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A lot of different factors have influenced our evolution. Socialising drove the development of our brain. Tools shaped our hands. Now, research has revealed that the simple act of carrying stuff may also have influenced our evolution.

Specifically, the evolution of our arms. Scientists have long known that the arms actually play a pretty key role in walking; helping keep our body balanced.

After recreating the lives of our ancestors for a day (or at least, carrying stuff through environments they would have traveled) researchers revealed that lugging rocks around has a pretty big impact on our arms ability to help us walk.

King of the swingers

When walking we swing our arms from side to side. It’s pretty hard to stop this. Even if you shove your hands in your pockets they often subconciously re-emerge a short time later.

This is because swinging our arms plays a pretty big role in helping us walk. Which seems odd, as they’re at the opposite end of the body to our feet. But this puts them in a great location to help counteract various unwanted movements during walking.


Note the rotation in the trailing foot

Despite trying to move forwards, walking generates a lot of rotation. Our hips rotate relative to our body to move each leg forwards. As a result of this, the foot on the ground (the trailing foot) also has to rotate, to still move forwards despite the hips’ rotation. The rotation of the feet generates a lot of torque. Since we’re actually trying to move forwards, this extra torque is (a) wasted energy and movement and (b) a potential source of instability.

Swinging your arms helps counteract this torque. Hence why your arm swings with the trailing foot. You can still walk without the arm swing, but its a lot more unstable and inefficient. Various computer simulations (and weird experiments on humans, involving taping their arms to their side) confirm this.

Interestingly, when carrying out these computer simulations scientists didn’t actually have to teach it to swing its arms. They just told it to minimise the torque in the trailing leg and the computer figured out the best way to do that was through arm swing.

Carrying rocks

So we have these lovely helpful arms that counteract the rotation in our legs as we walk. So obviously we might expect them to evolve in order to carry out this task really well; becoming the optimal size and weight.

It turns out that this is around the same size and weight as the legs; allowing the arms to exactly counterbalance them. Our early ancestors (and living apes) have arms around this size. Sometimes they’re a bit bigger, because living apes use their arms to climb so need the extra reach. But what happens when they start tinkering with their arms’ size and weight by carrying stuff?

Well, if they’re already the perfect weight then even the smallest change would tip them out of the ideal zone. More computer simulations (and weird human experiments) confirm that it doesn’t take much to throw things off. If Lucy or a chimp had picked up a rock to make a stone tool (like she may well have done) this would have really harmed her swinging ability.

However, shorter arms have a bit more wiggle room. They can carry heavier stuff before the arm itself gets too heavy. Those computer simulations revealed that an arm ~2/3 the size of the leg is optimal. That way it’s heavy enough to still counteract torque when empty; but has enough wiggle room to allow you to carry some pretty large rocks.

And our arms just so happen to be almost 2/3 the size of our legs.


How much someone can carry (Y axis; relative to their arm weight) versus the weight of their arm (X axis; relative to leg weight). AL 288-1 is Lucy, whilst the other bunch of numbers is a Homo erectus

Days of future past

So, there’s a lot of evidence that our arms evolved “wiggle” room to allow us to carry stuff. Their current size matches computer predictions about the ideal size; whilst fossils show that size evolving as carrying stuff would have become more common.

But the real icing on the cake came in November 2015; when scientists published the results of their recreation of an ancient journey hominins took. They found flint tools at an important Spanish site (Sima de los Huesos) that had been sourced from a nearby outcrop. So they got some volunteers, measured their limbs and got them to carry some rocks from this outcrop back to the site. Just as the hominins did more than half a million years ago.

Except the modern volunteers had a better fashion sense.

Except the modern volunteers had a better fashion sense.

Sure enough; as the computer models predicted those with the “optimal” arm weight found the journey the easiest. Someone with an arm 2/3 the weight of their legs used almost half the energy of lanky gits with longer arms.  This drastic difference confirms that these factors are strong enough to have driven some arm evolution.


Our arms play a pretty big role in walking; and carrying stuff has a pretty big impact on our arms ability to do this. Evolution may well have shaped our arms so they can still help us walk even when carrying stuff.


Collins, S.H., Adamczyk, P.G. and Kuo, A.D., 2009. Dynamic arm swinging in human walking.Proceedings of the Royal Society of London B: Biological Sciences, p.rspb20090664.

Park, J., 2008. Synthesis of natural arm swing motion in human bipedal walking. Journal of biomechanics, 41(7), pp.1417-1426.

Vidal-Cordasco, M., Mateos, A., Prado-Nóvoa, O., Terradillos-Bernal, M. and Rodríguez, J., 2015. Shorter arms count: The energetic costs of raw material catchment in a new experimental approach at Sierra de Atapuerca. Quaternary International.

Wang, W.J., Crompton, R.H., Li, Y. and Gunther, M.M., 2003. Optimum ratio of upper to lower limb lengths in hand-carrying of a load under the assumption of frequency coordination. Journal of biomechanics, 36(2), pp.249-252.

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Tim · 17th December 2015 at 5:38 pm

Pseudoscience. You have no evidence anyone ever carried any rocks.

    Adam Benton · 17th December 2015 at 6:09 pm

    There are thousands of Palaeolithic sites that feature rocks not from the immediate area. The clustering, density, presence of multiple materials, and so forth of the sites confirms they were produced by humans (or hominins) and not natural causes.

    As an interesting aside, in some cases there’s even (tentative) evidence of trade. There’s one French or Belgium site that had access to ochre; and another that had access to high quality rock. They appear to have traded these two resources back and forth. Other sites appear to show a similar thing going on with shells. Groups living in South of France somehow got their hands on Atlantic shells, whilst those living on the Atlantic coast somehow got their hands on shells from the South of France. However, the fact we’re dealing with hunter-gatherer groups – who by their nature are highly mobile – introduces a host of confounding factors. Hence why I called it tentative.

    But whilst we might not be able to confirm trade, we can confirm that things were being carried.

      Tim · 17th December 2015 at 7:35 pm

      I agree with your presumption that they carried rocks, but I am only trying to keep you honest that it is a presumption and presumptions are not science.

        Adam Benton · 17th December 2015 at 8:55 pm

        It is an idea. From an idea we can generate predictions about what we might expect to find at various archaeological sites. We then go and investigated and identify if those predictions are borne out. If they are it lends support to that idea; elevating it beyond presumption.

        And that is how science works.

        Tim · 18th December 2015 at 3:15 pm

        I’d call that weak science or hypothetical perhaps. You don’t think it might be a case of seeing what you’re looking for. Again, I’m not really saying you’re wrong about people carrying rocks per say – but wrong about the technique. It’s a good idea, but I think we should stay as far away from it in science as we can in order to preserve the integrity of science to give us truth. We don’t really know it’s truth from this method we just know our assumptions are lining up with our interpretation of what we see. But that is far from truth and very easily biased or influenced.

        Adam Benton · 8th January 2016 at 3:30 pm

        Seeing what you’re looking for – also called conformation bias – is always a problem. Fortunately, the scientific method also helps deal with that.

        As well as looking for positive evidence for an idea, you must also look for potentially falsifying evidence. Observations that, if true, would demonstrate your idea wrong.

        At many sites such evidence has in fact been found. Commonly, water or erosion produces the accumulation of stone; rather than humans. But crucially, at many more we only find evidence consistent with human involvement.

        This helps ensure that we’re not just being led by assumptions and that it is legitimate science.

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