Author : Wahid Ahmad
Hair is
a common feature among mammals, and it serves several important functions, such
as keeping us warm, protecting us from the sun's harmful rays, shielding us
from germs and bugs, and even helping us express our emotions by raising the
hairs on our skin. However, some mammals have evolved to be hairless.
Naked
mole rats are hairless because they live underground and stay close together to
keep warm. Cetaceans, like dolphins and whales, have very little hair because
it helps them swim faster. Large land animals often have less hair because it's
hard for them to get rid of excess heat with lots of fur. And then, there are
humans, who are unique because we have very little body hair compared to our
closest relatives, chimpanzees.
The
question is, why did we humans lost so much body hair in our evolution? That's
what scientists are trying to figure out. Some believe it was because we needed
to stay cool while walking on two legs, some think it was to reduce the risk of
parasites, and others suggest it might have been related to social behavior.
Researchers are studying when this hair loss happened and how it connects to
these different ideas.
Human Hair
Hair is something that mammals, including humans, have. It grows from little pockets called hair follicles just below our skin's surface.
In
humans, all our hair follicles are connected to tiny glands. Some are related
to sweat, and others make an oily substance that helps keep our hair healthy.
Each hair is made up of three parts: the outside layer (cuticle), the middle part (cortex), and the innermost part (medulla). The cortex gives our hair its color, and different shapes of hair can make it straight or curly.
Humans
have different types of hair all over our bodies. The big, visible, and darker
hairs are called "terminal hairs." These are in places like our
scalp, armpits, and pubic areas. Then there are the tiny, soft, and usually
colorless hairs called "vellus hairs." They are on places like our
eyelids and foreheads, and sometimes where we don't have much hair, like bald
scalps. These vellus hairs can turn into terminal hairs when certain hormones
are active. There are also "intermediate hairs" that are in-between
in terms of thickness and color, often on our arms and legs.
We have
millions of hair follicles on our bodies, and both males and females have about
the same number. However, males seem hairier because hormones make their hair
thicker.
Some of
our hair might have evolved for different reasons. Hair on our heads could have
been influenced by how attractive it made us look. Eyebrows might help show our
feelings. Facial hair might be about communicating and attracting mates. Armpit
hair might help in mating by trapping pheromones, chemical signals related to
attraction. And pubic hair might also help with pheromones and indicate
sexual maturity.
The
density of hair follicles can change based on where people live and their skin
color. Darker-skinned people might have fewer hair follicles. Things like
hormones and sexual attraction might have something to do with these
differences, but it's still not completely clear.
The
Evolution of Hairlessness in Humans
Human
hairlessness, is a result of an evolutionary shift from having long, thick, and
pigmented body hair (like the hair on our head) to shorter, finer, and
non-pigmented body hair. This change didn't mean we lost hair follicles; humans
have about the same number of hair follicles as chimpanzees, our close
relatives. The reason behind this shift in appearance has several theories.
Ectoparasite
Hypothesis
One
idea is that humans lost a lot of body hair to combat parasites like fleas,
ticks, and lice. Fur can be a cozy home for these pests and their eggs, which
can lead to infestations. Some of these parasites can also carry diseases.
However, this theory has some issues. Many other animals also deal with
parasites but haven't lost their hair. Even though humans have less body hair,
we can still get parasites in places like our scalp, armpits, and pubic areas.
Additionally, having less body hair might expose us to other disease-carrying
insects like mosquitos, which fur could protect against to some extent.
A
variation of this theory suggests that ticks, in particular, might have driven
the loss of body hair. The idea is that our common ancestor chimpanzees
lived mainly in trees and occasionally on the ground. Around 5 to 8 million
years ago, climate changes caused forests to break up into smaller patches,
leading to an increase in tick populations. This forced our ancestors to walk
on the ground, where they would interact with ticks more. Those with less hair
could see and remove ticks better, which could have been a big advantage
against tick-borne diseases. This theory suggests that hair loss and the shift
to walking on two legs happened around the same time, leading to the emergence
of hominins (early humans).
Another
variation of the parasite theory suggests that as humans began to live in more
permanent settlements, especially females and their children, there was a
higher risk of parasite infestations. So, natural selection favored less hairy
females because they were less likely to get infested. This might explain why
women have less body hair than men. This theory implies that hair loss started
around 1.8 million years ago when humans began to establish more permanent
settlements.
Vestiary
Hypothesis
The
vestiary hypothesis suggests that humans lost body hair because they developed
and started using clothing for warmth. The idea is that our early ancestors
learned to use things like animal skins and fabrics to stay warm at night,
which allowed them to lose their body hair. Shelter and fire were also used to
stay warm. This theory proposes that this change happened around 500,000 years
ago in the Homo erectus population.
The
Thermoregulatory Hypothesis
One
widely accepted idea that it may be because of our efficient cooling system,
specifically our ability to sweat through eccrine glands. It's thought that our
ancestors needed to stay active during the heat of the day in the savanna, so
being able to sweat effectively was a big advantage. Sweating helped us cool
down and enabled us to walk long distances during the day when predators were
most active. This made it possible for our ancestors to survive alongside
dangerous animals. The increased ability to get food resources, like meat,
through these activities further supported this theory. Our unique capability
for endurance running, which is the ability to run long distances without
overheating, also likely played a role in this.
The
reason why humans needed to lose most of their body hair was to maximize the
effectiveness of sweating. It's because when sweat evaporates directly from our
skin, it cools us down more efficiently than if we had a lot of body hair.
Having hair would require us to use more water to stay cool in hot conditions.
So, the loss of body hair saved our ancestors from losing too much water in dry
weather.
The
Naked Love Hypothesis
Another
theory suggests that the transition to walking on two legs (bipedalism) played
a crucial role in the evolution of hairlessness. Primates, including early
humans, have a strong bond between mothers and their infants. In most primates,
infants can cling to their mother's fur. However, as our ancestors evolved to
walk on two legs, they lost the ability to grasp their mother's fur with their
feet, making it difficult for mothers to carry their infants.
Skin-to-skin
contact between a mother and her baby is very important and pleasurable. When
human mothers have direct skin-to-skin contact with their infants, it enhances
their connection and helps in child development. Because mothers had less body
hair, it would have been more enjoyable to have this skin-to-skin contact,
making mothers more willing to carry their infants even when they needed to
travel or gather food.
This
hypothesis suggests that as our ancestors became more bipedal, hairlessness
developed to strengthen the emotional bond between mothers and infants, making
it more pleasant for mothers to care for their children
The
Aquatic Ape Hypothesis
This
idea suggests that our early ancestors, about 5 to 8 million years ago, spent a
lot of time in the water, like the coast of East Africa. Some proponents of
this theory point to the fact that certain parasites that affect humans depend
on contact with water. However, this hypothesis has a few issues. It doesn't
explain why humans would have lost their hair due to an aquatic lifestyle,
especially since we can't swim very well. It also doesn't clarify why we would
have retained these traits once we left the water, or how we would have dealt
with aquatic predators like crocodiles.
The
Melanin-Competition Hypothesis
Melanin
is the substance in our skin and hair that gives us our color and helps protect
us from the sun's harmful UV radiation. Some researchers believe that as our
early ancestors evolved to have darker skin to protect against the sun, this
also reduced their hair growth. Dark-skinned individuals tend to have fewer
hair follicles and shorter hair. So, according to this theory, hair loss would
correspond with the development of dark skin in our lineage.
Sexual
and Cultural Selection Hypothesis
Charles
Darwin proposed that hair loss might have happened because it made some
individuals look more attractive to potential mates. It's like how some people
find a clean-shaven look more appealing. But this theory doesn't explain why
everyone in a population would lose their hair.
Another
idea is that hair loss could be linked to cultural factors. This means that,
over time, people wanted to distinguish themselves from others, like
Neanderthals, who had lots of body hair. As a result, humans who had less hair
became more preferred as mates. This cultural selection might have driven the
emergence of hairlessness, which could have happened as late as when Homo
sapiens (modern humans) first appeared, about 200,000 years ago
Timeline
of hairlessness
Molecular
Evidence from Lice
Scientists
can use parasites like lice to study the history of our species. Human lice are
small insects that live on our bodies, and different types of lice prefer
different parts, like the head, body, or pubic hair. By looking at the genetic
differences in these lice, researchers found that the pubic louse that infests
gorillas likely crossed over to our human ancestors about 3.32 million years
ago. This implies that humans and gorillas lived in the same environment at
that time.
Molecular
Evidence from MC1R Locus:
The
MC1R gene is responsible for our skin and hair color. Darker skin has evolved
in humans to protect against harmful sunlight. Genetic evidence suggests that
dark skin has been present in the human lineage for at least 1.2 million years.
This timeline corresponds with the idea that humans lost their hair when they
were exposed to strong sunlight.
Anatomical
and Physiological Evidence from the Fossil Record
The
fossil record can tell us when various anatomical changes occurred in early
humans. One significant change was the transition to walking on two legs
(bipedalism), which started around 6.8 to 7.2 million years ago.
Bipedalism
was closely tied to hairlessness because sweating, a trait that helps humans
run in the heat, requires hairlessness. Various skeletal features like the
Achilles tendon, foot structure, long legs, and skeletal strength gradually
evolved in early humans, culminating with Homo erectus, who had body
proportions that allowed for endurance running similar to modern humans.
Conclusion
In
summary, anatomical and molecular evidence suggests that human hair loss
occurred between 3.32 million and 1.2 million years ago.
While
some hypotheses have been debunked or require further research, the
"melanin-competition hypothesis" could provide insights into both
hair loss and global hair distribution.
Based
on current knowledge, the "thermoregulatory hypothesis" is the most
convincing explanation for the evolution of hairlessness in humans, but other
factors may have reinforced this trait.
References
Rantala, M. J. (2007). Evolution of nakedness in Homo sapiens. Journal of Zoology, 273(1), 1-7.
Giles, J. (2010). Naked love: The evolution of human hairlessness. Biological Theory, 5, 326-336.