Some newborns have lots of hair and some are almost bald. Some have blond hair only to have brown hair as adults. Some babies are born with curls but may not have curls as adults. Infants will lose hair after about 6 months because of a change of hormones they are no longer receiving through the placenta from their mothers.
Babies are born with all the hair follicles they’ll need in their lifetimes. On average, people come into the world with about 5 million hair follicles. Between 10-14 weeks of pregnancy, those follicles start growing tiny stands of hair called lanugo. This is shed between weeks 24-28. The hair of a full term baby is grown in the last trimester.
Why do humans have hair and why do we have hairless skin in certain parts? Hairless skin is found only on the lips, nipples, genitalia and the bottoms of the hands and feet. Humans are actually as hairy as apes but their hair is much wispier and fainter. Hair is unique to mammals and serves a multitude of purposes providing a layer of insulation, a reflector of heat, as camouflage and as a tool of seduction.
Hair color, as is skin color, is determined by a variety of pigments, the most important is a molecule known as melanin. Melanin in found throughout the living world in feathers, fish scales, squid ink. It colors our hair and as the production slows down as we age, the hair turns gray. The color of hair and skin is very complicated and not a simple you have it or you don’t. There are over 120 genes involved in the pigmentation of mammals. With this complexity two blond adults could produce a dark haired baby or one dark haired and one blond.
Melanin is a great natural sunscreen. It is produced in cells called melanocytes. Interestingly no matter what skin color a human has it has the same number of melanocytes. It is believed that hair and skin developed darker or lighter color because of evolutionary benefits related to sun and Vitamin D. However this also is not clear cut as the lighter-colored hair and eyes evolved somewhere around the Baltic Sea about 6000 years ago with no clear Vitamin D metabolism benefit. Nina Jablonski in Living Color: The Biological and Social Meaning of Skin Color discusses that “people act as if skin color is a determinant of character when all it is is a reaction to sunlight”.
Human hair comes with all sorts of colors, textures and shapes. African hair is more coiled and dry; Asian hair is straighter and thicker; and Caucasian hair is somewhere in between with around 45% having straight hair, 40% having wavy hair, and 15% having curly hair. These variations are determined by many genes and combination with environmental factors (and not in the least by hair stylists!). There is great truth to the quote “you can’t tell a book by its cover.”
Sally Robinson, MD
Professor, UTMB Dept. of Pediatrics
Keeping Kids Healthy published June 2023
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