The Structure of Hair
There are 3 basic types of human hair; African, Caucasian and Asian. While they're virtually identical, genetically, there are small, structural differences. These were minor evolutionary adaptations to environmental changes.
African Hair Caucasian Hair Asian Hair
African Hair is the original Human Hair type. It evolved in a moist, subtropical environment, where moisture retention in hair wasn't an issue. African Hair has a smaller cortex. The shape of the this cortex is what causes it to kink. It also doesn't retain oils as well, leaving it more susceptible to frizz and dryness; particularly when permed, colored or chemically treatment.
Caucasian Hair's oval shaped cortex produces it’s natural wave. While the cortex is slightly larger (in part, a response to colder, dryer northern climates), it isn't great at retaining moisture, either. This allows Caucasian Hair to become dry and brittle, and lack natural shine; particularly when permed, colored or chemically treatment.
Asian Hair is the strongest, most structurally tough type of Human Hair. This round structure is what makes the hair so straight (It also makes it harder to perm). Asian Hair has the largest cortex of any type. This allows it to retain more natural moisture. There is more surface area to the hair which results in more cuticle area. So it reflects more light, giving it a natural shine.