Thursday, October 25, 2012

Selam and australopith mosaics

Interesting news in the latest Science for those of us interested in hominin fossils.  It's been widely acknowledged among evolutionists and creationists alike that the australopiths represent a kind of mosaic form, with upper bodies that resemble modern apes and lower bodies that are much more human-like.  Thus, they appear to have had a unique form of locomotion, with legs clearly adapted to walking around on two feet (like us) but arms potentially more suited to climbing in trees.  In my estimation, that makes for a curious sort of intermediate form.  Rather than having intermediate features, we instead see features that resemble one or another modern form mixed together in the same organism.  I don't know if that's really counterintuitive under an evolutionary model, but it's certainly curious.

Back in 2006, Alemseged et al. announced the discovery of a juvenile Australopithecus afarensis skeleton, the same species as the famous Lucy fossil.  The new fossil came from Dikika, Ethiopia, and was named Selam.  This week, Alemseged is back with a much more detailed analysis of the scapulae (shoulder blades) of Selam.  Since shoulder blades attach to the muscles of arms, they should be able to tell us a lot about how Australopithecus muscles attached and consequently how Australopithecus moved.

The results are pretty much what we expected.  Selam's shoulder blades resemble the shoulder blades of gorillas, which even this biochemist can see from the photos in the article.

Figure 1 from Green and Alemseged, Science 338:514-517.

By looking at different life stages of Australopithecus fossils, the researchers could also tell that the development of the shoulders appear to follow a more ape-like growth pattern.  Taken altogether, the evidence appears to support the idea that australopith arms were suited to climbing.

As I said, this is something we already knew, but it's nice to get a bit of confirmation from new discoveries.

Green & Alemseged. 2012. Australopithecus afarensis Scapular Ontogeny, Function, and the Role of Climbing in Human Evolution. Science 338:514-517.

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