[...] a handful of scientists think that these ultra-marathoners are using their bodies just as our hominid forbears once did, a theory known as the endurance running hypothesis (ER). ER proponents believe that being able to run for extended lengths of time is an adapted trait, most likely for obtaining food, and was the catalyst that forced Homo erectus to evolve from its apelike ancestors. Over time, the survival of the swift-footed shaped the anatomy of modern humans, giving us a body that is difficult to explain absent a marathoning past.

Our toes, for instance, are shorter and stubbier than those of nearly all other primates, including chimpanzees, a trait that has long been attributed to our committed bipedalism. But a study published in the March 1 issue of the Journal of Experimental Biology, by anthropologists Daniel Lieberman and Campbell Rolian, provides evidence that short toes make human feet exquisitely suited to substantial amounts of running. In tests where 15 subjects ran and walked on pressure-sensitive treadmills, Lieberman and Rolian found that toe length had no effect on walking. Yet when the subjects were running, an increase in toe length of just 20 percent doubled the amount of mechanical work, meaning that the longer-toed subjects required more metabolic energy, and each footfall produced more shock.