Extended Male Growth in a Fossil Hominin Species. 2007. C. A. Lockwood et al., Science 318: 1443-1446.
Researchers examined fossils from 1.5 to 2 million years ago and found that in one of our close relatives, Paranthropus robustus, the males continued to grow well into adulthood, just as they do in gorillas. This resulted in a much bigger size difference between males and females than we see today.
“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 pattern of growth also gives a better understanding of who is male and who is female in this sample of skulls and it turns out that there are far more males in the fossil sample. Because fossils from the most prolific site, Swartkrans, are thought to have been deposited by predators such as leopards and hyenas, it appears that males were getting killed more often than females.