Hair

Hair is a special kind of protein filament that grows from tiny pockets called follicles in our dermis, the layer just under our skin. It is one of the things that helps set mammals, like humans, apart from other animals. Our human body is mostly covered with these follicles, which can make thick, noticeable hair or very fine, almost invisible hair.

People often pay a lot of attention to hair because of how it grows, the different types there are, and how to take care of it. Hair is also very useful for scientists because it is a kind of biomaterial made mostly of a protein called alpha-keratin.

Different cultures and times in history have had very different ideas about hair. How people style their hair or whether they remove it can show many things, like how old they are, their gender, or what they believe in terms of religion. Hair can be a way to express who we are and where we come from.

Overview

The word "hair" refers to two main parts:

1. The part under the skin, called the hair follicle. This is found in the dermis and helps hair grow back after it falls out. It also helps the skin heal after a wound.
2. The hair shaft, which is the hard part that sticks out above the skin. It is made mostly of a protein called keratin.

Hair has several layers. Starting from the outside, there is the cuticle, made of thin cells. Inside that is the cortex, which contains bundles of keratin. At the center is the medulla, a loose area.

The word "hair" comes from old languages like Middle English, Old English, and Old Norse. It is related to words for hair in languages such as Swedish, Dutch, and German.

Description

Each strand of hair is made up of the medulla, cortex, and cuticle. The medulla is the innermost part and may not always be present. The cortex is the main part of the hair and gives it strength and color. The cuticle is the outer layer that helps protect the hair. The shape of the hair follicle decides whether hair is straight or curly. Straight hair has a round shape, while curly hair is more oval.

Hair growth starts in the hair follicle. The part we see is called the hair shaft and does not change once it leaves the follicle. The base of the hair, called the bulb, is where new hair is made. Hair color comes from pigments called melanin. Brown and black hair have more of one type of melanin, while red hair has more of another type. Blond hair has less pigment, and gray hair happens when melanin production slows down.

Hair grows on most of the body but not on the palms, soles, or lips. There are different types of hair, like fine hair and thicker hair, each with its own purpose, such as keeping us warm. Hair goes through growth stages, and the speed can change with age and other factors. The world record for longest hair is held by Smita Srivastava from Uttar Pradesh, India, whose hair measured over 7 feet long.

Hair can be straight, wavy, curly, or coily, depending on the shape of the hair shaft. The Andre Walker Hair Typing System is a popular way to classify hair types, ranging from straight (Type 1) to coily (Type 4). Each type has its own care needs.

Main article: Human hair color

Main article: Human hair growth

Main article: Andre Walker Hair Typing System

Composition

Hair is mostly made of a special kind of protein called keratin, along with other proteins called keratin-associated proteins. Our bodies can make many different types of these proteins, which help give hair its strength and texture. In some animals, like echidnas, these extra proteins make up a much larger part of their hair or quills.

These proteins form long, thin structures inside hair cells, creating strong bundles called macro-fibrils. The cells themselves are held together by a thin layer of proteins and fats, which helps keep the hair firm and protected.

Functions

Many mammals have fur and other hairs that serve different purposes. Hair helps keep animals warm, hide them from danger, and send signals to others. It also helps them feel things outside their skin. For humans, hair on the head helps keep us warm and cool, and it protects us from the sun's strong rays.

Hair can also protect animals. Some animals, like hedgehogs and porcupines, have special hairs that turn into hard spines to scare away danger. Our eyebrows and eyelashes help keep dirt and sweat out of our eyes and also help us show feelings like sadness or surprise. Eyelashes help us feel when something is too close to our eyes, and they can even make our eyes water to wash away dust.

Evolution

Hair began to appear in the common ancestor of mammals, the synapsids, about 300 million years ago. We do not know exactly when synapsids first developed traits like body hair and mammary glands because fossils rarely show soft tissues. The oldest clear evidence of hair comes from fossils of animals called near-mammals from the late middle Jurassic period, about 220 million years ago. Some studies suggest that even older non-mammalian synapsids might have had fur.

Most mammals have special glands called harderian glands in front of their eyes, used to care for their fur. Early mammals also had these glands, but their ancestors did not.

The fur of modern animals connects to nerves, acting as a sensory tool. Fur may have started from sensory hairs like whiskers. Advanced therapsids might have had a mix of naked skin, whiskers, and scutes. Full fur likely developed during the transition from therapsids to mammals. Today, many mammals have areas of skin without hair, such as the palms of hands and soles of feet.

Primates, including humans, have less hair than many other mammals. Humans evolved in East Africa about 2 million years ago, developing endurance running and better ways to stay cool in the heat. Losing body hair helped with this, as air currents next to the skin helped with cooling through sweating.

Humans also show traits from youth, called neoteny. For example, women keep fine, soft hair (vellus hair) that men lose as they develop thicker, coarser hair (terminal hair).

Different hair textures exist, such as curly hair, which may have helped protect early humans in Africa from strong UV light. Curly hair might let in less UV light compared to straight hair, which could have been important when early humans lost their body hair. Later, as humans moved to areas with less UV light, straight hair may have developed to allow more UV light into the skin.

Recent studies show that a genetic change at the EDAR locus, which affects hair texture, likely caused the straight hair common in East Asians to develop within the last 65,000 years.

Evolutionary variation

The EDAR locus

Disease

See also: Hair diseases

Ringworm is a fungal disease that affects areas of the skin with hair.

Premature greying of hair is when people's hair turns grey earlier than usual, depending on their background.

Hair care

Hair care is all about keeping your hair clean and looking nice. It includes taking care of hair on your head, face, and body. Different people have different ways of caring for their hair based on where they live and what their hair is like. You can change your hair in many ways, like coloring it, cutting it, or even removing it completely.

There are several ways to remove hair. Shaving uses a razor to cut hair close to the skin, making it feel smooth. After shaving, you might notice new hair growing back quickly, and it can feel a bit thicker because the ends are blunt. Waxing pulls hair out from the root using sticky wax and a strip of paper or cloth. This can keep skin smooth for weeks, and over time, the hair may grow back thinner. Laser hair removal uses a special light to target hair growth, which can reduce hair over many visits. Hair on the head is often cut with scissors or trimmers to keep it at a nice length. Sometimes, cut hair is used to make wigs.

Social role

See also: Hairstyle

Hair is very important to humans. It can grow on most parts of the body, except for the palms of the hands and the soles of the feet. People often notice hair on the face, ears, head, eyebrows, legs, and armpits, and these are the areas that are usually trimmed, plucked, or shaved.

Different hairstyles can show things about a person, such as their group or beliefs. For example, during the English Civil War, people who supported Oliver Cromwell had short hair to show they were different from others. Some groups, like hippies or punks, have special hairstyles that show they belong together. Hair can also show a person's age or health. For instance, white or gray hair usually means someone is older. Some people choose to dye their hair to cover this, while others let it be natural.