What is the evolution of hairs? Did they evolve from scales?
From a little research I have been able to find some of the hypotheses and speculations but no papers expressly confirming or denying the matter.
One theory displayed in "The evolution of mammalian keratinized structures" (abstract only) is that mammalian hair developed along the following pathway:
- Hair follicles develop between reptilian like scales.
- A granular layer formed in the pilosebaceous canal (see diagram of modern human skin)
- The granular layer extended into the epidermis. I read this as the point where scales started to more resemble mammalian skin. It is suggested that this was as a result of a need for increased flexibility and agility.
- Hairs began to protrude through the follicles (as seen in rat tails today). Adaptations to organisms living in cold conditions led to the production of wool to help conserve heat.
So this paper would suggest that hair production evolved separately from but parallel to the movement away from reptilian scales.
"Some Speculations on the Evolution of the Vertebrate Integument" (Paul F. A. Maderson) suggests an entirely different origin of hairs. He proposes that hairs ultimately evolved from sensory appendages that were involved in the detection of temperature in early synapsids. The extension of these micro-limbs into hairs was selected towards as a result of their thermally insulating effect by-product rather than for their sensory role, however.
As to the early evolution of mammalian hair, Rowe et al. (2011) hypothesized that the primitive function for hair was not thermoregulatory, but rather for tactile sensation (contra the hypotheses of Spearman and Maderson). Rowe et al. say:
Body hair develops as migrating neural crest cells induce patterns of tiny placodes that mature into hair follicles equipped with mechanoreceptors. These include lanceolate endings (velocity detectors excited by hair deflection), Ruffini receptors (tension receptors activated as hair is bent), and Merkel cells (slowly adapting sensors). In ontogeny, hair is first sensory, and only later does it insulate, as underfur thickens and thermoregulation matures.
So the developmental argument is in favor of tactile sensation.