ScienceDaily (Nov. 30, 2007) -- New research shows that some of our
closest extinct relatives had more in common with gorillas than
previously thought. Dr Charles Lockwood, UCL Department of
Anthropology and lead author of the study, said: "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. 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 research used 35 fossilised specimens of Paranthropus robustus, an
extinct relative of Homo sapiens which existed almost two million
years ago. The fossils came from the palaeontological sites of
Swartkrans, Drimolen and Kromdraii, all of which are in South Africa's
Cradle of Humankind World Heritage Site near Johannesburg.

The research was inspired by earlier discoveries at Drimolen by Dr
Andre Keyser, one of the co-authors of the study. Dr Colin Menter,
from the University of Johannesburg and co-director of current
fieldwork at Drimolen, explains: "Discoveries at this site showed us
that sex differences in Paranthropus robustus were greater than we had
previously thought. While there are some specimens from Drimolen that
are just as large and robust as those from other sites like
Swartkrans, there is a complete female skull that is distinctly
smaller than the other, well-preserved specimens of the species."

Jacopo Moggi-Cecchi, based at the University of Florence and an expert
on fossil teeth, participated in the study and says: "It takes large
samples of fossils to ask questions about variation and growth, and
it's really a tribute to fieldworkers such as Robert Broom and Bob
Brain [who worked at Swartkrans] that this research could even take
place. It's also an example of why we need to continue to look for
fossils after we think we know what a species is -- more specimens
allow us to answer more interesting questions. Even isolated teeth can
give us new insights into what variation means."

Dr Lockwood adds: "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.

"Basically, males had a high-risk, high-return lifestyle in this
species. They most likely left their birth groups at about the time
they reached maturity, and it was a long time before they were mature
enough to attract females and establish a new group. Some of them were
killed by predators before they got the chance."

A final point made by the researchers is that not all fossil hominin
samples show the same patterns, and it is quite possible that further
work will reveal clear diversity in social structure between human
ancestors, 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 human social structure evolved.

This research was published in the journal Science on November 30,
2007.

Research at Drimolen has been funded by the Leakey Foundation, the
Department of Science and Technology in South Africa, the Italian
Ministry of Foreign Affairs, and the Italian Cultural Institute in
Pretoria. The Royal Society supported Charles Lockwood's work in South
Africa.

Fossils are housed at the University of the Witwatersrand in
Johannesburg and the Transvaal Museum (Northern Flagship Institution),
Pretoria.