Straight Hair Happened Twice

Here is a fact that should be stranger than it sounds: tightly curled hair — the kind common across sub-Saharan Africa — is, by the account of the researchers who study it, found on no other non-domesticated mammal on Earth (Lasisi et al. 2023). Not on apes, not on monkeys, not on anything that wasn’t bred by humans into the shape. We are also the only mammal with a functionally naked body and a hair-covered patch on top of the head. Whatever happened to human hair, it happened to us, specifically, and the obvious question is what it was for.

The instinct — mine, probably yours — is to assume each hair type is an adaptation. Curly hair for the tropics, straight hair for the cold, every variant a solution selected because it worked. That instinct is half right, and the half it gets wrong is the more interesting half.

The adaptive star

Start with the curl, because that’s the part where the adaptation story holds up.

The naive version says curly hair keeps you cool by trapping a cushion of air, the way a sweater keeps you warm — which would make it a terrible idea in the heat. A 2023 experiment took the question off the armchair and put it on a thermal manikin: a metal head wired to measure heat flux, dressed in human-hair wigs ranging from straight to tightly curled, under lamps simulating midday equatorial sun (788 W/m² at 30 °C). The result inverts the sweater intuition.

Hair does cost you some evaporative cooling — sweat works a little less efficiently under a covered scalp. But that cost is dwarfed by what hair blocks: incoming solar radiation. And the tighter the curl, the more it blocks, because tightly curled hair stands off the scalp and widens the air gap, the way the depth of a fur coat — not its insulation — is what shields a desert animal from the sun. The payoff lands in the one currency that mattered most on an African savanna with scarce water: a scalp under tightly curled hair needs the least sweat to stay in heat balance (Lasisi et al. 2023).

So the function isn’t “the hair cools you.” It’s: the hair shields the brain from the sun, so you spend less water protecting the most heat-sensitive organ you own. Sitting directly over a large, fast-overheating hominin brain, that’s a passive sunshade that buys you time before your next drink. The authors frame it as plausibly enabling the big brain — lifting a thermoregulatory ceiling on how large the thing on top could get. (They’re honest about the limits: it’s a static head model, the timing in the fossil record is unknown, and whether it shifts whole-body temperature is unsettled.) The curl itself is built at the root — African follicles grow physically bent, a curved bulb shaped like a golf club, dividing asymmetrically, and they keep producing curls even when grown in a dish (Thibaut et al. 2005).

Good story. Curly hair earns its place as an adaptation. Now watch it fall apart for the other half of the world’s heads.

The trait that happened twice

East Asians and Northern Europeans both have high frequencies of straight hair. The lazy assumption is that this is one trait — inherited from some shared straight-haired ancestor, or selected by some shared northern pressure. It is neither. East Asian straight hair and European straight hair are made by completely different genes.

In East Asians, straightness tracks a single variant of the EDAR gene, V370A, that is “almost absent in Europeans and Africans” (Tan et al. 2013). In Europeans, it tracks variants of TCHH, trichohyalin, which are rare in East Asia and peak in Northern Europe (Medland et al. 2009). When you look at admixed populations like the Uyghur, both genes are present and both contribute, independently, with no interaction — they’re two unrelated machines that happen to output the same look (Wu et al. 2016).

Same phenotype. Two genes. Two populations. Independent origins. That is convergent evolution, in the textbook sense.

What convergence does and doesn’t prove

I wrote a post recently — “The Convergent Skeleton” — arguing that when four independent observers using incompatible toolkits recover the same structure, the parsimonious explanation is that the structure is real, out there, not imposed by any one observer. Convergence as evidence of something true and constrained.

Straight hair is the case that keeps that argument honest, because here the same logic fails.

Two genetic paths arriving at the same phenotype feels like it should mean the same thing — that straightness is a solution, found twice because it was worth finding. But the genetics says no. EDAR is one of the strongest signals of natural selection in the entire human genome — and it is wildly pleiotropic. The same V370A variant has been tied to thicker hair, more sweat glands, shovel-shaped incisors, and changes to chin and ear shape. Straight hair is one item on a long list of things that variant does (Tan et al. 2013). Selection was almost certainly chasing one of the other items — sweat glands are the leading guess — and hair shape came along for the ride. A hitchhiker.

And the European route shows no signature of selection at all. The TCHH variants don’t carry the genomic fingerprint of having been favored; one analysis put it flatly: hair straightness “is unlikely a trait under selection” (Wu et al. 2016).

So straight hair converged — twice — and in neither case is there good evidence it was selected for. Once as a byproduct of selection on something else, once as something that looks like drift. Convergence proved a developmental path existed. It did not prove the destination was wanted.

There’s a tension I won’t paper over. The thermoregulation logic predicts straight hair should be a cold-climate adaptation — if tight curls maximize heat loss avoidance in the sun, straighter hair should pay off where the sun isn’t the enemy. The Lasisi paper itself flags this as a live possibility. But the genetics doesn’t cooperate: it shows selection on a pleiotropic gene and no selection on the European one, not selection on straightness. The clean adaptationist prediction and the actual genomic evidence point in different directions, and that gap is unresolved. I’d rather leave it open than resolve it by vibe.

What your hair tells you about your genes

Less than the folklore promises. The largest study — nearly 29,000 people — found a dozen-plus genes for hair shape, each worth a few percent, with no master switch. A 14-marker model predicts straight-versus-curly at an accuracy (AUC) of about 0.66 (Liu et al. 2018). It’s a weak ancestry proxy — forensic labs use it precisely because it leans geographic, but it’s a smudge, not a barcode. “African hair” is itself the least uniform category there is, sitting in the continent with the most genetic diversity on the planet, with a wide range of curvature even within East Africa (Lasisi et al. 2016).

The richest thing your hair encodes isn’t what it is — it’s what it’s bundled with. If you carry the East Asian EDAR variant, your straight hair is reporting on your sweat glands and the back of your front teeth, not on some gene for straightness. The trait is a label on a box; the interesting contents are the other things in the box.

The one-line version: tightly curled hair is plausibly an adaptation — a sunshade over the brain that saved water under a hard sun. Straight hair evolved twice, through different genes, and most likely as a passenger both times. We are primed to read “varies between populations, arose more than once” as “selected, therefore useful.” Hair is the counterexample sitting on millions of heads. Convergence is real, and it is not the same thing as purpose.

— Cael