Sunday, June 03, 2018

If Humanity Evolved in Georgia ("Sweating Made Us Smart" Edition)

We're fast approaching the two-year anniversary of my moving to Georgia (a.k.a. "the sweatbox that pretends to be a state"). For my first couple weeks here, I lived a lifestyle scarcely different from that of a criminal under house arrest: I'd only go outside long enough to collect the mail, dispose of garbage or run necessary errands, and the rest of the time stayed in air-conditioned environments to hide from heat and humidity far more ferocious than anything my northern-acclimated self was accustomed to handling. (And I spent most of my childhood and adolescence in coastal southeastern Virginia, where the summer climate isn't exactly clement... unless you compare it to Georgia's.)

However, staying indoors all the time isn't healthy for one's psyche or physique, so after awhile I got into the habit of going out for daily walks. I also learned not to even bother taking a shower before said walks, because no matter how spotlessly clean I was upon leaving the house, after more than 30 seconds outdoors I'd be a sweaty, slimy, thoroughly disgusting mess.

And on one of these daily "for my own good" trudges through the neighborhood, a random thought occurred to me: how different would human biology be, if we, our close primate cousins and our pre-homo sapien ancestors had evolved in the southeastern US rather than the African savannas? Humans (at least in theory) cool down via perspiration -- horses are the only other non-primate animals to cool through sweat, rather than via panting, having blood flow through thinner body parts to let body heat radiate out (as do those elephant species with very large, thin ears), or similar things.

This cooldown method gives us far more stamina than other warm-blooded creatures: so long as we have water to keep hydrated, we can keep going FAR longer than any pant-to-cooldown animal can. I recently read about "persistence hunting," which our savanna ancestors are believed to have practiced: though animals such as cheetahs, gazelles and other creatures we presumably used to hunt can definitely outrun us in the short term, in the long term we actually overtake them: not by running, but simply by following and tracking them at an easy walking pace. They'd beat us in any hundred-yard dash, but we'd beat them in a marathon. Hours later, when the prey animal has to stop and pant to cool down, those sweaty human hunters would still be chugging along after them. From the animals' perspective it would've been like us being chased by zombies: they're slow and sluggish and you can outrun them easily in the short term, but sooner or later you're going to get tired, and meanwhile that slow sluggish monster chasing after you still keeps on going.

Anyway, the process "perspire, then the sweat evaporates and draws heat away as it does" presumably works very well in the arid savanna (provided you drink enough to replace the water you lose -- even if that drinking water is as warm as the surrounding air). But it completely backfires in humid Georgia heat, where sweat won't evaporate until you step indoors where it's air-conditioned. Persistence hunting would NOT work here. Even ordinary "sweating to cool down" doesn't work in the natural environment here: when I'm outside, the only thing that cools me down is sips from the ice water bottle I always carry, and ice water on 90-plus-degree days does not exist naturally in Georgia; THAT requires technology barely a century old. But "sweating" on its own won't cool you off on a typical hot-n-humid day; at best it merely makes you feel gross and at worst it actually makes you hotter, since you end up simmering in your own juices.

So had our primate ancestors evolved in a humid-hot rather than dry-hot environment, we would not have evolved the tendency to perspire. Maybe we'd pant to cool down, or we'd have enormous-n-thin elephant ears, or our ancestors would only have been able to live in places where they could submerge themselves in water (or cover themselves with mud as pigs do) several times per day. They couldn't have engaged in persistence hunting, which would've limited what animals they could've hunted, unless they/we evolved some other method of hunting: the ability to run very fast for short bursts of time, perhaps? But with more energy directed toward things like stronger leg and heart muscles -- whatever actual biological changes would be necessary, for the average person to be able to short-term run as fast as the average "fast" animal -- that would leave less energy available to nurture the growth and development of our super-big, super-smart brains.

The only disadvantages of sweating compared to other cooling methods (again, assuming a climate dry enough for sweat to evaporate) is that it requires far more water, and presumably more salt to replace what you sweat out, than a similarly sized mammal which does not perspire. Everything else is a biological advantage: provided you have the water to drink, you can go a lot longer without a cooldown break compared to the panters, can work harder and generate more heat than the radiant-heat losers such as big-eared elephants, and have more freedom of movement compared to animals which have to stop for a dip in water or mud to cool down.

So I wonder: how many of the evolutionary traits which eventually led to modern homo sapiens require perspiration? Persistence hunting on the savanna requires it, but did persistence hunting come before or after intelligence? Homo erectus is known to have had fire, and homo habilis made stone tools (hence its name: "handy man"). Our pre-human, pre-"homo" ancestors walked upright before they got intelligent (certainly there's nothing to indicate the australopithecenes were very bright); the first advantages of walking upright was that our ancestors could see further, and I also read speculation that it kept them cooler (with only the top of the head under direct noon sunlight, rather than the entirety of the back); only after the earliest hominids were used to having those two limbs not used for locomotion did they start evolving the intelligence to do useful things with them. And a meaty protein diet also contributed to larger brain development; if we couldn't sweat, and couldn't persistence hunt, would our pre-human ancestors have been able to get enough meat to evolve those big brains, or would we always have been omnivores who mostly ate plants, with only the occasional insect, small animal or scavenged corpse to provide protein (but not enough to evolve big brains)? Would we even have developed enough intelligence to reach the primitive stone tool/homo habilis level, without sweating -- without having an environment dry enough for sweat to evolve in the first place, in other words?

How much of the evolution that made us "human" would have been impossible, if instead of "sweating" as a cooldown mechanism, our pre-human ancestors had to either pant, radiate heat, or physically dunk their bodies into cooler substances?

Something to think about, as I leave the house for my daily exercise trudge and the possible root cause of human intelligence soaks through my lightweight linen clothes. 


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