Humans are evolutionarily adapted to caloric restriction resulting from ecologically dictated dietary deprivation imposed during the Plio-Pleistocene period.

Humans are evolutionarily adapted to chronic undernutrition as a consequence of ecologically dictated dietary restriction. Increased aridity, cooler temperatures and increased climatic oscillation effected an alteration of the quantity and quality of vegetation upon which hominids depended for food during the Plio-Pleistocene period. Hominids responded physiologically to climate-induced caloric curtailment in the same way organisms respond to experimentally imposed caloric restriction: by reducing the rate and/or altering the manner in which they metabolized fuel. Such metabolic alterations are mediated principally by the hypothalamus and it is herein hypothesized that the human hypothalamus was subjected to substantial selective pressure, promoting an energetically conservative hypometabolic state. Moreover, the most salient phenotypic characteristics typifying the human species - long lifespan, low reproductive potential, lengthy development and high brain/bodyweight ratio - are effectuated in organisms undergoing caloric restriction. These phenotypic/physiological characteristics - herein termed the quadripartite complex - can be modulated by metabolic rate, which is, in turn, modulated by the hypothalamus. An appreciable alteration in climate occurred between 2.0 and 1.5 million years ago, a juncture at which one hominid lineage (Paranthropus) went extinct. Paranthropus was characterized by such external adaptations as robust cranio-facial morphology and pronounced enamel deposition, indicative of subsistence on tough, low-quality vegetal foods. Conversely, the Homo lineage responded to its marginal dietary repertoire through internal means, centering on metabolic suppression. It is herein hypothesized that this adaptive metabolic alteration, enacted in response to ecologically imposed caloric restriction, produced the defining morphologic attributes of Homo and enabled the evolutionary success of the human species. Among the implications of this line of thinking is that modern humans may be particularly sensitive to the deleterious effects of excess energy intake and, concomitantly, particularly amenable to the ameliorative effects of caloric restriction.