In addition to our upright posture and distinct mode of communication, one of the first distinctions an alien culture could notice if they lined up humans in a row with all the other primates would be their apparent lack of fur.
In actuality, humans are remarkably hairless in comparison to the majority of mammals (granted, with the exception of the occasional individual). This trait is shared by a small number of other mammals, including naked mole rats, rhinos, whales, and elephants. But how precisely did we get to this desolate state?
Does it serve us today in any way? And how do we explain why some areas of our bodies have thick, dense hair?
Humans do, in fact, have a lot of hair; on average, our body surface contains about five million hair follicles. But practically all of the hair follicles on the human body produce vellus hair, which is small, short, and fuzzy and comes from shallow follicles as opposed to the deeper, thicker terminal hairs that are only present on the head, our underarms, our pubic areas, and—mostly in men—our faces after puberty.
According to Tina Lasasi, a biological anthropologist at the University of Southern California who specialises in the science of hair and skin, we theoretically have hair on every inch of our body; it's simply miniaturised hair follicles. Yet it has been so drastically reduced in size that it no longer effectively insulates us.
Experts are unsure of the cause of this transition from thicker, coarser fur to very fine vellus hairs, as well as the precise timing of it. Yet, a number of hypotheses have been put forth as to what might have precipitated the loss of our hair. The so-called "body-cooling" idea, also referred to as the "savannah" hypothesis, is the prevalent theory among scientists. This suggests that early humans' increasing need to thermoregulate their bodies was a contributing factor in fur loss.
Homo erectus and later hominins began persistence hunting on the open savannah throughout the Pleistocene, pursuing their prey for long periods of time in order to exhaust it without the use of complex hunting tools, which only emerge in the fossil record later.
The loss of fur would have allowed them to sweat more effectively and cool off more quickly without the need for breaks, which would have prevented overheating during this endurance exercise.
Studies that have identified switches for some genes that control whether particular cells grow into sweat glands or hair follicles provide more support for this notion. The developmental pathway for each of these items is related, according to Lasasi. We can roughly estimate with confidence that 2-1.5 million years ago, people would have likely lost their body hair if we take a look at it in addition to some of the things we are able to deduce about genes that boosted human skin colour. According to a related argument proposed in the 1980s, moving to an upright bipedal position would have reduced the benefits of fur for reducing radiation emitted by human body (bar on the top of our heads). Without fur, we can sweat more effectively, hence
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