Two hominines extinct, Australopithecus Afarensis And Australopithecus Africanuswere significantly more dimorphic than modern chimpanzees and humans; Australopithecus Afarensis were also significantly more dimorphic than Australopithecus AfricanusAccording to Dr. Adam Gordon, paleoanthropologist at the University of Albany and the University of Durham.
Reconstruction of the medico-legal face of Australopithecus Afarensis. Image credit: Cicero Moraes / CC by-SA 3.0.
Dimorphism of sexual size is not only a physical trait – it reveals something deeper in behavior and strategy of evolution.
In accordance with sexual selection theory, high sexual dimorphism in living primates is generally correlated with strong male-masculine competition and social structures allowing polygygyneous mating systems, where one or some large males monopolize reproductive access to several women.
On the other hand, dimorphism with a low sexual size can be found in any species, but tends to be found in those with social structures linked to the pair and low competition for mating opportunities.
Modern human populations have an average of a low to moderate sexual dimorphism, where males tend to be slightly larger than women on average but with substantial overlapping in size.
Fossil data is often fragmentary and determining the sex of ancient individuals is almost impossible.
To get around this, Dr. Gordon used an average geometric method which allows an estimate of size from multiple skeletal elements – including humerus, femur, tibia and others.
He then applied re -exchange techniques to simulate thousands of comparisons between fossil hominins and modern primates, ensuring that statistical models reflected the incomplete and unequal nature of real fossil samples.
The data from gorillas, chimpanzees and modern humans with known sex and complete skeletons were used to build a comparative frame.
Unlike previous studies, which have sometimes interpreted the low or non -conclusive statistical results as evidence of similarity, Dr. Gordon’s methods have revealed clear and significant differences even during the use of relatively small fossil samples.
To exclude the possibility that body size changes in Australopithecus Afarensis Evolutionary trends reflected rather than sexual differences, Dr. Gordon has also tested chronological trends through a fossil duration of 300,000 years of Hadar training in Ethiopia.
Its analysis has not found any significant increase or decrease in size over time, indicating that the variation observed is better explained by the differences between men and women – and not by evolutionary drift or long -term increases in the average size.
“They were not modest differences,” said Dr. Gordon.
“In the case of Australopithecus AfarensisMen were considerably larger than women – perhaps more than in any large living monkey. »»
“And although these two species of disappeared hominins have specific differences in size than modern humans, they were also more different from each other in this regard than species of living monkeys, suggesting a greater diversity of evolutionary pressures acting on these species closely linked to what we had previously appreciated.”
Australopithecus Africanus. Image credit: JM Salas / CC by-SA 3.0.
Dr. Gordon’s previous research suggests that high sexual dimorphism in living primates can also be associated with intense stress of resources – when food is rare, small healthy women can get enough food to meet their own metabolic needs and store energy for faster reproduction than the biggest difference for men and women, which led to the size of women.
High sexual dimorphism identified in both Australopithecus The species suggest a high degree of competition between men, similar to that of chimpanzees or even gorillas, while the difference between the two fossil species can be due to a difference in intensity of these sexual selection forces and / or to a difference in the intensity of the stress of resources in their environment (for example, a difference in the duration of dry seasons with low fruit availability) and its impact on the female body.
In any event, dimorphism of high sexual size in these fossil hominines contrasts strongly with the more balanced size observed in modern humans and offers an overview of a different model of the life of early hominines – a large size can have been a key factor in the success of male reproduction for competitive reasons, and small size may have been a key factor for women energy.
The implications of the results are varied. Australopithecus Afarensiswhich lived between 3.9 and 2.9 million years, is largely considered as a direct ancestor of modern humans or a species very closely linked to a direct ancestor.
However, his high degree of sexual dimorphism suggests that the first hominines may have lived in social systems that were much more hierarchical and competitive than we thought.
Meanwhile, the least dimorphic Australopithecus Africanus – who rushed over time with Australopithecus Afarensis But the first appears and appears for the last time in the fossil file slightly later, between approximately 3.3 and 2.1 million years – can represent a different evolutionary branch of the hominin tree, or perhaps a stage of transition in the development of more human social behavior.
“We generally place these first hominines together in a single group called the Australopiths graciles, a group of species that would have interacted with their physical and social environments in a very similar way,” said Dr. Gordon.
“And while that true to a certain extent-the evidence suggests that booth these specties may have social organizations More like gorillala Than Modern People-The meaning different in the Amount of Dimorphism in these Two Extinguits Species Suggests that these Closely-Related Hominin Species working 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 Lights and Close Relative Interagi With Le Monde.
The results appear in the American Journal of Biological Anthropology.
_____
Adam D. Gordon. 2025. Sexual size dimorphism in Australopithecus: Postcrranian dimorphism differs considerably between Australopithecus Afarensis,, A. AfricanusAnd modern humans despite low -power re -enchanting analyzes. American Journal of Biological Anthropology 187 (3): E70093; DOI: 101002 / AJPA.
