Eye color for a plausible argument stating that some eyes can or do change, based on chemical reactions and hormonal changes within the body. Studies on Caucasian twins, both fraternal and identical, have shown that eye color over time can be subject to change, and major demelanization of the iris may also be genetically determined. Most eye-color changes have been observed or reported in the Caucasian population with hazel and amber eyes. Carleton Coon created a chart by the original Martin scale. The numbering is reversed on the scale below in the (later) Martin–Schultz scale, which is (still) used in physical anthropology.
American anthropology future behavior; and they cooperate in problem-solving. Nevertheless, the term "culture" applies to non-human animals only if we define culture as any or all learned behavior. Within mainstream physical anthropology, scholars tend to think that a more restrictive definition is necessary. These researchers are concerned with how human beings evolved to be different from other species. A more precise definition of culture, which excludes non-human social behavior, would allow physical anthropologists to study how humans evolved their unique capacity for "culture". Chimpanzees ("Pan troglodytes" and "Pan paniscus") are humans' ("Homo sapiens") closest living relatives; both are descended from a common