What if our ancestors were not as similar to modern humans as we thought? A recent study published in The American Journal
of Biological Anthropology reveals that certain fossil species of hominids had a sexual dimorphism of size much more marked than that observed in Homo Sapiens. In other words, males were significantly larger than females, a characteristic that we find today in gorillas or orangutans, but not with us. These results, obtained thanks to a new method of fossil analysis, could well transform our understanding of social structures, reproduction and even the survival strategies of our distant ancestors.
A difference in size that says a lot
Size sexual dimorphism (DST) is one of the keys to understanding the social dynamics of a species. In modern humans, it is relatively low: on average, men are a little larger than women, but with wide overlap. On the other hand, in some large apes such as gorillas, males are much more massive than females. This contrast is not purely biological: it reflects deeply anchored social behavior, in particular the competition between males for access to females, and the way in which the resources are distributed within the group.
It is precisely this track that Adam D. Gordon, anthropologist at the University of Albany, explored. By examining the fossil remains of two well -known species of the genus Australopithecus – A. (of which the famous fossil “Lucy”) is a part A. africanus
– He highlighted a marked difference of body size between the sexes, even greater than that observed in gorillas.
Fragmentary fossils, but innovative methods
One of the major challenges of paleoanthropology remains the scarcity and incompleteness of fossils. To estimate the body size of people who have been missing for millions of years, researchers often have to be satisfied with bone fragments: a femur, a tibia, sometimes a humerus. However, determining sex from these remains is extremely difficult, and the samples are often too small to allow solid conclusions.
To get around these obstacles, Gordon has developed a statistical re -enchanting method based on the geometric average of several bone measurements. This approach was tested on modern specimens (humans, chimpanzees, gorillas) whose sex and size are known, then applied to the Australopithecus fossils.
The result is final: the males of A. et A.
africanus were much larger than females, in proportions that far exceed those of the current human species.
New research by anthropologist Adam D. Gordon of the University of Albany reveal an important sexual dimorphism in some of our first human ancestors. Credit: Ken Zirkel, Natural History Museum.
What it reveals about their lifestyles
But why such a difference in size? According to the theory of sexual selection, a high DST is often the sign of a strong competition between males. In societies where dominant males can monopolize access to several females, large size becomes an evolutionary advantage. This is the case in gorillas, where the male alpha imposes his domination thanks to his physical strength.
The study suggests that the first hominids lived in similar social systems, where large males competed for access to reproduction. Conversely, the females would have been subjected to different pressures: in forced environments, a smaller size could be an energy advantage for pregnancy and breastfeeding.
Thus, the combination of strong sexual competition and high environmental stress could explain the DST pronounced in these species.
Two species, two scalable trajectories
If the two species studied have an important DST, they differ however on this point. A.which lived between 3.9 and 2.9 million years in East Africa, shows an even more marked dimorphism than A. africanuspresent in southern Africa between 3.3 and 2.1 million years.
This difference could reflect separate evolutionary pressures: for example, a more intense competition in males ofAfarensisor stronger environmental constraints in females ofafricanus. It could also suggest that the two species occupied different social and ecological niches, despite their evolutionary proximity.
These conclusions question the idea that all the Australopithecus shared similar lifestyles. On the contrary, they indicate a greater behavioral and social diversity than we imagined so far.
A new perspective on human evolution
By revealing that our ancestors perhaps lived in more hierarchical societies, with strong competition between males and energy pressures on females, this study opens the way to a rereading of human evolution. It shows that the trajectory which led to the emergence of Homo sapiens – with its more egalitarian and less polygynes – was not inevitable societies, but the fruit of progressive changes in social and environmental dynamics.
In short, these ancient bones tell us not only of biology, but also of society. And they remind us that the paths of evolution are multiple, sometimes surprising, and always more complex than you think.