Skin. According to professor and head of anthropology at Pennsylvania State University Nina Jablonski, it is one of our most underappreciated organs, and one to be celebrated. It is at once a connector and protector, allowing us to feel and experience our world while protecting us from many of its harsh elements. It plays an active role in our culture, serving as a biological billboard of make-up, tattoos, paints and piercings.
As the interface between our bodies and our environment, skin has also played an extremely important role in our evolution. As the earliest humans began to disperse out of equatorial Africa into higher latitudes, skin’s pigmentation adapted to our new environments, leading to the mosaic of skin tones we see today.
At her upcoming Field Museum lecture on Saturday October 1st, Jablonski will discuss the many roles our skin plays in modern life, and how its evolution has affected who we’ve become. We asked her for a preview.
You’ve said that skin is the evidence of evolution on our bodies. What do you mean by this?
Skin, like the rest of our bodies and our behaviors, is the product of evolution. Most of these things we take for granted. For instance, when we hear about new fossil discoveries, we think about the evolution of the skeleton, so we think about bones as being the product of evolution. But often, in fact most of the time, we don’t think about things like skin and hair, which are usually not fossilized, as being products of evolution, even though they are. [They] are just as important to our understanding of human evolution as bones.
Skin is the most important interface between [our bodies] and the environment. It bears the brunt of dealing with many environmental stresses—everything from sunshine and wetness to the chemical environment to abrasions and insect bites [to] microbes. It has been scrutinized by evolution to as great if not greater an extent than other organs because it serves this unique function of protection, and yet it must be sensitive—it can’t just be [a] sort of armor plate of protection; it has to be somewhat porous, so that certain things can get in and certain things can get out. So it’s this very interesting, semi-permeable, resilient interface, and it has undergone tremendous scrutiny by natural selection in evolution.
The earliest humans, who lived near the equator, all shared very dark skin pigmentation. What were the evolutionary benefits of this?
Dark skin pigmentation is produced by the pigment melanin—more specifically, eumelanin, which is a dark brown polymer pigment that is tremendously good at absorbing ultraviolet (UV) radiation as well as scattering some UV radiation and chemically neutralizing the harmful effects of UV radiation on cells. Things that are produced in cells as a result of bombardment from UV and other highly energetic radiation can be very damaging to the body. Melanin is good at being right at the forefront and absorbing and neutralizing those harmful rays and harmful effects.
One of the specific things that melanin does is it helps to prevent damage to DNA, which is very important, and it helps to prevent loss or damage to another biomolecule called folate. Folate is a B vitamin that is essential for making DNA.
Repeatedly in nature, high amounts of melanin have evolved to be protective under high regimes of UV radiation. Many organisms that live under tropical sunlight, for instance, have dark skin, or they have dark exposed areas on their body that are filled with melanin. Humans simply became huge melanin factories under the influence of natural selection when they lost most of their [ape-like] body hair—a very important step in the evolution of humans. The loss of that body hair meant that naked skin was exposed, [so] how to protect the skin became a big problem. The major protection against UV radiation was the evolution of melanization.
As early humans dispersed and populations moved away from the equator, lighter skin tones emerged from natural selection—why?
Melanin is a wonderful natural sunscreen, but it slows down the process of making vitamin D in the skin. Vitamin D is made from UVB radiation impinging on the skin. At high latitudes, UVB radiation is less common and more highly seasonal. If you have very darkly pigmented skin, [your] ability to make vitamin D is lessened [significantly]. I have moderately pigmented skin for a European. If I’m standing next to someone with very darkly pigmented skin and we’re both out in the middle of the summer sunshine, I will be able to make five or six times as much vitamin D in my skin in any time period as my darkly pigmented friend.
At the equator, where the sun is really intense, a darkly pigmented person will be able to make enough vitamin D through casual exposure to satisfy their physiological needs, because there’s simply so much UVB in the sunlight. But as soon as they get outside the tropics, where the UVB is less plentiful, they must spend longer and longer [periods of time in the sun] in order to get enough vitamin D to satisfy their physiological needs. Vitamin D [is] incredibly important for human health and reproductive success. Vitamin D-rich diets, including marine mammals and fish, were eaten by humans later in our evolutionary career—after we had the technology to be able to harvest foods from the sea. But early in our evolutionary career, we mostly didn’t make use of high vitamin D foods. We got our vitamin D from the sun through our skin.
As humans dispersed outside of the tropics, there was [an] intense evolutionary pressure to lose pigmentation. Modern genetics, and specifically genomic studies, have shed a lot of light on this process. People like me, who are interested in the history of evolution, or the history of adaptation—we’ve made inferences based on the fossil record [about] what we thought probably happened in human evolution. But the genomics experts have actually been able to establish unequivocally that there were independent genetic mutations that occurred in the ancestors of Western Europeans and Eastern Asians that led to loss of pigmentations as people dispersed into those areas. That’s really interesting, because both of these groups today have lightly pigmented skin, but it’s not from the same set of genetic changes. It’s from independent genetic changes that brought about loss of melanin pigment in the skin.
And, not only do we see different pathways in modern Europeans and modern Eastern Asians, but [researchers have] been able to look at the ancient DNA in Neanderthals and diagnose that they, in fact, had de-pigmented skin that was due to yet a different set of mutations. So the evolutionary pressure to lose pigment at high latitudes was great, and in at least three different instances—two groups of modern humans and one group of fossil relatives—we see three different genetic trajectories for achieving loss of pigmentation.
Darwin, the original authority on evolution, didn’t believe that human skin pigmentation was based on climate. How has this thinking changed over the course of the last 200 years?
Darwin dismissed the idea that pigmentation was due to climatic factors because he felt that, on the basis of what he knew at the time, the correlation between coloration and climate wasn’t strong. We have been able to come [our] conclusions largely as the result of having new kinds data—better data—on UV radiation at the Earth’s surface, for instance, as well as new kinds of genetic and genomic data, [both of] which were not available to Darwin. I think that if Darwin had the data that we do now, he would entirely rethink his stance on this.
Another major evolution related to skin—sweating—allowed humans to evolve larger brains. How are these connected?
Different lineages of mammals keep cool in different ways. If you have a dog, think about your dog: your dog can sweat a little bit, but mostly it keeps cool by panting. Sweating is the method for keeping cool that is used by all higher primates—we don’t pant at all. We lose some heat through our respiratory passages, but most of the heat lost is through the skin.
What was really important as humans evolved in tropical Africa is that they evolved a very efficient method of keeping cool. When we start running, or doing a lot of muscular work in a hot environment, our bodies build up a tremendous amount of internal heat that has to be dissipated. Chimpanzees, our closest relatives, have a moderate density of sweat glands on their bodies, and they are able to lose some heat through the evaporation of sweat. But, if a chimpanzee runs for maybe a few hundred meters, it will become exhausted because it can’t lose enough body heat fast enough to allow it to continue to exert itself.
[However], if we lose our body hair, that affords a much greater surface area for us to lose heat. If we can increase the number of sweat glands, that makes it possible to lose even more heat. Basically, those were the two things that happened in early human evolution: we lost hair [and] we gained sweat glands.
The most temperature-sensitive organ in the body is the brain. Imagine this hot chimpanzee that I introduced a few seconds ago—if his or her brain gets within two or three degrees Celsius of its resting body temperature, that animal begins to be stressed and suffer the effects of heat stroke. Bring that up four or five degrees, and you have an animal that’s in severe distress.
When you increase the number of sweat glands and increase the sweating capacity, you are increasing the cooling capacity of the whole body and the cooling capacity of the brain. As sweat evaporates, it lowers the temperature of the skin on the whole body. The cool blood going from the surface of the body back to the heart is not tremendously cooler—it’s maybe a degree Celsius cooler than the arterial blood—but it’s importantly cooler. That makes a huge difference when you need to pump cooler blood up to the brain to be able to maintain normal physiological activity.
Basically, the evolution of a good, efficient sweating apparatus made possible the expansion of the brain, which became one of the most conspicuous features of our evolution. There are lots of good behavioral reasons why we evolved larger brains, but there also had to be real, physiological parameters that made a large brain sustainable. And by being able to keep a large brain operating within its thermal tolerances, we had an important breakthrough in evolution.
Jablonski’s lecture, “Evolving Skin: The Remarkable History of Our Largest Organ” will take place at 1:00 PM on Saturday October 1st at the Field Museum. It is free with basic admission. Call (312) 665-7400 to reserve your seat. Presented in partnership with The Leakey Foundation.