為什麼我們不像其他哺乳動物那樣渾身都是毛茸茸的？再進一步，人為什麼只在頭上長出了長長的頭髮？人和人之間的頭髮為什麼不太一樣？
來自美國密歇根大學（University of Michigan）的生物人類學家蒂娜·拉西西（Tina Lasisi）以及她近年來從事的工作或許可以幫助我們解開這些謎題。她的團隊正致力於研究“人類表型變異的進化與遺傳基礎，重點關注色素沉著和毛髮”。換句話說，她希望從演化、遺傳學、人類學乃至熱力學的角度，探索人類的皮膚和毛髮多樣性的成因。
為何人們的頭髮和皮膚會存在如此多的變異？蒂娜解釋稱，原因既在於自然選擇，也在於自然選擇的缺失。大約兩百萬年到一百萬年前，人屬（Homo）出現了，人類開始完全直立行走，某個時刻起我們的毛囊數量開始減少，並失去了長長的體毛，取而代之的是細小的絨毛。然而，原有的毛髮是一層重要的屏障，既能保暖，也能讓我們免受紫外線輻射的傷害。為了彌補缺失的屏障，我們的祖先就在選擇壓力下演化出更深的膚色，用皮膚中的黑色素保護自己。
至於頭髮的故事，講述起來要複雜得多，蒂娜介紹道，頭髮與皮膚都無法形成化石，這讓我們對其知之甚少，只能透過構建假設，基於過去的證據進行大量推斷，才能得出一些結論。
比如，在2023年發表於美國科學院院刊（PNAS）的一項研究中，蒂娜提出了一個關鍵的假設：緊密捲曲的頭髮能夠減少我們從太陽輻射中獲得的熱量，並透過實驗證實了這一點。蒂娜和同事使用熱量人體模型和不同形態的人發假髮，在模擬不同風速和不同太陽輻射的條件下，測量頭皮的對流、輻射和蒸發熱通量資料。結果表明，頭髮能顯著減少到達頭皮的太陽輻射量，以及頭皮為了平衡太陽熱量須要蒸發的汗液量，而且頭髮的捲曲程度越高，對太陽輻射的防護效果就越好。
從演化的角度看，蒂娜在論文中給出了一個可能的解釋，古人類生活在乾旱、炎熱且日照強烈的地區，在這種情況下，演化會傾向於適應節約用水。這種捲曲的頭髮可以隔熱、減少水分流失，同時延長人們飲水後從事劇烈體力活動的時間。而且，體溫調節的能力會限制大腦的演化，因此，緊密捲曲的頭髮可能會減少古人類的被動熱負荷，進而緩解大腦尺寸增大和體溫調節壓力增加帶來的體溫調節限制。
鑑於古人類已經出現了直立的姿勢，頭髮的出現或保留可能在取得了一個最佳平衡，既能讓身體大面積的熱量散失最大化，又可以最小化頭皮小面積接收的太陽熱量。與直髮相比，緊密捲曲的頭髮可以減少熱量流入。因此，緊密捲曲的頭髮的演化可能代表了古人類在腦容量日益增大時，面臨的新的體溫調節挑戰的綜合演化反應的一部分。
對於普通人來說，當科學解答了一個與每個人息息相關、但你可能從未意識到的問題時，一定會感到十分興奮。但對於蒂娜這些研究人員而言，做到這一步還不夠。他們須要找到一些精確的方法將這些人與人的差異進行描述並分類。
比如測量膚色，以往的方式須要透過侵入式的皮膚穿刺以及各種化學分析，才能精確測量黑色素的含量和型別。
在大規模人群研究中，這種方法顯然不適用，語言描述也具有很大的侷限性。隨著反射分光光度計的興起，研究人員終於擁有了一種能夠真正輕鬆且無創地測量皮膚顏色的工具。於是在 2017 年和 2018 年，眾多與之相關的研究論文紛紛發表，科學家們發現，膚色中存在著之前未曾意識到的巨大多樣性。
對於頭髮的測量，蒂娜也面臨著類似的困境。有時我們使用的詞彙以及我們自認為觀察到的變化，與實際測量的結果並不一致，因此，在研究中找到合適的測量工具很重要。比如，雖然有直髮、波浪卷、捲髮等各種語言上的描述，但這些並不能涵蓋所有的頭髮形態種類，沒有單一的標準可以用來定義頭髮的形狀。
加之頭髮的很多形態都值得被測量，比如，如果聚焦到單根髮絲的層面，過去的一百多年來，研究人員通常會將頭髮切成兩段，觀察它的橫截面，測量頭髮的粗細和橫截面的形狀。但頭髮的捲曲度很難測量，因為這種彎曲涉及三維空間的定位。
Where Did Curly Hair Come From?
Rachel Feltman: Have you ever really thought about the hair that grows out of your head? I mean, I’m sure you’ve thought about your hair—in terms of which way to get it cut and how to get that one really wonky piece to behave itself—but have you ever considered why it is the way it is?
For Scientific American’s Science Quickly, I’m Rachel Feltman. My guest today is biological anthropologist Tina Lasisi, an assistant professor at the University of Michigan. She leads a lab that studies the “evolution and genetic basis of human phenotypic variation, with a focus on pigmentation and hair.” In other words she’s figuring out why human skin and hair comes in so many gorgeous varieties.
Thanks so much for joining us to chat today.
Tina Lasisi: Great to be here.
Feltman: So I’ve been a fan of your research for a few years now ’cause, among other things, you’re really asking and answering questions about hair that I don’t think anyone else is tackling. How did you get interested in your field of study, and would you tell our listeners a little bit about it?
Lasisi: Absolutely. So I got interested in this when I was an undergrad. I did my undergrad at the University of Cambridge, where I was studying archaeology and anthropology, which there consists of studying archaeology, biological anthropology and social anthropology.
And I was always someone who really liked culture and traveling, so I thought I was gonna be a cultural anthropologist, but I got this lecture in the evolution of human skin color that really had me intrigued, and it was a lecture where they showed, you know, those really famous map pairings where you see the distribution of skin color around the world and the distribution of UV radiation, and it was just like this [makes explosion noise], you know, brain-exploding moment of like, “Wow, like I never thought about that,” and learning more about evolution and how there’s all these processes that can shape the way that humans are—the way that a lot of different species are, right—that really got me intrigued, and I felt like, “Okay, now I understand why my skin is the color that it is,” but my immediate next question was: “Well, why is my hair curly?”
Feltman: Hmm.
Lasisi: And there wasn’t a great answer at the time, and I was lucky enough to be in a really supportive environment, and I had a mentor who said, “You know, why don’t you just go into the science side of anthropology and study this?” And so, what year are we in—2025? Okay, 14 years later, here I am [laughs] still working on that.
Feltman: Yeah, well, and, you know, it sounds like the nature of your work is pretty interdisciplinary. You know, how would you summarize everything you’re looking at to someone who’s not familiar with your work?
Lasisi: That’s such a great question. I’m actually teaching an introduction to anthropology class right now, and I’m trying to explain to the students, like, “Anything can be anthropology, and everything can be anthropology.” You can use so many different methods. So right now, I would say I am definitely an evolutionary biologist. I work on human biology. I also work on—thermoregulation is work that I’ve worked on. I’ve worked with thermal engineers. I also have worked on genetics; that’s a big part of what I do. I’m also in a Department of Ecology and Evolutionary Biology. So all of those little bits and pieces, they give a different insight into the question that you can ask, and so everything that I do involves sitting [laughs] behind a computer, mostly, but also collecting samples from people and measuring things with various instruments and a lot of computer imaging, basically [basically].
Feltman: Very cool. And so, broadly speaking, why is it that people have so much variation in their hair and skin?
Lasisi: Mm-hmm. So the reason is simultaneously because of natural selection and because of the absence of natural selection. So the story that we’ve been able to piece together for skin color is that very long ago—somewhere between, you know, two to one million years ago—as the genus Homo was emerging, we were completely bipedal and at some point would have started losing our body hair, so really reducing those hair follicles so that we have like, this, tiny peach fuzz all over our body. And by doing that we have lost a really important barrier, right? So a lot of people can associate hair with keeping you warm, but it can also protect you from UV radiation. And so those ancestors probably would have been under selective pressure to evolve darker skin because by having more melanin in your skin, that’s another way that you can protect yourself from that UV radiation.
The story afterwards is one of adaptation to different environments. So it turns out that having all of that wonderful melanin to protect you is great when there’s a lot of solar radiation, but if you’re in an environment with not a lot of radiation, you end up running into issues with being able to produce enough vitamin D…
Feltman: Hmm.
Lasisi: Which is something you can only do in your body with the power of solar radiation that helps you convert it into an active form. Now, there are, of course, exceptions to that because there are places in the world where people have diets that are rich in naturally occurring vitamin D, like in the Arctic.
And since all those times we’ve moved to so many different places, and you have all of this variation that’s evolved because of that. And in the last, let’s call it 200 years what’s really nice is that we have developed all of these cultural ways of adapting to different places. So instead of being someone who maybe doesn’t have a lot of melanin and going to a place that is very, very sunny and being like, “Well, geez, I’m gonna have to wait a couple of generations for evolution to fix it for my ancestors,” we now have sunscreen and all of these other things that we can do. We have vitamin D supplementation.
Now the story with hair, it’s much more complicated to tell because we really don’t know. The thing about hair and skin is that in both cases, they don’t fossilize, and so we’re having to infer a lot from the past. And we do that by putting together hypotheses and saying, “Well, if this is the reason that natural selection would have selected for this kind of hair or that kind of skin, what’s the distribution that we expect to see?” And with hair we don’t have a lot of thoroughly tested hypotheses, but some of the work that I did in my Ph.D. that got published a few years ago was asking the question: “Well, does tightly curled hair reduce how much heat we might gain from solar radiation?” And I found in my experiments that, yes, it really does have this role. And so now the question is: “Can we also use genetics to ask, ‘Well, how did this happen? What’s the history of this? And what’s the story for every group of people around the world?’”
Feltman: Yeah, that’s so cool. I loved that study. It’s not apparent ’cause it’s pulled back and bleached within an inch of its life, but I have very curly hair [laughs]. And I was like, “I’ve always wondered why when I get a blowout, I feel [laughs], I feel like my head is gonna sweat right off.” Meanwhile, when people are like,”‘I don’t know how you live through the summer with that long hair,” and I’m like, “I don’t know what you’re talking about [laughs]. It’s fine.” So I love when the science answers questions I didn’t even know I had.
So a lot of the ways that we’ve historically categorized different variations in hair and skin are, of course, really lacking and sometimes quite racist. What factors are actually at play that lead to differences in the makeup of our skin and hair, and how has your work changed the way you think about how we might describe or categorize those variations?
Lasisi: Mm-hmm, that’s really an interesting question. So there’s a number of factors that we can tease apart there, right? We can ask the question of: “What are the mechanisms and the biological processes that contribute to this variation?” When it comes to skin color, we’ve known for a long time that it’s melanin, but measuring how much melanin is in someone’s skin is actually [laughs] really invasive. It’s really invasive—like you’d have to have a skin punch, you’d have to do various chemical analyses to measure exactly how much melanin and what kind of melanin is in there. So that’s really difficult, and people need a shorthand, especially if you’re doing population-wide studies. So people have tried to come up with really good descriptions, but descriptions can only go so far, and measuring something is so much better.
So with the rise of reflectance spectrophotometers, we finally had a tool that could really easily and noninvasively measure the color of skin. So this can be done at various levels of detail.
You can have one that is specifically trying to estimate the visible range of melanin, and it can give you something called melanin index, which is something that’s been developed to say, “Okay, well, how much melanin is in someone’s skin?” And so that really helped us collect a lot of accurate data, and in 2017, 2018 there were a lot of papers that came out saying, “Oh, wow, look at all of this variation in skin color that we didn’t realize existed in Africa.”
And so that’s where you have this really interesting insight of, “Oh, sometimes the words that we use and the variation that we think we’re seeing doesn’t align with what it is that we’re measuring,” which is why it’s so important to have tools that measure things. With hair we suffer from a similar problem, where, okay, well, we have all these descriptions of straight, wavy, curly, but is that really what the range of the variation is?
However, there isn’t a single thing that you can measure to define hair shape. There’s a lot of things that you can measure—if you are narrowing down to the level of a single hair fiber, in a single hair fiber you can get a cross section. You can slice that in half, look at that cross section and say, “Well, how thick is that hair fiber? What shape is it?” And that’s something that we’ve been doing for over 100 years, and we’ve noticed that there’s a variation there. But when it comes to the curl it’s really difficult because hair curves in three dimensions.
So that is the thing that I actually worked on the longest—it took me 10 years to develop a method that I’m, you know, remotely happy with. And it involves…[full transcript]
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