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.
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.
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.
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.
tl;dr
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.
References
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.