Before we get down to specifics, let's consider the main subject. What's the source of the strange things people do? Where did we even get all of these bad ideas?
Instinct
Many of the things we do are a result of our biology. Even if we're not 'animals' (those of you who recall what the average college apartment looks like may disagree), our bodies are. We have that flight or flight reflex, which anybody who's ever been asked out on a date is personally familiar with. We still wrinkle our noses up at bad smells, crave cuddles when it's cold out, and gawk at attractive rear ends.
Still, genetics, chemicals, and physiology alone don't makes us 'human'—at least, not in the terms which we think of it today. We have what we need to achieve such a state, but culture is what sculpts the raw material of humanity. It provides us with a link between what we are and what we can become.
Consider Tarzan for a moment. He is, unfortunately, much like Santa Claus. He's a charming and magical image, but also an ultimately unrealistic romanticization. A human raised by apes would end up nothing like Tarzan. (For one, if you smiled at him, he'd probably try to kill you. Never grin at a gorilla. It's like playing 'chicken' with a mac truck being driven by a kid who's played “Grand Theft Auto” way too make times.)
Man Vs. Animal
A man or woman utterly isolated from human influence, behavioral standards, and anything else related to the trappings of culture would hardly be 'human' in any sense other than the purely biological. Unable to speak any known language, wear clothing, relate, or (goodness gracious) use toilet paper, such a person would probably be more akin to monkey than man.
While they'd still be human, there wouldn't be much human about them. They'd seem closer to what we think of as animal. Perhaps a highly intelligent, curious, very strange looking one—to be honest, we are pretty much the naked mole-rats of the primates—but still unlike what we think of as human.
We're not a different species from our ancient ancestors. Some thousands of years of evolution may separate us from them, the turn of generations breeding changes within our genetic code; but the people who walked barefoot and illiterate and ate raw meat with their fingers were just as human as you or I. In fact, many of us still struggle with illiteracy, run around barefoot, and eat with our fingers. (You know who you are.)
So when, then, did our ancient lineage make the transition from what we define as 'animal' to what we think of as 'human'? We can line up skulls on a bench, measure the different shapes and sizes, and engage in furious academic debate over the types of teeth. But there's no a distinct moment in time, a clear line, at we might point and say, “This is human. That is not.” (Well, not unless you put Wolverine from X-men on one side and a cute little girl in pigtails on the other. But that's just cheating.)
Culture
Was it something we did, something we thought? The first attempt at language? Art? Curiosity, humor, writing... Burial rituals, or the telling or stories to explain those things which we didn't undersand? Was it the discovery that when we break wind upon a flame, it explodes into a conflagration—or the realization that said phenomenon is really funny, especially when someone lights their ass on fire?
We have it in us to be an amazing and wild variety of things, from simple hunter-gatherer to astronaut. But it is our culture—our body of knowledge, of technology, of ritual and language and meaning—which actually draws out that potential. 'Culture' allows us instant and constant access to the summation of what all of those before us have achieved and become in their lives. This shapes us into something different, something more, than what we could be without it. (Or, in some cases, something less. For instance, without language, we would never have come up with the excuse “It seemed like a good idea at the time.” And I'm still not sold on the idea that us figuring out how to light stuff on fire was a good thing.)
We—you, I, and everyone else—are authorities on the subject of what it is to be human. We have the ultimate first-person perspective. (Unless, of course, you're either a dog that has learned to read, or a sentient robotic brain. No offense meant to either party.) So we're all intimately familiar with the urges to do crazy things, and with the need to handle those urges in a way that obeys the laws of society.
Our instincts are, by and large, responsible for many of our oddities. The chemical reactions and instincts that govern many of our natural inclinations evolved in a setting far from modern society. Instinct, when shaped by self, and poured into the mould of modern culture, births a multitude of strange and varied phenomena.
For example, before we had the tools and technology to defend ourselves, leaping a foot in the air, screaming like a little girl, then attempting to climb the nearest tree was a perfectly reasonable reaction to sudden, loud noises. Taken out of context, however, people who do that these days look like complete lunatics.
We cram our feet into four-inch wedges and strap on skirts that binds our thighs together. Not only does this sometimes look ridiculous, but it seems completely contradictory to survival instinct. (Think about trying to run away from a grizzly bear in high heels and a miniskirt.) But when viewed through the lens of cultural ideas about sexual appeal, it's just another way of displaying virility and attracting a mate. Tight, showy clothing is one way that members of our culture have been conditioned to approach the issue.
We are, as individuals, part biology, part culture, and part ourselves. There are some things that are ours by nature, some that result from our experiences, and some that neither culture nor instinct nor plain old idiocy can yet explain.
Much of this was inspired by Clifford Geertz's 1966 paper, “The Impact of the Concept of Culture on the Concept of Man”. I highly advise checking it out.
Quote: “When seen as a set of symbolic devices for controlling behavior, extrasomatic sources of information, culture provides the link between what men are intrinsically capable of being and what they actually, one by one, in fact become. Becoming human is becoming individual, and we become individual under the guidance of cultural patterns, historically created systems of meaning in terms of which we give form, order, point and direction to our lives.” (Geertz, The Impact of the Concept of Culture on the Concept of Man)
