Our male ancestors had harems of females

Our male ancestors had harems of females that they would jealously guard from the attention of love rivals, according to a study published today.

This is one implication of the discovery that one of our recent relatives was surprisingly like gorillas in that males took much longer to mature than females.

	The BBC's Walking with Cavemen series recreated the life of our prehistoric ancestors

These findings could help researchers understand how these early human ancestors lived and socialised together, since gorillas live in harems controlled by a "dominant" silverback male.

Charles Lockwood of University College London and his colleagues shed new light on the lifestyle of Paranthropus robustus by analysing a large number of skulls.

They looked at how worn down the teeth were to determine how long the individuals had been alive. The researchers then measured the size and shape of the skull to determine how mature the individuals had been, and also to figure out whether they were male or female.

The results, published today in the journal Science, indicate that P. robustus males grew for a longer period than females. This is also true for some living primates, such as gorillas.

"When we examined fossils from 1.5 to 2 million years ago we found that in one of our close relatives the males continued to grow well into adulthood, just as they do in gorillas," he says.

"This resulted in a much bigger difference between males and females than we see today."

In gorilla populations, individual mature males, the large silverbacks, live with a group of females, mating with them and protecting these harems. P. robustus may have lived a "polygynous" lifestyle like this too, according to Dr. Lockwood and his colleagues.

"It's common knowledge that boys mature later than girls, but in humans the difference is actually much less marked than in some other primates.

"Male gorillas continue to grow long after their wisdom teeth have come through, and they don't reach what is referred to as dominant silverback status until many years after the females have already started to have offspring.

"Our research makes us think that, in this fossil species, one older male was probably dominant in a troop of females. This situation was risky for the males and they suffered high rates of predation as a result of both their social structure and pattern of growth."

The work dovetails with a recent study that concluded that the reason that women outlive men by an average of around five years is due to sex, harems and violence in the Stone Age.

Our prehistoric male ancestors kept female harems and fought over them to procreate: because the potential number of offspring was greater for males, competition for mates was severe. As a result, evolutionary forces focused on making males big and strong, rather than long lived.

Prof Tim Clutton-Brock of Cambridge University and Dr Kavita Isvaran of the Centre for Ecological Studies, Bangalore, India found that the difference in lifespan between males and females in creatures such as red deer, prairie dogs, lions, baboons, geese, mongooses, wild dogs, beavers and others grows in direct proportion to the degree to which an animal's society is polygynous, that is a society where one male enjoys the attentions of several female breeding partners.

Thus the very fact that men age faster and die younger than women suggests human Stone Age society was polygynous.

Scientists believe that further work on the differences in the way our ancestors matured will reveal clear diversity in their social structure in the same way that one sees differences among apes such as chimpanzees, bonobos, gorillas, and orangutans.

This research will help us to understand how the structure of today's human societies evolved.