Which component of energy expenditure is the most variable from one individual to another? adaptive thermogenesis thermic effect of food basal metabolism energy cost of physical activity?

Out of all the activities that lead to energy expenditure in the human body, the one that varies most from one individual to another is the basal metabolism energy or basal metabolic rate. 
The BMR is the energy expended in order to maintain bodily functions such as respiration, circulation, homeostasis, hormone secretion and growth. The basal metabolic rate of a person is highly dependent on the person's lean body mass.

  like australopithecus afarensis; probably the direct ancestor of this group. africa: 5.8 to 4.4 million years ago.