New research by UAlbany anthropologist Adam D. Gordon finds substantial sexual dimorphism in some of our early human ancestors. Credit: Ken Zirkel,the Museum of Natural History
A newly published study has found that males of some of our earliest known ancestors were significantly larger than females. The pronounced difference in body size present in both Australopithecus afarensis (the East African species that includes the famous fossil “Lucy”) and A. africanus (a closely related southern African species) suggests the ancient hominins may have lived in social systems marked by intense competition among males, leading to the substantial size disparity among the sexes.
The research, led by University at Albany anthropologist Adam D. Gordon, appears in the July issue of the American Journal of Biological Anthropology. Using a novel approach that overcomes the limitations of incomplete fossil records, the study reveals that both A. afarensis and A. africanus were more sexually dimorphic than modern humans—and in some cases, even more than gorillas.
“These weren’t modest differences,” said Gordon, an associate professor in 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.
“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 results add new depth to interpretations of the fossil record. Earlier studies had reached differing conclusions about dimorphism in A. afarensis, with some suggesting it resembled the relatively low levels seen in modern humans. Furthermore, direct comparison between fossil species had not previously been made because investigations were limited by incomplete fossil samples and insufficient statistical power to detect real differences.
“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,” said Gordon.
“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.”
Why sexual size dimorphism matters
Sexual size dimorphism (SSD) isn’t just a physical trait—it reveals something deeper about behavior and evolutionary strategy. Consistent with sexual selection theory, high SSD in living primates typically correlates with strong male–male competition and social structures allowing for polygynous mating systems, where one or a few large males monopolize reproductive access to multiple females.
In contrast, low SSD can be found in any species, but tends to be found in those with pair-bonded social structures and low competition for mating opportunities. Modern human populations exhibit low to moderate SSD, where males tend to be slightly larger than females on average but with substantial overlap in size between the sexes.
In addition, Gordon’s previous research suggests that high SSD in living primates can also be associated with intense resource stress—when food is scarce, small healthy females can get enough food to meet their own metabolic needs and store energy for reproduction faster than larger females can, leading to more offspring with smaller mothers in the next generation and a resulting greater difference in male and female size.
The high SSD identified in both Australopithecus species suggests a high degree of competition among males, similar to that of chimpanzees or even gorillas, while the difference between the two fossil species may be due to a difference in the intensity of those forces of sexual selection and/or a difference in the intensity of resource stress in their environments (e.g., a difference in the length of dry seasons with low fruit availability) and its impact on female body size.
In any event, the high SSD in these fossil hominins contrasts sharply with the more balanced size seen in modern humans and offers a glimpse into a different model of early hominin life—one where large size may have been a key factor in male reproductive success for competitive reasons, and small size may have been a key factor for females for energetic reasons.
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How the research was conducted
Fossil data are often fragmentary, and determining the sex of ancient individuals is nearly impossible. To work around this, Gordon used a geometric mean method that allows for size estimation from multiple skeletal elements—including the humerus, femur, tibia and others. He then applied resampling techniques to simulate thousands of comparisons between fossil hominins and modern primates, ensuring that the statistical models mirrored the incomplete and uneven nature of real fossil samples.
Data from modern gorillas, chimpanzees and humans with known sex and complete skeletons were used to build a comparative framework.
Unlike past studies, which sometimes interpreted weak or inconclusive statistical results as evidence of similarity, Gordon’s methods revealed clear and significant differences even when using relatively small fossil samples.
To rule out the possibility that body size changes in A. afarensis reflected evolutionary trends rather than sex differences, Gordon also tested for chronological trends across a 300,000-year span of fossils from the Hadar Formation in Ethiopia.
His analysis found no significant size increase or decrease over time, indicating that the observed variation is best explained by differences between males and females—not by evolutionary drift or long-term increases in average size.
Rewriting history
The implications of Gordon’s findings are wide-ranging. Australopithecus afarensis, which lived between 3.9 and 2.9 million years ago, is widely regarded as either a direct ancestor of modern humans or a species very closely related to a direct ancestor.
Yet, its high degree of sexual dimorphism suggests that early hominins may have lived in social systems that were far more hierarchical and competitive than once thought.
Meanwhile, the less dimorphic A. africanus—which overlapped in time with A. afarensis but first shows up and last appears in the fossil record slightly later, between roughly 3.3 and 2.1 million years ago—may represent a different evolutionary branch on the hominin tree, or perhaps a transitional stage in the development of more human-like social behavior.
“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, highlighting the diversity of ways that our extinct ancestors and close relatives interacted with the world.”
More information:
Adam D. Gordon, Sexual Size Dimorphism in Australopithecus: Postcranial Dimorphism Differs Significantly Among Australopithecusafarensis, A. africanus, and Modern Humans Despite Low‐Power Resampling Analyses, American Journal of Biological Anthropology (2025). DOI: 10.1002/ajpa.70093
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Early human ancestors showed extreme size differences between males and females (2025, July 29)
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