Thalassemia is another anemia associated with protection against malaria. Genetic differences in the structure of adult hemoglobin. Occurs in childhood because fetal hemoglobin gets one through until about the age of 2. Lowered efficiency of RBC metabolism may combat the malarial parasite. Color depends on melanin content in skin.
Melanin absorbs UV radiation and protects skin from cancer.
Temporary increase in melanin leads to suntan. May also have to do with Vitamin D synthesis, in that people with darker skin were thought not to manufacture as much Vitamin D as people with fair skin, which may also account for skin color distribution so that people with darker skin live in sunnier, equatorial regions. This has been questioned because there is no paleopathological evidence of rickets. Also, recent work has shown that people with dark skin make as much Vitamin D as those with lighter skin. As body size increases, surface area increases as the square, but body mass increases as the cube.