<span>Human skin is one of the more important cooling systems. Unlike many other mammals, humans have sweat glands. These cool the surface of the skin by evaporating water. Body heat is carried to the surface of the skin by the circulatory system. Humans also have finer hair than other animals. We are actually as hairy as chimpanzees, but our hair is so fine it makes us appear to have none. This lack of dense hair allows the skin to evaporate sweat more efficiently. Another human cooling mechanism is our posture. Walking upright means only a small portion of the body is exposed to direct sun during the hottest part of the day. Our ancestors were thought to scavenge during this period because preditors had to escape the sun. Walking on 4 legs meant the entire surface of the back was exposed to the sun.A uniquely human adaptation to heat is the scalp. Blood vessels run directly from the surface of the brain through pores in the skull and out across the head and face. This serves to cool the brain. Humans have enromous brains which generate quite a lot of heat. This system of veins serves like a kind of radiator, keeping the brain from overheating. It also explains why head injuries tend to bleed so profusel</span>
Long strands of double-helical DNA fit into the nucleus of a single cheek cell, the double helix is packaged and wrapped around proteins, folded back onto itself and coiled into a compact chromosome. 4.
