Endothermy, i.e. the endogenous production of metabolic heat, has evolved multiple times among vertebrates, and several strategies of heat production have been studied extensively by physiologists over the course of the twentieth century.
The independent acquisition of endothermy by mammals and birds has been the subject of many hypotheses regarding their origin and associated evolutionary constraints.
let me repeat this one – ” The independent acquisition of endothermy [warm blood] by mammals and birds has been the subject of many hypotheses”
yeah, i can imagine why …