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If all life relies on water, and water freezes at 32F and boils at 212F.

Then why isn't the nominal temperature for life 122F?

122F will literally kill some animals, including us.

Do our biochemical reactions need "cold" (most people prefer ~72F) for some reason?

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    $\begingroup$ A large part of life's 'operating temperature range' is directly linked to the effect of the sun on the Earth and Earth's distance from the sun. A range of temperatures exist on Earth and evolution will take over from there. Seems like a good question for SE:Biology. $\endgroup$ – LordStryker Sep 12 '14 at 19:55
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    $\begingroup$ I don't think there is any reason, $a priori$, for the preferred temperature to be exactly half way between freezing and boiling. The thermodynamics of stability for things like liposomes, proteins, and DNA are not tied to water directly that I can tell. $\endgroup$ – Jon Custer Sep 12 '14 at 20:00
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    $\begingroup$ generally, there are bacteria, capable of operating at -5 C (and survive absolute zero) and there are some, thrieving at 100 C $\endgroup$ – permeakra Sep 12 '14 at 21:14
  • $\begingroup$ I think this question has a lot to do with enzymes, which are proteins that act as biological catalysts. Enzymes have a specific temperature range, too cold and they function too slowly, but if it's too hot they get 'denatured' and cease to function. This doesn't answer your question - because the point remains: how is this temperature optimum determined? I was just commenting this to add specificity. $\endgroup$ – surelyourejoking Sep 13 '14 at 6:57
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Here's a link to a brief Scientific American article that presents the results from a study of this question. Basically, for every $\ce{1^{o}C}$ increase in body temperature, 6% fewer fungal species can survive - so the hotter, the better (the longer you're likely to survive). The downside of "hot" is the increased metabolic rate required, which translates into more time spent consuming food. If you were to plot the effect of these two competing effects on longevity versus body temperature you would get something like an inverted (upside-down) parabola. The study found a maximum longevity at 98.1 F, not far off from the observed average body temperature of 98.6 F.

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