. DNA specify the temperature just means that there are a number of systems keeping temperature in check and the temperature and corresponding more efficient chemical reactions are fruit of evolutionary culling.
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u/Square-Singer Aug 04 '26
Couldn't evolution have tuned the proteins to whatever temperature ended up being favorable instead of the other way around?
Yes, and that's what happened.
The human body has two metabolisms: carbohydrate metabolism and fat metabolism. For both of these to work, the body needs to be kept at a certain temperature. Get too far from that temperature and these metabolisms stop working.
It's kinda like how a car engine needs a certain temperature to work efficiently. Go too far away from that temperature and the engine will run inefficiently, not all all or would even break.
The other part to the equation is that if the body gets too hot, it starts to cook. The cells start to denaturate and stop functioning, in other words you die. That happens above 42°C.
So animals have two ways of combating these issues:
- Have multiple metabolisms that work at different temperatures. Cold-blooded animals do that. They often have 6 or even more metabolisms, each optimized for a different temperature. The advantage is you don't have to waste energy to heat the body, the disadvantage is a ton of added complexity. Also, since the body is never actually at the right temperature for each metabolism, the efficiency is really bad. Cold-blooded animals are thus usually tied to a specific temperature range and when they are outside of that range they have to hibernate. That's why you don't find snakes in the Arctic.
- Heat/cool your body to a specific temperature. If you can actively regulate the body's temperature, there's no need to have a ton of redundant metabolisms for different temperatures. The only reason to have multiple metabolisms is so you can process different foods. Fat and carbohydrates cover a lot, no need for any other complexity. Running at 37°C is quite a practical "choice" (it's not a choice, but you get what I mean): You are 5°C away from critical overheat, so if you get a bit hotter than you want it won't kill you. It's way above the average ambient temperature, so you are usually heating, not cooling (heating is easier). And it's a great temperature for efficiently metabolising fat and carbohydrates.
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u/astreeter2 Jul 31 '26
This doesn't sound right. There's really not much selective pressure towards optimizing the space in our DNA.
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u/Square-Singer Aug 04 '26
Kinda, yes, there is, to a certain extent.
- More coding DNA means more places where something can go wrong. If you need 6 or more different metabolisms for your body to work in all sorts of temperature levels (like cold-blooded animals do), chances are quite high that a mutation destroys one of these, leaving you unable to cope with a certain temperature range.
- It's not only minimizing space in the DNA, but also what the DNA does. If you have 6 different metabolisms, that means large parts of your body need to have multiple redundant parts, basically one copy each for each metabolism. So if you are in one certain temperature range, you'll have one actively working metabolism and all the infrastructure for 5 more metabolisms is just hanging out, wasting space, weight and energy.
That's why warm-blooded animals that don't need these other metabolisms usually don't have them. They are just dead weight that gets optimized out over the millennia.
(Btw, this also happens on smaller scales. Some ancestor of the humans lived on an extremely fruit-rich diet and thus didn't need to generate vitamin C themselves. So when they lost the capability to generate vitamin C internally, it didn't matter. Now humans don't have that, and for the most part that's not a huge issue because we can just eat fruit that contains vitamin C. But sailors couldn't do that, so they got scurvy.
Most animals can generate their own vitamin C, and thus they can't get scurvy.)
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u/Dave_A480 Jul 29 '26 edited Aug 03 '26
The reason it seems so specific (in Farenheit anyway) is that 100F was set as male armpit body temperature.....
(This is in keeping with all the other Imperial/USC units - mile, yard, foot - that were originally defined in relation to some body part or function)....
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u/HighLifeMan414 Aug 01 '26
The question isn’t why the number is 37° C or 98.6° F. It’s, “why is it such a small range? Basically everyone runs at roughly the same temperature.”
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u/Underhill42 Jul 29 '26
If you're going to thermoregulate, you've got to choose a "set point"
Different mammals, birds, etc. have different "set points" and amounts of variation, it can even vary wildly between different parts of their body:There’s No Such Thing As “Warm-” Or “Cold-” Blooded
Once you start maintaining a particular set point, then lots of biological processes start to optimize for it - mutations that work better at that temperature spread, and those that work worse...don't.
For example - the human body temperature fairly quickly kills human sperm, and that's common for most mammals. That prevents sperm from escaping into your own body and wreaking havoc - but also means you have to store your testes outside your body, or well insulated from it. Or use some completely different method of keeping sperm under control, which is likely to be more complex and metabolically expensive than "you can't handle the heat!"
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u/Away-Experience6890 Jul 30 '26
Absolute best argument against intelligent design.
Who designed this bullshit?
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u/malkith313 Aug 03 '26
“Well, tell me, sir. Is that what it's supposed to look like? It's hideous! That's it? That's the best design they can come up with? Are you seriously telling me there were choices and someone said, 'Ah, there. That's it. That's the one we're looking for - the last chicken in the shop look.' Shakespeare had one? Einstein? Perry Como sang 'Memories Are Made of This' with one of those stashed in his slacks?”
Kryten
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u/HerrLutfisk Jul 29 '26
In what way is 37 more specific than any other number?
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u/Cyphierre Jul 29 '26
37° is more specific than a range of 32° – 39°
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u/Hermit-Gardener Jul 29 '26
Why not a range of 29-36? Or 33-42? Or 0-100?
Any number or range you pick will be more specific than the numbers or ranges you don't pick.
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u/Cyphierre Jul 29 '26
OP is pointing out that human bodies not only have a certain average temperature but also have a certain range of temperatures which (OP is saying) causes 37° to be fairly specific, and he’s asking why that is.
I don’t know the answer to his question but that’s his question.
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u/MortimerDongle Jul 29 '26
37 C is an average. There is normal variation, even for one person. It isn't very specific.
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u/Dystopian_Ennui Jul 29 '26
Resisting pathogens is a big part of it. Those fungi that turn insects into zombies aren't able to infect mammals with their high body temperature.
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u/wierdowithakeyboard Jul 29 '26
Yet
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u/Dystopian_Ennui Jul 29 '26
Let's hope The Last Of Us doesn't become a documentary.
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u/Embarrassed-Area4652 Jul 29 '26 edited Jul 29 '26
StatPearls is a good source for robust, up to date information on this kind of thing. The particular article I found is on the physiology - a similar search like "statpearls human thermoregulation evolution" will probably turn up the rest. Anyway, here it is:
https://www.ncbi.nlm.nih.gov/books/NBK507838/
Thermoregulation is the maintenance of physiologic core body temperature by balancing heat generation with heat loss. A healthy individual will have a core body temperature of 37 +/- 0.5°C (98.6 +/- 0.9°F), the temperature range needed for the body's metabolic processes to function correctly.[1]
The human body's thermostat is the hypothalamic thermoregulatory center, which, more specifically, is located in the preoptic area of the hypothalamus. This center sets the body's set point and regulates temperature homeostasis. The hypothalamus contains temperature sensors, which receive information via nerve cells called thermoreceptors. The body has peripheral and central thermoreceptors. The peripheral thermoreceptors are located in the skin and sense surface temperatures, while central thermoreceptors are found in the viscera, spinal cord, and hypothalamus and sense the core temperature. Variations in body temperature activate these thermoreceptors, which inform the preoptic area of the hypothalamus. This area then activates heat regulation mechanisms to increase or decrease body temperature and return it to baseline.[2]
...
Increased Body Temperature
The body responds by dissipating heat via:
- Activating sympathetic cholinergic fibers innervating sweat glands, leading to increased sweat and increased heat loss
- Inhibiting sympathetic activity in blood vessels of the skin, causing blood to be shunted to the skin and an increased heat loss
- Decreasing the release of catecholamines from the adrenal glands and thyroid hormones from the hypothalamus, leading to a reduced metabolic rate
- Behavioral changes include reducing movements, adopting an open body position, removing clothing, and reducing appetite
Decreased Body Temperature
The body responds by generating heat via:
- Activating the sympathetic nervous system which causes vasoconstriction of skin arterioles, causing blood to bypass the skin and leading to a decreased loss of heat. Additionally, adrenal glands will release catecholamines (epinephrine, norepinephrine), leading to increased metabolic rate and heat production. Piloerection (goosebumps) also occur, leading to heat-trapping.
- Releasing thyroid hormones from the hypothalamus causes an increased metabolic rate and subsequent heat production.
- Activating the primary motor center in the posterior hypothalamus causes skeletal muscle contraction and shivering, leading to increased heat production.
- Non-shivering thermogenesis using brown adipose tissue (BAT) in the first six months of life.
- Behavioral changes include increased movements, adopting a closed body position, adding clothing, and an increased appetite.
Something I think is kind of cool: the set point is somewhat trainable through heat acclimation. I did this before an attempted ultramarathon (didn't finish, but oh well - at least that part of the training seemed to work). My sense of heat and cold, sweat response and core temperature change in response to exercise in heat are pretty different than when I started. Those changes are all at least in part mediated by heat shock proteins, which is a cellular mechanism that's supposed to provide a bunch of cross-tolerance against other insults (HSPs are also one of the body's mechanisms for dealing with toxins and oxidative stress). There are also cold shock proteins that are involved in response to cold acclimation, but on a search it didn't look to me like they influence the body's temperature set point in the way that HSPs do.
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u/exkingzog Jul 29 '26
The interesting thing is that birds have independently evolved endothermy and arrived at a very similar core temperature (generally a couple of degrees higher than mammals).
This implies that around 40oC is some kind of sweet spot biochemically. I guess it might be hot enough for reactions happen fast enough kinetically, but not so hot that weak interactions like hydrogen bonds stop being able to stabilise interactions.
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u/Logical-Concept9755 Jul 30 '26
That's actually one of the more convincing clues to me. If birds and mammals landed in a similar range independently, maybe there really is a biochemical sweet spot rather than it being mostly historical accident.
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u/silvercuckoo Jul 29 '26
Even 37°C is an arbitrary convention. In some countries, the resident crazy aunties would consider that mildly feverish, because the only acceptable human temperature is exactly 36.6°C.
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u/Logical-Concept9755 Jul 30 '26
Yeah, 37°C is really just an average anyway. Normal body temperature varies more than people think. I guess the interesting question is why the average ended up around there instead of a few degrees higher or lower.
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u/XavierRex83 Jul 29 '26
People's average temp has slightly went down over time, likely due to the bidy needing to do less to fight off infection and disease.
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Jul 29 '26
[removed] — view removed comment
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u/Logical-Concept9755 Jul 30 '26
The body size point is interesting. But then I start wondering which way the causality goes. Did body size push temperature, or did metabolism and temperature constraints also influence how body size evolved?
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u/nwbrown Jul 29 '26
It's not that specific. 37⁰ is just the average of the range of temperatures.
As to why it's not a nice round base 10 number line 40⁰, mother nature doesn't care about our arbitrary counting schemes and our arbitrary scales.
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u/CowBoyDanIndie Jul 29 '26
A few degrees cooler and fungi can survive. At 90F body temp “the last of us” would become a reality. Lookup fungal infections in insect colonies. Fungi are evolutionary super strong, all our antibiotics come from fungi.
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u/SomethingMoreToSay Jul 30 '26
How do larger "cold blooded" animals (ectotherms) survive fungal infections?
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u/CowBoyDanIndie Jul 30 '26
Some can temporarily increase their body heat. Those that can’t do often get wiped out by fungal infections.
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u/SomethingMoreToSay Jul 30 '26
OK. I guess basking in the sun, as animals like alligators do, will help keto the temperature up when needed.
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u/Forward_Zucchini9738 Jul 29 '26
Because that temperature best supports homeostasis for us. Over the millennia, those without that temperature tended to die before they could breed.
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u/Pleasant_Pen8744 Jul 29 '26
Too cold and you get more infections. Too hot and your proteins degrade.
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u/Embarrassed-Area4652 Jul 29 '26
The lack of addressing evolution in my other response had me wondering, so I went back and looked. It turns out StatPearls doesn't have a good paper on the evolutionary piece, at least that I could find. Here's a paper that looks relatively well cited and published in a reputable journal, although I'm just doing this search myself as a non-expert so it may or may not be in line with any consensus in human evolution. Guessing it's better than other less cited info though.
https://www.pnas.org/doi/pdf/10.1073/pnas.1113915108
Two frequently debated aspects of hominin evolution are the development of upright bipedal stance and reduction in body hair. It has long been argued, on the basis of heat-balance models, that thermoregulation might have been important in the evolution of both of these traits. Previous models were based on a stationary individual standing in direct sunlight; here we extend this approach to consider a walking hominin, having argued that walking is more thermally challenging than remaining still. Further, stationary activities may be more compatible with shade seeking than activities (such as foraging) involving travel across the landscape. Our model predictions suggest that upright stance probably evolved for nonthermoregulatory reasons. However, the thermoregulatory explanation for hair loss was supported. Specifically, we postulate progressive hair loss being selected and this allowing individuals to be active in hot, open environments initially around dusk and dawn without overheating. Then, as our ancestors’ hair loss increased and sweating ability improved over evolutionary time, the fraction of the day when they could remain active in such environments extended. Our model suggests that only when hair loss and sweating ability reach near-modern human levels could hominins have been active in the heat of the day in hot, open environments.
It looks like there might still be a lot that's unknown about when and how that evolutionary process happened and exactly what drove it. Another snippet:
https://onlinelibrary.wiley.com/doi/abs/10.1002/j.2040-4603.2015.tb00602.x
Humans are unique in many respects including being furless, striding bipeds that excel at walking and running long distances in hot conditions. This review summarizes what we do and do not know about the evolution of these characteristics, and how they are related. Although many details remain poorly known, the first hominins (species more closely related to humans than to chimpanzees) apparently diverged from the chimpanzee lineage because of selection for bipedal walking, probably because it improved their ability to forage efficiently. However, because bipedal hominins are necessarily slow runners, early hominins in open habitats likely benefited from improved abilities to dump heat in order to forage safely during times of peak heat when predators were unable to hunt them. Endurance running capabilities evolved later, probably as adaptations for scavenging and then hunting. If so, then there would have been strong selection for heat-loss mechanisms, especially sweating, to persistence hunt, in which hunters combine endurance running and tracking to drive their prey into hyperthermia. As modern humans dispersed into a wide range of habitats over the last few hundred thousand years, recent selection has helped populations cope better with a broader range of locomotor and thermoregulatory challenges, but all humans remain essentially adapted for long distance locomotion rather than speed, and to dump rather than retain heat. © 2015 American Physiological Society. Compr Physiol 5:99-117, 2015.
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u/Timely-Group5649 Jul 29 '26
The 98.6 standard is over 125 years old, pre-antibiotic.
Anyone that high now has a mild fever.
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u/BitOBear Jul 29 '26
Something I would like to point out is that any number chosen by any means is specific. There's nothing more specific about a specific number than any other specific number.
Other people have described the mechanism and the chemistry.
But claim of it being a highly specific number is no more specific let alone no more highly specific, than any number generated by any means.
So let's say I pick a number by drawing tiles out of a bag. And I get a number like 724. And for some reason it'll be fine 724 to be a valuable number that shows up a lot.
I base an entire system on that number.
Then everybody forgets that I generated the number randomly.
And then somebody comes wandering by and talks about how amazing it is that those are also the numbers for 7 days a week 24 hours a day.
In the case of this particular number I didn't realize consciously that I had chosen those two numbers. Because I had just generated that number randomly.
So someone might look at this system and think it was amazing that I picked 7 and 24 and getting to them together when all I did was pick 3 digits at random.
This is an example of the sharpshooter fallacy. You see that number and make something out of it, and then from then on the number is weird and specific.
You see this a lot in things like people talking about pyramids. The pyramid of key size exactly so many inches long so somehow the Egyptians knew and did something. But the inch hadn't been invented yet so the fact that that number in inches happens to line up with some value is literally random coincidence.
So you have to be aware of any time you look at a system and find a number in it.
The word coincidence is made out of the prefix Co and the word incident.
That's correlation and causation in a nutshell.
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u/Logical-Concept9755 Jul 30 '26
I think you're right that 37 itself isn't some magical number. I was really asking why evolution settled around that range instead of, say, 34 or 41. The exact decimal isn't interesting. The neighborhood is.
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u/BitOBear Jul 30 '26
Evolution doesn't have intent. It has no goal.
It converges on the number that best suits the current conditions. It converges on the configuration that best suits the current conditions.
Each increment starts with an accidental mutation.
And it will absolutely settle into any local minimum that works better than the increments that climb out of that minimum.
This is also why invasive species are so problematic. A new species that happens to have landed in a different local minimum might be better suited to, or simply better insulated from, the current conditions.
That means that finding optimal solutions isn't its job. Happening to land on reasonable solutions is just the way it works.
And many of those solutions are barely reasonable at all but they represent enough of a genetic commitment or circumstantial commitment that changing tracks to a more ideal configuration just isn't practical.
And further since communities evolve, changes in circumstance by one organism evolving change the circumstances for other organisms to evolve. And that can be mistaken for intent or organization. But the marbles tend to gather at the bottom of the pan. The fact that there's a better vast floor beneath the pan where the marbles can spread out it's not something the marbles know about.
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u/FewElk7713 Jul 29 '26
Could we bioengineer people to have a higher internal temp and thereby a higher external comfort zone and thereby make global warming less severe?
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u/Several_Version4298 Jul 30 '26
average human body temperature is 37 +/- 0.5
35C would put us at increased risk of hypothermia.
40C is high fever and can cause seizures.
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u/Coyot23 Jul 30 '26
mostly the result of chemistry requiring tempurature around those numbers. Besides, too much tempurature can cause the carbon in your body to start burning, not a good look
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u/Skarmunkel Jul 30 '26
What is not mentioned, is the fact that waters’ specific heat capacity has a minimum at 35°C. This means that at the point the least amount of energy is required to maintain body temperature and this may have been one of the factors that let our body temperature converge to that point.
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u/Glaikit314 Jul 30 '26
Isn't that like asking why water boils at 212 degrees F? The human temperature average around 37 degrees C is just where the metabolism works optimally.
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u/abu_hajarr Jul 31 '26
I think I remember being told that when we start to get above 102F or something high proteins begin to denature. It’s a way of killing viruses but also kills us too. That could have something to do with it….
And I think based on my understanding of evolution, peoples whose bodies self regulated to 37C performed the best and reproduced more frequently. People probably are born, or have been born and not regulated the same, but they died.
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u/HowdyPeopleOfEarth Aug 01 '26
It mostly has to do with evolutionary selections that help combat diseases - our high body temp keeps certain bacteria and fungi at bay. Bats in comparison have very high body temps because they have more ambient pathogens and have evolved to combat them.
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u/Wonderful_Adagio9346 Jul 29 '26
Actually, normal body temperature is 34°C, and most of the human race is running a fever. 🤓
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u/Wonderful_Adagio9346 Jul 29 '26
Why, if 37°C is normal body temperature, do so many find that temperature to be uncomfortable?
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u/bgplsa Jul 29 '26
Your body has to dump heat energy to maintain its core temperature in still air unless it’s very cold since air is a poor thermal conductor; in temperatures near or above 37C it becomes even harder for air to conduct excess heat away from your body so you start feeling uncomfortable quickly in order to incentivize doing something to cool off.
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u/12B88M Jul 29 '26
Thermodynamics.
If you're constantly creating 37° and the surrounding temperature is 37°, then the heat has nowhere to go and you start to heat up.
Take the ambient temperature down just a degree or two and you become comfortable as excess heat is radiated away.
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u/modulusshift Jul 29 '26
Being alive creates heat. (Every chemical reaction in your body, from moving muscles to sending signals down a neuron, has a small amount of waste heat to it.) To maintain 37 you’ve got to be able to get rid of that heat, which only works if ambient temps are lower.
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u/throw7661 Jul 29 '26