Recently, I posted a bit of news regarding an article that appeared in The Journal of Experimental Biology. Well, I promised I would look into that matter further, and so here I am. The article, authored by Evie Vereecke and Peter Aerts, analyzes the mechanics of the gibbon foot in hopes of better understanding the mechanism of bipedal locomotion in organisms that lack a pedal arch. I specifically mentioned gibbons in a previous article on plantar fasciitis because they are the most bipedal of non-human primates (10-12% of their locomotive repertoire), and they look damn cool while doing it. While the authors probably lack my enthusiasm for the “look” of the bipedal gibbon, they are certainly enthusiastic about a potential model organism when they see one.

A little background first: gibbons, family Hylobatidae, are the most distant relatives of humans within the ape lineage. They are primarily arboreal and move mostly by means of brachiation. Having branched from the lineage that eventually led to humans around 16-23 million years ago (MYA), according to various molecular studies, it would appear that there is relatively little to learn from them in terms of human evolution. The authors of this particular paper, however, are arguing this simply isn’t true. While the gibbon certainly isn’t an exact representation of our last common ancestor with chimpanzees, by any stretch of the imagination, they argue it offers particular insight below the ankle.

This argument is based on the fossil evidence available regarding the evolution of human bipedalism, particularly of the human foot. According to various papers, the specialized bipedal foot likely did not evolve completely until at least less than 5 MYA, and perhaps even as recent as 1.8 MYA when early Homo appeared on the scene. While I would put the date closer to 3.5 MYA, around the time of the Laetoli footprints, the date at this point isn’t particularly relevant. What is relevant is that the human foot evolved from one that was primarily of an arboreal nature, to one that was a sort of a hybrid, and finally to the very flexible, yet simultaneously force absorbent locomotive mechanism adapted to bipedalism.

What the authors believe the gibbon can add to the equation is an understanding of the biomechanics of a foot of a creature that is regularly arboreally and terrestrially bipedal, much like ancestral hominoids are hypothesized to be. To do this they took data from three adult gibbons at the Wild Animal Park Planckendael in Belgium using a high speed video camera to analyze the internal joint mechanics (see image below) and matched this up to force plate data from a previous study (Vereecke et al., 2005a). Specifically, they wished with this technology to investigate whether the connective tissue and muscles on the plantar (bottom) of the foot in gibbons contribute to propulsion via elastic recoil, as the human foot does, or whether they contribute to mechanical energy loss. In human bipedalism 52% of the stretch and recoil energy of the connective tissues of the feet is recovered during locomotion. Surprisingly, the researchers find that the same recovery mechanisms present in human locomotion are also present in gibbon bipedal locomotion, albeit with slight deviations in anatomical source.

If the gibbons have such a similar elastic energy recovery mechanism to humans, then, why aren’t they predominantly bipedal like us? Well, apart from it being a anthropocentric fallacy that we are the zenith of evolution to which all life strives to be, it comes down to simple selective advantage. The authors argue that the pedal morphology exhibited in anatomically modern humans (AMH) would have only been selectively advantageous in a predominantly terrestrial environment, as a more rigid foot has a greater capacity for generation of propulsion. What we see in gibbons is a clear mechanical disadvantage for forward propulsion when compared to humans, which should be expected of an organism that locomotes ~90% via brachiation as gibbons do.

So, to recap, what the authors demonstrate with this study isn’t that gibbons are extant representations of ancestral humans. What they do demonstrate, however, is that while an arboreal ‘compliant’ foot may not be the most effective mechanism for push-off during bipedal locomotion, particularly when compared to the more rigid arched foot of humans, it does contribute to propulsion in as much as it uses stored elastic energy from passive stretch and recoil of the tendons and ligaments on the plantar side of the foot. With this knowledge scientists are one step closer to a complete understanding of the biomechanics of locomotion (Pun intended). Therefore, research such as this is not merely important for its implications in the human evolutionary story, it is important in and of itself.

(References)

Vereecke, E & Aerts, P, 2008, 'The mechanics of the gibbon foot and its potential for elastic energy storage during bipedalism', Journal of Experimental Biology, vol. 211, no. 23, pp. 3661-3670. 10.1242/jeb.018754