Humans are unique among primates in having mostly bare skin. But evidence suggests early humans were far hairier than we are today. Understanding why our ancient ancestors were so hairy can provide insight into human evolution.
When did humans lose their body hair?
Humans began losing their thick body hair around 3 million years ago. Before that, earliest human ancestors like Australopithecus afarensis, famous for the fossil Lucy, were covered in hair much like modern apes. But by 1.2 million years ago when Homo erectus emerged, body hair had reduced substantially.
Today most humans have fine, sparse hair covering the body. We also have coarser hair concentrated in places like the scalp, eyebrows, eyelashes, armpits and pubic region. But we lack the thick fur of most mammals. So when and why did humans lose their ancestral coat?
Theories on why humans became less hairy
There are several theories proposed by anthropologists on why humans shed most of their body hair:
- Endurance running – Early humans adapted to run long distances in pursuit of prey on the hot, open savannas of Africa. Thick fur would have caused them to overheat during strenuous exercise in the heat. Losing body hair helped better regulate body temperature and enabled persistence hunting.
- Thermoregulation – Being largely hairless allowed humans to cool their bodies efficiently when temperatures rose by sweating. Fur would have trapped heat and made it harder to keep cool.
- Parasites – Thick fur provides a home for parasites like lice and ticks. Losing hair may have reduced parasite loads. Parasites can also cause hair loss as part of the immune response.
- Sexual selection – Sparse body hair may have evolved through mate choice. Hairlessness could have been seen as more aesthetically pleasing by early humans.
- Aquatic ape hypothesis – Controversially, some argue that humans were semi-aquatic and lost hair for swimming and diving. However, there is little evidence to support this theory.
The leading explanations focus on thermoregulation. Losing a fur coat allowed better cooling through sweating and helped prevent overheating during long distance travel and hunting.
Evolution of human hairlessness
When did the various waves of human hair loss occur? Research suggests:
- 4 million years ago – Early humans like Ardipithecus ramidus were still quite hairy but had bare palms for grip and possibly bare soles for walking.
- 3 million years ago – Australopiths like Lucy had lost most fur on the arms and legs but were still furry overall.
- 2 million years ago – Early Homo had moderately hairy bodies but had lost fur on the arms, legs and chest.
- 1.2 million years ago – Homo erectus showed major hair loss. It had sparse, fine body hair much like modern humans.
So human hairlessness evolved gradually over millions of years. The most dramatic changes occurred with Homo erectus, when expanded hunting and gathering in hot, open environments likely drove further hair loss.
Differences in male and female hair loss
An evolutionary curiosity is that human males are hairier than females, but males of most ape species are not. Why did human males evolve more body hair?
Proposed explanations include:
- Sexual selection – Added hair developed as a signal of maturity and masculinity attractive to females.
- Pheromone distribution – More hair dispersed male pheromones, chemical cues that altered female physiology and behavior.
- Aggression – Males used hair to appear larger and more threatening to competitors.
Male secondary sexual characteristics like beards may have evolved for a combination of these reasons. But scientists are still studying why male humans diverged from other apes by becoming hairier.
Functions and features of human hair
While humans lost most of their ancestral body hair, the hair that remains serves important functions:
| Function | Description |
|---|---|
| Warmth | Hair on the scalp, face, and pubic region help retain heat and warm vital areas. |
| Protection | Eyelashes, nose hairs and eyebrows protect eyes and airways from debris. |
| Sensation | Fine hairs enhance touch sensation. Hair follicles connect to nerve endings in skin. |
| Display | Hair signals youth, health, and other traits. It can influence attraction and social status. |
| Pheromones | Some apocrine glands release pheromones from hair that elicit social/sexual behaviors. |
| Excretion | Hair facilitates excretion of wastes and excess sebum from skin. |
So while humans are naked apes overall, the hair that remains plays crucial roles in our health, sensory experience, and social interactions.
Hair patterns among human populations
There are some variations in hair patterns across human ethnic groups adapted to different environments. Key differences include:
- Higher hair density in cold climates to insulate.
- Lower hair density in hot climates to cool easier.
- Darker, thicker hair near equator to prevent sun damage.
- Straighter hair in cold, wet climates to repel rain.
- Tighter curled hair in drier climates to retain moisture.
Certain hair traits evolved relatively fast as humans dispersed into new habitats. For example, straight hair spread among East Asians within last 10,000 years as populations adapted to rainforest environments in Southeast Asia.
Evolution of human skin and sweat
Related to hair loss is the evolution of human skin and sweat glands. As humans shed fur, exposed skin with numerous sweat glands helped prevent overheating during long distance travel and running. Key evolutionary changes to the skin included:
- Darker melanin pigment protected against intense UV radiation in Africa.
- Many more sweat glands than other mammals increased cooling capacity.
- Loss of fur allowed free evaporation of sweat to cool the body.
- Dense networks of blood vessels in skin aided heat dissipation.
These adaptations allowed humans to perform vigorous physical activities during the heat of the day better than most mammals.
Human eccrine sweat glands
Humans have around 2-4 million eccrine sweat glands distributed across the body. These glands produce a watery sweat that evaporates from the skin, removing excess heat. The number of sweat glands in humans compared to other mammals includes:
| Species | Sweat gland density (per cm2) |
|---|---|
| Humans | 150-600 |
| Chimpanzees | 9-32 |
| Rhesus monkeys | 32-50 |
This massive increase in sweat glands was a key adaptation for human thermoregulation and endurance in hot environments.
Humans re-domesticated hair
While wild ancestors like Homo erectus had lost most body hair, modern humans seem to have re-domesticated hair in some ways:
- Clothing re-introduced artificial fur for warmth and decor.
- Hairstyles evolved into signals of identity and status.
- Shaving, styling, dyeing became part of social routines.
- Some sexual selection favoring hairiness may have re-emerged.
So humans evolved as naked apes thanks to natural selection. But cultural evolution brought hair back as a social and expressive feature in new ways.
Conclusion
In summary, humans evolved extensive hair loss mainly for thermoregulation advantages. Thick fur impeded cooling in hot environments critical for endurance running and hunting. Hair diminished gradually over millions of years but reduced substantially by the emergence of Homo erectus.
Secondary sexual characteristics led male humans to become hairier than females, unlike other apes. But overall, modern humans are unique as largely naked, sweaty primates. Our naked skin aided adaptation but also made us dependent on behavioral innovations like clothing, shelter and fire.