1. Why does the human body look smooth if we have as many hair follicles as chimpanzees?
The human body has hair follicles at roughly the same density as chimpanzees. One study compared the skin of humans, chimpanzees, and macaques (relatives of Japanese macaques). It found no significant difference in follicle density between humans and chimpanzees, while macaques had higher density than both [1]. What changed was the type of hair. The fur of chimpanzees is made of "terminal hairs," which are thick and colored. Human body hair is mostly "vellus hair," which is fine and colorless, visible only under a microscope. The lack of visible body hair is suggested to be caused by terminal hairs turning into vellus hairs, rather than a reduction in the number of follicles [1]. A researcher involved in this study explained, "We have follicles at the same density as apes have follicles for fur" [3]. If the number of follicles hasn't changed, but the hair is thinner, how does the human body handle the insulation and heat release that fur used to provide? One clue is sweat.
In the 2018 study comparing the skin of humans, chimpanzees, and macaques, macaque hair follicle density was reported to be higher than in humans or chimpanzees, showing that humans are not uniquely low in follicle count [1].
2. How do sweat glands cool a body with thin hair?
There are several types of sweat glands, but "eccrine sweat glands" are the ones that produce watery sweat to lower body temperature. The average density of human eccrine sweat glands was about 10 times that of chimpanzees and macaques. The density of sweat glands in these two primate species was reported to be very similar to each other [2]. The figure of 10 times refers to the average density, not that humans have 10 times more glands in total. It means that in the same area of skin, sweat outlets are arranged at 10 times the density on average. Sweat removes heat from the body as it evaporates. On a body with thick fur, sweat gets tangled in the hair and has trouble evaporating. When hair becomes thin, sweat is more exposed to the air, making it easier for heat to escape, according to one explanation [4]. Regarding the order of evolution, this study presents a hypothesis. Hair density decreased in the ancestor of humans and great apes, and then sweat gland density increased in humans. There is evidence that hair and sweat glands evolved along separate paths [1]. Humans became bodies that cool down with sweat by thinning their hair. So, in what environment did human ancestors live that led to this change?
3. Why did thin body hair survive? The savanna theory and the endurance running hypothesis
A commonly cited reason for thinning hair is the "savanna theory." For human ancestors who expanded their living space from trees to grasslands, environments with few shade trees and high, dry temperatures favored bodies with thin vellus hair and exposed sweat glands, according to this idea [4]. This theory is influential but has not been completely proven [4]. It is explained that head hair remained to protect the head from strong sunlight (ultraviolet rays) [4]. Another theory is the "endurance running hypothesis." In 2004, Bramble and Lieberman argued that the human genus (Homo) evolved through natural selection for the ability to chase prey by running after about 2 million years ago. Bodies that release heat by evaporating sweat from the head and skin are thought to have supported this running ability [3]. It is important to note that the main subject of this paper is not the loss of body hair itself. It does not state that humans "could run because they had no hair" [3]. Both theories overlap in the direction that "bodies that release heat with sweat were useful in hot grasslands." However, because hair does not remain in fossils, it is impossible to directly verify when hair was lost [3]. Other clues are needed to know the timing.
One influential hypothesis is the savanna theory, which suggests that for human ancestors moving from trees to grasslands, bodies with thin vellus hair and exposed sweat glands were advantageous in hot, dry environments with no shade.
4. When did human body hair thin out? Gene clues point to about 1.2 million years ago
Hair does not remain in fossils. Neither do sweat glands [3]. Therefore, studies have looked for timing clues in genes. In 2004, Rogers et al. examined the MC1R gene, which is related to skin color. They estimated that the variant associated with dark skin in Africa arose at most about 1.2 million years ago. This is used as a lower bound estimate for "before this time," not a confirmed date for when hair was lost [7]. This is because the need for darker skin is linked to being exposed to sunlight after losing hair [7]. However, about 1.2 million years ago is the upper end of a wide estimate and a "lower bound guideline," not a confirmed year. The endurance running hypothesis suggests after about 2 million years ago, MC1R suggests before about 1.2 million years ago, and the savanna theory offers another perspective. Each clue has a range. Not deciding on a single year is closer to the current state of research. So, what about clothes? Did humans start wearing clothes after their hair thinned out?
5. When did people start wearing clothing? What louse DNA tells us
Lice provide clues that indicate when clothing was born. The "body louse," which lives on clothing, is thought to have split from the ancestor of head lice and moved to the new habitat of clothes. In a 2011 analysis by Toups et al., the split of body lice from the ancestor of head lice was dated to at least about 83,000 years ago, or as early as about 170,000 years ago. The estimate range is wide, with a 95% interval of about 29,000 to 691,000 years ago. Clothing use may date back to the time when Homo sapiens were in Africa [8]. Earlier, in 2003, an analysis by Kittler et al. estimated the split to be within about 72,000 years ago (±42,000 years), suggesting clothing might be a surprisingly recent invention [9]. These two estimates differ. All these numbers are much more recent than the about 1.2 million years ago for MC1R. This leads to the view that it is highly likely hair thinned out before clothing. However, there is still no direct evidence linking hair and clothing. The year lice split is not necessarily the exact year humans started wearing clothes. It is safer to view these estimates as including ranges, based on several assumptions.
6. How can you check your own arm hair and sweating at home?
Near a bright window at home, compare the outside and inside of your arm. Compare the thickness and color differences of the hair on your arm. If you have a magnifying glass, use it to look closely at the hair. After exercise, try to remember which areas are more likely to get wet with sweat. This article's story about "follicles not decreasing" and "sweat glands increasing" can help you think about where this is happening in your own body. To trace the research, the comparison of follicles and sweat glands starts with the 2018 paper by Kamberov et al., the endurance running hypothesis with the 2004 paper by Bramble and Lieberman, and the louse study with the 2011 paper by Toups et al.