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.
You may have noticed that our shoulder is a very flexible joint. It flexes and extends (arm straight out in front and straight out behind) abducts and adducts (arm straight sideways and hanging loose by your side) and rotates (try holding your arm straight out sideways with your palm upwards then rotate your hand so that it faces the ground. This movement won’t have rotated your shoulder much. Now continue rotating your arm in that direction and try to get your palm facing upwards again. Notice how rotation now takes place at the shoulder joint).
The shoulder blade (scapula) and the muscles which originate from it have a large part to play in this flexibility, and also in preventing such a flexible joint from dislocating. The four muscles of the rotator cuff originate from different parts of the scapula and hence are more important in stabilising the shoulder joint when the arm is in different positions. The relative importance of these muscles depends on whether the shoulder is habitually abducted (raised) as in chimpanzees, or adducted (lowered) as in humans.
Since chimpanzee upper limbs are more habitually abducted, the supraspinatus muscle, which stabilises the shoulder joint in this position, is much larger and stronger than the equivalent in humans. This means that the areas above and below the scapular spine in chimpanzees are much more evenly sized (fig. 1).
The scapula also shows a number of other specialisations for a habitually abducted position for the upper limb. Figure 1 also shows that the glenoid fossa where the scapula articulates with the head of the humerus is more upwardly (superiorly) orientated in chimpanzees than in humans, making it that bit easier to hold their arms above their heads all day.
The relative orientation of the widest and longest dimensions of the scapula (fig. 2) is also an important factor in determining how easy it is hold the upper limb in extreme abduction.
A scapula where the longest dimension is in a more similar orientation to its width provides a more advantageous insertion for the trapezius and serratus anterior muscles, which rotate the scapula when we abduct our arm past 90 degrees so that the glenoid fossa faces superiorly.
On the other hand, it seems like some things never change. In terms of the anatomy of the humerus at the shoulder there are some remarkable consistencies seen between humans and the African apes.
For a start, lets consider the orientation of the humeral head which participates in the shoulder joint compared to the distal end of the humerus which participates in the elbow joint (fig. 3).
Compared to the macaque and the Asian apes, it can be seen that the humeral head axis is much more closely aligned with the elbow joint in the gorillas, chimpanzees and humans, as shown by the shaded segment circles. The reduction of this angle may have occurred in the common ancestor of gorillas chimpanzees and humans somewhere between 8 and 16 million years ago when the head of the humerus twisted so that it faced more medially (towards the midline; left in fig. 3) rather than more posteriorly as in macaques.
However, it also possible that this torsion of the humeral head occurred separately in the lineages leading to gorillas, chimpanzees and humans where is represented an adaptation to weight-bearing during knuckle-walking in the African apes, and possibly to tool use in the hominin lineage. The fossil evidence I’ll review in a later post should tell us which of these possible scenarios is correct.
The main differences between the elbow of the African apes and humans are not concerned with upright walking in humans, but more to do with adaptations to knuckle-walking in gorillas and chimpanzees. At the distal end of the humerus lie the trochlea for articulation with the ulna of the forearm and the capitulum which articulates with the radius (fig. 4).
From inferior view, a lateral ridge is prominent as the border of the trochlea which is used to stabilise the ulno-humeral articulation during knuckle-walking, and also when the chimpanzee is hanging from branches. The olecranon fossa on the posterior of the humerus receives the olecranon process of the ulna when the elbow joint is straight (extended). The steep lateral margin in apes helps stabilise the ulno-humeral joint in extension and weight-bearing as in knuckle walking.
In inferior view the chimpanzee captiulum seems to be relatively larger than in humans, and appears to be relatively slightly taller. This allows for a greater range of the radius when the elbow is extended, allowing the forearm to be position directly below the humerus to help with weight-bearing during knuckle walking.
The morphology of the ulna and particularly its olecranon process is also important in determining the range of movement posssible at the elbow. The olecranon process is the attachment site for the triceps muscle which acts to extend the elbow. It also bears the trochlear notch for articulation with the trochlea of the humerus. Different orientations of the trochlea are suited to different habitual positions of the elbow joint (fig. 5).
The variation described in fig. 5 shows whether the cup-shaped trochlea notch is facing completely upwards or is more vertically orientated and facing forward (anteriorly) when the elbow is extended. An angle of 20 or 30 degrees from horizontal is normal for African apes and is also variable in humans, depending on what activity the person does most during their lives, e.g. carrying, using tools, climbing, etc.
In terms of the whole arm, the major differences is overall length, which is greatly reduced relative to leg length in humans compared to the other apes, including orang-utans and gibbons (fig. 6).
The wrist and hand.
The first thing to notice about the wrist is that it is largely identical in the African apes and humans. All of the same bones are present and the same mechanisms for stability are used whether you’re climbing, carrying or using tools. (For more information on wrist anatomy, see my poster on gibbons).
Differences between the chimpanzees and humans are again thanks to adaptations in the former to knuckle walking (fig. 7).
The bony ridges seen in the chimpanzee wrist and hand (and also in the gorilla hand – but not wrist!) limit hyper-extension of the joints when the apes knuckle walk. this is more efficient than constantly keeping muscles tensed to son the same job. It is notable that the gorilla does not employ exactly the same mechanism of knuckle walking – this suggests that it evolved separately in the ancestors of gorilla and chimpanzees. Which would mean that the last common ancestor of gorillas, chimpanzees and humans was unlikely to have used knuckle walking to get around.
And if that doesn’t make you think that we didn’t evolve from chimpanzees, then I’ll have try to try and show you some of the fossil evidence next time.
Aiello, L., & Dean, C. (1990). An introduction to human evolutionary anatomy (p. 596). London: Academic Press.
Kivell, T. L., & Schmitt, D. (2009). Independent evolution of knuckle-walking in African apes shows that humans did not evolve from a knuckle-walking ancestor. Proceedings of the National Academy of Sciences of the United States of America, 106, 14241–6. doi:10.1073/pnas.0901280106.
Wikimedia: http://upload.wikimedia.org/wikipedia/commons/4/49/Ape_skeletons.png. Accessed: 18/04/14. Freely available.