I preface a lot of my blog posts by saying I am not an expert on X, Y, or Z and keep that in mind when interpreting my opinions, and that applies here as well. But I’m gonna do something I enjoy doing, which is wild, conspiracy minded speculation about subjects I have ancillary but not in depth knowledge of. So here we go! I am not a physical anthropologist or an evolutionary biologist, and I am speaking purely out of my lay understanding of these fields. I am not deliberately getting any FACTS wrong, but I may do so without realizing, and my speculations absolutely are just wild speculations on my part and not anything scientific.
Okay, so, sweat glands. We have them. Do you know who doesn’t have them? Reptiles. And, by extension, anything which evolved from reptiles, like birds. Which are technically reptiles. I think a lot of people get a very linear progression perspective on evolution from popular media, and its important to realize thats bullshit. Why do reptiles not have sweat glands and we do? Its not because we are more advanced. Our sweat glands are homologous - i.e., evolved directly from and are largely similar to slime glands in use by amphibians, i.e. those forms of tetrapod life which never fully broke off from living in the water, and those slime glands in turn evolved from slime glands fish have which let them exude slime to protect their bodies from the surrounding water, parasites, and injury. Basically, like fish and amphibians, mammals are slimy boys. Reptiles are not - they subsequently lost their sweat glands in order to retain water better, after they split off from our most common ancestor. This I think really highlights the flaws in calling primitive mammals “mammal like reptiles” - making it seem like we evolved from and are superior to reptiles, when they’ve been doing their own thing for millions of years.
Anyway, thats a bit off the point I am making. Not only are mammals slimy boys, like amphibians and fish - animals which notably are generally very poor at retaining water and surviving in dry areas - but humans in particular are extremely wet for mammals. What do I mean we are extremely wet? Well, what I mean is, we sweat. A lot. Yeah, all mammals sweat, but most mammals have primarily apocrine sweat glands, which release waxy, oily sweat that actually helps waterproof hair and help retain moisture. Humans, however, have an unusually high number of eccrine sweat glands, which produce far more watery sweat with less oil and more water, to use to cool down. Most mammals pant to cool down, we instead drench ourselves with sweat, which evaporates to cool us down. This is far more effective than panting, meaning we are less prone to overheating than other mammals - but it also means we hold the dubious claim of being the sweatiest mammal. Its also important to note, this is unique to us, even compared to chimpanzees - we are extremely sweaty, way sweatier than our closest relatives or any other mammals, with one possible exception. That exception is the horse, who also has many eccrine glands and also produces latherin a surfactant which renders their sweat even more effective at cooling.
This seems likely an adaptation to heat and long periods of physical exertion, which lines up with humans practicing persistence hunting, a style of hunting where we essentially just slowly follow animals until they are so tired they give up which is often thought to be the dominant form of hunting humans practiced historically. However, it also seems to imply something else of note - that humans evolved in an environment where our survival was not sharply limited by the availability of water. If it was, why would we have evolved a system which wastes so much of it, comparatively? My understanding is we are thought to have evolved on hot, arid savannah lands, which does not inherently sound like an area where water is abundant. Is it possible to reconcile these two things? Or do they even need to be reconciled - after all, this is a dilemma I basically just made up and I’m not even a scientist.
There is a widely debunked theory called the aquatic ape hypothesis which posits many of our traits which have been explained as part of humans evolving from forest dwelling arboreal apes to savannah dwelling bipedal apes are better explained by us spending large amounts of time in water. I always used to find it very interesting since a lot of its points seem to hold up and even make more intuitive sense than conclusions made based on fossil evidence - but fossil evidence has the advantage of actually being true. But I do wonder, how important was proximity to water in early human development?
We clearly need a lot of water to survive based on how sweaty we are. Our subcutaneous fat, hairlessness, body hair patterns, innate infant swimming skill, etc all also point to possible affinity for water as well, even if all are circumstantial. We also often suffer iodine deficiency unless we regularly eat seafood or live on lands which have a lot of iodine in the soil due to being formerly submerged under ocean water, causing our food to be naturally high in iodine. Also, if you look at patterns of human settlement, we show an enormous preference for settling near rivers. Now, you may say, “Of course we do, water is necessary for life and agriculture,” and sure, it is, but do you think that if somehow Thorned Devils evolved into an agricultural civilization, that they would have such a high affinity for water? To me, it really seems like humans have always chosen to settle near water and depended on its widespread availability to survival, because we seem to have a lot of adaptations which require its abundance or make us better at living near or functioning in it. But this definitely isn’t something I see widely discussed in human evolution or early anthropological science I’ve encountered, aside from the debunked aquatic ape hypothesis. That being said, its entirely likely 1) it is discussed and I’m just not expert enough to see those discussions, or 2) its not discussed because the connections I am pointing at are stupid and I’m wrong. But hey, I milked a blog post out of it? eh? eh?
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