Temperatures on Earth

[lifesagift]

But you're essentially correct: -50C and +50C are at the extreme limits of human endurance.

Boru

Human normal metabolic function adds heat and yet human body can not operate more the 2-3 degrees above its normal operating temperature of 37C. So human body must be able to shed heat. This become impossible when ambient temperature exceeds 40-45C on a sustained basis, so the human would die without mechanical cooling.

Likewise humans can only produce so much heat per hour and the speed with which humans lose heat increases with lowering ambient temperature. Without extensive thermal insulation humans would gradually suffer lowering body temperature and eventuaally die when ambient temperature drops below 0C.

So the real operating temperature range for humans without additional artificial insulation and cooling is more like 0-40C.

If you add artificial insulation and artificial cooling, the range expands dramatically. The best example is a space suit. A human in a space suit can survive -150C to 150C, sometimes both at the same time.

With great application of industrial resource it is conceivable humans can live on a sustained basis between -150C and 150C.

Plane and simple really... it's our normal body temperature that determines the conditions in which we operate best.

So that's begs another question.... why 37.2 deg C?

Wouldn't blood run quicker at 53.4 deg C, so making oxygen distribution much quicker, and hence humans much fitter, faster etc?

[The Grand Nudger]

We'd need a hell of alot more food....for starters - and bleeding out faster isn't such a hot idea.

In any case...what If I had wings? Well fuck, why don't I? Wouldn't I be fitter and faster? I think that in this example, what's begging the question is any given notion of what may be fitter or faster in a vacuum, not the tech specs of the human body. Rabbits run hotter than we do - can't remember the last time I saw a human being on a rabbits plate.

[lifesagift]

We'd need a hell of alot more food....for starters - and bleeding out faster isn't such a hot idea.

In any case...what If I had wings? Well fuck, why don't I? Wouldn't I be fitter and faster? I think that in this example, what's begging the question is any given notion of what may be fitter or faster in a vacuum, not the tech specs of the human body. Rabbits run hotter than we do - can't remember the last time I saw a human being on a rabbits plate.

But they breed more effieciently than we do?

[Anomalocaris]

Plane and simple really... it's our normal body temperature that determines the conditions in which we operate best.

So that's begs another question.... why 37.2 deg C?

Wouldn't blood run quicker at 53.4 deg C, so making oxygen distribution much quicker, and hence humans much fitter, faster etc?

It's much more complicated. Most metabolic function involve some form of protein reaction. Different protein reactions occurs best at different temperatures. As a result, organisms adapted to different metabolic temperatures uses a different suite of proteins to support their metabolic functions. Even the same metabolic function often uses a different protein in a cold adapted organism than in a hot adapted organism.

The proteins an organism can synthesize and uses are deeply encoded into its genes. For highly complex organisms like mammals many proteins are required to support life. One could not easily change out some proteins and leave others in place without throughly disrupting life critical functions. So once the protein suite, and their coding genes, of a family of highly complex organisms become adapted to a certain metabolic temperature, the hurdle to changing that temperature is very high. A whole suite of genetic changes must all occur more or less at once to successfully change the metabolic temperature of members of that family without killing it.

This is why most placental mammals, including humans, have almost the same metabolic temperatures. Once the first ancester of placental mammals perfected a suite of genes and proteins suited for metabolism at 37C, it became very difficult for any of its descendants to adopt a different metabolic temperature without compromising some life critical function. Birds as a group, whose ancesters probably evolved endothermic independently of mammals, also have mostly similar metabolic temperature of around 40C.

As to why the first ancester to adopt 37C Metobolic temperature adapted that particular temperature and not any of the other possibilities, the specific details of answer is probably hard to determine from about 100 million years later. But we can be fairly certain the general outline of an answer would be something like:

1. There is benefit to maintaining a steady metabolic temperature. This enables an animal metablize efficiently all the time and to remain active and alert when it is cold as when it is hot. So this gives the animal the ability to outcompete another which becomes lethargic when it gets cold due tout diminished metabolic rate and efficiency.

2. But There is a high energy cost to high metablic temperature. A placental mammal with a fixed body temperature of 37c must eat 10 times more calories than a cold blooded reptile of similar size to keep up the body temperature. So an animal would not tend to evolve metablic temperatures higher than necessary to gain from 1.

So the first placental ancester to evolve 37C metablic temperature probably found that temperature to be the best attainable compromise between 1 and 2 in its living environment. Once it's genes and metablism settled into 37C, it became difficult for its descendant to change it. Consequently virtually all subsequent placental mammals kept the 37C metabolic temperature even though their have diversified from the arctic to the tropics.

[The Grand Nudger]

But they breed more effieciently than we do?

Do they? By what metrics? I'm a big fan of the "quantity is a different kind of quality" line of thought...it just doesn't seem to be working for the rabbits. Efficiency is another one of those things that I'd call a "metric in a vacuum", like a higher temp or wings as they apply to human beings.

[Anomalocaris]

But they breed more effieciently than we do?

Do they? By what metrics? I'm a big fan of the "quantity is a different kind of quality" line of thought...it just doesn't seem to be working for the rabbits. Efficiency is another one of those things that I'd call a "metric in a vacuum", like a higher temp or wings as they apply to human beings.

Rabbit metablic temperature is almost the same as humans, within about 1 degree. A human running rabbit temperatures would be barely feverish. A human running typical bird metabolic temperature would be seriously ill and on the verge of brain death.

Amongst mammals only the monotremes, the most primitive mammal group still surviving and most distant mammalian relations to placentals, has substantially different metablic temperatures from placental mammals. A human running the same temperature as a platypus would be unconscious and deeply hypothermic.