A lot of excitement surrounds the ‘transitional’ hominins as these are the members of the hominin clade which appear to bridge the gap between genus Australopithecus (see my previous two posts here and here) and our own genus Homo. Of course, with excitement in the world of palaeoanthropology comes controversy. A large part of this controversy centres around whether any of the transitional hominins actually deserve to be included in genus Homo, or whether they all should be placed in the umbrella-like genus Australopithecus (also here).
Meet Australopithecus sediba (sediba means ‘fountain’ in the local seSotho language; Berger et al., 2010) which was present in South Africa between 1.8 and 2.0 millions years ago (Mya).
Most people agree that Au. sediba should be placed in Australopithecus. The upward tilt of the glenoid fossa of the scapula, relatively long arms and curved finger bones with strong muscle attachments (see pt. v) suggest that Au. sediba still frequently used trees for protection or food, or that these features didn’t hinder efficient bipedalism so were not lost rapidly through natural selection.
So how do we know that Au. sediba was an efficient biped? The pelvic fragments suggest that the ilia were flared and the distance between the sacro-iliac joint and the acetabulum were shorter than earlier bipedal australopiths – this would have helped decrease stresses in the pelvis caused by the weight transfer from spine to hip joint and on to the leg (see pt. i and pt. ii).
The femoral fragments show a relatively large femoral head (good for weight bearing) and a marked bicondylar angle to bring the knee and therefore the foot close to the midline for better balance during the single-foot stance phase of the walking cycle (see pt. ii and pt. iii).
The orientation of the joint between the tibia and the talus is perpendicular to the long axis of the tibia, suggesting efficient weight transfer to the foot, although specific morphology of the talus and calcaneus appears represents a mixture of apparently arboreal and bipedal characters, even down to the signs of muscle attachments and weight-bearing lumps and bumps (pt. iii; Zipfel et al., 2011). These bones are not particulary ape-like or modern human-like but somewhere in between.
The remaining fossils which fall within the time period of the transitional hominins (and aren’t robust australopithecines) are sometimes referred to as Homo habilis (handy man) and sometimes split between Homo habilis found in and around Olduvai Gorge sites mainly in Tanzania, and Homo rudolfensis (named for Lake Turkana, formerly Lake Rudolf, Kenya). Some would even prefer Au. habilis and Au. rudolfensis!
For simplicity here I’ll just refer to them all as H. habilis (fig. 3; fig. 4) which wandered the African planes between 1.4 and 2.4 Mya.
The position of the foramen magnum in H. habilis is very much underneath the skull – an excellent position to balance the head on top of an upright spine. The foot bones collectively known as OH8 show a mixture of ape-like morphologies and more modern morphologies. Broadly speaking, the proximal bones, including the talus at the bottom centre in fig. 3, are more ape-like than the other bones, with the more bones such as the long metatarsals at the top being more human-like. You may remember that this is the same pattern seen in South African Au. africanus, (Kidd & Oxnard, 2005) despite H. habilis being found in east Africa, which was home to an opposite pattern of foot bone similarity before this point in Au. afarensis.
This results in OH8 possessing a lateral longitudinal arch for efficient weight transfer along the foot, and a big toe which is more in line with the other toes.
A separate fossil of an isolated first metatarsal has large joint surfaces also suggests that the big toe would have been heavily load bearing in H. habilis (Zipfel & Kidd, 2006), although it is distinct from human first metatarsals in a more comprehensive shape analysis. This means that H. habilis bipedalism was not yet fully modern in terms of weight transfer or possibly even used as extensively as in modern humans.
The Kidd & Oxnard paper was actually very important when it showed that east African H. habilis and South African Au. africanus had similar bipedal adaptations in the foot since it meant that, along with some more accurate dating of the South African fossils, there was only one pattern of foot bones present in Africa at the time. Before 2005, the fact that there were 2 separate patterns of adaptation was very troubling to some palaeoanthropologists – which one gave rise to our morphology? Now that question doesn’t seem to matter quite so much.
Next week, we’ll take another step closer to Homo sapiens. We may even leave Africa for the first time! Bring your hat and compass.
Berger, L. R., de Ruiter, D. J., Churchill, S. E., Schmid, P., Carlson, K. J., Dirks, P. H. G. M., & Kibii, J. M. (2010). Australopithecus sediba: a new species of Homo-like australopith from South Africa. Science (New York, N.Y.), 328, 195–204. doi:10.1126/science.1184944. Freely available.
Kidd, R., & Oxnard, C. (2005). Little Foot and big thoughts—a re-evaluation of the Stw573 foot from Sterkfontein, South Africa. HOMO – Journal of Comparative Human Biology, 55, 189–212. doi:10.1016/j.jchb.2004.07.003
Pontzer, H., Rolian, C., Rightmire, G. P., Jashashvili, T., Ponce de León, M. S., Lordkipanidze, D., & Zollikofer, C. P. E. (2010). Locomotor anatomy and biomechanics of the Dmanisi hominins. Journal of Human Evolution, 58, 492–504. doi:10.1016/j.jhevol.2010.03.006
Zipfel, B., DeSilva, J., Kidd, R., & Carlson, K. (2011). The foot and ankle of Australopithecus sediba. Science, 333, 1417–1420. doi:10.1126/science.1202703
Zipfel, B., & Kidd, R. (2006). Hominin first metatarsals (SKX 5017 and SK 1813) from Swartkrans: a morphometric analysis. Homo: Journal of Comparative Anatomy, 57, 117–31. doi:10.1016/j.jchb.2006.01.001