Compared to other large mammals on Earth, we humans have relatively little hair. This obvious fact is not so strange until we look at our closest living relatives in the animal kingdom – chimpanzees and other great apes. Why, might we ask, have we evolved to be almost hairless when all other primates are still covered in fur? In fact, bizarre as it may seem, our lack of hair has been the key to our evolutionary success as a species.

The question why our earliest ancestors lost their hair when fur would have been beneficial to them – for keeping warm on cold nights, for example – was pondered by Charles Darwin, the famous evolutionist: "No one supposes that the nakedness of the skin is any direct advantage to man." He concluded that the reason hominins lost their hair was sexual selection. In other words, we prefer our partners to have little hair. But what caused our ancestors to start losing their hair before this preference set in?

The most likely hypothesis to explain the loss of hair is that it became necessary when early hominids moved to a more open savannah habitat around 2 to 3 million years ago, when they started to hunt big game.Hunting in open savannah meant being exposed to the strong heat of the sun for several hours in the day and being in danger of overheating. Being covered in hair prevented our ancestors from losing heat fast enough. So, as Peter Wheeler, of Liverpool John Moores University in the UK, explained, losing hair allowed them to develop the ability to sweat and therefore to cool down. Early humans could, as a consequence, be out at midday hunting and foraging. "It would be [an] enormous advantage to be able to spend the entire midday foraging, finding mates or fighting enemies", Tamás Dávid-Barrett of Oxford University says. "Sweating allows that, and for sweat to be efficient you need to be mostly hairless". The meat from hunting animals gave early humans the energy to fuel their growing brain.

Humans are the sweatiest primates on Earth. They have up to 5 million sweat glands,which produce about 12 litres of sweat per day. It's this propensity to sweat that accounts for our hairlessness and has allowed us to thrive. It might help to remember this next time you're surrounded by sweaty hairless torsos in the gym!
词汇表
primates 灵长目动物
bizarre 奇怪的，离奇的
species （动植物的）物种
to ponder 思索，考虑
evolutionist 进化论者
nakedness 裸露
hominins 古人类
sexual selection 雌雄淘汰
ancestor 祖先
hypothesis 假说，假设
hominids 原人
savannah 稀树草原
to hunt 打猎
game 野禽
early human 早期人类
foraging 觅食，搜寻食物
find a mate 寻找配偶
gland 腺
propensity 习性，倾向
to thrive 长得健壮
torso （人体的）躯干