Easier-read version of "Bipedal Wading in Hominoidae past and present) Page 1 Page 2 Page 3
They very rarely stepped into the moat that surrounded their enclosure, only ever doing so if the food which children threw to them fell short. This amounted to only 28 seconds in two hours of focal study (where an individual is tracked non-stop.) But when they did go in it was almost always on two feet, even if the water was shallow enough to go in on all fours. One of the workers there, who had tracked the bonobos every day for two months told me that my observations were no fluke. He had seen individuals go into the water every day and always bipedally. These findings are exactly what the model for a wading origin for bipedalism would predict. Only once were they seen to go in on all fours. (Click this link to see video clips of this behaviour.)
Redy (male, left ) and Hermien (female with infant, right) wading bipedally at Planckendael, near Brussels.
Of course we did not evolve from extant apes, so what evidence is there that our putative ancestors waded too? Certainly they could have. If one examines the paleohabitats of the earliest bipeds - Orrorin tugenensis (5.6-6.3 mya), Ardipithecus ramidus (5.8-4.4mya), Australopithecus afarensis (3.2-4.1 mya) and A. anamensis (3.9 mya) one finds that, generally, they were wet and wooded, not dry and open. Each of them has been associated with significant bodies of water: either lakes or rivers. There are other hominid sites, whose paleohabitats seem to have been more arid, but they are young compared with the sites of Orrorin and Ardipithecus. Again this evidence is only what the wading model would expect. It certainly can’t be argued that it refutes the hypothesis.
How did Lucy Move?
So they could have waded, but did they? If the earliest bipeds were wading apes one might expect to find some kind of evidence for it in their morphology. The most complete and famous fossil record is AL 288-1 (Lucy), and it is no surprise that she has been extensively studied. Much has been written about her curious morphology but rather than a consensus view forming, the interpretation of her structure has caused much disagreement.
Some, like Owen C. Lovejoy and Robin Crompton, have argued that she walked in a fully upright, purely terrestrial, human-like way, others such as Jack Stern and Christine Berge say that it was kind of waddling gait with a 'bent-hip-bent-knee' posture, indicating that she lived a more arboreal lifestyle.
Berge has claimed that she couldn't have walked upright because she wouldn't have been able to keep her balance as her anatomy is not geared to a fully erect posture. Crompton and his collaborators have retorted that it couldn't have been a bent-hip-bent-knee gait because she would have used up too much energy and become too hot.
The arguments have certainly generated a great deal of heat but both sides have been making the same, big, dry assumption. If Lucy was a wading ape then most of their contradictions could be resolved. Buoyancy would have made it easier for her to move with a bent-hip-bent-knee gait and the cooling effect of water would have stopped her overheating whilst doing so. On the other hand, maintaining an upright posture would have been no problem for an ape as long as she was wading in water that was deep enough.
Another piece of evidence recently cast new doubts on the way she moved. In 2000 Brian Richmond & David Strait discovered that Lucy's distal radii (the ends of the arm bones nearest to the wrist) had traits analogous to ones found in chimpanzees and gorillas associated with knuckle-walking.
Because it wouldn’t make sense that she would have had two separate ways of moving on the ground (bipedalism and knuckle-walking), a rather convenient argument has been put forward explaining that these traits were evolutionary baggage from the past - even though A. africanus, a putative descendent of A. afarensis that lived only about 0.5 my later, no longer had those traits.
The most parsimonious explanation of Lucy’s knuckle-walking traits has to be that Lucy was a knuckle-walker. And, if that was the case then, logically, the simplest explanation for it was that she had three modes of locomotion for three different substrates: She climbed in trees, knuckle-walked on the ground and waded in water.
Those hips
Lucy has a very peculiar post-cranial morphology. In particular her pelvis has a characteristic platypelloid (flat front-to-back, wide side-to-side) shape with large iliac arches. They were odd enough for Charles Oxnard to conclude that she was probably not even an ancestor of ours or the African apes, but was instead evidence of a failed experiment of bipedalism. He contemplated that her mode of locomotion may have been an odd mosaic of behaviours seen in extant primates or else it was “a totally new and unknown manner of locomotion which would be unique in its own right.”
What could those traits really have been for? If we cast aside our assumption that she was a terrestrial walker and consider the possibility, instead, that Lucy was a wading ape - what explanation might be found for those traits then?
I approached the problem with a series of experiments with human subjects in a variable-depth swimming pool (or multi-pool). I wanted to find out what factors impact on wading speed. Predictably, it slowed down as the water got deeper and, specifically, I found that speed was directly related to the area of the submerged body profile. From this one would predict that any ape specialised to wading would evolve behaviours and traits to minimise this profile.
Sideways Wading
The really fascinating observations came from comparing different wading techniques. Intriguingly, I found that sideways wading, which was predictably slower than wading forwards at the shallowest depths, was actually at least as fast when the water reached chest height. Waddling, a kind of ice-skating mode, was intermediate between the two. This, remember, is from a species that can hardly be said to be adapted to moving sideways. I had never even considered the idea until my fourteen-year-old son, one of the volunteers, suggested it!
The concept might sound a little fanciful at first but extant apes have been seen to do it too. A worker at Planckendael told me that, on several occasions, he had witnessed bonobos running bipedally and sideways to get out of the rain quickly.
