New research reveals that males of some of humanity’s earliest known ancestors were significantly larger than females, suggesting their societies were shaped by intense competition. The study, published in the July issue of the American Journal of Biological Anthropology, found that both Australopithecus afarensis—the species of the famous fossil “Lucy”—and the related A. africanus showed a pronounced difference in body size between the sexes. This finding challenges previous theories that some early hominins exhibited size differences similar to those of modern humans, and points to a more complex and diverse social past than previously believed.

The key to this discovery lies in sexual size dimorphism, or SSD, which is the average size difference between males and females of a species. In living primates, high SSD, such as that seen in gorillas, is often associated with social systems where large males compete fiercely for multiple mates. In contrast, species with low SSD, such as modern humans, tend to have less intense male competition. The new analysis reveals that both Australopithecus species exhibited high dimorphism, surpassing that of humans and chimpanzees, indicating that male competition and environmental pressures, such as food scarcity, played a significant role in their evolution.

Led by University at Albany anthropologist Adam D. Gordon, the research team overcame a significant obstacle in paleoanthropology: the incomplete and fragmentary nature of the fossil record. Traditionally, comparing dimorphism between fossil species has been statistically difficult. Gordon’s team used a novel iterative resampling method, which creates thousands of simulated fossil samples from complete skeletons of modern humans, chimpanzees, and gorillas. This technique enabled a robust comparison that mirrored the missing data patterns observed in ancient hominin fossils, revealing apparent differences where previous studies had found none.

These findings suggest that early hominins were not a uniform group. While both species were highly competitive, A. afarensis males were “dramatically larger” than females, possibly more so than any living great ape, according to Gordon. The significant variation in dimorphism between A. afarensis and A. africanus indicates that these closely related species were shaped by a surprisingly diverse set of evolutionary pressures. This glimpse into our ancestors’ lives reveals a world where large size was a key factor in male reproductive success, providing a different perspective on early hominin social dynamics.

References

• Gordon, A. D. (2025). Sexual size dimorphism in australopithecus: Postcranial dimorphism differs significantly among australopithecus afarensis , a. Africanus , and modern humans despite low‐power resampling analyses. American Journal of Biological Anthropology, 187(3), e70093. https://doi.org/10.1002/ajpa.70093

• Parker, M. & University at Albany. (2025, July 29). Early human ancestors showed extreme size differences between males and females. Phys.Org; University at Albany. https://phys.org/news/2025-07-early-human-ancestors-extreme-size.html