Australopithecus sediba is a possible human
ancestor discovered in South Africa in 2010. The discovery was made at Malapa,
a fossil-bearing cave located about 15 km (9.3 miles) NE of the well-known
South African hominid-bearing sites of Sterkfontein and Swartkrans and about 45
km (28 miles) NNW of Johannesburg (Berger, et al., 2010). It is situated
within the Cradle of Humankind World Heritage Site. The recovery effort was led
by Lee Berger, a paleoanthropologist at the University of the Witwatersrand,
Johannesburg. The find was made when Matthew, Lee’s 9 year old son, discovered
hominin collar bone embedded in a rock (Balter, 2010).
The find comprised two extremely well-preserved
partial skeletons that were initially thought be somewhere between 1.78 and
1.95 million years old (Dirks, et al., 2010), later revised to
1.977 million years (Pickering, et al., 2011). These belonged to a
juvenile male (MH1) aged 12 to 13 at time of his death and an adult female
(MH2) (Berger, et al., 2010). They were found
together buried in alluvial sediment, deep within the Malapa cave, part of an
eroded cave system. Also found were the remains of wildcats, hyenas and a
number of other mammals. On the ground above the cave are a number of ‘death traps’,
or long vertical shafts. The smell of damp issuing from the shaft would have
attracted animals. The pair – possibly mother and son – may have fallen to
their deaths while searching for water. The sediments imply that subsequent
high-volume water inflow, perhaps the result of a large storm, caused a debris
flow. This carried the still partially articulated bodies deeper into the cave,
to deposit them along a subterranean stream (Dirks, et al., 2010).
MH1 and MH2 were assigned to a new
australopithecine species, Australopithecus
sediba. The word ‘sediba’ means ‘fountain’ or ‘wellspring’ in the Sotho
language. The more complete cranium of the juvenile MH1 has a capacity of
420cc, probably at least 95 percent of adult size. The remains share numerous
similarities with Australopithecus
africanus in the cranial vault, facial skeleton, lower jawbone and teeth,
but there are also significant differences in the cranial, dental and
postcranial anatomy. Homo-like
features include smaller molars and premolars and less pronounced cheekbones.
Certain features of the pelvis are similar to those seen in Homo erectus. The lower-to-upper limb
bone proportions are also similar to those of later Homo, and unlike the more apelike proportions of Homo habilis. The anatomy of its hip,
knees and ankles suggest that Australopithecus
sediba was a habitual biped. Overall, it was claimed that Australopithecus sediba shares more
derived features with early Homo than
it does with other australopithecines. However, Berger was reluctant to place
the new discovery within Homo,
preferring to classify it as an australopithecine (Berger, et
al., 2010).
The initial announcement of Australopithecus sediba attracted
extensive news coverage, but not everybody was convinced by the claims made for
it. Australian anthropologist Darren Curnoe was reported (MacKnight, 2010) as claiming that Australopithecus sediba is in the wrong place at the wrong time to
be a human ancestor. He noted that Homo
habilis emerged in East Africa well before the time of Australopithecus sediba. However, his argument does assume that Homo habilis is indeed an early human. This may not be the case. It is also possible
that at least some of Australopithecus
sediba’s humanlike features could have evolved independently, and may not
necessarily imply shared ancestry (Wood & Harrison, 2011).
Nevertheless, subsequent studies do support Berger’s
initial claims. They suggest that aspects of the brain, dental morphology,
pelvis, hand and foot of Australopithecus
sediba could be interpreted as incipient humanlike features. A virtual
endocast of the brain, obtained from synchrotron scanning, revealed an
australopithecine-like size and pattern of convolutions. However, the
orbitofrontal region showed possible development towards a humanlike frontal
lobe. Possibly some neural reorganization of the brain preceded its later size
increase in early humans (Carlson, et al., 2011).
The teeth of MH1 and MH2 are a mosaic of
primitive and derived traits. Cladistic analysis of 22 dental traits suggest
that Australopithecus sediba was a
sister species of Australopithecus africanus
(i.e. the two shared a common ancestor) and that the two were further evolved
in the direction of Homo than were
the australopithecines from East Africa (Irish, Guatelli-Steinberg, Legge, de Ruiter, & Berger, 2013). The lower jawbone
morphology reduced dentition (especially canines and premolars) confirms that Australopithecus sediba was a distinct
species to Australopithecus africanus
and not merely a late-surviving form of that species (de Ruiter, et al., 2013).
The upper ribcage of Australopithecus sediba exhibits an apelike funnel shape, unlike
the barrel shape associated with Homo.
The funnel shape, as noted above, may be an adaptation to under-branch
suspensory locomotion. The barrel shape may be associated with the increased
chest volume and lung function necessary for endurance walking and running. The
lower thorax, however, appears less flared than that of apes and more closely
approximates the morphology found in humans (Schmid, et al., 2013). The spine is long
and flexible, a form that has more in common with early Homo than with other australopithecines. Curvature of the lower
spine is a hallmark of walking upright (Williams, Ostrofsky, Frater, Churchill, Schmid, & Berger, 2013).
The upper limbs were still predominantly
apelike, suggesting the retention of substantial climbing and suspensory
abilities (Churchill, et al., 2013). The hands show a
mixture of australopithecine and human features. They retained adaptations for
tree-climbing, but there was also a long thumb and shorter fingers. These
suggest precision gripping of the type associated with tool manufacture and use
(Kivell, Kibii, Churchill, Schmid, & Berger, 2011).
The pelvis and foot presented a mosaic of
apelike and humanlike characteristics. These suggested adaptations to a more
efficient (albeit not entirely human) form of bipedalism, at the expense of
reduced arboreal efficiency (Kibii, et al., 2011; Zipfel, DeSilva, Kidd,
Carlson, Churchill, & Berger, 2011). The bipedal
mechanics differed from those reconstructed for other australopithecines,
suggesting that there may have been several forms of hominin bipedalism at this
time. The adaptations of Australopithecus
sediba may have enabled it to both walk and climb reasonably well and thus
survive in a dual arboreal/terrestrial world (DeSilva, et al., 2013).
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