Humans are primates, but we differ from our close relatives in many ways. One of the most obvious is how we move. Humans are bipedal, whilst our cousins are commonly climbers or quadrupeds, or a bit of both. At least, that's the impression you get if you only look at the adults.
Only studying the grownups would lead you to think these primates walk upright very rarely. For instance, in the trees, orangutans only spend 2% of their time walking on two legs. However, young primates muddle this picture up a bit. Scientists are finding more and more cases of primate species spending lots of time on two legs, but only when they're young.
Perhaps these young monkeys and apes provide key information on bipedal evolution. How did humans get so weird at walking?
Bipedal younglings
How does a baby move? Crawling might leap to mind (pun intended), but that's just the tip of the iceberg. Young humans can also roll, scooch, prop themselves up on furniture, and that's not even getting into all the different modes of crawling they have.
Infant primates are similar, experimenting with a whole range of movement styles before settling on the adult form of movement. Climbing, swinging, walking on their hands, walking upright. . . these are all methods that young primates try. However, hidden within this noise is an interesting fact: bipedalism is often much more common than in adults.
Common chimps, for example, are typically knuckle-walkers. Yet this doesn't become the dominant form of movement until they're ~2 years old. In the meantime, they use a whole range of movement styles, including bipedalism 15% of the time. That's more than 3x as often as the adults! But that's nothing compared to gorillas. In adulthood, their large size restricts the amount of climbing and walking they can do; but that doesn't stop them when they're young. They spend 20% of their time upright in their youth, whilst adults are bipedal closer to 2% of the time.
Other young bipeds include olive baboons, many species of monkey, and really I could just go on listing primate species. But that would just be hard to remember, so I'll leave it there.
But before I do I must leave the caveat that this trend isn't true of all primates. Bonobos, for instance, seem to miss out on the cool hip youth trend of walking upright during childhood. Maybe they just need someone to sit the wrongway round in a chair with a backwards baseball cap to explain to their kids how tubular walking upright really is.
Insights into evolution
So, primate youngsters are fussy creatures, not sure how they really want to walk. How does that impact us? Well, the key insight comes when you look at how this changes over time or rather, how it doesn't.
Despite experimenting with all these different ways of walking, these primates ultimately figure out their grownup way of moving around. Whilst this might result in changes to how frequently they're bipedal, it really doesn't impact their skill at walking upright. Nor does all this time walking upright seem to detract from their ability to use their grownup way of movement.
Remember those olive baboons from earlier. Well, even when they grow up and shift away from bipedalism they'll still return to it sometimes. And they haven't lost any skill, walking as far as fast as they used too. And their skill at walking like a grownup isn't harmed if they walked upright a lot as a kid. In other words, all these different sorts of locomotion seem to use the same brain bits.
This means that when our ancestors were evolving new ways of walking, their brain bits wouldn't have had to change much either. The primate brain seems quite flexible when it comes to walking. So that's one less thing we'd have to evolve as we became bipeds, making the whole process a bit easier; from evolutions perspective. It might also explain why there's so much variety in primate locomotion. It's easy for us to evolve new ways of moving.
Our early ancestors seem to bear this out. Although many were shifting towards bipedalism, they still practised a range of ways of moving about. All made possible by our flexible locomotive brain bits. Now we seem to have settled on bipedalism, but I'd wager we still have that cognitive flexiblity.
Who knows how it will move us in the future.
References
Crompton, R.H., Sellers, W.I. and Thorpe, S.K., 2010. Arboreality, terrestriality and bipedalism. Philosophical Transactions of the Royal Society of London B: Biological Sciences, 365(1556), pp.3301-3314.
Doran, D.M., 1992. The ontogeny of chimpanzee and pygmy chimpanzee locomotor behavior: a case study of paedomorphism and its behavioral correlates. Journal of Human Evolution, 23(2), pp.139-157.
Doran, D.M., 1997. Ontogeny of locomotion in mountain gorillas and chimpanzees. Journal of Human Evolution, 32(4), pp.323-344.
Druelle, F., Aerts, P. and Berillon, G., 2017. The origin of bipedality as the result of a developmental by-product: The case study of the olive baboon (Papio anubis). Journal of Human Evolution, 113, pp.155-161.
