Why don't humans have fur?

Actually, humans do have fur. The hair on our bodies is the same as the fur of other mammals. The current theory is that fur (or body hair) is a protective mechanism, designed to help the body maintain a constant temperature. It warms the body in cold weather, and protects it from harmful sunlight in hot weather. When it gets too hot the body has other methods of reducing the heat. Humans reduce their body temperature by sweating (something that other fur covered mammals do not do). However having a lot of fur interferes with the heat loss via sweat, so humans evolved to have much less hair on their bodies.

Another (newer) theory is that humans evolved to have less body hair in order to get rid of bugs like fleas, lice etc. This apparently made them more attractive to members of the opposite sex, and so having less body hair became an evolutionary advantage. Personally I don't find this theory as acceptable as the body heat theory.

I found this link to an article that specifies a number of theories regarding the reason humans don't have fur, but unfortunately, the link is in Hebrew. Sorry.

The article states, among other things, that the heat-reducing advantage of bare skin was proven to be false, as sweat evaporates from fur twice as fast as from bare skin.

Good question.

Nobody knows for sure - evolutionary theory tells us that somewhere along the line, humans (human ancestors, to be exact) without fur were able to survive and reproduce better than their furry rivals, and that this caused us, as the survivor's descendants, to be furless as well.

As to why not having fur was an advantage at the time, there are several theories, none with enough proof behind them to be conclusive. Among them:

1. The "African savanna" theory: Our ancestors went through a period of living in the hot savanna, where having less hair meant that you were less likely to die of heat stroke
2. The "hygienic" theory: Less hair meant that parasites such as lice have less place to hide - having less hair meant that you were less likely to get a disease transmitted by parasites
3. The "mating" theory: Humans found hairless mates more attractive than hairy ones, leading to the latter having more descendants.

None of these theories are perfect, but I feel that the truth lies along these lines, perhaps a mixture of several factors.

Seeker

The big brains of homo sapiens need a far greater protein nourishment than lesser apes, such as chimpanzees. Thus our distant big-brained and pre-agriculture ancestors had a far greater dependence on eating meat than chimps. Most of human history has been in ice ages, and as ice sheets advanced, the ice pushed food animals southward, compressing their numbers in a band just south of the ice. It is here that our hunting-tooled ancestors found their best hunting for the protein-rich meat they needed to survive. But it was cold in this band of plenty, and some of these hominids had the imagination to clothe themselves with the fur skins of their kills to keep warm. Designing and making effective clothing requires imaginitive intelligence, manual dexterity and cooperative living - which in turn encouraged the evolution of greater intelligence.  Such clothing gave early humans a great adaptive advantage for maintaining correct body heat as seasons changed or they migrated widely.  Once they habitually wore animal skins, they lost most of the bulk of their own body fur and evolved the perspiration method of body temperature control in order to more efficiently manage body heat in a clothed condition. 

Subsequently, the imaginative intelligence and dexterity that early humans evolved for making animal-skin clothing enabled them to make houses for greater protection from weather and beasts, and then enabled farming to ensure much more reliable sources of protein for their big brains.

Eventually this human imaginative intelligence evolved, due to its survival challenges from intense human activities, to where it put men on the Moon. So we traded our fur for a space suit.