Final answer:
Natural curl is believed to be determined by the structure of keratin strands in the hair and related disulfide bonds. These can be temporarily or permanently altered through styling processes that affect hydrogen or disulfide bonds. Additionally, the evolution of curly hair may have offered protective benefits or improved temperature regulation.
Step-by-step explanation:
At present, natural curl is believed to be the result of the configuration of keratin strands within the hair and their bonds. The predisposition for hair to curl is primarily determined by genetics, which dictate the type of hair fibers produced including their shape and the way they emerge from the hair follicle. Curly hair tends to form as a result of the follicle's shape and the way the keratin strands are held together by various types of bonds within the hair strand.
More specifically, disulfide bonds play a critical role in the natural curl of hair. They are strong links between sulfur atoms in two different parts of a keratin strand and they help to define the hair's strength and shape. The natural shape can be temporarily or permanently altered by manipulating these disulfide bonds through chemical treatments for waving or straightening hair.
In the case of temporary waves, when hair is wet, water molecules penetrate the keratin fibers, breaking some hydrogen bonds that assist in holding the hair's shape. As the hair dries and new hydrogen bonds form, the hair can maintain a new temporary shape. However, over time or when exposed to moisture again, these temporary bonds can break, returning the hair to its natural state.
As for the evolutionary aspect, it's thought that curly hair may have evolved to provide better protection from UV light or because less hairy skin allowed sweat to evaporate more efficiently, enhancing the body's ability to regulate temperature. Regardless, hair has significant social, cultural, and biological meaning across different human societies.