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Evolution/Body Temperatures


Good day.  I have two main questions.

First:  how did evolution give us a resting body temperature of precisely 98.6 degrees?  And why?  Are there inherent benefits to that temperature, as opposed to 95 or 103 degrees?  Could you explain the evolutionary reason and mechanism of our body temperature?

I see that animals have different body temperatures.  Why do different mammals have different body temperatures?  Whatís that bout?  Animals seem to have higher temperatures than us.  Could you explain why that is, and the evolutionary implications of the varying body temperatures of mammals?

Second, why, if our body temperature is 98.6 degrees, does air or water that is 90 degrees feel hot and why don`t we feel cold when it`s 70 degrees outside?  Why does  staying in water thatís 98 degrees for an extended period of time (if we keep drinking water) cause hyperthermia?  

Any additional observations or interesting info on the subject would be welcome too.


Cole LaValley

Dear Cole,

Your questions are really in two areas:  evolution and physics.  I'll try my best to answer, but a lot of this is going to be hypothesizing.

First, it's important to remember that some evolutionary processes are random.  Natural selection is not.  But genetic drift is, and this can contribute to traits that seem "random".  (For an overview of genetic drift, please read: )

Human bodies average 98.6 Farenheit (there's a little bit of individual variation, but it's very small) because our enzymes work optimally at that temperature.  That may sound like a circular argument, and in some ways it is.  But if you consider that any trait we have is either:

adaptive:  increases our chances of survival (and reproduction, leaving the genes coding for those traits to the next generation)

maladaptive:  decreases our chances of survival/reproduction

neutral:  does not affect our chances of survival/reproduction any given environmental context, then it starts to make more sense.

At some time in our distant, distant ancestral past, a reptile-like ancestor inherited a mutation (or set of mutations) that set us (and our mammal cousins) on the road to homeothermy (the ability to regulate our body temperature metabolically, and not have it controlled by the environment) and endothermy (the ability to generate heat metabolically, instead of collecting it from the environment/sun).  The body temperature of the first "warm blooded" mammal precursors was probably not 98.6.  But at its higher temperature, it didn't have to rely on the sun for heat, and it didn't slow down at night.  This new physiology enabled these animals to be nocturnal and to react more quickly when environmental temperatures were cold.  All that is adaptive, and so it was passed on from generation to generation to us.

Why 98.6?  That might be a matter of genetic drift.  Mammals range in body temperature from about 35oC - 43oC, depending on species.   The ancestor of all humans likely had a body temperature close to that 37oC, which is what we have now.  Why?  It might have been a matter of chance and genetic drift.  But because our ancestors' enzymes and other metabolic processes had, in essence, co-evolved with that setpoint, it might have been maladaptive (or lethal) to inherit a mutation that would not allow an individual to maintain a body temperature at which its metabolic processes could proceed normally.  Those individuals would be out of the gene pool, leaving only those with the most functional homeostatic temperature setpoint.

The same could be said for any mammalian lineage.  Body temperature variation across species is likely a product of both genetic drift and natural selection, just the same as any other trait.


Now for the physics part.

Our bodies produce heat via metabolic reactions.  But they *maintain* a constant temperature by dissipating and generating heat constantly, as our internal thermostat directs.  Just like an oven.  An oven doesn't stay on constantly when you set it to 350o.  It generates that temperature with its coils, then shuts off the coils when it reaches 350.  As heat radiates away (following the Second Law of Thermodynamics), the thermostat detects the variation and turns on the coils again, to maintain the oven at 350.

Our own thermostats work the same way, heating and allowing heat to escape constantly to maintain 98.6F/37C.  Our bodies perceive temperatures cooler than our body temperatures as warm because our nervous systems are sensing that the heat exchange rate is slowed down, compared to that when we are in air that's much colder. Not every human feels 90oF as uncomfortably warm; a lot depends on where that human lives and grew up.  (Some research has shown that if you spent your toddler years in a cold climate, you will never be comfortable in a warm climate as an adult, and vice versa.)

Not sure if that answers all your questions, but I hope at least it gives some interesting jumping off point for thought.



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Dana Krempels


I can answer questions about evolutionary mechanisms and theory, including genetic drift, mutation, natural selection, etc. I also can clear up misconceptions about evolution as it's sometimes talked about by those not well-versed in the subject (e.g., some politicians and many religious fundamentalists).


I have a Ph.D. in Biology, and presently teach Evolution and Biodiversity, Genetics, Botany, and Zoology at the University of Miami.

House Rabbit Society Beta Beta Beta Biological Honor Society

Exotic DVM Magazine (veterinary journal)

B.S. in Biology B.A. in English Ph.D. in Biology

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