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Since the publication of Charles Darwin's The Descent of Man and Selection in Relation to Sex in 1871 (1), there has been a vigorous debate about the meaning of sexual dimorphism for a range of physical attributes in numerous animal species, including primates and humans, extinct and extant. Key points of discussion are how to interpret size dimorphism in past humans and human-like ancestors and what inferences can be drawn about the evolution of human mating systems and social organization. In this issue of PNAS, Reno et al. (2) report on their investigation of sexual dimorphism in the three-million-year-old Australopithecus afarensis, an important and well known hominid, ancestral to the genus Homo (3). Insight into dimorphism in this taxon has important implications for social behavior and organization in later and present-day humans. Body mass dimorphism varies dramatically among primate species, both present and past. For most anthropoids, males are bigger than females (4–8). Humans today display relatively limited sexual dimorphism (≈15%), whereas some of the other hominoids (gorillas and orangutans) are highly dimorphic (>50%) (5, 9). Body mass is easily determined in living species. For past nonhuman primates and human ancestors, mostly represented by fragmentary fossil remains, body mass is far less accessible. Recently, the femur head (the ball of bone at the top of the femur that fits into the hip joint) has been invoked as a source for estimating body mass in early hominids, Homo, and its evolutionary predecessor, Australopithecus (10, 11). Comparisons of body mass in fossil hominids reveal that general levels …

Equality for the sexes in human evolution? Early hominid sexual dimorphism and implications for mating systems and social behavior