Big males, small females: A window into prehistoric power struggles

The fossil skeleton nicknamed Lucy has long been the poster child for Australopithecus afarensis – a small, bipedal human ancestor that walked East Africa roughly three million years ago.

For decades, researchers have debated whether males and females of A. afarensis differed sharply in body size or whether, like modern humans, the sexes overlapped substantially. A newly published study now answers that question – and upends some familiar assumptions.

“These weren’t modest differences,” said lead author Adam D. Gordon, an associate professor of anthropology at the University at Albany. “In the case of A. afarensis, males were dramatically larger than females – possibly more so than in any living great ape.”

His work also shows that a close southern relative, Australopithecus africanus, shared a similar but slightly less extreme pattern.

Together, the findings paint a picture of early hominin societies in which big-bodied males competed intensely for mates and smaller females faced their own evolutionary pressures to conserve energy.

Size gap in early humans

In modern primates, sexual size dimorphism (SSD) is not merely cosmetic. High SSD, as seen in gorillas, typically signals strong male–male competition and social systems in which one or a few large males monopolize access to several females.

Low SSD, characteristic of modern humans, aligns with more egalitarian or pair-bonded structures. Gordon’s results place both A. afarensis and A. africanus at the high end of that spectrum – and even suggest that A. afarensis may have exceeded gorillas in the male–female divide.

What makes the study novel is not just quantifying SSD in each fossil species but comparing them directly while accounting for the frustratingly incomplete nature of the fossil record.

Earlier efforts produced conflicting estimates – some pegged A. afarensis as almost human-like in its parity – partly because individual fossils are fragmentary and often cannot be sexed with confidence.

New tools for old bones

To overcome those hurdles, Gordon devised an iterative resampling method. First, he gathered measurements from multiple skeletal elements – humerus, femur, tibia, and others – in as many Australopithecus fossils as possible.

He then built a comparative framework using full skeletons of living gorillas, chimpanzees, and humans.

By randomly deleting measurements in those modern skeletons to mimic the missing data pattern in the fossils, he could simulate the same limitations researchers face.

Repeating that process thousands of times allowed him to generate robust expectations for SSD under different scenarios.

Unlike earlier studies that sometimes treated inconclusive statistics as evidence of similarity, Gordon’s approach revealed clear differences. It showed statistically significant gaps between the sex-specific size of the two fossil species and among all three living apes.

“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,” Gordon said.

“This allows the inclusion of multiple fossil individuals even when those individual specimens are fragmentary.”

Size stayed, sexes differed

Could the size variation within A. afarensis simply reflect gradual body growth over 300,000 years rather than a male–female split?

Gordon tested that possibility using fossils from the well-dated Hadar Formation in Ethiopia.

However, his analysis found no consistent trend toward larger or smaller bodies over time, ruling out a species-wide size shift and strengthening the case for pronounced dimorphism.

Environment shaped their bodies

Although both Australopithecus species show greater SSD than modern humans, they are surprisingly unlike each other. That contrast is unusual: among living apes, closely related species such as chimpanzees and bonobos exhibit similar levels of dimorphism.

The stark dimorphism gap between the two extinct hominins suggests they faced unusually different selection pressures, highlighting the diverse ways our ancestors interacted with the world.

One plausible driver is variation in environmental stress. Earlier work by Gordon links high SSD to habitats where food scarcity penalizes large females more than small ones.

If A. afarensis faced harsher dry seasons than A. africanus, selection could have driven the sexes apart -favoring ever-smaller females and ever-larger, competitive males, and widening the size gap.

Alternatively – or additionally – male competition for mates may have been fiercer in East Africa than in southern Africa.

Unequal societies emerged early

Whatever the causes, the implications are striking. A. afarensis males may have enjoyed exclusive breeding rights, similar to silverback gorillas today. Females likely lived in groups dominated by these towering males.

The slightly lower – but still high – dimorphism in A. africanus hints at a somewhat different social structure, perhaps with less monopolization or different foraging pressures.

Either way, the findings dispute the long-held view that gracile Australopitheci formed a uniform ecological and social bloc.

Rewriting the story of human evolution

Australopithecus afarensis is often cast as a direct ancestor of humanity, while A. africanus may represent a side branch. Yet the new evidence suggests that both lineages experimented with social strategies far removed from the relatively egalitarian norms of Homo sapiens.

That realization complicates any tidy story of linear progress toward modern human sociality. Instead, early hominins display a “greater diversity of evolutionary pressures … than we had previously appreciated,” Gordon noted.

By illuminating the powerful interplay between sexual selection, ecological stress, and body size, the study invites paleoanthropologists to re-examine other fossil species with Gordon’s statistical toolkit.

As new fragments emerge from African sediments, researchers can now ask sharper questions: Were big males always best? Did female body size shrink in response to environmental crunches? How and when did the more balanced human pattern evolve?

Size reveals ancient human roles

For now the message is clear: three million years ago, being male in an Australopithecus troop meant being massive. Being female meant staying small, perhaps to stretch scarce calories farther during pregnancy and lactation.

And for scientists today, these size differences between the sexes serve as a crucial proxy for the behaviors and conflicts that shaped our remote ancestors long before the genus Homo took its first steps.

The study is published in the American Journal of Biological Anthropology.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–