A new study shows dramatic differences in size between male and female Australopithecus afarensis. This ancestor of modern humans walked the Earth from between 3.9 million to 2.9 million years ago. These findings show that these ancient hominins displayed a high degree of sexual dimorphism. This trait must have directly shaped their social systems and mating ecology. This study reinforces once again the evolutionary importance of Australopithecus afarensis. It not only examines its relationship with our own species, Homo sapiens, but to another species, Australopithecus africanus.
To calculate body sizes, researchers applied a new geometric mean method. Their estimates were grounded in a range of skeletal elements, such as the humerus, femur and tibia. This new, creative approach allowed researchers to get a more detailed picture of the morphology and features of these prehistoric species. The research sheds new light on how size differences affect competition between males. Additionally, it explores possible intergroup mating tactics in early hominin societies.
Sexual Dimorphism in Australopithecus afarensis
For example, Australopithecus afarensis is widely considered to be a direct ancestor of modern humans. Or, it could be a sister, but still closely related, species of those ancestors. The species is especially known for its extreme sexual dimorphism, where males were up to 25 times larger than females. This feature closely resembles the size dimorphism seen in extant primates, including species like chimpanzees and gorillas.
That big size difference would have meant that males were almost certainly competing fiercely for mates. High sexual dimorphism is frequently associated with polygynous mating systems, one male mating with several females. When it came to Australopithecus afarensis, this competitive environment could likely have been shaped by resource distribution and ecological stressors.
The male size advantage was so pronounced that larger males were likely to have been more successful in winning reproductive opportunities. These dynamics created a rigid social structure that probably dictated their social behavior and mating interactions.
The Relationship with Australopithecus africanus
Australopithecus afarensis wasn’t just hanging on, it flourished in its time. It lived alongside Australopithecus africanus, which lived from around 3.3 to 2.1 million years ago. Australopithecus africanus could be the only australopithecine twig on the hominin tree. Or, it might just be an intermediate step in developing genuinely human-like social behavior.
Fossil evidence suggests that Australopithecus africanus were much more dimorphic sexually than humans today. This indicates that it found just as competitive a landscape as its predecessor had, though. Both species were characterized by an extreme sexual dimorphism, even more than that of gorillas. This unexpected finding bolsters long-standing hypotheses regarding their social structures and mating strategies.
The coexistence of these two species raises questions about their interactions and ecological roles. The same competitive dynamics we see in Australopithecus afarensis were likely to have been present in Australopithecus africanus, driving similar behavioral adaptations.
Implications of Size Disparity on Social Behavior
The clear, marked sexual dimorphism in body size between Australopithecus afarensis and Australopithecus africanus implies a high level of male-male competition. This competitive dynamic evolved the social structures of these early hominins. A high sexual size dimorphism is usually indicative of a high level of competition between males. This struggle forces them to compete with one another to take control and win breeding chances.
Competition was a major driving force behind the evolution of our ancestors. Resource availability was just as important in determining their physical dimensions. The new study argues that extreme levels of resource stress likely played a key role in creating sexual dimorphism. Smaller, healthier females are able to do so just as effectively. They rapidly replenish their reproductive storage, providing them an advantage in harsh and resource-poor environments.
The overall effects of resource availability on physical development in early hominins are elucidated by these findings. They critically illuminate the ship’s effect on macro social organization and micro primate reproductive ecology. An appreciation of these dynamics helps researchers make comparisons between extinct species and contemporary primate behavior.