UCL scientists report the first look at early hominid social structures, finding that our close relatives had more in common with gorillas than previously thought.

Ed Yong

A single fossil can tell you about the shape, diet and movements of an extinct animal but with enough specimens, you can reconstruct their social lives too.

Charles Lockwood of University College London used an unusually large collection of fossils to peer back in time at the social structures of one of our closest extinct relatives, Paranthropus robustus, which inhabited southern Africa between 1.2 million and 2 million years ago.

Lockwood’s research is published in this week’s Science and shows that while P. robustus walked upright like a human, it had much more in common with modern gorillas.

Supersized males

“The size difference between the two sexes was very similar to that of gorillas and unlike that of chimps and humans,” explains Lockwood. “The males were much larger than the females because they continued to grow well after reaching sexual maturity and long after the females stopped.” Gorilla males also grow during early adulthood and the oldest ‘silverbacks’ are often twice the size of females—a pattern called dimorphism. “They don’t reach dominant silverback status until many years after the females have already started to have offspring,” says Lockwood.

This simple sex difference also provides a clue about the social structure of P. robustus. When males continue to grow during adulthood, it’s a sign that they fiercely compete with each other over the sole right to mate with large harems of females. Young adults only have a chance of successfully reproducing if they grow large and strong enough to challenge the ranking male. “This is the first time that we’ve had a clear look at the social structure of any early hominid species,” said Lockwood.

Chris Stringer from the Department of Palaeontology at London’s Natural History Museum, called the results “fascinating”. He said, “It has long been suggested that these creatures were ape-like in many aspects of their biology and behaviour, and that males were significantly larger than females. This study confirms that and gives us insights into the probable troop structures of these near-human forms.”

35 Fossils are better than one

Lockwood stressed that his study was only possible because of the large numbers of P. robustus fossils found in South Africa. “Even for species that are well known, every new fossil matters,” he said. “Obviously, discovering new species is very important but you can only answer questions about variation and behaviour when you have lots of fossils.”

His team analysed 35 adult specimens taken from three sites in South Africa’s Cradle of Humankind World Heritage Site. The fossils ran the gamut from young adults to senior citizens, and were grouped by age on the basis of tooth erosion.

Among these fossils, older males were generally larger than younger ones and had more strongly defined facial features, implying that individuals continued to grow after they came of age. Females on the other hand remained the same size after young adulthood.

The 35 specimens come mostly from males. It is thought that while males waited to grow large enough to challenge for dominance, they lived alone or in small bands near the fringes of the group, where they made easier pickings for predators. In contrast, females lived well defended lives under the protection of the dominant males.

Other species, like Australopithecus africanus, whose fossils are also found in South African caves, have much fewer differences between the sexes, just like chimpanzees and modern humans. This shows that our extinct relatives were a very diverse bunch. “Evidently, they had as much variation in their social groupings as modern African apes do”, said Stringer.

Image courtesy of Charles Lockwood