Easier-read version of "Bipedal Wading in Hominoidae past and present)                                         Page 1 Page 2   Page 3

Sideways wading clearly creates significantly less drag than moving frontally. The excellent "Life in Moving Fluids" by Steven Vogel provides the basic fluid mechanics behind this observation but actually you only have to put your hand outside a moving car to feel the effect for yourself. Placed front-on, it gets buffeted heavily by the wind. Place it sideways and it cuts through the air like a knife. Sideways certainly would be the optimal mode of wading at all depths if humans were able to propel themselves with as much force as they do frontally.

An ape specialised in wading might therefore be expected to adopt a sideways gait to maximise its efficiency. The notion seems far fetched at first but if you dispel any pre-conceived notions and look at the facts dispassionately, I think you'll agree that the evidence that Lucy might have moved in this way is quite staggering.

Firstly, what evidence is there that Lucy would have been able to propel herself sideways with more force than we can? Quite a lot, actually. Moving sideways in water requires a fair amount of complex muscle co-ordination. The volunteers who participated in the wading experiments found that kicking the back leg out sideways (involving a strong abduction of the femur) and alternatively clawing back the front leg (adduction) seemed the best technique for the fastest sideways wading. So, we'd expect to see evidence that Lucy was stronger in this kind of movement than we are.

The skeletal and muscular structures involved in this kind of movement are illustrated in the diagram below. Basic lever theory would predict that for A. afarensis to have a stronger sideways movement than us, it should have either:

1. A greater pulling force applied (W) from the iliac arches to the greater trochanter of the femur (for abduction) or from the femur to the ischiopubic ramus (for adduction).

2. An increase in femoral neck length (N) or

3. A decreased femur length (L).

but Lucy would appear to have had all three.

Australopithecines have always been associated with long femoral necks and short femurs, large iliac arches and extended ischiopubic ramus.

Berge noted that “it appears that the australopithecine adductor musculature must have been more powerful than that of humans” and Lovejoy and others have suggested the same for her abduction.

What is the evidence that she was streamlined for sideways wading? That evidence is even stronger. The A. afarensis pelvis is often contrasted with Homo as being very much wider than ours. However this is the result of scaling up her size to human proportions and can therefore be a little misleading. If one ignores scaling and measures the pelvis itself one finds that it is not so much that her pelvis was wide laterally but that it was shallow (less than 65% of the human thickness) dorso-ventrally.

Human wading experiments show that submerged body profile is significantly reduced when wading sideways (about one third less). It follows that any ape specialising in such a mode of locomotion would evolve a body shape that reduced this profile to a minimum. Lucy’s sideways profile appears to have been less than half of her full frontal area

The morphology of A. afarensis would appear to have been remarkably well suited to sideways wading although it is, of course, possible that she actually waddled through the water as predicted by Berge. No other explanation, hitherto suggested, fits a putative mode of locomotion so perfectly.

So if Lucy was a wading ape, can we imagine a scenario where her ancestors might have needed this ability? As young adolescent apes, if they had been forced to wade quickly across stretches of waist deep water even just once a week before reaching sexual maturity, it would still represent hundreds of wading events, each yielding an opportunity for selective pressure to eliminate the traits that hindered orthograde posture and bipedality. In this model the actual percentage time in the water need not be high. The threat of predation, often used as an argument against the wading-origin idea, would paradoxically significantly increase the adaptive pressure and simultaneously explain why adaptations specifically for fast wading would have evolved.

There are many habitats that would provide such opportunities for these selective pressures to work. Alister Hardy originally envisioned a coastal habitat. Elaine Morgan refined this by suggesting it was the flooding of ancestral forests leaving island niches. Others, such as Marc Verhaegen have proposed coastal mangrove swamps. I personally prefer that idea that Lucy was a “river ape” inhabiting gallery forests that surrounded rivers and lakes. This putative habitat is certainly more consistent with the fossil record and could even work in arid zones, as forestation inevitably shrinks closer to water sources as the climate becomes drier.

The view that bipedalism may have originated in apes that were adapted to a water-side habitat is still not a popular one. However if the theory has been dismissed it is not clear on what basis this has happened because all the serious research in this area has yet to be done. Charles Oxnard wrote “though numbers may prevail in democratic institutions, in academia and in science they do not. One fact that controverts an hypothesis is more powerful than a thousand that support it.” In my study I actually found nothing to rebut the idea that bipedalism may have had a wading origin and a great deal to support it.

 Algis Kuliukas
(September 2001)

Read the full thesis here.