The most striking difference between human populations, or "races" (a loaded term I used inDISTANT), is the skin color. As we all know, and as the following graph shows, populations from tropical areas have darker skin than populations from higher latitudes:
Fig. 1. Distribution of skin pigmentation in human populations (fromBritannica-Enzyklopädie)
What causes this variation? Several theories have been proposed, including sexual selection for different skin tones in different locations, but due to the strong correlation between pigmentation and the amount of sunlight, biologists have suggested that differences in pigmentation are the result of selection caused by different conditions. lighting in different locations. it becomes local. 🇧🇷 Here is the amount of light (measured as annual total UV) in different regions of the world (To use: The original number here was wrong: the source suggested that it was an annual incidence of UV, but it was actually only for the month of November, which explains its asymmetry around the equator).
Fig. 2 (from Jablonski and Chaplin 2010): mean annual UVA (380 nm). Intensity is indicated by gradations from dark to light in the range of 65 to 930 Jm−2on 10 levels with partially darkened oceans.
The notion that pigmentation evolved as an adaptation to light conditions is supported by the observation ofOptionalChanges in pigmentation that occur with increasing exposure, e.g. B. Browning. Fair-skinned people get darker with more sun exposure.
These differences between populations almost certainly represent more than one evolutionary event. First, although we don't have fossil skin from our African hominid ancestors likeman standing, it is likely that they were obscure, as African populations are today. But earlier ancestors could also have been lighter. If you look at our closest relatives, chimpanzees and gorillas, their skin (at least the parts below the hair) is not pigmented. Only exposed areas are pigmented. Therefore, the ancestral color of humans was probably light (but not as light as the Swedes say), and as we became "naked apes" we evolved towards a darker shade. (The development of baldness in our species is another matter, perhaps related to our ability to sweat.)
Then, as populations of supposedly black people migrated to the Middle East and Europe, they evolved lighter complexions. But when these populations colonized Australia, the skin color darkened again. This also almost certainly happened when humans migrated from northern Asia across the Bering Strait into the Americas: populations that arrived in Central and South America likely re-evolved dark pigments.
What were the selective compulsions that caused these changes? I have long accepted the "classic" story taught in school: populations that receive more sunlight develop darker skin as protection against UV-induced melanoma and the toxic effects of excess vitamin D3, which is only produced when sunlight hits the skin. 🇧🇷 In areas of low light, the skin develops a lighter hue because we need significant amounts of vitamin D3 to build strong bones (without vitamin D3, children develop rickets, which is why foods like milk often contain vitamin D). For example, dark-skinned ancestors in the tropics had lower vitamin D toxicity and fewer melanomas, while light-skinned ancestors in temperate zones had stronger bones. This could result in mortality or differential reproduction, which would explain differences in pigmentation.
The problem with this story is exactly that: it is a story. While this scenario sounds plausible, there wasn't a lot of hard evidence for it, at least not when I was in school. A recent article inPNASby Nina Jablonski and George Chaplin summarizes the latest discoveries and comes to some different conclusions about the evolution of human skin color. Your findings:
Why tropical populations are darkerProbablynofor melanoma. The authors conclude that:
Sunburn and skin cancer have negligible effects on reproductive success (7, 18). Non-melanoma skin cancer is common in older people in modern, lightly pigmented populations living in sunny climates, but it rarely leads to death or disability (20). Melanoma affects younger people and is often fatal, but it is much rarer than non-melanoma skin cancer.
They also point out that most human skin cancers are due to white people moving to the tropics, which would not have happened to our ancestors. They conclude that "the effects of skin cancer on reproductive success in humans are modest today and likely were statistically insignificant prior to rapid and long travel and migration."
Jabonski and Chaplin also criticize the idea that excess vitamin D is another selective force: they say that "the overproduction of vitamin D as the main cause of the development of dark pigmentation was refuted by the finding that hypervitaminosis D caused by sun exposure It's physiologically impossible." ." due to photochemical regulation."
This was all new to me and I was happy to hear it. I'm not going to tell my students a lot about melanoma and vitamin toxicity when I'm talking about skin color development.
If not skin cancer or hypervitaminosis, what were the selection pressures? Jablonski and Chaplin propose that the amount of folate (folic acid), one of the B vitamins, plays a crucial role in biosynthesis, including DNA synthesis and repair. In an article published in 2000, which I had not seen before (quote below), the authors presented the idea that the skin darkens because sunlight can destroy the folic acid in the blood vessels of the skin to prevent the folate from destroying it. 🇧🇷 In the present article, they specifically mention that a lack of folate in human embryos can cause neural tube defects (NTDs), which act as strong selection pressures.
I find the evidence for this theory intriguing but somewhat sparse. The authors say there is evidence for a "protective effect of dark pigmentation against folic acid deficiency", but in reviewing the cited studies, I see only a correlation between "racial" origin and neural tube defects. This could be due to factors other than pigmentation. The authors seem to be on more solid ground when they claim that folate protects against neural tube defects, with several studies showing that supplementing women's diets with folic acid results in significantly less production of neural tube defects in their offspring.
Why populations with less sun exposure are lighter.Here, the authors accept the idea that selection is based on the need for vitamin D3, which not only strengthens bones, but also plays a role in immune response, cell proliferation, and brain, heart, and brain function. and the lungs, the pancreas. As folic acid is produced in the epidermis and dermis, it seems plausible that the skin was lightened to ensure adequate levels of this important vitamin. But again, the evidence is inconclusive. The strongest, cited in the 2000 paper, is that recent black immigrants from tropical to temperate regions (Ethiopia to Israel, India to the UK) tend to be vitamin D deficient.
Why is there a tan?Jablonski and Chaplin propose that tanning was an adaptation that evolved not to protect long-distance migrants (since they did not exist among our ancestors), but to protect populations inintermediaryLatitudes where there are much greater seasonal variations in the amount of UV. Seasonally variable pigmentation would be useful to protect against folate deficiency during high UV seasons by allowing vitamin D synthesis during low UV periods.
Jablonski and Chaplin's work is fascinating, I will definitely tell my students about the "folate hypothesis" and also about the evolution of tanning. But I insist that these ideas are provisional. Studying the adaptive significance of variation in the human race is a difficult task for two reasons. First, we cannot perform experiments on humans that are not medically related, such as administering folic acid to pregnant women. We certainly cannot move people from one place to another and observe the connection between their characteristics and their physical condition, although in some cases we can take advantage of random migrants. Using fruit flies, which are larger in northern regions than in the tropics, we can conduct experiments and show that large bodies develop in the laboratory when the flies are reared in cold conditions.
The other problem is that the traits that characterize human populations evolved a long time ago (60,000 to 10,000 years ago, when humans left Africa and colonized North America and Australia), and we can only speculate about the selective forces that took place. long time. 🇧🇷 so long. (However, the geographic distribution of ultraviolet light hasn't changed much since then!)
It is strange but understandable that we know so little about the evolution of population differences in our own species. We know a little about skin color, but when it comes to nose shape, hair texture, eye wrinkles and body structure, the other characteristics that characterize the human population, our ignorance runs deep.
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Jablonski, N.G. and G. Chaplin. 2010. Human skin pigmentation as adaptation to UV radiation. perc. nat. US Academy of Sciences107(Supplement): 8962-8968.
Jablonski, N.G. and G. Chaplin. 2000. The evolution of human skin coloration. J. Human evolution39:57-106.