The Juvenile Australopithecus afarensis

Back in August, 2005, I wrote a post on Dinosaur Embryos, Growth and Human Evolution. One section of the post discussed attempts to study the growth patterns of KNM-WT 1500, in particular, and Homo erectus in general. I followed the discussion with:

More intriguing is the possibility that this type of analysis could be extended to earlier fossils. Interesting fossils have been found in Drimolen, South Africa (about five years ago). The finds consisted of the bones of two infants. One was 2-3 years of age, the other 8-10 months. One is tentatively assigned to the genus Homo, the other to Australopithecus robustus (the interesting thing about the A. robustus infant was that a lot of the robustus traits were clearly visible on the fragments – indicating that even early in life their are species differences among the australopithecines). Although the finds were fragmentary and only few bits of the crania were found one wonders if in a few years the above type of analysis could be extended to these fossils as well.

So, I am happy and somewhat surprised that the above types of analyses are in the process of being performed on a juvenile Australopithicus afarensis discovered in 2000 at Dikika, Ethiopia.

Kambiz has an excellent overview of the find, as well as links to the Nature articles, newstories and pictures.
The find (DIK-1-1) comes from the Sidi Hakoma member of the Hadar formation and dates to 3.35-3.31 million years ago. From Alemseged et al 2006:

Sandstones that yielded the specimen were deposited on a subaerial delta plain. This depositional setting, combined with the remarkable preservation of many articulated faunal remains lacking evidence of preburial weathering, most likely indicates that the juvenile hominin was buried as an intact corpse shortly after death during a major flood event.

Added slightly later: National Geographic has a great video concerning the find…
Consequently, the preservation was fantastic. How remarkable is the preservation? An intact hyoid was discovered!

Prior to this the oldest intact hyoid was a Neanderthal hyoid dated to about 60 kya found at Kebara.
The individual was about three years old (based on dental development) and may be female (based on tooth size – I should point out I’m a little skeptical of the sex determination). Overall, the morphology is an interesting mix and reinforces A. afarensis’ status as a transitional fossil. The hyoid, for example is more similar to apes than humans, as is the superior orientation of the glenoid (part of the scapula) which resembles that seen in gorillas . Additionally, the semicircular canals resemble that seen in apes. On the other hand, most of the bipedal features seen in other A. afarensis lower limbs are present in DIK-1-1. Consequently, the debate about the locomotory behavior of A. afarensis is sure to be intesified. Alemseged et al 2006 point out:

The DIK-1-1 skeleton confirms the functional dichotomy of the body plan of A. afarensis: a more derived lower body adapted for bipedal locomotion, combined with an upper body that is, in many respects, ape-like. The functional interpretation of these features is highly debated, with some arguing that the upper limb features are non-functional retentions from a common ancestor only, whereas others propose that they were preserved because A. afarensis maintained, to some degree, an arboreal component in its locomotor repertoire. Now that the scapula of this species can be examined in full for the first time, it is unexpected to find the strongest similarities with Gorilla, an animal in which weight-bearing and terrestrial knuckle-walking predominately characterize locomotor use of the forelimbs.[bolding mine-afarensis] Problematic in the interpretation of these findings is that the diversity of scapula architecture among hominoid species is poorly understood from a functional perspective.

There are several interesting points to be made about this quote. First, is anybody really surprised that a hominin experimenting with bipedalism would have morphology similar to that of the gorilla – the most terrestrial of the apes? Second, extent hominids (i.e. members of the family hominidae – the group that includes apes and humans) are a poor reflection of the variation that existed in the Miocene and Pliocene. Consequently, modeling past locomotor behavior is apt to be difficult because we lack proper models.
Added Later:
Nature has an interesting series of videos on the find. The Nature video clears up my skepticism on the sex determination. Apparently they CT scanned the skull and measured the adult canines…
National Geographic has a video as well…