KW: How did you diagnose this specimen as A. afarensis?
ZA:Afarensis in general can be described as a bipedal, megadont [large-toothed], small-brained species. And it's transitional between the earliest part of our history, such as Ardipithecus ramidus, and the later genus Homo. So when you look at this individual, what is interesting is that key features that are diagnostic of A. afarensis are already established at the age of three. If you look at the snout, for example, which is called the nasoalveolar clivus, it's bi-convex. This distinguishes it from the Taung child [which belongs to another australopithecine species known as A. africanus], in which that area is flatter. And when you look at the lateral margins of the nasal cavity, they are sharp, as in afarensis. In Taung, by contrast, those margins are blunt. Likewise, the nasal bones are not wide as in Taung. Rather, they are narrow and apelike, as in afarensis. The mandible, too, looks like the lower jaws of afarensis found previously in Hadar.
KW: One of the things that is so spectacular about this fossil is that you have so many elements from a single individual. But that being said, what does this fossil add to the picture of afarensis, which is already a pretty well known species?
ZA: A lot, actually. I'll highlight the main points. First, even though afarensis was well known based on the adult specimens, we had little idea about the juveniles. So yes, we are looking at afarensis, but we are looking at afarensis when it was a child. This is a new dimension. It adds a lot to our understanding not only of afarensis, but also many other early hominin species. (By the way, to my knowledge there is no associated skeleton of a juvenile hominin older than the Neanderthals [who lived between 250,000 and 25,000 years ago].)
So we will now be able to tell the morphology of different skeletal elements of A. afarensis at age three. Understanding these features is a goal on it's own, but it will also allow you to compare them with the adult elements, the same skeletal parts, and tell how they changed during development.
But when you go the specifics, this individual not only is the most complete early juvenile hominin, but it includes previously completely unknown or very little known skeletal elements. The two examples here are the shoulder blades (the scapulae) and the hyoid bone. And the relevance of the shoulder blades is huge. There was a fragmentary piece of scapula from Lucy, but it was not really possible to analyze this bone. So this is the first and the earliest shoulder blade known?and we have both of them. Analyzing these bones will enable us to address critical questions about the locomotor behavior of afarensis.
KW: In the 1980's there was quite a bit of debate over how afarensis moved around. Can you explain what the controversy centered on?
ZA: There was no doubt that Australopithecus afarensis was a biped. There were discussions about whether it was a perfect biped like us, or whether it climbed trees as well. The question was, How do we interpret the primitive features on the upper part of the body [the curved fingers and certain characteristics of the humerus, for example]? Some researchers think that these features were just primitive retentions, with no relevance to the locomotion of Australopithecus afarensis. [Proving otherwise would require] demonstrating that those primitive features on the upper part of the body were compromising bipedalism. But the other group argued that if a species like afarensis preserved for many years these features, then they were maintained for reasons that were relevant to the adaptation of the species.