After the interest in last week’s post, I thought it worthwhile to follow up with a little more detail. Although we are focusing on brachiation, the implications are wider as we are really discussing form and function. All but the most stubborn designers know form and function are intertwined. In the natural world selection pressure influences shape over time – when a novel arrangement of tissue creates more success, it is likely to survive into future generations. As the new form becomes refined it can also lead to environmental niches for species. The familiar bird species in the accompanying image (from the Wiki page ‘Bird Flight’) allows us to appreciate this dynamic. Each bird has flight characteristics determined by wing size and shape. In turn, wing size and shape influences, and is influenced by, the bird’s lifestyle, whether it is the long soaring glide, high speed dives or agile turns necessary to find and capture each bird’s prey.
We can use a similar approach to compare the scapulae of different ape species to appreciate the relationships between form and function of their shoulders. In a 2015 study Young et al. used complex morphometric analysis tools to compare the various shapes of scapulae from a range of primate species.
Although the methods are complex, the resultant output of geometric morphometrics is relatively easy to understand and can be seen in the second image above. The graph created by Young et al. shows the principal directions of change in scapular shape between species and also illustrates how, despite variation between individuals, each species neatly clusters together. Two directions of change are plotted on the graph. From left to right (x-axis) we see an increase in length of the scapula along the superior border and a change in the angle of the glenoid fossa from superior-lateral to almost purely lateral. On the other axis (y), we see an increase in depth in the supraspinatus fossa and an increase in size of the superior angle.
Each species is plotted according to overall morphology of the scapula and we see the brachiating specialists (siamang and gibbon) in the bottom left and the Homo species in the top right – almost as opposite one another as possible.
While these comparisons may seem quite theoretical, they have important functional and evolutionary implications. The paper was able to conclude that brachiation was an off-shoot locomotor development from the African ape lineage – a specialisation for the gibbon branch and not a direct part of the Homo heritage (see the third image – Hylobates = gibbons). Although that conclusion will undoubtedly be questioned by some, the functional implications are less easy to overrule.
The gibbon scapula on the left of the graph is long and slender, oriented along a narrow and acute upward angle. Once the rotator cuff muscles are placed onto the bone, they will have a very narrow range of fascicular direction. Contrast that to the two scapulae on the upper right of the graph, where we see a broader bone with more depth for the supraspinatus.
The knuckle-walking chimps and gorillas are both higher than Human along the y-axis for supraspinatus development and that makes sense because of the role supraspinatus plays as a shoulder stabiliser during quadrupedal gait – helping stabilise against adduction in the same way gluteus medius does for our hips.
The human scapula does not fall at one extreme or the other on the graph, showing that we have a more generalised scapula appropriate for a range of uses. The supraspinatus fossa is neither deep nor shallow, the blade is broad and orients laterally to allow forward-facing manipulation (such as tools!). The wide span of the scapula gives a broader range of fascicular directions for finer control over a greater range of glenohumeral motion. Compare this to the gibbon scapula, whose musculature is focused up and out to support the arm as the body swings underneath it in a wide arc which requires little glenohumeral range.
The conclusion we can draw from this is that while we are not evolved from brachiators nor are we ‘designed’ to brachiate, we can brachiate if we really want to because we are generalists. Our ‘design’ allows us to manipulate, to throw, to hug and perform handstands. Since we lack the specialised scapular alignment, focused muscle concentration and muscular strength of brachiators, we must launch ourselves into brachiating with care and appropriate training!