New research published in the American Journal of Biological Anthropology highlights the extreme sexual dimorphism in early human ancestors, revealing that some species like Australopithecus afarensis and A. africanus exhibited striking size differences between males and females. This newly uncovered information, led by anthropologist Adam D. Gordon of the University at Albany, challenges previous assumptions about early hominin social structures and behavior, offering fresh insights into their evolution and ecology. The study, titled Sexual Size Dimorphism in Australopithecus, not only sheds light on the physical differences between the sexes but also uncovers the evolutionary pressures that shaped these early ancestors. Gordon’s research, published in July 2025, is transforming the way we think about human evolution, bringing a new perspective to the social dynamics of our ancient ancestors.
Extreme Size Differences in Early Human Ancestors: A Game-Changer in Evolutionary Understanding
The newly published study reveals that early hominin species, such as Australopithecus afarensis, which includes the famous fossil Lucy, and A. africanus from southern Africa, displayed sexual dimorphism that surpassed what is seen in modern humans—and in some cases, even in gorillas. Male individuals of these species were significantly larger than females, suggesting that male competition for mates played a major role in shaping physical traits. “These weren’t modest differences,” said Gordon, an associate professor at the College of Arts and Sciences. “In the case of A. afarensis, males were dramatically larger than females—possibly more so than in any living great ape.” This revelation redefines the understanding of early hominin social organization, with major implications for how we view their evolutionary history.
The Role of Male Competition in Shaping Early Human Evolution
The substantial size differences between male and female A. afarensis and A. africanus suggest that these early hominins may have lived in social systems dominated by intense male-male competition. In species with high sexual size dimorphism, such as chimpanzees and gorillas, the largest males often monopolize access to females. Similarly, the high degree of dimorphism found in these ancient hominins points to similar competitive dynamics. Gordon emphasizes, “And although both of these extinct hominin species exhibited greater sex-specific size differences than modern humans do, they were also more different from each other in this respect than living ape species are, suggesting a greater diversity of evolutionary pressures acting on these closely related species than we had previously appreciated.” The findings suggest that male competition for mates was a significant factor driving the evolution of physical traits in these species.
Unveiling Evolutionary Pressures: Why Size Matters in Early Human Ancestors
The discovery of extreme sexual size dimorphism in early human ancestors provides new insights into the ecological and social pressures they faced. Gordon’s research demonstrates that both male competition and resource stress, particularly during pregnancy and lactation, played critical roles in shaping the size differences between the sexes. “This study provides strong evidence that sex-specific evolutionary pressures—likely involving both male competition for mates and resource stress acting more intensely on female size due to the metabolic constraints of pregnancy and lactation—played a larger role in early hominin evolution than previously believed.” The study offers a more nuanced understanding of how early human ancestors interacted with their environment and one another.
Reassessing the Social Structures of Early Human Ancestors
The findings of Gordon’s research challenge the way scientists have historically grouped early human ancestors. Prior to this study, A. afarensis and A. africanus were typically placed together as part of the gracile australopiths, a group of species thought to have had similar social and physical interactions. However, the extreme differences in sexual dimorphism between the two species suggest they may have experienced distinct evolutionary pressures. “We typically place these early hominins together in a single group called the gracile australopiths, a group of species that are thought to have interacted with their physical and social environments in very similar ways,” Gordon said. “And while that’s true to a certain extent—the evidence suggests that both these species may have had social organizations more like gorillas than modern people—the significant difference in the amount of dimorphism in these two extinct species suggests that these closely related hominin species were subject to selection pressures more distinct than the selection pressures applied to any pair of similarly closely related living ape species.”
The Methodology Behind Unveiling Sexual Dimorphism in Ancient Fossils
One of the most innovative aspects of Gordon’s research is his approach to analyzing fossil data. Fossil records are often incomplete, making it difficult to draw clear conclusions about sexual dimorphism in extinct species. To overcome this challenge, Gordon used a novel iterative resampling method to simulate comparisons between fossil hominins and modern primates. This approach allowed him to work with fragmented fossil remains while still producing statistically significant results. “This analysis overcomes these issues by using an iterative resampling method that mimics the missing data structure in both fossil species when sampling from skeletal material of living species, allowing the inclusion of multiple fossil individuals even when those individual specimens are fragmentary,” Gordon explained. This breakthrough methodology enabled a more thorough exploration of size differences in early human ancestors.
Source link
