What drove the evolution of endothermy?

The metabolic production of heat and high, stable body temperatures characteristic of birds and mammals was a major step in the evolution of vertebrate animals; however, the 1979 paper by Albert Bennett and John Ruben (Bennett and Ruben, 1979) represented a fundamental shift in thinking about the problem of the evolution of endothermy. They argued that endothermy based on thermoregulatory considerations alone (e.g. Bogert, 1949; Cowles, 1958) was not the sole selective advantage for endothermy and perhaps not the initial factor for its evolution. Instead, Bennett and Ruben postulated that the evolution of endothermy was directly linked to the selection for high levels of activity sustained by aerobic metabolism.