Why do we have body hair?

Some have more of it than others, but what is the purpose of our hairy patches?

(Image source: Pixabay)

Body hair is not there by chance – it has played a vital role in our species’ survival. As our prehistoric ancestors ditched swinging from the treetops to stand on two legs and pace the Savannah plains, thick fur posed a problem for heat regulation and so the majority was lost, with the exception of a few patches, most notably on our heads. Evolutionary shedding of thick body hair also removed the threat of parasitic hitchhikers.

Today, as two-legged ‘naked apes’, the most exposed part of the human body to the Sun’s ultraviolet rays is the head, which retained hair for protection. We have also evolved the ability to sweat to cool our bodies down in the heat from glands called apocrine sweat glands. These glands not only release a cooling fluid but pheromones too. Patches of our body hair are conveniently located where these glands are found to trap the sweat and hold on to the pheromones in order to attract a mate.

However, though we may appear to be a bald version of our primate cousins, humans and other apes still share the same amount of hair follicles, which is on average around 5 million. Humans simply no longer grow thick coats.

Werewolf syndrome

Typically, hair will continue to grow to a set length depending on its location on the body. The hair on the head can continue to grow for a metre or so, whereas underarm hair will cease new growth after a few centimetres. However, there is a rare genetic condition known as congenital generalised hypertrichosis, or ‘werewolf syndrome’ as it is commonly known, whereby excessive hair growth appears in patches or covers the body when compared to others of a similar age, gender and race. 

This is believed to the result of an extra collection of genes on the X chromosomes. Though different collections have been recorded in different patients, their location on the chromosome remains the same. This has led researchers to believe that this extra collection of genes activates other hair-growth genes.

This article was originally published in How It Works issue 122

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