Fossil cranium of SK Swartkrans number 48 : Wiki Commons
In the Gauteng Province of South Africa lies a cave called Swartkrans, which is one of the most important caves in the famous Cradle of Humankind. This cave has provided an extensive amount of fossils which have been attributed Paranthropus robustus. The genus Paranthropus is composed of three separate species i.e. Paranthropus aethiopicus, Paranthropus robustus and Paranthropus boisei.
This genus has never been discovered outside of the continent of Africa and lived from about 2.6 to 1.4 million years ago. P. aethiopicus and P. boisei have to date only ever been found in eastern Africa, including Tanzania, Ethiopia and Kenya, while P. robustus has only been uncovered in South Africa. Paranthropus is Latin for ‘near human’ a name created by the famous Scottish palaeontologist Robert Broom in 1938.
The cave of Swartkrans is one of many palaeocaves in the Gauteng region, with other famous caves such as Kromdraai and Sterkfontein located in the vicinity.
A tiny 4% of the fossils uncovered at Swartkrans are attributed to the genus Homo, while the remaining assemblage has been attributed to P. robustus. The stratigraphic layers of Swartkrans were given the label ‘Member’ and it is from Member 1 that the new fossils were retrieved. This strata has been dated to between 1.9 to 1.8 million years ago (Ma), thanks to the work of Pickering et al., (2011). These discoveries comprise one partial right femur and 13 isolated teeth representing both deciduous and permanent teeth. Nine teeth were determined to belong to P. robustus, three were attributed to Hominidae gen. et sp. indet and the final tooth was assigned to cf. Homo. The single femur was attributed to cf. Paranthropus.
Hominidae gen. et sp. indet: It is an extinct (in this case) great ape, but the genus and species cannot be determined. (Hominidae is short for great apes (humans, orang-utans and chimpanzees), while gen. et sp. indet stands for genus and species indeterminate)
cf. Paranthropus/ Homo: It resembles but is not identical to any species of Paranthropus or Homo (cf. is short for confer)
The teeth were described, measured (where possible), and taxonomy assigned.
The fossil femur catalogued as SWT1/LB-2, is of particular interest because few fossil bones of the Paranthropus leg have been uncovered. Using measurements of the partial femur, the body mass when the individual was alive, was estimated to be 37.1 kg and is thus significantly larger than the mass of female P. robustus. This suggests that P. robustus exhibited a significant difference between the size of males and females. This difference is called sexual dimorphism.
There is growing support among palaeoanthropologists for the idea that if you measure the size of the head of a femur this tells us something of the body size sexual dimorphism (BSSD). For example, P. robustus has a BSSD that is greater than modern day chimps and humans, but smaller in modern day gorillas and orang-utans. In other words, P. robustus had a larger size difference between males and females than chimps or humans and less so in comparison to gorillas and orang-utans. Cranial size of P. robustus also suggests the same idea.
Primates have quite intriguing social systems (Van Schaik and Van Hooff, 1983). Sexually dimorphic populations have a higher young male mortality rate than other groups of primates with differing social systems. Females band together, while the males exist at the periphery occasionally becoming the high ranking male of the group.
The periphery males will only contest a high ranking male position, if the chances of success are high. Young male Gorillas don’t become socially adult until 5 to 6 years after females have done so, thus delaying competition. They are at a disadvantage by spending time being alone or banding with other males and so would represent fair game to the carnivores.
Lockwood et al., (2007) examined the fossil skulls of Swartkrans and Drimolen revealing a high proportion of young males Paranthropus in the combined sample, suggesting a sexual competition side effect. While Susman et al., (2001) noted a slightly higher proportion of females to males, suggesting that females were much easier prey than the larger males.
It must be said that there is a certain amount of uncertainty in seeing differences between fossils of males versus females. Also the postcranial samples were excluded from the research of Susman et al (2001) and Lockwood et al (2007). It would be very difficult to assign the sex of a particular postcranial bone. New research using a method of Strontium analysis suggests that the presumptive females originated from a distant part of the landscape. Implying that the social pattern was the opposite to the one previously discussed.
In summary, Pickering et al (2012) are not confident of the causal relationship hypothesis between the BSSD of P. robustus and the differential male-female predation vulnerability.
LOCKWOOD, C. A., MENTER, C. G., MOGGI-CECCHI, J. & KEYSER, A. W. 2007. Extended male growth in a fossil hominin species. Science, 318, 1443-6.
PICKERING, R., KRAMERS, J. D., HANCOX, P. J., DE RUITER, D. J. & WOODHEAD, J. D. 2011. Contemporary flowstone development links early hominin bearing cave deposits in South Africa. Earth and Planetary Science Letters.
PICKERING, T. R., HEATON, J. L., CLARKE, R. J., SUTTON, M. B., BRAIN, C. K. & KUMAN, K. 2012. New hominid fossils from Member 1 of the Swartkrans formation, South Africa. J Hum Evol, 62, 618-28.
SUSMAN, R. L., DE RUITER, D. & BRAIN, C. K. 2001. Recently identified postcranial remains of Paranthropus and early Homo from Swartkrans Cave, South Africa. J Hum Evol, 41, 607-29.
VAN SCHAIK, C. P. & VAN HOOFF, J. 1983. On the ultimate causes of primate social systems. Behaviour, 85, 1, 91-117.