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Australopithecus sediba has taken us on a rollercoaster of a ride. First discovered back in 2008, this South African hominin looked like it might be a close relative of ours, perhaps even an ancestor. It combined features seen in earlier, chimp-like Australopithecus and our own genus Homo; prompting countless newspapers to report it as the “missing link”.

MH1; one of the Australopithecus sediba skeletons found
MH1; one of the Australopithecus sediba skeletons found

However, it’s status as the missing “apeman” was short-lived. In 2011 later it was discovered Australopithecus sediba was only ~1.9 million years old1. Since the first members of Homo lived over 2 million years ago2,  it obviously couldn’t have been their ancestor (unless they had a Delorean. That always leads to weird family relations).  

But still, that mixture of traits means they were still a close relative. Right?

Stitching together Australopithecus sediba

Since them, follow up research has continued to find Australopithecus sediba had a combination of features from classic Australopithecus and more recent Homo. Like a modern mouth with some ape traits. When wanting to sound fancy, scientists refer to such a mixture as a “mosaic”3.

Perhaps the best examples of this come from their arm; which was long like an ape so they could grab branches4. However, their fingers were short like ours5; which are less great for grabbing but good for tool use. Was this a sign Australopithecus sediba was good with their hands?

The hand of sediba, with short stubby human-like fingers

Almost every aspect of Au. sediba‘s has this mix of human and Australopith traits; from the jaw through to their backbone. Put them all together and you wind up with image below. Definitely a chimp-like Australopith, but with some key human traits3.

With one key exception: how they walked.

A modern human (left); sediba (middle) and chimp (right)
A modern human (left); sediba (middle) and chimp (right)

Australopithecus sediba struts!

Australopithecus sediba walked on two legs, like every known member of our family. However, their gait was unique. As their foot hit the ground it twisted, so the outside edge landed first. This is called “hyperpronation” and no other members of our family walk like this6.

Well, some modern people actually do. The resulting damage to their joints was also found in Au. sediba fossils; confirming this was also how they walked. But whilst only some modern people have hyperpronation, every Au. sediba fossil we’ve found did6.

The damage seen in Australopithecus sediba consistent with hyperpronation

As such, it looks like this was their normal way of walking; not a medical condition. What’s up with that?

Given that most other hominins walk the same, this could mean Australopithecus sediba‘s branch split off early; before our normal gait evolved. This would have happened early in our revolution, potentially pushing their origins back to before even Lucy lived7!

If this is the case then what’s the deal with all of their similarities to us? Were they independent adaptations in a lineage that was evolving along a similar path to us?

A possible reconstruction of the human family tree, taking these new findings into account
A possible reconstruction of the human family tree, taking these new findings into account

Although that’s a fascinating possibility, not all the researchers agree. The sheer number of traits they share with us has led some to conclude they have to be a close relative. Maybe then, they were an “evolutionary experiment” with bipedalism. A weird, short-lived offshoot that tried something different8.

Or maybe something about their lifestyle made them extremely likely to develop foot problems. Without more fossils to study, a mundane explanation like that remains plausible.

References

  1. Pickering et al, (2011). Australopithecus sediba at 1.977 Ma and implications for the origins of the genus Homo. Science 333, 1421
  2. Aiello, L. C., & Wells, J. C. (2002). Energetics and the evolution of the genus Homo. Annual Review of Anthropology, 323-338.
  3. Berger, L. R. (2013). The Mosaic Nature of Australopithecus sediba. Science,340(6129), 163-165.
  4. Churchill, S. E., Holliday, T. W., Carlson, K. J., Jashashvili, T., Macias, M. E., Mathews, S., … & Berger, L. R. (2013). The upper limb of Australopithecus sediba. Science340(6129).
  5. Kivell, T. L., Kibii, J. M., Churchill, S. E., Schmid, P., & Berger, L. R. (2011). Australopithecus sediba hand demonstrates mosaic evolution of locomotor and manipulative abilities. Science333(6048), 1411-1417
  6. DeSilva, J.M., Holt, K.G., Churchill, S.E., Carlson, K.J., Walker, C.S., Zipfel, B. and Berger, L.R., 2013. The lower limb and mechanics of walking in Australopithecus sediba. Science340(6129), p.1232999.
  7. Schmid, P., Churchill, S.E., Nalla, S., Weissen, E., Carlson, K.J., de Ruiter, D.J. and Berger, L.R., 2013. Mosaic morphology in the thorax of Australopithecus sediba. Science340(6129), p.1234598.
  8. Irish, J. D., Guatelli-Steinberg, D., Legge, S. S., de Ruiter, D. J., & Berger, L. R. (2013). Dental Morphology and the Phylogenetic “Place” of Australopithecus sediba. Science 340(6129)

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19 Comments

Robert Oerter · 17th April 2013 at 2:48 am

Just came across your blog and had a question that’s been bothering me for some time. You write,
“Such hopes were short lived however, as it was discovered Au. sediba was ~1.9 million years old (Pickering et al., 2011). Since the first members of Homo lived 2.3 million years ago (Aiello and Wells, 2002), Au. sediba obviously couldn’t have been their ancestor….”

I read this sort of thing all the time in discussions of evolution, and it just seems wrong. OK, we have fossils of sediba that date to 1.9 Mya. But that only tells us they were around at that time. It doesn’t tell us when they arose or when they died out. IIRC, it’s not unusual for a species to be around for a million years or so, right? So sediba could easily have arisen 2.5 Mya or earlier, no? And therefore could have been ancestral to Homo.

Any chance you can clear this up for me?

    Adam Benton · 17th April 2013 at 1:59 pm

    The problem of “there may be earlier/later examples” applies to just about every discovery in palaeoanthroplogy. There may be an earlier sediba, an earlier first stone tool, earlier first art etc. However, most scientists refrain from spending much time speculating about these possibilities since, although it’s likely we will discover earlier/later examples, any claims we make about them would be baseless until we’ve actually found them.

    However – even if we did discover an earlier sediba that lived at the right time – the fact they walked differently to us suggests they weren’t our ancestor after all.

Robert Oerter · 17th April 2013 at 2:11 pm

Thanks for the reply, Adam, and yes, I get that there are now other reasons to rule out sediba.

It’s the logic of ruling out one species as an ancestor based on very incompletely known dates that bothers me. OK, we shouldn’t speculate where we don’t have evidence. But to use the dates to rule out ancestry is itself a form of speculation. It requires speculating that sediba WASN’T around 500,000 years earlier. That’s just as unjustified as speculating that it WAS around.

Sorry to harp on this, as I said, it’s sort of a bugbear for me. I’ve even seen actual published papers that say “X couldn’t have been ancestral to Y because they existed AT THE SAME TIME.” That’s like saying my mom can’t be my mom because we are both still alive.

    Adam Benton · 17th April 2013 at 2:53 pm

    The thing to remember is that they don’t use this data to rule out various ideas for certain. Scientists will happily adapt their ideas with little fuss as new evidence is revealed.

ashley haworth-roberts · 18th April 2013 at 1:35 am

This looks like rhetoric by and large:
http://crev.info/2013/04/divorce-spats-between-lucy-and-designated-replacement/

    Adam Benton · 18th April 2013 at 10:04 am

    “oooo there’s debate, raising questions over the validity of the whole thing”

    Which is basically one of my predictions for the creationist response to sediba

Jeshyr · 19th April 2013 at 3:17 am

I’m another blow-in via Pharyngula, so apologies that I’m sure this is such a basic question it’s quite silly … but reading this I’m confused about what the ‘rule’ is that divides Homo and non-Homo. I know there were multiple species that fit the class, though H.Sapiens and H.Erectus are the only two I recall from my education-via-BBC-documentary, but how do you all decide that something is a Homo not an Australopithecus or whatever??

    Adam Benton · 19th April 2013 at 8:30 am

    No worries, it’s a good question I may use for next weeks “question of the week”. It’s always worth reminding me not everyone has read everything I’ve ever written (and I don’t take that as a personal slight).

    At any rate, the traditional distinguishing features between us and Austro was stone tool use and large brains (our earliest members had around 900cc brain capacity, comapred to 350/400 for Austro). Then we found Homo habilis, which used stone tools but only had a brain ~700cc. So the “big brained” part of the definition of Homo was dropped. Then we recently found some clearly Australopithecus specimens using tools! So now that part of the definition does’t work either.

    So now they’re separated by shades of grey rather than the black and white of things like “tool use”. How flat was their face? What size was their teeth? How long were there legs etc? This gradation is what you’d expect to see if they evolved, so although it’s confusing for us it’s a nice bloody nose for any creationists watching.

    At any rate, all of this means it’s hard to give a specific definition of what makes a creature Homo. The upshot of this is that some early members of our genus (like Homo habilis) are being reconsidered, with some arguing they’re actually Austro!

    Sorry I can’t give a properly definitive answers, I hope you decide to stick around nonetheless.

ashley haworth-roberts · 21st April 2013 at 4:28 am

Adam
I was thinking of sending this comment to AiG – unless you think it inaccurate:
“http://www.answersingenesis.org/articles/2013/04/20/news-to-note-04202013#fnMark_1_2_1 (item 1)

“Other evolutionary researchers have suggested that Lucy (Australopithecus afarensis)—a competitor in this race to the root of the human evolutionary tree—did not have a human sort of bipedalism, but a different “australopithecine bipedalism” altogether. (In other words, the tree-swinging arboreal ape wasn’t really bipedal. See A Look at Lucy’s Legacy and Lucy, the Knuckle-walking “abomination”? to learn more.)” UNTRUE. ‘Lucy’ was bipedal.

“On the other hand, the special issue of Science devoted to sediba shows that it was well-designed for climbing and swinging. It was an ape. No amount of wishful thinking can make it the “transitional form” the researchers claim that it is.” UNTRUE.
http://www.sciencemag.org/site/extra/sediba/index.xhtml
http://www.sciencemag.org/content/340/6129/163
“The less well-preserved elements of the lower rib cage suggest a degree of human-like mediolateral narrowing to the lower thorax, indicating a rather unsuspected mosaic anatomy in the chest that is not like that observed in Homo erectus or H. sapiens.”
http://www.bbc.co.uk/news/science-environment-22108784
“The new analysis shows this species – Australopithecus sediba – had a human-like pelvis, hands and teeth, and a chimpanzee-like foot”.”

    Adam Benton · 21st April 2013 at 10:08 am

    As I predicted at the end of this article, creationists would likely respond to it by playing up the ape-like characteristics an ape/human intermediary naturally has. AiG have fulfilled this prophecy superbly.

    For example, they talk about how well suited her arm was to climbing, missing out the fact their fingers were comparatively short and more human like; which isn’t a particularly useful tree climbing trait. That’s not to say that they didn’t climb trees, just that they’d sacrificed some of this arboreality in favour of a dextrous hand, a fact AiG glosses over.

    Their cherry picking of data doesn’t end there. The papers notes how sediba walked with a strange gait, then note how humans who walk with such a gait suffer from muscle strain etc. AiG cites both of these fact as evidence against it walking upright but conveniently omits the very next line from the article which goes “although hyperpronation can have pathological consequences in modern humans, we are proposing here that the skeleton of Au. sediba reveals a suite of anatomies that are adaptive for…this kind of walking”

    So by only citing certain aspects of their anatomy, AiG can make sediba look like a regular arboreal, non-bipedal ape. Big whoop, you can do the same thing to humans. Except if you cherry picked humans like that, everyone would call you a liar. Can we say the same thing about AiG because they’re doing it to sediba?

andre salzmann · 21st April 2013 at 7:43 am

It would be very interesting to have a interactive web site/page where all known “finds” and present digs in progress are indicated, on a map, of Europe, Africa and Asia. On which, if you could enter a dig site with you arrow (on the computer), you have access to basic data retrieved on that site. Then, for example, if Lucy’s name came up- where she was uncovered, one could click on her and on specifics about her. Selected archeologists should then be requested to enter and update data as it is clarified and once processed ?
It seems now that even western chimps ( 300cc brains) use tools. As well as starlings with 5 cc (?) brains. Most probably it will not be size, but ability, deduced from “cultural” discoveries to come, that will distinguish Homo from Hominid. Hominid from Ape? To put it simply.
Clearly, even studies in primate and human behavior are helping to clear our minds. It is one very large mosaic to be pieced together.

    Adam Benton · 21st April 2013 at 10:11 am

    That would certainly be an interesting and useful resource. If you make the website I’ll happily start plugging in data. I’ve already gathered a bit for research I’m doing.

    As for brain size’s relationship to tool sue, it’s worth noting that brain size relative to body size seems to be the key, not just absolute brain size. For example, although crows have a small brain, compared to their body it is relatively the same size as a chimps. Crows are also the most adept tool users in the bird family.

ashley haworth-roberts · 21st April 2013 at 6:20 pm

In reply to Adam at 10.08 am, I’m now sending my comments to AiG, together with a link to this blog post (hope that’s OK).
If A sediba managed to walk bipedally some of the time, despite the hyperpronation issue, the AiG claim that A afarensis must have been a knuckle-walker looks increasingly untenable.

    Adam Benton · 21st April 2013 at 6:25 pm

    I look forward to their more in detail post on the subject.

ashley haworth-roberts · 21st April 2013 at 6:26 pm

My comment as just sent appears in the ‘rabble rouser’ thread here:
http://forums.bcseweb.org.uk/viewtopic.php?f=1&t=2967&p=45465#p45465

    Adam Benton · 21st April 2013 at 7:14 pm

    You should get your own blog. I’ve found it much easier than making forum posts (that’s how EvoAnth started out)

ashley haworth-roberts · 25th April 2013 at 2:00 am

Just seen, only skimmed:
http://www.answersingenesis.org/articles/aid/v8/n1/sashaying-to-throne-sediba?utm_source=aigsocial04242013lucy&utm_medium=social&utm_campaign=facebooktwittergooglelinkedin
“Yet a closer look at the latest analyses reveal that sediba is a fine illustration of ape diversity.”

    Adam Benton · 25th April 2013 at 9:30 am

    Their ultimate conclusion rests on the old “common features evidence of common designer” claim. That’s an unfalsifiable hypothesis making it, and their refutation of sediba as part of the human family, unscientific.

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