Mammals were the first animals to evolve body hair and it is still one of their common features today. Even mammals like whales and dolphins are born with hair above their mouths, even though the adults are hair-less.
Humans appear to have very little hair compared to most mammals. However, there are as many hairs on a human as on our closest living relative, the chimpanzee. The hairs are much smaller and thinner, which is why we appear not to have fur.
Fur, or body hair, is a way of stopping our body heat from escaping. It isn’t just the fur itself that keeps us warm but the layer of air that is trapped between the hairs. The more air you can trap next to your body, the more insulation it provides.
Insulation is particularly important for mammals because they are warm-blooded. This means they keep their body temperature at about the same level whatever environmental conditions are like.
When the weather is cold, mammals have to burn energy to heat their bodies. This would be a very inefficient use of energy if the heat were to immediately escape out of their skin, which is why they need fur. The advantage is that it allows them to remain active even during cold weather.
Reptiles and amphibians are cold-blooded and don’t have body hair to keep them warm. They depend entirely on the sun for warmth and when the temperature is low they have to remain inactive for long periods of time.
Birds are also warm-blooded, but they evolved this feature separately from mammals. They have feathers instead of hair to help retain their body heat.