The skeletons of Lucy (left) and Kadanuumuu (right). Both belong to the early human species Australopithecus afarensis. (Images not to scale.)
I never fully appreciated how small Lucy was until I saw her bones for myself. Photographs and restorations of her and her kin within the species Australopithecus afarensis had never really given me a proper sense of scale, and when I looked over her incomplete skeleton – formally known as specimen A.L. 288-1 – I was struck by her diminutive proportions. In life she would have only been about three and a half feet tall. Her physical stature seemed to be inversely proportional to the influence her bones have had on our understanding of our origins.
As it turns out, Lucy was small even compared to members of her own species. Although it is unlikely to diminish her notoriety, this week a team of paleoanthropologists led by Yohannes Haile-Selassie have released the description of another, older partial skeleton of Australopithecus afarensis discovered by Alemayehu Asfaw in the famous Afar region of Ethiopia. The discovery of this skeleton marks only the second time that parts of the forelimbs and hindlimbs of one individual A. afarensis have been found together, and it provides some new insights insights into how early humans moved.
Technically called KSD-VP-1/1, but informally dubbed Kadanuumuu (meaning “Big Man”), the newly described skeleton is represented by parts of the leg, arm, pelvis, neck, ribs, and shoulders. It is not as complete as Lucy, but there is enough of the skeleton present to know that it belonged to the same species. This specimen lived about 3.6 million years ago, almost around the same time as the A. afarensis individuals in Laetoli, Tanzania (home of the famous hominin trackway) but about 0.4 million years before Lucy. Time is not the only factor that differs between the two partial skeletons.
As the nickname suggests, “Big Man” was of a larger stature than Lucy. In general it fell within the upper range of estimates of A. afarensis body size, standing between five to five-and-a-half feet tall. Given this difference in size and characteristics of the pelvis (one of the few parts of the skeleton useful for determining the sex of an individual), the authors propose that KSD-VP-1/1 was a male, though the authors do not comment at length about sexual dimorphism between male and female members of the species.
X-rays of the scapulae of a) a modern human, b) KSD-VP-1/1, c) a gorilla, and d) chimpanzee. From Haile-Selassie et al, 2010.
Yet the most remarkable aspects of KSD-VP-1/1 relate not to size, but parts of its anatomy not seen before. Paleoanthropologists have found bits and pieces of scapulae (shoulderblades) from adult A. afarensis before, but never one as complete as in KSD-VP-1/1. When compared to the shoulderblade of our species, a gorilla, and a chimpanzee, the scientists found that the KSD-VP-1/1 scapula is “intermediate” between ours and that of a gorilla, with the chimpanzee shoulderblade being greatly different from the others (again hinting that chimpanzees have undergone a good deal of anatomical specialization in the last six million years or so and cannot be taken as perfect proxies for what our earliest hominin ancestors were like). Hence the scapulae of A. afarensis are unique – they are not quite like ours but they are quite different from those of chimpanzees – and the researchers behind the description suggest that either 1) the shoulderblades underwent significant modification once hominins started walking on the ground, or 2) that they represent a unique mode of locomotion unlike any seen among the few living hominids.
Likewise, the authors also propose that A. afarensis may have had a different thorax shape than previously thought. On the basis of the species’ relative proximity to the split with the chimpanzee lineage and the ribs of Lucy, it had previously been hypothesized that A. afarensis had a funnel-shaped ribcage (narrow at the top, wide at the bottom) which helped the thorax house a big, vat-like stomach in which large amounts of plant food could be processed. The new paper disputes this hypothesis on the basis of five ribs from the upper part of the new specimen’s chest. While the upper ribs of some gorillas were similar to the upper ribs of KSD-VP-1/1, overall the top ribs in the thorax were more like ours than those of living African apes. On this basis the authors state “there is clearly no evidence that the early hominid thorax was ‘funnel-shaped’ as previously claimed”, but this is not necessarily so. The upper part of the thorax may be more like ours – or at least intermediate between ours and that of some gorillas, as was the case with the scapula – but we still do not have a clear picture of what the lower ribcage was like. Given that the references the authors cite related to this point are books and no page numbers are given it is difficult to see how their findings relate to previous research on the subject (such as the restorations made by Peter Schmid which became the basis for the funnel-ribcage restoration). At this point either ridcage hypothesis – “human-like” or “funnel-shaped” – is a hypothesis which requires future discoveries and analysis to support or reject.
In terms of arms and legs, KSD-VP-1/1 appeared to confirm to what had previously been proposed about proportions. Compared to us it would have had long arms, but compared to a chimpanzee it would have had long legs, again highlighting the disparity between the upper and lower body. Even then, the legs of A. afarensis and its relatives only appear to be moderately more elongated than those of their predecessors, and the authors take this to mean that the evolution of slightly longer legs was of limited importance to walking around on the ground.
Looking at KSD-VP-1/1 as a whole, its anatomy was well-adapted to upright walking on the ground. The transition from life in the trees to upright walking had happened long before, and while A. afarensis was still capable of climbing trees it appears to have been more at home on the ground. As stated by the authors, after the initial transition to bipedalism, which preceded A. afarensis, it seems that “highly derived terrestrial bipedality enjoyed a long period of stasis punctuated only occasionally by additional modifications to the postcranium.” Notice two key words there – “stasis” and “punctuated.” As we learn more about human evolution, the stop-and-go pattern of punctuated equilibrium becomes more important to understanding the branching patterns seen in our own family tree.
Haile-Selassie, Y., Latimer, B., Alene, M., Deino, A., Gibert, L., Melillo, S., Saylor, B., Scott, G., & Lovejoy, C. (2010). An early Australopithecus afarensis postcranium from Woranso-Mille, Ethiopia Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1004527107