Why Do We Have Different Hair Types?

Stylist: Hey, can I help you?

Tina: Hey, I'm sorry, I'm new here.

I'm looking to get my hair done.

Do take walk ins?

Stylist: I don't know if we have anyone who works with, uh, textured hair.

Tina: Textured hair?

Stylist: You know what— I have so many friends that have curly hair, I could probably, I could probably do your hair.

Just go and sit down and I will be with you in just a second.

Client: Oh my god, I love your hair.

It's so poofy.

Can I touch it?

Tina: You're probably wondering how I ended up in this situation.

We'll talk about that later.

But first, let's talk about why we even have scalp hair in the first place.

And why does it look like this, this, this, and this?

If you think about it, it's pretty weird, right?

Humans stand out from other primates with their naked bodies.

But for some reason, one of the places that we still have hair is the top of our heads.

Well, looking at our species, we typically have a full head of hair as adults.

Sorry to all you bald-headed baddies out there.

It may seem weird that humans went for this non committal mix between being naked and being furry.

But it makes a little bit more sense if you consider it in the broader context of human evolution.

Humans most likely lost their body hair to evolve an impressive thermoregulatory system based on sweat.

This system relies on evaporative cooling, which happens when sweat evaporates directly from our skin.

So you might wonder why not be completely naked then?

Wouldn't it be important to cooler heads too?

Well, it might have something to do with protecting our really sensitive brains.

As animals that walk up right, our heads get the brunt of the solar radiation that comes down on us.

Brains generate a lot of heat and are very sensitive to overheating.

So waiting for our heads to overheat and then activating a sweat response might not be a great strategy.

Instead, a better option would be to use a barrier that reduces the effect of heat gain from solar radiation.

That's why most mammals living in the desert and other places with lots of solar radiation still have fur.

Fur actually blocks a lot of the radiation from reaching the skin but it becomes an insulating trade off.

You block heat from reaching the skin but also limit the heat that can escape.

Enter curly hair.

The tightly curled hair that we see in many human populations has this incredible structure that leaves lots of air pockets between hair strands and maximizes the distance between the top of the hair and the scalp.

This makes it a great solution to minimize heat gain from solar radiation while maximizing the amount of heat that can be lost from the scalp.

In other words, curls keep you cooler in the sun.

Client: Oh my god, your hair is so different.

Tina: Okay?

Obviously not all humans have the same hair.

So why is our hair so different?

The answer is evolution.

If the last common ancestor of all modern humans had tightly curled hair because of some thermo regulatory advantage, it means that natural selection had some effect.

A quick recap on natural selection.

It's the process through which populations of living organisms can adapt and change.

But even when natural selection crafts a super handy trait it may not stick around if the pressure to have that trait is no longer around.

So as humans migrated into all kinds of different climates, there may no longer have been an advantage to having tightly curled hair.

There are a number of plausible scenarios for what happened next: Option one, neutral evolution or genetic drift, the genes affecting hair no longer experience selection.

So in each generation, the genetic variants associated with decreasing hair curl were just still likely to be passed on as those variants that increased hair curl.

Option to selection in the opposite direction.

If tightly curled hair increases heat loss, there may have been cases where it would have been more favorable to retain heat.

So for populations who were in particularly cold environments, we might consider that there was some advantage to having straight hair.

Option three, the selection of traits that are genetically linked to hair.

The thing about our genome is that there isn't a one to one relationship between traits and genes.

Many traits are influenced by multiple genes, and many genes influence multiple traits.

That means that in some cases, we're not really sure what the target of selection was.

To learn more about this, make sure to check out our previous episode on skin pigmentation.

For example, looking at the Ectodysplasin A Receptor Gene (EDAR), we see evidence of selection in East Asian populations.

The variant that was selected makes hair thicker and straighter, which might make you think that natural selection favored that kind of hair, but this variant also has an effect on the shape of your teeth and mammary glands, among other things.

So the reason that thicker straighter hair might be more common in those populations could be because of a seemingly completely unrelated trait.

To sum this all up, because different populations have different evolutionary histories, we see a lot of variation in hair around the world.

Client: Just like a little sheep, I love woolly hair.

Stylist: Excuse her.

It's not woolly.

She actually has a 3C 4A curl pattern.

Client: ABC 123, whatever.

I love afros.

Tina: We talk about hair and subjective categories like straight, wavy, curly, or we try to come up with classification systems like type four, type three, but hair doesn't exist in multiple discrete categories.

What we know is hair texture, hair shape, or hair morphology is actually continuous variation.

Looking at an individual hair strand, we can distinguish two aspects of its shape, cross section, and curvature.

If you take a hair, cut it in half and look at the surface, you can see its cross-sectional shape and size.

People often talk about fine and coarse hair, but what they really describe is variation in the cross sectional size of a hair fiber.

Also, not all hair strands are perfect circles hair can be circular oval, sometimes cross sections can look weirdly triangular, or even kidney-shaped.

But the type of hair variation that we most often refer to is curvature.

The range of variation from straight to tightly curled hair is a consequence of the curvature of individual hair strands.

That's not to say that there's no place for describing hair textures using a typing system.

One of the most widely used hair typing systems is the Andre Walker system.

Black women in the natural hair movement have adopted this framework and developed it even further to give language to the huge range of variation in our hair because science didn't bother to do that.

Until we have widely available tools to easily measure our hair curl objectively, it's reasonable to use the best descriptive language that we have, even if it's a bit subjective.

Client: So what are you looking to do with your hair?

Tina: Well, I'm I'm hosting this new science show and I'm trying to figure out my look, I'm not really sure what I want, but I'm open to suggestions.

Stylist: So do you maybe want something like the Rachel, or Ariana Grande, or maybe even Taylor Swift with bangs?

Tina: Are you sure you know how to work with my hair?

Stylist: Oh yeah, absolutely.

And now that I take a closer look, it doesn't even look that frizzy.

You're so lucky that you have good hair.

[slow-mo] Good Hair Tina: What's a scam that's become so normalized, we don't even realize it's a scam anymore?

Good hair versus bad hair.

The idea of good versus bad hair is so insidious that it's normalized the expectation to take drastic measures to alter your hair texture, if it doesn't fit biased standards of beauty, even if it causes severe consequences to your health.

Discrimination against certain hair textures affects everything from mental health to whether people are perceived as professional in the workplace.

We've even had to put policies in place to protect people's rights to wear their hair, the way it grows out of their scalp.

But did you know that this runs so deep that it bleeds through into the scientific literature.

One example is this "real" medical condition called "woolly hair syndrome," where the "disease" is defined as strong coiled hair in "non-Black people."

There is a very long history of people trying to present their biased opinions as scientific fact.

Especially in terms of presenting some people as inherently better than others.

If we ask people around the world what they consider to be beautiful hair, we'd see that there are many different opinions hair might have evolved for a very functional reason like protecting our heads from solar radiation, but we now use it as a medium for expressing our identities and creativity.

Sometimes, that means finding someone who knows how you want to express your identity.

Braider: so what do you think?

Tina: I love it.

Braider: Now tell me what you learned.

Tina: Always read the reviews online Braider: Please do.

Tina: Sometimes cross sections can lurk.

*chuckles* Can lurk.

*Snort* Everything is going so well.