Why Do Humans STILL Grow Hair in the Weird Places?

Humans are the only primate that walks around looking freshly plucked. The fine "peach fuzz" on your arm isn't fur — it's the ghost of it. Why we went bald is genuinely unsettled, with three competing hypotheses. The thermoregulation idea (modeled by Peter Wheeler, championed by anthropologist Nina Jablonski): bare skin lets us sweat and cool efficiently — humans carry roughly 2–5 million eccrine sweat glands, about ten times the density of a chimpanzee. The endurance-running idea (Dennis Bramble of the University of Utah and Harvard's Daniel Lieberman, Nature, 2004): we evolved to run prey to death across the heat ("persistence hunting"), which is impossible under a fur coat. And the parasite-avoidance idea (Mark Pagel & Walter Bodmer, 2003, "A naked ape would have fewer parasites"): fur is a hotel for ticks, fleas, and lice. The timing is fuzzy — likely early Homo / Homo erectus, roughly two million years ago, but the estimates stretch from ~1 million to over 3 million years. The tufts that earn their keep. Every patch we kept is doing a job. Scalp hair shades the brain from the midday sun (upright posture aims the sun at the top of the head). Eyebrows divert sweat from the eyes — and are crucial for face recognition: a 2003 MIT study led by Pawan Sinha found that erasing the eyebrows from famous faces hurt recognition more than erasing the eyes. Eyelashes are an aerodynamic filter: a 2015 Georgia Tech study (Amador & Hu, J. Royal Society Interface) found lashes about one-third the eye's width minimize both dust and tear evaporation — and longer actually backfires. The weird ones — the unsolved fight. Nobody knows why we kept armpit and pubic hair. Three competing, unproven theories: scent-signaling (Desmond Morris, The Naked Ape, 1967) — except there is still no confirmed human pheromone (Tristram Wyatt, 2015); friction reduction; and a maturity signal (it erupts at puberty). The gorilla secret. Humans uniquely carry three kinds of lice — head lice, body lice, and pubic ("crab") lice — each gripping a different hair type. David Reed (Florida Museum, BMC Biology, 2007) sequenced their DNA: the head louse's closest relative lives on chimps, but the pubic louse's closest relative lives on gorillas — humans acquired it from gorillas about 3.3 million years ago, almost certainly through shared sleeping nests or scavenging carcasses, not sex. And the twist: the body louse evolved to live in clothing, so lice DNA actually dates the invention of clothing — to somewhere between roughly 80,000 and 170,000 years ago (Kittler et al., 2003; Toups & Reed, 2011). Finally, the reversal: we went naked, then invented razors — ancient Egyptian priests shaved their entire bodies every other day so, per Herodotus, "no louse could adhere."