Humans grow underarm hair primarily because it serves as a surface for dispersing scent signals produced by a dense cluster of specialized sweat glands in the armpit. While humans lost most of their body hair over the course of evolution, the patches in the armpits (and groin) persisted because they sit directly over apocrine glands, which secrete oily compounds tied to body odor and, potentially, chemical communication between people.
What Underarm Hair Actually Does
Your armpits contain an unusually high concentration of two types of sweat glands: eccrine glands, which produce the watery sweat that cools you down, and apocrine glands, which secrete a thicker, oily fluid directly into hair follicles. Apocrine secretions are initially odorless. The familiar smell of body odor only develops when bacteria on the skin break those secretions down.
The hair itself acts like a wick. It increases the total surface area available for these secretions to evaporate from, which helps volatile compounds become airborne. How far your scent carries depends on several factors: clothing layers, the temperature of your armpit, total hair surface area, arm movements, and how close someone’s nose happens to be. In evolutionary terms, this scent dispersal likely played a role in signaling reproductive fitness or individual identity to others, much the way it still does in other mammals.
The Bacteria Behind Body Odor
Armpit hair doesn’t just spread scent. It also provides a warm, moist habitat for a thriving community of microorganisms. The armpit microbiome is typically dominated by staphylococci and corynebacteria, with occasional propionibacteria mixed in. Of these, corynebacteria are the key odor producers. They break down the oily apocrine sweat into the pungent compounds most people recognize as “B.O.” Staphylococci, despite being abundant in the armpit, don’t appear to generate that characteristic smell.
This bacterial ecosystem exists in a carefully maintained balance with the skin’s immune and barrier systems. The hair follicles themselves are part of that ecosystem, giving bacteria a physical structure to colonize. Removing the hair changes the microbial landscape, which is one reason shaving or waxing can temporarily reduce body odor.
Why Humans Lost Body Hair but Kept It in Certain Spots
The broader question is really about the mismatch: why did humans lose the fur that covers nearly every other mammal while keeping hair in the armpits, groin, and scalp? The leading explanation ties hair loss to heat regulation as early human ancestors moved into open, sun-exposed environments in Africa.
Research modeling the energy costs of fur versus bare skin in early hominins shows the tradeoff was complex. During the day, hairless skin made it far easier to shed excess heat, especially for active individuals in direct sunlight. But at night, especially at the higher altitudes where early human ancestors like australopiths lived, the cost of being hairless was enormous. At just 1,000 meters above sea level, a hairless female australopith would have needed roughly 3,500 extra calories per day to offset nighttime heat loss, while a male would have needed about 5,600. Given that their total daily energy needs were only around 1,250 to 1,740 calories, going fully hairless at those altitudes was simply not survivable.
The most likely timeline is that significant hair loss appeared with the emergence of the genus Homo around 2 million years ago, after climate cooling allowed these populations to move into lower, warmer habitats where nighttime heat loss was less punishing. Even then, hair was retained in specific areas where it served functions beyond insulation. Scalp hair protects against direct solar radiation. Underarm and groin hair sits over apocrine gland clusters and continues to play a role in scent signaling and friction reduction between skin surfaces that regularly slide against each other.
When Underarm Hair Starts Growing
Underarm hair doesn’t appear until puberty because it depends on rising levels of androgens, a group of hormones produced partly by the adrenal glands. The process begins with adrenarche, a gradual increase in adrenal androgen production that starts in early childhood but becomes noticeable around ages 8 to 13. As levels of the androgen DHEAS rise past a certain threshold, the body begins developing adult-type apocrine glands, oilier skin, and eventually hair in the armpits and pubic area.
A recently identified hormone called 11-ketotestosterone also appears to be an important driver of these changes. Body composition plays a role too. Children who gain weight rapidly in early childhood tend to have higher DHEAS levels, and hormones like insulin, IGF-1, and leptin have all been linked to earlier or more pronounced adrenarche. This is one reason some children develop body odor and body hair earlier than their peers.
The development of adult-type armpit odor is itself a marker of adrenarche, appearing as the newly matured apocrine glands begin secreting into hair follicles for the first time. This is why young children don’t have noticeable body odor in the way teenagers and adults do.
Is Underarm Hair Vestigial?
There’s an ongoing debate about whether underarm hair qualifies as vestigial, meaning a leftover structure that no longer serves its original purpose. The honest answer is that it falls somewhere in between. Humans no longer rely on scent signaling the way other animals do, and modern hygiene practices largely work to suppress the very odor that armpit hair evolved to help broadcast. In that sense, its original function has been culturally overridden.
But the hair still reduces friction between the arm and torso during movement, still provides a surface for moisture wicking, and still supports a microbial ecosystem that contributes to skin health. As one review in the International Journal of Trichology put it, even though human hair no longer has survival value in the Darwinian sense, it does serve useful functions and is “not completely vestigial.”
Medical Conditions That Affect Armpit Hair
Losing underarm hair unexpectedly can occasionally signal an underlying health issue. Alopecia areata, an autoimmune condition best known for causing patchy hair loss on the scalp, can in severe forms (alopecia universalis) cause complete body hair loss, including in the armpits. A rarer condition called Graham-Little-Piccardi-Lassueur syndrome specifically features a triad of symptoms: progressive scarring hair loss on the scalp, nonscarring hair loss in the armpits or pubic area, and rough, bumpy skin papules on the trunk or limbs.
Hormonal conditions that lower androgen levels can also thin or eliminate armpit hair. If you notice sudden or unexplained loss of body hair, it’s worth having your hormone levels checked, since it can point to adrenal or thyroid dysfunction that benefits from early treatment.