Modern hair biology tends to focus on the study of humans and a small number of model species. However, by exploring the structure and function of hair follicles in some lesser-known species, we can gain clues about the evolution and development of hair.

Hair is ancient. It evolved in our synapsid ancestors to become a key characteristic of mammals by the time of the divergence of monotremes some 180 million years ago. Yet many adaptations are much more recent. Coat structure and hair cycles can differ widely within mammalian families. Specific genetic mutations and artificial selection in domesticated species can radically alter hair phenotypes. Such variation is evidence of pliable factors within the molecular and developmental systems.

However, some recurring patterns are noteworthy. Follicle types and group arrangements arise from an ontological hierarchy of follicles in which the initial generations are usually dominant, e.g. in “double coated” species, the earlier developing follicles produce course guard hair whereas the smaller, later formed follicles grow finer underfur or wool. Branching morphogenesis results in compound follicles which might comprise only later forming secondary follicles as in goats and sheep, or include all follicle types as in mustelids. This follicle branching may continue into adult life (neogenesis), as in brushtail possums. Follicle growth is often exquisitely regulated to produce fibre structures that fulfil protective or insulative functions. Hair cycles of many mammals are responsive to hormones and environmental cues to renew coats according to season or sexual communication.

Homologous follicle group patterns are also evident in human skin, suggesting that the follicle population should not be regarded as uniform. Comparisons can be made between human alopecia and seasonal follicle regression in other mammals. By investigating the mechanisms underlying such phenomena, we might better understand the biology of human hair and its clinical conditions.