By the time “Lucy” walked the Ethiopian landscape three million years ago, her world might have been more competitive than we thought. A recent study shows that male Australopithecus afarensis, the species Lucy belongs to, were significantly larger than females. This size difference is even greater than that seen in modern humans and some great apes, like gorillas.
Interestingly, this finding also applies to Australopithecus africanus, a related species from Southern Africa. However, the size gap between the two species varies. Scientists suggest this could indicate different social structures among our early relatives.
Size Matters in Competition
Adam D. Gordon, an anthropologist at the University at Albany and the study’s lead author, notes, “These weren’t modest differences.” For A. afarensis, males were possibly much larger than females. This sexual size dimorphism often signals patterns in behavior. In many primates, a high level of dimorphism indicates fierce male competition for mating, often in polygynous systems where dominant males mate with several females. In contrast, lower levels of dimorphism tend to correlate with monogamous pair-bonding and less competition.
Modern humans sit somewhere in the middle. Males are generally taller and heavier, but the difference isn’t as stark. This new finding suggests that early hominins, like A. afarensis, leaned more towards the gorilla end, where males can weigh twice as much as females.
Gordon’s earlier research indicated that high sexual dimorphism in primates is often linked to resource stress. During tough times, smaller females may have thrived, requiring fewer calories to survive and reproduce. This could lead to a generational size difference favoring smaller females, amplifying male-female size differences over time.
A Unique Blend
Australopithecus afarensis displayed traits that combined both ape-like and human-like features. It had long arms suited for climbing and a small brain. Still, it walked upright. Footprints in Laetoli, Tanzania, from 3.66 million years ago, highlight the confident stride of our early ancestors.
Paleoanthropologists believe A. afarensis could be a direct ancestor of modern humans. With a timeline between 3.9 and 2.9 million years ago, its location in East Africa made it a likely predecessor to species like Australopithecus africanus and early Homo members.
New Insights
The debate over whether A. afarensis was highly dimorphic or similar to modern humans has been long-standing. A challenge has been the incomplete fossil record from that era. Gordon utilized a method to estimate body size from various bones, compensating for missing parts and running simulations. This analysis revealed significant differences when comparing fossils from the Hadar Formation in Ethiopia over 300,000 years.
The results leave gender as the probable explanation for the size differences. It paints a picture of A. afarensis living in groups dominated by larger males, while A. africanus, with its smaller size gap, may have endured different evolutionary pressures.
Gordon emphasized, “These closely related hominins faced selection pressures that were likely more distinct than those directing pairs of similar living ape species.” This suggests our evolutionary path wasn’t just a straightforward journey towards modern social structures but included varied strategies, where size differences could impact survival and reproduction.
In essence, this research adds a nuanced layer to our understanding of early hominin evolution, highlighting the complexity of social structures in our ancient past. For further reading on this subject, check the full study published in the American Journal of Biological Anthropology.
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australopithecus,Australopithecus afarensis,sexual dimorphism
