Thursday, September 22, 2011

The hand of Australopithecus sediba

Kivell et al, 2011 Fig 1 (Left view = palmar, Right view = dorsal)
Kivell and colleagues report on the nearly complete hand of Malapa Hominin 2 (MH2), which has been attributed to Australopithecus sediba, a possible ancestor to genus Homo from the site of Malapa in South Africa. By nearly complete, I mean it is only missing a couple of wrist bones and the distal phalanges of the four non-thumb fingers. And everything that is preserved is in amazingly good condition. 

The biggest question about the A. sediba hand fossils is: did this species make tools?  The authors' answer is yes: A. sediba had hand morphology which would allow it to make tools. How they arrive at this conclusion is interesting. One of the major hand fossils that they compare with this hand is the OH7 Homo habilis hand from Olduvai Gorge. H. habilis (handy man) has been considered a tool-maker  since these fossils were discovered over fifty years ago in direct association with Oldowan stone tools. Kivell et al. compare the morphology of the MH2 hand with OH7 and find that MH2 has some derived features such as "reorientation of the scaphoid and capitate intercarpal articulations" (Kivell et al. 2011, pg 1416) which are lacking in OH7. Thus, they argue, these features indicate that the MH2 hand is more derived than the OH7 hand, and might be a "better potential morphotype for the basal Homo hand morphology" than OH7 (Kivell et al. 2011, pg 1416). They make this argument even though A. sediba is neither assigned to Homo nor found in association with tools. One possibility that they raise is that OH7 is not H. habilis at all, but rather belongs to P. boisei.  Could be, but what is for certain is that the MH2 hand skeleton is going to be a very interesting part of the debate surrounding manual functional morphology, locomotion, and tool use in the future. 

Tuesday, September 20, 2011

Extinct lemurs and human evolution

Paranthropus boisei (left) and Hadropithecus (right) (Dumont et al., 2011:Fig1)

In a brief letter to PNAS entitled "Thinking outside the box: A lemur's take on hominin craniodental evolution" Lorrie Godfrey and colleagues respond to a recent article by Cerling and colleagues (discussed here). The Cerling article argued, based on stable isotope data, that the robust east African australopith Paranthropus boisei was likely a grazer.  The main point of the letter is the figure, which shows carbon isotope values for tooth enamel for a variety of hominins, Old World monkeys, and subfossil lemursP. boisei clearly has really high carbon isotope ratios.  The only other primates that can compete are Theropithecus (the grass-eating gelada baboon), and Hadropithecus, a recently extinct lemur that Dumont, Ryan and Godfrey argued just five months ago was unlikely to be a hard object feeder based on skull morphology and finite element modeling.  When you look at the skulls of these guys next to one another, the similarities are really striking! This comes full circle, back to Clifford Jolly's Theropithecus model for hominin diet from 1970, which suggested that australopiths might have had a diet based largely on grass leaves and seeds, instead of the prevailing "nutcracker" ideas. As summed up by Godfrey et al.
"there are important ways in which the diets of Paranthropus and Hadropithecus likely converged. Both probably consumed large quantities of “low-quality” foods (i.e., foods that were relatively high in structural carbohydrates and poor in nutrients and required heavy repetitive trituration). Such conclusions may have important implications for the evolution of the craniodental features that these species share."
UPDATE: 9/22/2012
I failed to notice the Cerling et al. reply to the Godgrey et al. letter to PNAS. They note that Hadropithecus could attain the observed 13C ratios by consuming CAM plants (cacti and suchlike which are common in Madagascar) instead of the C4 plants (grasses, sedges, etc) suggested for P. boisei. Thus, the isotopic and morphological similarities need not suggest any mechanical similarity in the diets of the species.  It seems like it would be useful to compare Hadropithcus with isotope values of lemurs in today's Malagasy spiny forests. Thanks for Chris Kirk for pointing out the omission.
Godfrey et al.'s isotope figure

Monday, September 19, 2011

The foot of Australopithecus sediba

A. sediba is a recently described species of gracile australopithecine from the site of Malapa in South Africa which dates to approximately 2Ma. A recent issue of Science contains several articles which describe in detail the anatomy of several very complete specimens. These fossils are important because they are so complete, and because their discoverers have suggested that the new species may be directly ancestral to our own genus Homo.

The paper by Zipfel et al describes the foot and ankle bones, including an articulated distal tibia, talus and calcaneus preserved encased in matrix in approximately anatomical position. Other foot specimens include a very complete distal tibia as well some metatarsals. The details of the ankle joint are pretty human-like except for a very robust medial malleolus. The real kicker is the calcaneus, which is quite gracile and lacks a weight bearing lateral plantar process (LPP). These features make the calcaneus look rather like a chimpanzee's. Coupled with the robust medial malleolus, this could mean that A. sediba retained a degree of arboreal climbing competence. I feel sure that the functional implications of this morphology will be heartily debated. 

What is especially interesting is that A. afarensis resembles modern humans in these details of the calcaneus and distal tibia. A. afarensis (the species Lucy belongs to) is the gracile australopithecine cousin of A. sediba which lived over a million years earlier and is widely considered the direct ancestor of genus Homo. Regardless of whether or not A. sediba is directly ancestral to Homo or not, this mosaic ankle morphology is interesting and suggests some level of independent evolution of modern human-like pedal morphology.