The Evolution of Brain and Body Size in Genus Homo

Humans, and most other late Homo species, are characterized by large brains and bodies. However, the discovery of two small-brained Homo species—H. floresiensis and Homo naledi—has cast doubts on large brain size as a defining feature of our genus. We reevaluated brain and body size scaling using data for 225 extant primates and 16 fossil hominid taxa, including one of the most diminutive species in genus Homo, H. floresiensis. Brain and body size are tightly correlated in genus Homo, varying along a positively allometric slope (R2 = 0.84, F(1,5) = 33, p < 0.01) that is significantly different from the slope characterizing extant primates (R2 = 0.94, F(1,222) = 3294, p < 0.001). Both small-bodied Homo floresiensis and Homo naledi have endocranial volumes (ECVs) that are consistent with their body size given the scaling relationship that characterizes genus Homo. Paired ECV and body mass estimates demonstrate considerable overlap of brain:body size proportions across fossil hominid taxa. Earlier hominids, Ardipithecus ramidus and Australopithecus anamensis, are characterized by ancestral brain:body size scaling; we discuss the hypothesis that a fundamental biological shift ca. 3 Ma altered the trajectory of encephalization—potentially linked to changes in fetal growth and gestation in Pleistocene fossil hominids—and may be directly implicated in the evolution of complex symbolic behavior in our lineage.