Dark skin (a result of increased melanin production in equatorial peoples) is likely a response to ultraviolet radiation, because uv radiation can cause:

Amino acids contain the elements carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur. of the 20 amino acids found in humans, 11 are called "nonessential" because they can be manufactured by the body when needed. which elements in these 11 amino acids are commonly obtained from the metabolism of sugar molecules?