SciCafe / Lectures Videos

[Fading up from black, blurred video footage of a presenter in front of a large screen appears. The American Museum of Natural History logo appears, then fades as the video sharpens to show TINA LASISI speaking to an audience.]

TINA LASISI (Post-Doctoral Researcher, University of Southern California): Have you ever wondered what a caveman mullet looks like? Or are you a normal, well-adjusted human being? Clearly not, because you came to this talk. And I appreciate that. But even if you don't go down these weird rabbit holes,

[Screen fades to a solid turquoise. Two still images of people looking in the mirror at their hair appear.] 

LASISI: I’m sure that we've all looked into the mirror at our hair and thought,

[Next to the two images, text appears: “What is this?”]

LASISI: “What is this?” But tonight, I want to encourage you to–

[The text and images fade away and are replaced with two different images of people looking in the mirror at their hair.]

LASISI: –unleash your inner evolutionary detective and ask,–

[Text appears: “Seriously, what is this?” Images and text fade and we see LASISI again.]

LASISI: “Seriously, what is this?” We're going to go on a epic adventure, that's going to start with asking why it was so important for our ancestors to keep cool. And it's going to end with, of course, robots. Now, you may be wondering, who is this dapper young scientist in front of you? Well, I am currently a post-doctoral researcher at the University of Southern California, where I work on – thank you! – projects on population genetics. And I do things from understanding the likelihood of finding a distant relative in a genetic database all the way to looking at genetics to understand complex traits like skin and, of course, hair. 

[Screen fades to turquoise and we see a picture of LASISI as a young college graduate in a graduation gown. Text appears: “Studying Biological Anthropology at Cambridge University.”] 

LASISI: But how this all really began is when I was an undergrad at Cambridge University in England. That is where I started studying biological anthropology. 

[Turquoise fades and LASISI returns to the screen.]

LASISI: And that's really the study of what it means to be human. Now, what I really love about doing research in biological anthropology is that I get to think not only about how different traits work. I get to ask why they evolved. 

[Screen fades to turquoise. A text quote appears: “Nothing in biology makes sense, except in the light of evolution,” attributed to Theodosius Dobzhansky.] 

LASISI: You see, nothing in biology makes sense except in the light of evolution. Put more simply, what was the reason for the evolution of various traits, including hair.

[Turquoise fades and LASISI returns to the screen.] 

LASISI: And that is our quest tonight. That is our million dollar question. Why do humans have hair on their heads?

[Screen fades to turquoise and text appears: “Enter: Furries.” Audience [LAUGHS]. Images of animals appear below text.]

LASISI: Our journey begins long ago, 

[Text appears: “Hair in mammals: 164 million years ago”]

LASISI: –at least 164 million years ago. That is when we find some of the oldest impressions of hair on skin that we can attribute to a mammal.

[Turquoise fades and LASISI returns to the screen.]

LASISI: Now, what's interesting about that is just like feathers pre-date birds, hair likely predates mammals. Now, we don't really know what the function was of those early hairs in those not quite mammals, but we know quite a bit about the functions of hair in today's mammals.

[Screen fades to turquoise and an image of two brown bears fighting and biting each other appears, with text next to it: “Physical Protection.”] 

LASISI: We know that it can help with physical protection. It can stop from abrasion to the skin. 

[Image and text fades and are replaced by an image of a white arctic fox on a snowy backdrop, with text: “Camouflage”]

LASISI: We know that it can do a thing or two for camouflage if you picked the right fur coat, 

[Image and text fades and are replaced by an image of grey monkeys with an orange baby monkey, with text: “Communication”]

LASISI: –and it can also communicate things. What you see right here are some really cute langurs. And they're this neutral, dark gray color when they grow up, but they start out bright orange. So that can tell you “I'm a baby.”

[Image and text fades and are replaced by an image a mountain goat on a snowy cliff, with text: “Thermoregulation”]

LASISI: And of course, thermoregulation. We know that a fur coat, whether store bought or home grown, can keep you warm.

[Turquoise fades and LASISI returns to the screen.] 

LASISI: And while all of these functions are interesting, we're going to focus on that last one today. So let's fast-forward–

[Screen fades to turquoise and an illustration of a hominid walking in a lush green ancient environment next to a modern human in clothing walking in a dry environment. Text reads: “About 2 million years ago”]

LASISI: –to about 2 million years ago. Imagine you are a Homo erectus gallivanting across East Africa–

[Turquoise fades and LASISI returns to the screen.]

LASISI: –with your newly evolved long limbs, and you are incredibly efficient at walking. Maybe you can even break into a little jog. You can cover a good distance. But the thing is, just because you can do something doesn't mean that it's easy. And the thing about being a biped and one that is active is that you generate heat. And when you combine that with a hot, arid environment, especially one that has a lot of solar radiation, things can get a little too toasty. And that is why we believe that around this time, our human lineage likely lost its body hair and grew a lot more high density of eccrine sweat glands. Now, if we want to get really technical, we didn't lose our body hair. We just miniaturized those hair follicles. But it is important to have naked skin so that you can sweat and evaporate that sweat efficiently. But I thought this was a talk about hair. So why am I talking about the benefits of losing it? Well, here's the plot twist. Hair can actually keep you cool. 

[Screen fades to turquoise. A diagram appears showing three sun’s rays hitting the skin and the layer of hair on top of the skin. One ray is bouncing back to the sky and reads “reflected.” One shows the hair getting warm and reads “absorbed.” The third ray warms the skin it touches.] 

LASISI: So you see, if radiation comes down on the skin and there is a nice layer of hair there, some of that radiation will get reflected before it reaches the skin. And some of that radiation will even get absorbed in the hairs instead of the skin. So that's a pretty good deal. 

[Turquoise fades and LASISI returns to the screen.]

LASISI: And that is why there are some studies that show that there are mammals out there, especially some types of squirrels and kangaroos that have very thick coats, even though they live in very sunny places. And that's because the bigger the coat is, the more the distance is between the top of the hair and the top of the skin, which can be pretty helpful. So from what we've learned so far, our ancestors lost their body hair so they could keep cool. But our ancestors also kept some of their hair so they could stay cool. Why would we have two solutions for seemingly one problem? That has to do with us being bipedal.

[Screen fades to turquoise. An illustration of a sweating and panting person standing in the sunlight appears. The sun’s rays are hitting their back and head.]

LASISI: You see an animal that stays upright gets the brunt of the solar radiation on its head. And when you decide to add a large heat-sensitive brain to the mix,

[Turquoise fades and LASISI returns to the screen.] 

LASISI: –you can see how our ancestors might have faced a unique set of challenges. And that unique set of challenges might explain why we are the only weirdoes who couldn't commit to being fully naked or fully hairy, but decided to go with the combo meal. Here's another special thing about us humans. We are very diverse, especially when it comes to the hair.

[Screen fades to turquoise. Several images of real humans with straight, curly, wavy, and tightly coiled hair appear.]

LASISI: There are a lot of different hair textures. We have everything from stick straight hair to tightly coiled hair. We've got it all. But let's talk about curly hair for a second.

[Turquoise fades and LASISI returns to the screen.]

LASISI: This is a uniquely human trait. I love to point out to people, “Have you ever noticed that most mammals have straight hair?” I often ask, “Do you know any mammals that have curly hair?” And people will say, “What about poodles?” I myself have a poodle, but they're not really curly. And the curls are nowhere near what we see in humans. Also, they're domesticated. And if you know dog breeds, you know, we've done a lot of funny things to them. So let's restricted to somewhat more naturally occurring animals. A lot of people will say, “What about sheep?”

[Screen fades to turquoise and a picture of a sheep grazing.]

LASISI: Surprise! Sheep do not have curls. We actually know a lot about sheep's wool and sheep's wool is crimped. 

[The sheep picture moves aside and wavy lines appear, stacked on top of one another. Beneath the lines, text appears: “Crimped, not curled.”] 

LASISI: It has a bilateral wave. So a wave that goes up and down.

[Turquoise fades and LASISI returns to the screen.]

LASISI: Now, you might think that I'm nit-picking here because you’ve got to nit-pick to do a Ph.D., right? But no, it's actually really structurally important because, you see, when you have this nice, neat bilateral wave, you can stack a lot of fibers next to each other. When you stack fibers next to each other in a super neat way, it means there's not a lot of airspace between them. On the other hand, if you've seen human curls, they can be pretty chaotic. Now that chaos is good, because that means there's a lot of airspace and basically you can maximize the distance between the scalp and the top of the hair without packing on a dense amount of fur. But whenever we see anything in nature that is unique in a species, we really have to pause and ask why. What is so special about this species that evolution had to come up with a brand-new solution when nothing else worked out of the box? Well, knowing that our ancestors were these bipeds who had these large brains, we're going to run with that. A unique thermal regulatory challenge. A unique challenge of keeping a precise body temperature relating to having this big brain and maybe not wanting to activate this sweat response. We'll get back to sweat later on.

[Screen fades to turquoise. Text appears above three boxes: “Putting it to the test.”]

LASISI: We have enough burning questions now that we can put something to the test.

[Text appears in the first box: “Hypothesis: Human scalp hair evolved as a thermoregulatory adaptation.”]

LASISI: Our hypothesis is that humans evolved scalp hair as a thermal regulatory adaptation. So as a solution to regulate our body temperature and specifically to stop from overheating. 

[Text appears in the second box: “Main Question: Does scalp hair protect us from heat gain?”]

LASISI: Our main question is, does scalp hair protect us from solar heat gain?

[Text appears in the third box: “Sub-Question: Does hair texture affect the ability to minimize solar heat gain?”]

LASISI: And one of our sub-questions is do different hair textures affect our ability to minimize solar heat gain? 

[Turquoise fades and LASISI returns to the screen.]

LASISI: So how do we go about testing that? Wigs on robots, obviously. 

[Screen fades to turquoise. A picture appears of a bright red humanoid-looking machine with a very curly wig on it, sitting in a wheel chair and hooked up via wires to computers. There are large propeller fans behind the humanoid machine.] 

LASISI: Before anyone gets me, yes, technically, this is not a robot. It's a thermal manikin. What is a thermal manikin?

[Turquoise fades and LASISI returns to the screen.]

LASISI: It is a nightmarish humanoid measuring device where they made a clear decision to have the plugs go into the eye sockets.

[LAUGHTER]

[Screen fades to turquoise. The same picture of the manikin appears on screen, slightly zoomed in to show that indeed, all the wires are plugged into where the eyes would be on a human body.] 

LASISI: Like, do you see what I'm saying? Like, that is a choice. That is a design choice that was made.

[Turquoise fades and LASISI returns to the screen.]

LASISI: But these are great tools for measuring the performance of protective gear or clothing and usually you do it in a climate-controlled chamber. Now, the thing about these thermal manikins is they don't come cheap. They cost a pretty penny. Hundreds of thousands of pretty pennies. But that's why it's important to have rich friends as a scientist! And my lovely collaborators at Loughborough University in the UK happened to have one of these bad boys lying around, and they let me put wigs on it, which is really nice. But let's talk a little bit more about how manikins work. How do they work?

[Screen fades to turquoise. A diagram appears, with a blue side marked “Interior” to the left of a thick black line, and an orange side marked “Exterior” on the right of the same line. The line is labeled “Surface.” In the center a text bubble says “34°C.”]

LASISI: Well, it starts with the surface temperature. You set the manikin’s surface temperature to a certain setting. And if an item of clothing is not very insulating, 

[In the exterior section of the diagram, a snowflake icon appears. In the interior section of the diagram, an arrow pointing up with the text bubble “Energy (W/m²)” appears.] 

LASISI: –and the outside temperature is colder, it will take more electricity to keep that set surface temperature. 

[In the exterior section of the diagram, a flame icon appears. In the interior section of the diagram, an arrow pointing down with the text bubble “Energy (W/m²)” appears.] 

LASISI: However, if the outside temperature is warmer or you have a very insulating piece of clothing, then it will require less energy to keep that set surface temperature. So that is how we take all of those measurements and learn about the thermal properties of different fabrics.

[Turquoise fades and LASISI returns to the screen.]

LASISI: But instead, I decided to put wigs on it. Why not? It should work the same, right? So I decided what I was going to test. What we have to do was to find human hair wigs. And so I got a bunch of human hair wigs from the Internet, of Chinese origin.

[Screen fades to turquoise and 3 white boxes appear.] 

LASISI: All of them were eight inches long.

[In the first box a photo of a straight black wig and an icon of a human with straight hair appear.]

LASISI: One of them was straight.

[In the second box a photo of a curly black wig and an icon of a human with wavy hair appear.]

LASISI: One of them was artificially curled with moderately large curls. 

[In the third box a photo of a very curled and frizzy wig and an icon of a human with very curly hair appear.]

LASISI: And the other one was artificially curled with very tight curls.

[Turquoise fades and LASISI returns to the screen.]

LASISI: Now, the reason I did this is we wanted to control as many variables as possible. And as I said before, humans are wonderful in the range of variation that they have in their hair. And hair varies in a lot of different aspects. You don't want to be dealing with all of those aspects in one experiment. So you want to hold as many variables constant as possible. And so by limiting and constraining our hairs from a particular source and just making sure that the only difference was not length related, was just the curl size, we were relatively confident that the results we're getting are relating to our variable of curvature–

[Screen fades to turquoise and a white box appears. Text appears: “Variables.” The three icons for straight, curly, and very curly hair appear along with an icon of a head that has no hair. Text appears below: “Wigs”]

LASISI: –so low, mid, and high curvature. But we also ran all of the experiments in the so-called nude condition. 

[The hair icons and “Wigs” text fades. They are replaced with icons of a lightbulb, and a lightbulb crossed out. Text reads: “Variables: Solar Radiation”]

LASISI: We looked at radiation, which we simulated with floodlights. That is what we're really interested in, right? Not just ambient temperature, because the way that heat transfer works in radiative heat is quite particular.

[The lightbulb icons and “Solar Radiation” text fades. They are replaced with icons of a person standing still, walking, and running. Below the icons, respectively, text reads “0.3 m/s”, “1 m/s”, and “2.5 m/s”. Text also reads: “Variables: Windspeed”.]

LASISI: We looked at a wind speed to simulate the effect of kind of standing still outside of walking and of running.

[The movement icons and text fades. They are replaced with icons of a water droplet and a water droplet crossed out. Text reads: “Variables: Evaporation.”]

LASISI: And because we know that evaporative cooling and sweating is so important to humans, we also wanted to make sure that we looked at evaporation. So we ran all of the tests with the manikin dry and with the manikin scalp soaked wet so that we could understand about the evaporative cooling effect.

[Turquoise fades and LASISI returns to the screen.] 

LASISI: And we simulated everything for 30 degrees Celsius, also known as 86 degrees Fahrenheit, because this is America. And now we're going to look at figures because science. Don't worry, guys. I got you.

[Screen fades to turquoise. Text appears: “Without Solar Radiation.” Below the text are two graphs for “Dry” and “wet”, with wind speed on the X-axis and heat on the Y-axis. To the right of the graph are the icons for nude, straight, curly, and very curly hair in different colors.]

LASISI: So the legend over here with the different colors shows you the different head coverings that we use. So orange on the top there is the nude. The green is low curve or straight, the blue is mid curve or moderately curly. And then the purple at the bottom is high curve or tightly curled. So our X-axis here is the wind speed and the Y-axis is heat flux. Anything over zero means we have heat loss. That's what we're looking for, right? Anything under zero means we have heat gain. So let's ask, without solar radiation, what did we find in our experiments? 

[The graphs fill in with lines representing the different hair curl textures. On the “Dry” side, all the hairs are close to zero (aka thermoregulatory balance) and the nude condition is slightly higher, with more heat loss.]

LASISI: Well, we found that there wasn't a lot of difference between the different hair textures. We could see that clearly. You had more heat loss with the nude scalp. Okay, that's good. What about evaporative cooling without any solar radiation?

[On the “Wet” side, there is a similar patter, but the nude condition has even more heat loss than the different hair curl textures.]

LASISI: Well, we just had that pattern exaggerated. We can clearly see that you can evaporate a lot more water off of your scalp if there's no barrier on it. And that increase that you see is what we would expect. The more wind speed you have, the more that contributes to heat loss and evaporation. So what we're really interested in is solar radiation, so let's bring on those results.

[The graph fades and a new set of “Wet” and “Dry” graphs appear, with text above that reads “With Solar Radiation.” The “Dry” graph has different hair textures close to thermoregulatory balance, with the low curve slightly overheating. The nude condition is overheating a lot.]

LASISI: Completely different! Now, you might panic for a second and think we're in the red zone. That's the danger zone. It is. As you can see, a nude scalp is well on its way to overheating, regardless of what wind speed we're talking about. And in fact, the only condition in which we're constantly at thermal balance, meaning we're not losing or gaining heat, is the high curve or tightly curled wig. And in fact, the tightly curled wig at the highest wind speeds is even losing heat. That's fantastic. It's exactly what we wanted to see, even though we don't want to see things because we're scientists and we're objective and all that good stuff. Anyway, let's see how evaporative heat loss works with solar radiation.

[The “Wet” graph looks similar to the “Wet” graph without solar radiation, where all hair textures have heat loss and the nude condition has the most heat loss.]

LASISI: Oh, okay. Well, it's back to what we had before. We see that it's reversed again with solar radiation. And again, we're seeing that having no barrier on the scalp means that you will evaporate the most. Now, does this mean that our hypothesis needs to go in the trash? Not necessarily. Let's look at this another way. 

[Turquoise and graphs fade and LASISI reappears on screen.] 

LASISI: You may recall that I explained the way that we did evaporative heat loss. It was by running these experiments with a dry thermal manikin scalp and then soaking the scalp. And as I say this, I realize I probably should have explained how we did that. So a thermal manikin doesn't have skin, but you put a sort of cotton onesie all the way on it and we soaked the scalp of that cotton onesie. That's obviously not how human sweat works, but we can still do some conversions, so we can answer some more humanlike questions,

[Screen fades to turquoise. Another graph appears with text above it: “How much sweat?”]

LASISI: –so we can ask what was the maximum evaporative potential? What is the maximum of sweat that we could evaporate in any of these conditions per hour?

[The nude condition’s line is at the top of the graph, while the straight, curly, and very curly hair lines appear in that order further down the graph, showing that the nude condition has the ability to evaporate the most sweat at any windspeed.]

LASISI: Obviously, having no barrier on the scalp allows you to evaporate the maximum amount. But what's even more interesting is once we did our calculations and conversions, we saw the differences in the different hair texture. With straight hair, you can evaporate more sweat than you can with any of the different levels of curl. But sweat is not free. 

[Turquoise and graph fade and LASISI reappears on screen.]

LASISI: In nature, you do not just soak your scalp and hope for the best. You have to actually work up that sweat and sweat it out. And for a hunting hominin, we're not just talking about water loss. We're talking about electrolyte loss, which without Gatorade stations can be a very dangerous thing. So what we really wanted to understand– 

[Screen fades to turquoise. Another graph appears with text above it: “How much sweat?”]

LASISI: –was when we put it all together for an ambient temperature of 30 degrees Celsius on a day with a lot of solar radiation, what is the minimum amount of sweat that is required to bring the body back to thermal balance so that it's no longer gaining heat?

[The nude condition appears to need a medium amount of sweat to bring the body to thermal balance. Straight hair requires less, followed by curly hair and very curly hair at the bottom with the least amount of sweat needed.]

LASISI: And what we see is that with tightly curled hair all the way at the bottom, you don't even need to sweat. You don't need to sweat at all. And while the nude condition allows you to evaporate the most amount of sweat, it also requires the most amount of sweat.

[Turquoise and graph fade and LASISI reappears on screen.]

LASISI: So to sum it all up. Our results show that hair minimizes heat gain from solar radiation.

[Screen fades to turquoise. Two columns show the no hair icon next to the straight, curly, and very curly icons. To the right of these columns we see a diagram of the sun’s rays being absorbed and reflected by straight and curly hair.]

LASISI: We looked at no hair versus hair and found that in a lot of cases, having a barrier such as hair stops you from evaporating as much sweat as you could, and it stops you from losing as much heat as you could. However, in the context of radiation, you kind of want that barrier because you want to stop that radiation from generating heat on your scalp in the first place.

[The diagram showing the sun’s rays disappears and is replaced by a blue arrow pointing up, with text next to it that reads “Curvature”, and an orange arrow pointing down that has text next to it that reads “Solar Heat Gain” and “Need for Sweating.”]

LASISI: What was also very exciting and never shown before is this correlation between increased hair, curvature or higher curl and decreased solar heat gain as well as a decreased need for sweating.

[Turquoise and graph fade and LASISI reappears on screen.]

LASISI: So what's next? Even though I enjoyed working with the Terminator-looking a nightmare, I'm not much of an experimentalist. What I really want to do is some simulations because wouldn't it be cool to take all of this data that we've generated on thermal properties of different hair textures and plug it into an algorithm, as the kids do these days? We would be able to take all kinds of data, including paleo-climatic data, and try and answer questions about what hair texture would have been optimal for our evolutionary ancestors. That's one plus, but another plus is with the right help, I could probably make an app so that you could plug in your variables and figure out what your next haircut should be. Then there's genetics. Now, I know I really opened this up by saying, “Hey, we found this impression of hair on skin from 164 million years ago.” I want you to understand that that is rare. They got really lucky. Usually soft tissue like hair and skin does not preserve. And so the best way to get an evolutionary understanding of how things changed can be genetics. We can look at different modern human populations. And if we understand what genes are involved in hair morphology, we can look at the evolutionary history of those genes and ask, “Did natural selection shape any of them?” Can we say that natural selection did anything for modern humans and even Neanderthals? Please ask me about Neanderthals. I love to talk Neanderthals, but there are of course many more hair questions like, “Do I have the cure for baldness?” I don't. Otherwise I would not be here before you today. I'd probably be on Mars. I will be answering all of those questions in my own lab starting this fall so you can follow along with my research there. Thank you.

[APPLAUSE]

[Credits roll.]