An open access paper [pdf] published recently in PNAS delves into one of the most integral questions to the understanding of energetics and evolution. According to the abstract, “A fundamental but unanswerable biological question asks how much energy, on average, Earth’s different life forms spend per unit mass per unit time to remain alive.” Unanswerable? Perhaps. Scientists, however, are stubborn.

Using a database of over 3,000 species the authors demonstrate that, despite huge differences in body mass, physiology, biochemistry and ecology, there is marked homeostatis in the metabolic rate of life. Although there is a 30-fold variation in basal metabolic rates across life (0.3 to 9 W kg ^-1), this does not come close to the 4,000 to 65,000-fold difference between the metabolic rates of the smallest and largest organisms observed if life conformed to the conventional allometric scaling laws.

From these results the authors postulate that natural selection has favored a metabolic rate within this ‘physiological window’, and that this window of convergence may be the optimal metabolic rate for life as a whole.