The hairy timeline of evolution – Fellows’ seminar by Nina Jablonski

27 January 2020

“Human beings are unique among mammals in their possession of visible hair on select parts of the body. The loss of most body hair in the course of early human evolution is now well understood as an adaptation for keeping cool by sweating under conditions of sustained exercise in hot environments,” said Nina Jablonski of the Department of Anthropology at Pennsylvania State University. “But why didn’t we lose all of our hair? Why did we retain hair in just a few places, especially on the top of our heads?”

“Why do we have it where we have it and why in specific forms and colours?”

Permanent STIAS fellow Jablonski described her current book project as a topic born out of a challenge posed at STIAS in 2012.

STIAS Fellow Nina Jablonski during her seminar on 23 January 2020

“It’s a topic that has been relegated to the periphery academically which is rich for exploration,” she said. “Hair is a medium of human expression. Hair is its own language.”

In her presentation, she explored some of the mysteries surrounding the evolution of human hair, including the origins of different hair forms and ever-growing scalp hair, and the development of complex social signalling through hair styles and colour.

“Considerable human creativity and technological innovation have been focused on the management and appearance of hair,” she said, “and the study of these phenomena reveals that ‘hair culture’ has been an important driver of human evolution.”

She pointed out that the Homo genus started to lose body hair about 2 million years ago taking us on a different trajectory from our chimp-like ancestors. Early members of the genus Homo engaged in long periods of sustained physical activity in hot and sunny environments. We had to increase our sweating capacity and keep our brain and body cool by whole body cooling – unlike other mammals, humans don’t have the ability to pant. Hair impedes the evaporation of sweat and therefore cooling. So the loss of hair was for thermoregulatory reasons caused by the sustained assault of solar radiation experienced in equatorial Africa.

But why did we keep the hair on our heads?

“Because of human’s upright stance there was a smaller area in the path of overhead solar radiation,” said Jablonski. “The hair was used to either dissipate or absorb heat.”

Jablonski and one of her PhD students, who is collaborating with investigators at Loughborough University, used thermal manikins to test the effect of different hair types – straight, wavy and tight curly – on solar radiation falling on the top of the head.

The tight curls showed minimum heat gain. Tightly curled hair disseminates heat and resists heat gain – making it more suitable for warmer conditions. Straight hair was inferior at dissipating heat but enhances heat gain – thus is more suitable for cooler conditions.

Turning to colour, Jablonski pointed out that hair owes its natural colour to two basic chemical building blocks. These are melanin – eumelanin and pheomelanin. Different colours have different capacity to absorb ultraviolet and visible light. There is considerable variation within shades.

“Not all black is the same,” said Jablonski. “There is huge variation in the melanin content leading to different light absorption. For example, African and Asian black hair has significantly more absorption potential than black hair in Europe.”

“White hair is the absence of pigment – caused by aging and stress – the melanin-producing cells in the hair follicles age and disappear.”

A genetic crapshoot

“Blonde and red hair has been accorded some degree of ‘specialness’ but it’s really just a genetic accident,” she added.

“Redheads have different forms of the MCIR receptor gene while blondes have the G allele of the KITLG gene. Blondes are most prevalent in Northern Europe but also occur in Aboriginals in Australia, in Melanesia and even in the San people in Africa. Blonde hair has evolved more than once, because of independent genetic events.”

“Approximately 70 000 years ago Homo made its first movements out of Africa in search of food. This was in small groups and led to genetic bottlenecks with decreased genetic diversity. So it was really a genetic crapshoot – an accidental assembly of genes which became fixed in small populations.”

Jablonski indicated that she is therefore hesitant to attach adaptive explanations to all the possible changes. “Many populations became extinct – in particular during the Ice Age,” she said. “So it might be accidental that some genetic traits were favoured over others.”

“We still don’t know why only some body hair grows. We also don’t know why long, ever-growing hair developed in the first place. Again it could be a function of a population bottleneck within Ice Age Africa. Such ‘genetic accidents’ then conferred other advantages, or at least they weren’t disadvantageous.”

And when and why did we start deliberately cutting and styling hair?

“Here we are at the mercy of the archaeological record,” she said. “There is no real evidence for human hair cutting on stone tools but no one has been looking for it.  I think there is every reason to believe that we have been doing it for many tens of thousands of years. This activity could have been done by stone tools or biodegradable tools, which were not preserved in the archaeological record.”

“Hair styles have been a big thing for a very long time. In animals they are used to identify species as well as facial expression, emotional state and intention,” she continued. “We have been deliberately styling for at least 100 000 years.”

“The Egyptians, for example, used elaborate styles and wigs as an important part of ceremonial activities.”

“Hair styles are one of the most copied phenomena in human history – for aesthetic reasons, to indicate economic or marital status and cultural appropriateness.”

Styling required time and was usually a collective social activity which is still a feature of hairdressers and barber shops today. Self-styling only became possible with the development of mirrors which occurred in Egypt, China and Japan roughly simultaneously.

“Male hair has received equal, if not more, attention.”

“Hair is still grounded in sexual selection and social acceptability,” she added. “Women are often held to higher standards, and the presence of hair in undesirable places and its link to femininity has changed over time.”

We instantly assess and judge based on hair style and colour and the politics of hair remain unavoidable.

Whether colour and style are used for statement making by sports stars or become a point of ridicule for politicians like Donald Trump and Boris Johnson – “The funny aside often precedes other more serious remarks,” said Jablonski.

“Hillary Clinton famously said ‘pay attention to your hair because everyone else will’,” she continued.

“Natural hair in the workplace has become more common and acceptable in the last ten years but is still not accepted overall. The natural hair movement has become important and empowering for women.”

“The racialised discussion of hair and skin differences is unfortunate,” said Jablonski. “However, I’ve worked in this space for 25 years and have encountered little negative feedback. I believe you have to start at the beginning with the evolutionary evidence. We have to tell a story of what happened and why to the best of our ability using non-pejorative language. We have to interpret, reinterpret, fix and counter some of the anguish that these issues have caused.”

Michelle Galloway: Part-time media officer at STIAS
Photograph: Christoff Pauw

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