A group of scientists working in the Afar region of Ethiopia have found a nearly complete female Homo erectus pelvis from around 1.8 million years ago.
The pelvis includes a sacrum, both os coxa, and a complete pubis. This pelvis is much more complete than the Turkana boy pelvis (KNM-WT 15000), which has been the main model for H. erectus pelvis and body shape. The Turkana pelvis has been reconstructed as tall and narrow, which is said to be an adaptation to long-distance running as it would’ve increased the efficiency. Turkana boy has been reconstructed to stand about 5′3″ at an age of about 11. Had he lived into adulthood, he would’ve been around 6′1″. Estimates of female pelvic shape derived from the Turkana boy have suggested that females would’ve had to have given birth to neonates with small brains, which would’ve required a long, human-like period of infant dependency.
The new pelvis from Gona permits a height estimation of between 3′11″ to 4′9″ based on the small size of the acetabulum (where the head of the femur articulates with the pelvis). A small acetabulum reflects a small femoral head, which reflects a small body size. If the Turkana estimates are correct, and the body size regression from the acetabular size is correct, sexual dimorphism in body height was probably greater in Homo erectus than has been previously estimated. Still, these estimates for body size seem reasonable when I think of dimorphism in modern humans.
The pelvic inlet in the new pelvis is within the size range of modern human females, despite the fact that the individual was very short. The outlet was even greater than that in modern humans. These dimensions can be used to estimate the brain size of a neonate. The new pelvis estimates brain size of 315 ml, which is around 30% greater than the estimates derived from the Turkana boy. These estimates indicate that neonatal brain size in Homo erectus was between 34-36% the size of adults. In chimpanzees, neonatal brain sizes are 40% that of adults, and in humans they are 28%.
An increase in brain size like that seen in Homo erectus requires an increase in size of the pelvis. This can be achieved by either increasing the size of the body overall, or increasing the size of the pelvic canal only, and especially in the female of the species. The new pelvis suggests that brain size was acting more on pelvic shape than on overall body size.
BSN49/P27 vs. AL288 (Homo erectus vs. Australopithecus afarensis) (From Simpson et. al 2008)
Modern human female pelvis
Since the Australopithecines, there have been many changes in pelvic shape. The pelvis has become more circular, with a decrease in bi-auricular breadth and an increase in anterior-posterior breadth. The pubic ramus has become shorter, which contributes to the decrease in biauricular breadth. The ilia have become less flared as well, contributing to an overall narrower body shape.
Some authors have suggested that Homo erectus displayed these adaptations as adaptations to long distance running. Characters such as tall body size, large acetabula (where the femur meets the pelvis), and narrow torsos and pelves are used as evidence that Homo erectus was increasing locomotor efficiency. However, the pelvis from Gona displays none of these adaptations. Her bi-iliac breadth is wide, her ilia are flared laterally, her acetabula are tiny, and her pubic rami are long.
Clearly, something is amiss. The body size dimorphism seems reasonable, but the pelvic shape suggests extreme behavioral dimorphism as well (males were out running long distances while the females waddled around bearing children at home). This may be reasonable, but it may not be. Other Homo erectus specimens, like the ones from Dmanisi provide evidence that small body size in erectus was not unusual. Perhaps our reconstruction of the Turkana pelvis requires modification, as it seems to be an outlier.
Sources:
Simpson SW, Quade J, Levin NE, Butler R, Dupont-Nivet G, Everett M, Semaw S. 2008. A female Homo erectus pelvis from Gona, Ethiopia. Science 322:1089-1092.
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