In a previous node on the cardiopulmonary equipment of avians, I might have come off as a bit condescending towards Homo sapiens (and all of Mammalia, for that matter). Sure, we may not have the complex system of air sacs and turbocharged heart that the order Aves is blessed with, but I'll tell you what we do have --- running, and more importantly sweating. There are very few beasts alive that can run or sweat nearly as well as H. sapiens, but it was sweating that proved our biggest advantage against the relentless mechanics of natural selection. Sure, sweating makes you feel gross and leaves dirty brown marks on your whites, but it was sweating, more than any other adaptation, that allowed hominids to spread throughout the world and take the niche in practically all environments as the ultimate apex predator, a status that doesn't appear to be changing anytime soon (for better or worse).

Sweating allows us to maintain high levels of aerobic activity in the noonday heat of the African savannah, where we evolved and spent much of our existence. It allows us to pursue prey until they literally drop from exhaustion. Our surplus of sweat glands must have evolved around the time of Homo erectus, who managed to colonize a massive swath of land from South Africa to Indonesia in a few thousand years — a paleontological eyeblink, and to allow such rapid expansion, H. erectus had to be able to move pretty efficiently. This not only meant bipedal locomotion (allowing for a more efficient walking stride and the ability to carry supplies), but also a high degree of social cooperation (after all, any lone hominid would wind up as Smilodon chow pretty quickly) and efficient thermoregulation. Homo erectus proved to be one of the most adaptable hominids, second only to H. sapiens; they colonized and thrived in environments that ranged from the hot, humid jungles of Java to the dusty and dry savannahs of Kenya, and all this was thanks to their oversized sweat glands, that allowed them to keep moving without dropping dead from heatstroke.

Running was another adaptation of H. erectus that proved to be of great advantage. H. erectus was the first hominid with a more-or-less modern postcranial skeleton; in other words, if you put a paper bag over its head, you wouldn't be able to tell the difference between an erectus and a sapiens, apart from a bit more body hair, and somewhat different limb and torso proportions, the latter of which falls rather neatly within the variation seen in our own species. Both H. erectus and H. sapiens were well-adapted to distance running, and sweating was just one of the many features that allow both erectus and sapiens to outrun predators and prey in the heat of midday.

Homo has some pretty odd limb proportions compared to the other great apes. Where most of the great apes have their forearms longer than their hind legs, we have that situation reversed entirely; our intermembral ratio is somewhere around 70. That means that our arms are about 70% the length of our legs. Furthermore, our legs are vastly more heavily-muscled than any other primate, and our arms comparatively weaker (but with better fine motor control than most). This is why humans have butts, and chimpanzees do not — our gluteal muscles are many times bigger than those of chimps and other primates, because we need them to provide propulsive force for our obligate bipedalism.

The legs are not the only place where we differ from our primate ancestors and relatives; our spines are curved in an S-shape, where most other primates have a more straight vertebral column. This allows us to place most of our mass along our center of gravity, and gives us better control over our balance. Running, after all, is nothing more than a controlled and continually-interrupted fall, and it helps to be able to shift our mass quickly and neatly. Our big toes also rotated forwards and lost much of their grasping ability, a feature that was already on its way out with the later australopithecines. Our feet became arched and springy, to better absorb the shock of supporting all our weight on our hind limbs, and to mitigate the violent shocks that running inflicts on our entire bodies with each step.

And the differences continue; we have a ligament in the back of our necks that helps to keep our massively-oversized heads level during running. Our pelvises narrowed and rotated forwards into a bowl shape, to give us both a better platform to support our internal organs (still a bit cranky from our transition to bipedalism). Our chest walls expanded to allow for more room to put a more robust cardiopulmonary system. Our skin lost its hair, something that facilitated the transition from panting to sweating to dispose of excess body heat. Humans became the best hunters on the planet, not because of any natural weapons we have, and not because we're especially fast or especially strong or especially robust, but because we could simply jog after our prey for long periods of time and wait for it to drop from exhaustion before we did.

Edison said it best when he said that genius is only 10% inspiration, and 90% perspiration...

Thanks to maxClimb and auraseer for pointing out a tiny error in the wording of my little bit on the intermembral ratios. My bad!

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