Mammals are endotherms capable of maintaining a relatively narrow range of variance in internal body temperature in the face of wide ranging environmental temperature changes.
With mammals, the closing of the cardiopulmunary loop and establishment of the four-chambered heart brings about a significant increase in blood oxygen content thereby endowing mammals with a burst of metabolic resources relative to their non-mammalian ancestors.
The acquistion of endothermy facilitated a shift to nocturnal lifestyle in early mammalian evolution, possibly as a means to avoid diurnal predators (Geiser et al. 2002).
The stabilization of internal body temperature has important implications from a biochemical and physiological perspective with respect to metabolic rate and the neural mechanisms of temporal synchronization of activity across widely distributed cell populations in the body.
It is well known that changes in temperature affect metabolic rates - a 10 degree celsius change in temperature results in a approximate doubling of the metabolic rate.
With the stabilization of temperature, selection pressures on metabolic temperature compensation mechanisms are released thereby lifting a set of previously conserved structural and kinetic constraints on protein evolution. .