How did we move before we walked upright?

There’s something of a paradox in the story of human evolution. Chimps are our closet living relatives and they knuckle-walk. Yet there’s no evidence of any of our ancestors ever knuckle-walking, which is what you might expect if we evolved from a chimp-like creature. In fact if you take a look at the earliest known human ancestor (which lived shortly after we diverged from chimps) the orientation of their spinal cord provides strong evidence they had an upright posture.

Since we only have the skull of this species, called Sahelanthropus tchadensis, we don’t know much about how they actually moved; except that it wasn’t like a chimp, their spinal orientation proves that. However, we do have some skeletal remains for a slightly younger species that lived ~6 million years ago (Sa. tchadensis & the chimp/human split being around 7 million years ago). It’s known as Orrorin tugenensis; and whilst we haven’t found a skull of theirs we have found some leg bones, so the species provides a great way to study how early humans were moving. 

And a study published by an international team of authors (including some people from the Stony Brooks department whose chimps were at the centre of a case to grant apes legal rights) does just that; comparing the femur of Orrorin to over 400 others; including living and fossil apes. They performed a principle component analysis of these bones, which is a fancy statistical technique that examines how similar they are too each other. Long, boring stats story short: the closer two species are to each other the chart, the more similar their femora. They then drew the consensus view of the family tree between these species on the chart, which allowed them to predict where the last common ancestor of humans and chimps should fall.

I already explained this in the post. If you're just skipping through to look at the pretty pictures the shame on you

I already explained this in the post. If you’re just skipping through to look at the pretty pictures the shame on you

This graph tells us 2 things. First, that humans and chimps appear to have been evolving in opposite directions since we split, evolving unique methods of movement. This would explain why no fossil humans have been found with evidence of knuckle walking: it’s a recent development in the chimp lineage. Secondly, it notes that Proconsul has a femur very similar to that of the last common ancestor between humans and chimps (but that doesn’t mean it was the last common ancestor, just had a similar leg).

Proconsul is a genus of primates that lived ~25 million years ago in Africa. Now, Proconsul was once thought to be the common ancestor of apes; given it shared many features with them. However, since then new fossils have been found which appear to have been even more similar, prompting Proconsul to be relegated to a more distant branch of the ape family. But as this study shows, it may still have moved like the common ancestor of humans and chimps, despite this distant relationship.

The family tree of the species examined in this study. Proconsul is all on its lonesome

The family tree of the species examined in this study. Proconsul is all on its lonesome

So, how did it move I hear you ask? Well, it was what’s known in fancy science words as a “pronograde arboreal quadruped” which basically means it had a horizontal body and climbed through the trees on all fours. I’ve even found a pretty picture of a reconstruction of Proconsul engaged in hardcore tree-on-primate action to help you , dear reader, visualise it.

wikipedia

Wikipedia can be really useful sometimes

But concluding the common ancestor also moved like this is based on the assumption that the similarities in their femora implies similar ways of moving. And that’s a fairly big assumption. Given we’re egocentric as a species and like studying our own evolution, we’ve identified some key traits in the femur that are strong evidence of a species moving bipedally. But no such “defining” characteristics have been linked to pronograde arboreal quadrupedalism (at least to my knowledge, but then I don’t spend much time looking at stuff >7 million years old).

And even if they have been, we haven’t actually found the femur of the last common ancestor to look for them. We’ve just got a prediction based on some statistics. Whilst this is a promising avenue of research that could shed a whole suns-worth of light on the evolution of our locomotion, there’s not yet enough to draw any real conclusions from it.

However, whatever follow-up research finds this study does prove one thing: human and chimp locomotion is evolving in opposite directions. But then, you already knew that didn’t you.

References

Almécija, S., Tallman, M., Alba, D. M., Pina, M., Moyà-Solà, S., & Jungers, W. L. (2013). The femur of Orrorin tugenensis exhibits morphometric affinities with both Miocene apes and later hominins. Nature communications4.

Ward, C. V. (1993). Torso morphology and locomotion in Proconsul nyanzae.American journal of physical anthropology92(3), 291-328.

Zollikofer, C. P., De Leon, M. S. P., Lieberman, D. E., Guy, F., Pilbeam, D., Likius, A., … & Brunet, M. (2005). Virtual cranial reconstruction of Sahelanthropus tchadensis. Nature434(7034), 755-759.

7 thoughts on “How did we move before we walked upright?

  1. If you go to the Creation Museum in Kentucky they will tell you that ‘Lucy’ (possibly not a direct human ancestor but nevertheless relevant) ‘knuckle-walked’ and was simply some kind of gorilla or chimp that went extinct sometime after the Genesis flood. It stands to reason – since evolution is ‘false’ because the Bible is infallible history, including of how humans came to be.
    I am being ironic.

  2. Interesting study but the shape of the pelvis is potentially far more illuminating given that the proximal femur, is comparatively similar even in widely differing innominates. Of course, the problem with analysing that part of skeletal anatomy is, as always, is the paucity of fossil evidence. When one does a principal component analysis of those pelves that are known, one finds that australopithecines are more distant from humans than this femoral data suggests.

    I apologise for mentioning this, yet again (it seems that some see it as heretical to do so) but a locomotor repertoire including wading through shallow water as well as vertical climbing trees in swampy habitats would act as the perfect precursor to both efficient upright walking and Pan/Gorilla knuckle-walking.
    The paleohabitats of a few candidate species that must be close to the Pan-Homo-Gorilla LCA (i.e. Sahelanthropus, Dryopithecus and Oreopithecus) were dominated by permanent water courses and Orrorin’s habitat was also pretty wet and wooded.

    Long, boring, history of science story short: Knee-jerk reaction to the mislabelled and misunderstood so-called “aquatic ape hypothesis” has led to a perfectly plausible idea on human origins (i.e. a wading component in bipedal origins) being ignored for over 50 years.

    Algis Kuliukas

  3. I hadn’t realized there were (non-human) hominids on the ark. If I recall correctly, the bible seems to have glossed over this extremely important matter. A little foresight would have saved an enormous amount of argumentation and gnashing of teeth down the track. To make matters worse, the hominids don’t appear in any of the historical paintings of the event either. Whether this is by design, or just general lack of attention to detail by the artists of the time will never be known. In any event, it is all rather unfortunate; in lieu of actual images of these guys walking around – or climbing on – the ark we have to rely on principle component analyses and related mathematical trickery which is always going to be highly unsatisfying.

    Me too.

    • Well non-human hominids had to have been on the ark, given we find them in the Pleistocene (which is the strata lain down by the post-flood ice-age). However, the human hominins, like Neanderthals, got onto the ark a different way. Their genome was carried forwards by Noah’s sons wives! One was married to a Neanderthal, the other a Denisovan…

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