Australopithecus afarensis: tree climber

ResearchBlogging.orgAustralopithecus afarensis is a bipedal human ancestor which lived in Africa from 3.9 – 2.9 million years ago. It’s particularly notable for being the first hominin to have a human-like foot, having replaced the chimp-like opposable toe of its ancestor Ardipithecus ramidus (who lived 4.4 million years ago) with a forward-facing human-esque toe. Although Ar. ramidus lacked this important adaptation, the evidence indicates that they were also bipedal, the key difference being that they still spent a lot of time in the trees. Their chimp-like big toe and curved fingers were excellent climbing adaptations but they were still walking bipedally. Perhaps they used their bipedal ability to walk from tree to tree; or maybe they walked along branches, a behaviour sometimes seen in modern orang-utans.

Curiously, Au. afarensis also possesses some climbing adaptations. They may have traded in their grasping big-toe for one better at bipedalism but they still retained the long arms and curved fingers which would make them be excellent climbers. This has resulted in a debate in the evolutionary anthropology community: is Au. afarensis still spending a lot of time in the trees or are their climbing adaptations simply unused archaic features which evolution has not removed because they aren’t harmful?

In 2000 palaeoanthropologists found a well preserved Au. afarensis specimen in Ethopia, a few miles away from that other famous Au. afarensis specimen: Al-288 1. They named this new discovery “Selam” and quickly identified her as 3 year old girl. Selam was remarkable for many reasons, including being the first to prove Au. afarensis‘ feet had arches like a human. She also is one of the few hominins to have their shoulder blade (scapula) preserved, providing a unique insight into her tree-swinging ability. An early analysis suggested that it was most similar to a juvenile gorilla’s scapula, but it quickly became apparent that this research overstated the similarity between the two.

A labelled scapula, on the off chance you haven’t extensively studied human anatomy.

Now a new analysis of Selam’s scapulas has been published, comparing them not only to living apes but also to all the other hominin scapulas we have. This allows us to understand how the scapula of Au. afarensis changes over the lifespan in an individual, which is important because human and ape scapulas mature differently. The shoulder joint itself orientates towards the head in humans whilst it doesn’t really change in apes. Similarly the scapula spine becomes more oblique as humans mature, whilst it doesn’t significantly change in apes. On the other hand, the region below the scapula spine (infraspinous fossa) does become wider as apes mature. This change occurs as apes grow up and spend more time knuckle-walking and less time swinging in the trees, mandating a slight shift in their anatomy. The exception to this being orang-utans, who don’t knuckle walk.

Right, so now you have a background in ape scapulas we can continue. Selam’s scapula looks more like an ape scapula than a human one. It also matures like an ape, lacking the changes in orientation a human scapula goes through. This ape orientation is most suited to suspensory behaviours, placing the muscles involved with hanging from a branch with the most efficient orientation possible. However, whilst Selam does look more like an ape it doesn’t cluster with the apes in a statistical analysis. Clearly there’s something unique going on here. Further, although it seems very similar to a juvenile gorilla (like the original analysis concluded), Au. afarensis does not seem to undergo the widening of the infraspinous fossa as they mature, like knuckle-walking apes do.

A statistical analysis of scapulas. The left clusters are apes, the right human (except for the circles which are orang-utans). L & R are Sema’s left and right scapula.

In short, Australopithecus afarensis has a unique scapula. It’s ape-like in the sense it has many adaptations for swinging through trees but lacks any signs they engaged in knuckle-walking. This would make sense given they were bipedal, so would be travelling on two legs rather than four. Whilst the retention of significant arboreal traits does seem to be strong evidence that Au. afarensis spent a considerable amount of time in the trees, there is still the (slim) possibility that it is simply an archaic feature which has yet to disappear. However, other research suggests that the increasing size of the infraspinous fossa is a result of changing activity as the ape matures, not “pre-programmed” maturation. In other words, the fact that the Au. afarensis spine matures the way it does is a result of the fact they are still spending a lot of time swinging through the trees.

Although many had suspected it for a while it would seem we can now talk about the lifestyle of Lucy, Sema and the other members of Au. afarensis with confidence. They walked upright from place to place, not unlike modern humans, but still spent a lot of time climbing in trees. Likely they were walking between forest groves in an increasingly patchy landscape and it this served as a selection pressure, driving them towards the modern human body plan.

Green, D., & Alemseged, Z. (2012). Australopithecus afarensis Scapular Ontogeny, Function, and the Role of Climbing in Human Evolution Science, 338 (6106), 514-517 DOI: 10.1126/science.1227123

9 thoughts on “Australopithecus afarensis: tree climber

  1. Pingback: Gravity – the god of our Age « Eye on the ICR

    Abstracts of two key scientific papers on ‘Selam’.
    (See also:
    Marc Surtees is a YEC – he flagged this article – but it actually states “”When we compared Selam’s scapula with adult members of Australopithecus afarensis, it was clear that the pattern of growth was more consistent with that of apes than humans.” At the same time, most researchers agree that many traits of the A. afarensis hip bone, lower limb, and foot are unequivocally humanlike and adapted for upright walking.” I think the evidence for bipedalism comes from other A. afarensis fossils other than Selam.)

    You may wish to see item 1 of AiG’s ‘News to Note’ for 17 November, entitled ‘Lucy was a real swinger’:
    I’m about to take a close look at it.

  3. (item 1: ‘Lucy was a real swinger’)

    My message to AiG (sent direct from within their website):

    “The theory of evolution is not a ‘lie’ but an honest interpretation of evidence. However, Answers in Genesis MISREPRESENT the science in an attempt to DISCREDIT it. (item 1)
    You link to this:
    Note that the fossil specimen of A afarensis is ‘Selam’ and NOT ‘Lucy’.
    The article states: “Although Lucy and her kin were no knuckle-draggers, whether they also spent much of their time in trees was hotly debated” and also quotes a key scientist as saying “While bipedal like humans, A. afarensis was still a capable climber”.

    AiG’s article – in support of its fraudulent depiction of ‘Lucy’ as a knuckle-walking extinct gorilla at the Creation Museum – is disgraceful cherry picking of evidence you like (whilst ignoring all other evidence).

    Your PROBLEM is that A. afarensis had ape-like shoulder blades and human-like foot arches (it also lived around 3 million years’ ago). Thus it SUPPORTS the PREDICTIONS of the theory of evolution – whilst undermining Mr Ham’s absurd claims (based on a reference in 2 Thessalonians 2 in the pre-scientific Bible) that the whole theory is just a ‘lie’.

    “If Lucy and her cousins could be shown to have abandoned the trees, so much the better.” It does not matter either way. Because YOU cannot show that the species was not bipedal. Except by ignoring the evidence for upright walking and deliberately depicting the species as a knuckle-walking ape in your so-called museum.

    “What is clear, actually, is that the evolutionists have found additional anatomical evidence that Australopithecus afarensis was just an ape.” No. YOU ARE A LIAR FOR JESUS. What about other partial specimens of the SAME species? Why are you ignoring these papers – despite all my messages bringing them to your attention:

    “Nothing about the results screams “human” or “human-in-the-making” but only “ape.”” You are ignoring other results. You are a LIAR FOR JESUS.

    “But if Lucy and Selam were not extinct, they’d just be another exhibit in the ape section of the zoo”. WHY are they extinct?

    Probably because the lived 3 million years ago – before Homo sapiens existed!

    YOU ARE ALL FRAUDS FOR JESUS. You HATE science – because it frequently undermines your primitive, literalist beliefs. So you engage in distortions and propaganda.

    By the way, Mr Benton DID reply to your previous article attacking his earlier blog:

    You are also either doubly dishonest or doubly incompetent or both. THIS is the Abstract of the original science paper from back in 2006 about ‘Selam’ – which you agree is the SAME species as Lucy:
    I quote: “The foot and other evidence from the lower limb provide clear evidence for bipedal locomotion, but the gorilla-like scapula and long and curved manual phalanges raise new questions about the importance of arboreal behaviour in the A. afarensis locomotor repertoire”.


    I am posting all this on Mr Benton’s most recent blog post on A afarensis dated 7 November and also at the British Centre for Science Education community forum. Thus, whilst not a blog, it will be in the public domain.


  4. Pingback: Answers in Genesis v EvoAnth #6:The conclusion « EvoAnth

  5. Pingback: Tree climbing and human evolution « EvoAnth

  6. Pingback: Did Australopithecus make stone tools? « EvoAnth

  7. Pingback: Pushing back the origin of meat | EvoAnth

You evolved too. Have a say.

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s