Via PubMed, an interesting, if highly speculative paper on the evolutionary origins of the health and longevity benefits of calorie restriction: "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. ... 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."

Link: http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=16406387