Why are there so many hair colours?
Humans, and mammals in general, can only produce two types of pigment, two types of melanin: eumelanin, which is dark brown, almost black, and pheomelanin, which is reddish-yellow and much lighter. The combination of the two pigments gives rise to the whole range of hair colours from black to the lightest blonde, including the absence of colour in people with some form of albinism, who are unable to accumulate pigment.
Today we know of about 700 genes (out of the 20,000 or so we have), about 3% of our total genome, that are directly or indirectly responsible for our hair colour – and our skin and eye colour. Most people on Earth have dark hair, close to black. This is the general trend in Africa, South America and Asia. In all these countries it is very rare to find a person who does not have very dark, black hair. This was probably the colour of the hair of our ancestors, the ancestral hominids that gave rise to the current human population, whether of African or Asian origin, as the palaeontologists José María Bermúdez de Castro and María Martinón-Torres of the National Centre for Research on Human Evolution (CENIEH) in Burgos (Spain) have recently pointed out.
The variety of lighter colours that we usually call red-brown or blonde, with an almost infinite range of shades, is essentially found in Europe. And, of course, it extends to countries that have historically been nourished by European emigration, such as North America and Australia.
Why does Europe have the greatest variety of hair colours? What is the difference that has caused the great diversity of brown, blonde, and red hair in Europe and not in any other part of the world? One possible explanation has to do with the adaptation of populations that migrated from Africa or Asia and gradually arrived in the areas of northern Europe, the British Isles and Scandinavia.
Ancestral hominids had dark skin and dark eyes. These were very useful features to protect them from the sun, which, without sufficient protection, would constantly burn their skin. In addition, the layer of pigment in the skin prevented the breakdown of a vitamin, B8, which is essential for proper embryonic development and reproduction. However, when these hominids arrived in northern countries, where they barely saw the sun for six months of the year, the dark skin that protected them in Africa became a problem. We need the sun to complete the synthesis of vitamin D. So those hominids who accumulated skin-lightening mutations had a clear advantage and left more offspring. Eventually, after many years, the traits that caused the dark skin colour to fade took hold and spread through the population.
The mutation of a single gene, MC1R, was probably the most influential of all. This mutation causes people to stop producing dark eumelanin and only produce light pheomelanin, resulting in a significant reduction in skin pigmentation. The skin becomes very white (and able to take advantage of the little sunlight they have in the north), the eyes also reduce their pigmentation and we see them as blue, and finally the hair takes on the characteristic pheomelanin colour of redheads. In fact, red-haired hominids appeared in northern Europe, where there are still areas where 30% of people have these characteristics. And if we mix red hair colour with the function of other genes that lighten the tone, we get a gradient of oranges from the lightest blonde to the darkest brown.
Blonde hair colours accumulate the most mutations of all. This is why blonde hair is relatively rare in general, but predominant in northern countries. Especially in women. There are more women with blonde hair than men, because some of these mutations are linked to sexual characteristics that we do not yet fully understand.
Question submitted by Quique López.
Answered by Lluis Montoliu, CSIC researcher, Deputy Director of the National Biotechnology Centre, and member of the CIBER Rare Diseases Networking Biomedical Research Centre (CIBERER-ISCIII). He has recently published a book, Genes de colores (Next Door Publishers), in which he describes in detail and in clear language the function of all those genes that determine the colour of our skin, eyes, and hair.
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