Laelaps

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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.

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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

Comments

  1. #1 Zinjanthropus
    June 23, 2010

    Hi Brian, nice post!

    I do have a few bones to pick (so to speak). You state “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,” but I think this is a bit of a misreading. The authors found that there were two (largely redundant) angles in which Kadanuumuu’s measurement turned out to fall in between human and gorilla measures (the angle between the glenoid and the lateral border of the scapula, and the glenoid and the bar which runs parallel to the lateral border). The PCA that the overall pattern for Kadanuumuu is more human-like.

    Second, the “funnel-shaped” thorax is mainly determined not by the lower part of the rib cage, but by constriction at the top of it. The lack of constriction demonstrated by Kadanuumuu’s second rib is pretty good evidence that the rib cage was not funnel-shaped.

  2. #2 CS Shelton
    June 24, 2010

    Hey, Zinj… Nice grammar for “an early hominin described as the largest of the Paranthropus species, that lived from about 2.6 until about 1.2 million years ago during the Pliocene and Pleistocene epochs in Eastern Africa.”
    (I gots wikipedia and fingers!)

    I read commenter “occamseraser” on The Panda’s Thumb say he had his doubts about attributing the remains to afarensis due to the lack of “craniodental remains” … What would that mean for this discovery then?

  3. #3 Zinjanthropus
    June 24, 2010

    Yes, no teeth is a problem when trying to diagnose a fossil into a certain species. I’m frankly happy that they didn’t just name a new species, as seems to be the trend nowadays.

    It’s either afarensis or anamensis, and both diagnoses would be strengthened by having teeth. I think people would complain no matter where they placed it. It’s at the right place and right time to be afarensis, so I think that’s probably the simplest diagnosis for now.

  4. #4 Brian Switek
    June 24, 2010

    Zinj – Thanks for the comment. As for the “intermediate” remark, I pulled it from this sentence in the paper: “In the two redundant angles (axillary–glenoid and bar–glenoid), KSD-VP-1/1g is largely intermediate between humans and Gorilla but lies above the latter’s interquartile range.” Perhaps I should have been more specific, but overall I found it interesting to see how there were some data points from gorillas which were close to KSD-VP-1/1.

    Thank you, as well, for your comments about the ribcage shape. While the top ribs may not indicate such a strongly funnel-shaped thorax as proposed by Schmid, whatever reconstruction we choose is still going to be a hypothesis until we find more fossils. Likewise, I would be interested to see if ribcage shape varied between small individuals (like Lucy) and large ones like KSD-VP-1/1.

    And this is not something directed only at the authors of the paper – even I have done this from time to time – but I have become frustrated by the habit of describing traits as more human-like or more ape-like based almost solely upon the anatomy of living hominids. Granted, many fossil hominins are fragmentary and may not preserve the same parts of the skeleton needed for detailed comparison, but how informative is it to say that a ~3.6 million year old hominin is more like a gorilla or more like our species when its closest relatives are not also considered? I am just thinking out loud here, but I have to wonder about how these choices for comparisons influence the way we interpret the paleobiology and relationships of these hominins.

    Again, thanks for calling me out on the points you mentioned. Like anyone else I don’t enjoy making mistakes or putting something down that is not entirely clear, but I learn more through the correction.

  5. #5 wolfwalker
    June 24, 2010

    As it turns out, Lucy was small even compared to members of her own species.

    Could this be interpreted as showing that Lucy was an adolescent female, not yet full-grown? And that some of the more primitive features of her anatomy are due to her juvenile physiology?

  6. #6 Allen Hazen
    June 24, 2010

    About those scapular photos…
    Not being equipped to do PCA, and just going by subjective gestalt impression…
    (1) In over-all shape, the chimpanzee is the outlier: much narrower.
    (2) Are all four illustrated in the same orientation? Taking the center of the dense region close to the glenoid as the center of the clock-face, it looks as if the glenoid is directed toward about 10:00 o’clock in three of the photos, but more toward 11:00 in the chimpanzee photo.
    (3) On the assumption that the orientations are appropriate, there is one striking feature in which KSD-VP-1/1 (easier to spell than it’s other name!) resembles H. sap rather than G.g. or P.t.: the two extant African apes seem to have a strong spine directed diagonally across the face of the scapula, towards 4:00 or even 4:30, but H.s. and our hero have their only really dense “lines” closer to parallel with the top edge: 3:15 or 3:30.

  7. #7 occamseraser
    June 24, 2010

    Lucy was fully grown. Her long bone epiphyses are all fused/closed. She’s an incredibly small female, shorter than the shortest modern human pygmies, and the “Big Man” comparison validates the consensus conclusion that this species was waaayyyyy sexually dimorphic.

    Yes, anamensis is another possibility, but anamensis-afarensis is probably an evolving lineage, and the division into chronospecies is perhaps arbitrary (but a decent compromise). Au. anamensis was also a biped, so no surprises here. Dare I mention the name Kenyanthropus?? Nice comment on the NatGeo website about attribution and affinities of the “Big Man”. Tim White and Co. opted not to attribute postcrania to Au. garhi even though they were found nearby precisely because there was no deinitive link to diagnostic craniodental bits.

    What do you think the authors meant in reference to the scapula as having unique (but unspecified) functional affinities? Hmmmm.

  8. #8 Danniel Soares
    June 24, 2010

    How I would like if graphic restorations of extinct organsims of all sorts didn’t attempt at a single result, “here, that’s how it looked”, but rather, or more frequently, exposed more emphatically a wider range of possibilities for their looks.

  9. #9 stripey_cat
    June 25, 2010

    That’s one heck of a sexual dimorphism (or possibly regional/niche specialisation?). Has anyone considered whether either might be pathological in some way?

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