In a paper published today in Nature, paleoanthropologist Zeresenay Alemseged of the Max-Planck-Institute for Evolutionary Anthropology in Leipzig, Germany, and his colleagues unveil the remarkable skeleton of an Australopithecus afarensis child who lived 3.3 million year ago. Scientific American.com editorial director Kate Wong called Alemseged at his office to talk about the discovery. An edited transcript of their conversation follows.
KATE WONG: How did you come to work in this part of Ethiopia?
ZERESENAY ALEMSEGED: The area is called Dikika, which actually means "pointed hill" or "nipples." It's a hill which is not far from the spot where we found the hominid. I started the project back in 1999, just after I finished my Ph.D. in Paris in 1998. This site is surrounded by many famous [hominin] sites, such as Hadar, Gona, the Midle Awash and others. It basically belongs to the same sedimentary basin, so I was not surprised that it yielded hominins. This site was actually found in the 1970s, along with the other sites. But it was not until 1999 that there was an expedition to this place, which I led.
When I decided to go there, I wasn't hoping to find the partial skeleton of a juvenile hominin--I had another set of scientific questions within paleoanthropology. My idea was to explore what happened before Hadar [the site at which the famous Lucy fossil was found] in terms of paleoenvironment, paleoanthropology, geology and stratigraphy. I went with a small group of Ethiopians in 1999 and we found lots of fauna--elephants, pigs, rhinos and what have you. And in 2000 I led a small group of Ethiopians there again. On the 10th of December I decided that we would survey this hillside. That afternoon the second person who was with me--another Ethiopian who was the antiquities officer (Tilahun Gebreselassie)--saw the hominin fossil first, sticking out of the sandstone. I was only a few meters away. Right away it was clear that it was a hominin, with the absence of the brow ridges, the lack of a postorbital sulcus, the small canine, the mandible's vertical symphysis--all those features. So I started looking around [at the surrounding ground] and right away I found part of the forehead. But with only four of us in the group, it was not possible right then to do the necessary excavation. We didn't even have screening materials. I had to go all the way back to Adis Ababa to pick up supplies first.
Of course there was another reason also to go back to Adis. I wanted to make sure this incredible discovery was safe. So I took it to Adis, put it in the safe and then went back to the site, where we recovered some more of the skull. We didn't do much. My goal was to make sure what was exposed was collected. With just a small number of people it's not a good idea to do a huge excavation because it gets out of control. So the idea was to make sure that everything that was eroded out of the sand and everything that was broken off the block of bones and sandstone was collected.
KW: Why has it taken so long to publish your findings?
ZA: When we found the fossil, the face was only partly exposed, and most of the other parts of the body--including the collar bones, the scapulae, at least 10 vertebrae and the ribs were covered by this hard and very compact sandstone matrix. Cleaning that took over five years. And [the work] is not finished yet. We also went back for four successive field seasons, spending two months every year on that spot--with more people--to recover more of the skeleton. The result is what is now the earliest and most complete juvenile Australopithecus afarensis ever found.
KW: How much of the skeleton do you have?
AZ: I would say it is more complete than Lucy. But in addition we have the face, which is not preserved in Lucy. So you can look at her and she looks at you also. We also have the lower jaw--which is still in connection with the upper jaw--and a full brain endocast, which is made of the sandstone impression. Part of the parietal, which is the upper part of the calotte, is gone, so you can see the endocast there. Squashed to the base of the skull are all the bones that I described earlier. So the upper part of body is there, except the arms, but we have a piece of the humerus.?
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.
How do you then answer these questions? One of the ways in my opinion is to find previously unknown skeletal elements such as the scapula, and show that there is an increasing number of skeletal elements from the upper part of the body and sometimes from the lower that are showing us that maybe afarensis climbed trees, for example. But the best way to address this issue will be to really understand the functions of each muscle, as we see them, as we interpret them from the bones. One clue regarding climbing that we have in our discovery is that the glenoid cavity [shoulder socket] is upwardly oriented.?
Apes have a glenoid cavity that is upwardly oriented. In humans it is more laterally oriented. Maybe a superiorly oriented glenoid cavity is telling you the individual was raising its hands above its shoulder. A primate does this when it climbs. So this could be one argument to support the climbing hypothesis. But we need more of these bones to see if that feature is representative of the species, or at least of the population it comes from.
KW: According to your paper, the scapula overall looks most like a gorilla's. But gorillas don't do much climbing, do they?
ZA: That is true, but when they are little they climb quite a bit. The big guys don't climb. I'm not saying afarensis climbed for sure at this stage, but I think the idea of climbing is again out there. So it has to be tested.
KW: You mentioned that the specimen preserves a complete natural endocast, an impression of the interior of the skull. What does this reveal about brain growth in afarensis?
ZA: The brain size of this three-year-old Australopithecus afarensis was only 330 cubic centimeters, which is the same as that of a chimp of comparable age. But, on average, adult afarensis had a higher cranial capacity compared to adult chimps, with specimen AL-444 from Hadar exhibiting a cranial capacity of 550 cc. When you compare [this baby afarensis brain size] to the female adult brain size, it had only formed only 63 to 88 percent of the adult value. In contrast, a chimp at this age would have reached greater than 90 percent of its adult size. This may indicate a slower rate of brain formation in afarensis, which is a characteristic of humans. We think there is some hint of that, but we don't know. And the reason we don't know is that at three years old, humans and apes overlap a lot when the values are compared between the young and the adults. Our take is that it's close to the average of humans, but that remains to be tested again in the future.
KW: In modern humans, the head and torso are decoupled, which aids in endurance running. Your paper notes that the upward-facing shoulder socket of the Dikika baby could indicate that the head and torso of afarensis were not decoupled. The paper also suggests that the semicircular canals of the inner ear may hold further clues to afarensis locomotion. How so?
ZA: The semicircular canal has a relationship with body balance and the morphology [in this specimen] is apelike. We're not making the argument that because they had a canal similar to that of apes, they climbed. You cannot argue that way. I think the functional analysis should come from the muscles. But we have these scapulae and the long fingers that seem to be primitive and may be relevant to climbing, and the canals are also telling us it's apelike. So it's tempting to say there's something there, that afarensis was climbing.
KW: The second paper in Nature, on the geological and paleontological context of this discovery, describes a mosaic of forested and open environments. It seems like that meshes well with the idea of a creature that spent time both on the ground and in the trees. A lot of these early hominins are turning up in more mosaic or wooded environments than people used to envision.
ZA: The fact that hominins lived in a mosaic environment is, I think, an established fact. The information comes from fauna, sediments, isotopes and what have you, so the environments were there. The big question is, which part of that environment was essential for their survival, for their adaptation? That is what we need to nail down. The earliest hominins lived in woodlands, in forested environments, for example. But it doesn't mean that they were completely out of the relatively open environment. I think they would venture into these environments when they needed to. There are recent works by Peter Ungar [of the University of Arkansas] concerning fallback resources and how hominin species might actually live in an environment when things are good. But when things go bad they fall back on another type of food, which means another type of environment.?
KW: There have been so many ideas over the years about why hominids became bipeds. Do you have any thoughts about this?
ZA: I believe we should just put the savannah theory aside. I think they basically became biped while they were living in a wooded, covered environment. The savannah could explain two other things. Actually, we should say grassland, because savanna is a more complicated term. But the relatively open environment was out there to be tried--be it later with Homo, when it was very dominant, or be it before, with the earliest hominins. Hominins were experimenting with all sorts of environments. But at some point they had some preferences. It's very hard to do, but only when we are able to [identify] their preferences can we talk about the mechanisms behind what triggered bipedalism, what triggered megadontia, what triggered brain expansion.
KW: One last question, about the hyoid. The only other fossil hominin hyoid on record is the one from the Kebara Neanderthal. There has been lots of speculation about whether that can reveal anything about Neanderthal language. What does this bone tell you about afarensis vocalization?
ZA: Since the relationship between articulated language and the hyoid bone is not established, any inference that you make about language based on this bone is not substantiated. However, we now know [based on this hyoid] that early hominins--at least afarensis--had these laryngeal air sacs, suggesting a voice box similar to that of chimps. But what does this mean when it comes to language? One has to functionally demonstrate first that this bone is actually relevant to language or the lack thereof before any conclusion can be drawn.?