The aorta in humans and African great apes, and cardiac output and metabolic levels in human evolution.

Humans have a larger energy budget than great apes, allowing the combination of the metabolically expensive traits that define our life history. This budget is ultimately related to the cardiac output, the product of the blood pumped from the ventricle and the number of heart beats per minute, a measure of the blood available for the whole organism physiological activity. To show the relationship between cardiac output and energy expenditure in hominid evolution, we study a surrogate measure of cardiac output, the aortic root diameter, in humans and great apes. When compared to gorillas and chimpanzees, humans present an increased body mass adjusted aortic root diameter. We also use data from the literature to show that over the human lifespan, cardiac output and total energy expenditure follow almost identical trajectories, with a marked increase during the period of brain growth, and a plateau during most of the adult life. The limited variation of adjusted cardiac output with sex, age and physical activity supports the compensation model of energy expenditure in humans. Finally, we present a first study of cardiac output in the skeleton through the study of the aortic impression in the vertebral bodies of the spine. It is absent in great apes, and present in humans and Neanderthals, large-brained hominins with an extended life cycle. An increased adjusted cardiac output, underlying higher total energy expenditure, would have been a key process in human evolution.