This week it’s the turn of the shoulder to get the long-standing debate treatment. It may not be obvious how changes in the anatomy of the upper limb are related to bipedalism and in fact that’s not really the point here. The most important thing to remember is that evolutionary theory states that features that are not used (selected for) will eventually cease to be.
Here, I’ll outline the differences between the shoulder and upper limb of humans and our closest relative, the chimpanzee, which happens to spend a lot of time climbing trees. A comparative look at the fossil evidence is for a later post. As ever, it’s important to remember that I’m not suggesting that we evolved from chimps – they just happen to be very useful when comparing a specialised morphology for arboreal locomotion to a less specialised upper arm morphology like ours.
The fact that they are closely related to us increases the chance that any differences have evolved relatively recently and are related to evolutionary gain (or loss) of certain features through positive (or a lack of) selection.