Following on from McHenry & Corruccini (1975) but using modern 3D geometric-morphometric techniques (see, e.g. Franklin et al., 2005 for other examples), a thorough analysis of the shape of the pelvis was conducted in great apes to determine the relationship between shape of the australopithecine pelvis and extant apes and modern humans.

See Supplementary Materials for details of the methods used.

The “data mining” technique, commonly used in the world of business and commerce by using software such as Microsoft Excel and tools included in it like Pivot Tables, allows a vast amount of data to be summarised in a few lines of data. From these summaries, interesting data can be “drilled through” to get to the detail of what is going on behind the big picture.

This was the approach used here to analyse the large data set produced by this study―more than 135,000 lever arm ratios.

The overall averages of the permutations of lever arms grouped by muscle action for australopithecines as compared with Homo sapiens are summarised in the table below―in the cells above and to the right of the diagonal. Below and to the left are the relative rank order of Pan troglodytes relative to Australopithecus afarensis and Homo sapiens, among the 12 species analysed.

So, for example, the ratio of the mean lever arm values for muscles associated with Flexion/Extension for australopithecines is approximately 75% of that for Homo shown in the red cell at the top right.

When compared to the 12 species analysed, australopithecines ranked lowest for this ratio, Homo highest and Pan troglodytes ranked 5^th―shown in the yellow cell in the bottom right.

The key finding is that the lever arms of muscles involved in flexion and extension, compared to those involved in rotation or abduction and adduction, are most different between Homo sapiens and australopithecines. The Australopithecus specimen AL 288-1 had an average lever arms ratio figure that was approximately 75% of that for Homo sapiens. It was ranked lowest of the twelve species studied, whilst Homo sapiens was ranked 1^st.

When lever arms for muscles pertaining to adduction and abduction are contrasted with those involved with rotation, there is relatively little difference between Homo sapiens and australopithecines, both ranked in the middle of the list of species.

Drilling down on those two data points, one can see the figures in context of the full set of species and more statistical information about them.

Drilling down further, one might next tease out more specific muscle groupings, some which give even clearer results, and others which give more ambiguous findings. For example flexion and extension can be split apart and compared with either lateral or medial rotation, or with adduction and abduction, individually.

Taking one example, if one compares lever arm ratios for muscles exclusively involved with flexion with those exclusively involved in medial rotation, one finds an even greater difference between Homo sapiens and AL 288-1.

Here the ratio is 0.648 (1.082/1.669). Below, in Table 4 is similar data for all the species and, again, it is summarised in Figure 8.

By contrast, if one compares the lever arms for extension with those for adduction, one finds very little difference between Homo sapiens and AL 288-1 (Ratio 0.951).

Detail, again, shown below in Table 5 summarised in Figure 9.

As one can see, there is a vast number of permutations of pairs of landmarks and landmark groups that could be used to perform analysis with this data. But considering the significant caveats about the data outlined earlier, perhaps it would be better to end at this point, to step back from the detail a little and to discuss what the results may or may not indicate.

A 3D GM analysis of the shape of the hip bone, sacrum and femur was consistent with earlier studies that indicated that australopithecines were similar to humans in many aspects pertaining to bipedalism, and supported the long-held conviction that they were almost certainly obligate bipeds, like us. The shape of the australopithecine proximal femur and sacrum, in particular, clustered well within that of Homo sapiens. However, consistent with other studies (e.g. Zuckerman et al., 1973 ), it was shown that sufficient differences in the shape of the hip bone between the AL 288-1 specimen and Homo sapiens were identified to indicate that australopithecines might have used a bipedal gait that was quite unlike the striding, “inverted-pendulum” gait of modern humans.

A more detailed investigation of load arms of potential muscle origins, grouped by major muscle action, was consistent with this and identified differences in lever arms in muscles pertaining to flexion in particular, as compared to rotation and abduction/adduction of the hip, as being largest between humans and australopithecines in the species studied.

Clearly, many individual muscle lever arm ratios could be analysed to increase the depth of this study and to derive quantitative data but even this limited study shows approximately a 25% difference in the lever arm ratio between muscles involved with flexion/extension compared to those involved with abduction/adduction and rotation.

The classic interpretation of the australopithecine pelvis is that it was not only a biped but an efficient one. Indeed, according to Don Johanson’s anecdote in his book “Lucy―The Beginnings of Humankind”, all he had to do was to show the fragments of the knee joint he had collected from Hadar to Owen Lovejoy and he pronounced that had come to that very conclusion, immediately. To Johanson’s question about its bipedality, Lovejoy apparently replied “My friend, he could walk upright. Explain to him what a hamburger was, and he’d beat you to the nearest McDonald’s nine times out of ten” (Johanson & Edey, 1981: p. 163) .

The anatomical feature Lovejoy was identifying was the bicondylar angle, which is indeed interpreted as being a trait indicative of an energy efficient gait. Australopithecines have a bicondylar angle that is greater than humans and this is interpreted as being in accordance with the fact that they had relatively wide pelves and relatively short lower limbs.

In humans the bicondylar angle indicates the placing of the knee directly under the center of mass during the stance phase (to reduce energetically costly swaying from side to side) and, if australopithecines had wider hips and shorter limbs, then this angle would need to be all the more to achieve the same thing.

There is also the evidence of the distal femur (from a different specimen) that is indicative of fully extended bipedalism―the condyles are more ovoid in shape from the lateral aspect than those of extant great apes, a trait interpreted as being an adaptation for walking with an extended knee, consistent with a striding gait, like modern humans.

However, there are some key problems with this interpretation that seem to have been skipped over.

The very short stature and short lower limbs of australopithecines are contra-indications to an efficient striding gait, even if they were walking on optimal surfaces: those that are flat, firm and vegetation-free. However the Hadar paleohabitat evidence appears to show almost the opposite. Far from being dominated by flat, open grassland, it appears to have been rather wet and wooded. Indeed it seems more likely to have been a wetland for a million or so years around the time of death of AL 288-1 (Johanson & Edey, 1981: p. 129) .

It is interesting to speculate about what form of locomotion might best account for such a radically different hip bone shape and to consider which type of muscle action would best be biomechanically favoured by the australopithecine hip as compared to the human form.

If the australopithecine pelvis indicates bipedality, but not our kind of bipedality, it begs the question: What other kinds might there be?

As the morphology of the australopithecine hip is most certainly unlike most of the (largely arboreal) more primitive forms analysed here, it seems unlikely that such a departure from the Primate “norm” could be explained by a peculiar combination of exclusively terrestrial and arboreal locomotion as suggested by Oxnard & Hoyland-Wilkes (1994: p. 22) .

It is argued here that the simplest and most satisfactory potential explanation for their pelvic shape anomalies is a significant wading component in their locomotor. A twisting, side-to-side gait (one perhaps likely in waist deep water) would be easier to perform with a hip which gave biomechanical advantage to rotation, abduction and adduction of the hip. The remarkably platypelloid shape of the australopithecine pelvis (Tague & Lovejoy, 1986) might similarly be explained as a streamlining adaptation to moving though water using such a twisting, side-to-side gait as illustrated in Figure 10.

Motion in water incurs far greater drag forces than in air and so it is logical that any hominid adapted to significant amounts of wading would be expected to have evolved traits to reduce that drag. In fluid dynamics, the drag coefficient is

usefully estimated to determine the force of drag that an object will be subjected to as it moves. The drag coefficient is associated with its surface area in the plane of motion and is contributed by two effects: skin friction and form drag. The degree of form drag is heavily dependent on the shape of the object. Classic “streamlined” shapes impart much lower drag coefficients than ones that are not streamlined (see Figure 11 below).

The area of the lateral profile of the australopithecine at the hip must have been significantly less than one would expect if they had a pelvis shaped like most apes or large primates. As drag is calculated to be proportional to the area of the shape, it can be inferred that having a lateral profile that has half the area of, say, a chimpanzee could be expected to reduce drag by a significant amount (Kuliukas, 2001) .

The side-to-side gait postulated would also predict a greater biomechanical premium on those muscles involved in rotation and adduction/abduction of the thigh. It is argued here that no other explanation of the peculiarly platypelloid shape of the australopithecine pelvis has been so succinctly argued.

Of course, such speculation could be criticised as being fanciful and/or circular: One layer of speculation placed on top of another. However, the fact remains that the australopithecine pelvis does indicate a form of bipedalism that is not quite like ours and it as form that is remarkably different from the “prototype” Primate form, one clearly strongly associated with arboreality.

If one ties this in with anecdotal evidence of observations of extant great apes moving in shallow water (see e.g. Karlowski, 1996; Bearder, 2000; Tutin et al., 2001; Myers-Thompson, 2002; Kuliukas, 2002; Breuer et al., 2005 ), the idea is greatly supported. There is no other scenario in the natural world where otherwise committed quadrupedal great apes will so predictably switch to moving (not just posing momentarily) bipedally―and remain doing so as long as the conditions prevail―than in waist deep water.

To the two falsifiable null hypotheses listed earlier to test the wading hypothesis, a third might be added here:

・ Any biomechanical advantage inferred by the anatomy of the australopithecine pelvis would not have given them any advantage whilst wading in waist deep water.

It is argued that the first two null hypotheses listed earlier have been falsified and that the third, listed above, if not falsified, appears inconsistent with what these results appear to show.

There are at least 30 distinct ideas about the evolutionary scenarios likely to have led to human bipedalism and they are described in a fairly voluminous literature (for a full review see Kuliukas, 2017 or http://www.tinyurl.com/BipedalModels). When a model or set of ideas is first described in the scientific literature, the paper begins with a summary of other ideas that have been published before. However, there never appears to be any systematic method for doing this. Usually, a small subset of the published ideas are quickly summarised by the author(s) before they begin in describing the evidence pertaining to their preferred model. Missing in the literature until recently, it would seem, is any attempt to systematically assess the models that have been published and come up with an evaluative framework against which all models might be rated.

Kuliukas, 2017 made such an attempt to provide such an evaluative framework, and compared all of the known published models on the origin of human bipedalism. In doing so, he noted the disparity between the apparent strength of wading hypothesis when contrasted with the lack of favourable reporting of it in university level text books.

This paper will therefore end with a summary of that evaluative framework as it relates to the wading hypothesis.

Of all the models of bipedal origins, only wading can argue, even in theory, that if a would-be hominin tried to move quadrupedally in the proposed scenario, it would die.

Many of the models of bipedal origins use a rather obvious benefit of bipedality in modern humans as the proposed driver. “Carrying” models are a good case in point. There is no doubt that carrying is one clear benefit of bipedalism for us today, but it more difficult to conceive of scenarios where it could have driven the origin of bipedality. Wading does not suffer from such problems. Even extant apes, that are committed to quadrupedalism on dry land will predictably switch to bipedalism in waist deep water.

Several models argue that bipedality would have improved food acquisition in the proposed early bipedal hominin. Similarly, wading can make such claims as wet, swampy habitats are known to have a rich biomass, especially in times of drought. Wrangham et al. (2009) argued that bipedal wading in places like the Okavango delta could have been a significant component of bipedal origins.

A good model of bipedal origins should either place hominins in relatively safe zone with few predators or at least offer a plausible counter argument to the issue of predation. Many savannah-based models can be criticised for this as they place vulnerable hominins, on the open plains, or adjacent to them―often whilst carrying meat or even more vulnerable infants. It could be argued that wading does this too, as there are several very dangerous aquatic predators in such habitats, notably crocodiles and hippopotami. However, the wading model proposes that the earliest hominins also were relatively arboreal and could have climbed trees to escape such predators. Also, some wading hypotheses propose coastal locations, where there are fewer predators.

A good model of hominin bipedal origins should not only account for our bipedality but also why chimpanzees and gorillas move quadrupedally as knuckle-walkers. This became particularly pertinent as fossil evidence emerged around 2000 that bipedalism was much older than we had thought, to the point that it may even precede the split with Pan and Gorilla. So the model should also be flexible enough to explain how the LCA might have been somewhat bipedal from which evolved one lineage that became more so, and two that “reverted” to quadrupedalism. Wading in swampy habitats offers the perfect scenario for this as even extant apes move bipedally in shallow water and being an “ex-wading” ape seems intuitively fitting as a way of explaining knuckle walking.

Kevin Hunt (1994, 1996) suggested that scenarios where bipedalism is exhibited in extant apes might provide big clues as to the sort of situation where our ancestors might have begun moving bipedally. Hunt’s extensive study of wild chimpanzees found that 80% of instances of bipedalism were in the context of postural feeding which led him to propose that this would have been a plausible scenario for the origin of our bipedality.

However practically every such recorded incident was arboreal and all of them were with the support of their upper arms. Also, this postural form of bipedalism, in any case, did not comprise any form of locomotion.

It begs the question: Is there a different, perhaps rarer scenario where extant apes are compelled not just to pose bipedally momentarily, but to move on two legs without the support of the upper limbs. Clearly wading through shallow water provides precisely this scenario. Indeed it is difficult to envisage any other scenario where a group of extant apes could be induced to move bipedally forever as long as the conditions prevailed.

Some models of the evolution of bipedality argue that it was somewhat driven by one gender. Hunting models are one example of this, including the recent fashion to promote endurance running as a significant factor of the evolution of human bipedality. They are not all male-biased. Some promote young mothers carrying infants as a key driver, or even female-driven sexual selection based on penile display.

There could, of course, be an element of truth in such ideas but I would argue that, all things being equal, a model of hominin bipedality that worked equally for males and females should be considered stronger than one that didn’t. In this regard then, of course, wading meets the criterion.

One of the motivations for writing this paper was the fact that very few papers have been written about the peculiar shape of the australopithecine pelvis. It’s extremely platypelloid is unique amongst the Primates, most species of which have almost the exact opposite ratio of A-P: lateral dimensions. Although this has been widely interpreted as being symptomatic of bipedality, few seem to have focused on what the platypelloid shape might tell us about how they moved and attempted to derive a model of bipedal origins around such scenarios.

One notable exception in John Kingdon’s (2003) “Squat feeding” hypothesis which goes into some detail about the shape of their pelvis and argues that living on the (outside) edge of gallery forests in woodland that started to open up into surrounding savannah grasslands there would be the possibility of collecting food that fell from trees, such as nuts and fruits, and this would have led to the phenomenon of “squat feeding”―a neat precursor to bipedality.

This paper provides compelling complementary evidence to part of Kingdon’s idea. Gallery forest habitats do indeed seem to have been inhabited by early bipedal hominins. However, rather than the pertinent scenario being on the outside margins of the gallery forest, this paper argues that the inner margins―i.e. close to or in the shallow water near the banks―offer a far stronger and more plausible scenario. A wading gait that involved a degree of lateral motion, perhaps with a side-to-side twisting motion, appears to offer a perfectly plausible adaptive explanation for the australopithecine pelvic shape. Its platypelloid shape would offer significant drag reduction whilst moving through water laterally and the shape of the pelvis generally seems to favour the muscle groups that would propel such wading―increased adduction, abduction and rotation.

One of the most obvious aspects of a plausible model of human bipedality is that it should be supported by strong evidence from the fossil record. At least the fossil evidence should consistent with it. So if, for example, the model promotes some aspect of life on the open plains of Africa was the driver of human bipedality, one would expect to find fossil evidence of putative early bipeds in geographic locations, in paleohabitats, consistent with that model.

Bearing in mind the taphonomy of the process of fossilisation suggests that most fossils are formed in depositional substrates, it is almost tautological that waterside habitats conducive to wading are likely to have been present when hominids died and became fossilised.

On top of this self-evident link, however, there are several key examples of the paleohabitats of early bipeds that appear especially conducive to the sort of swampy wetlands one would predict to be associated with wading-climbing apes. Sahelanthropus. Hadar. To name but two.

Very few of the published models on the origin of hominid bipedalism concern themselves with the sister problem―explaining the equally peculiar mode of locomotion of our great ape cousins, the chimpanzees and gorillas. Striding bipedalism is unique in the animal world, as is knuckle walking. It seems to me that a really good model of bipedal origins should explain both, simultaneously. If one considers bipedal origins as a very early phenomenon preceding the split of the Homininae, Paninae and Gorillinae and that this last common ancestor already practiced a kind of wading-climbing facultative bipedalism. I have outlined here how I consider wading-climbing to be an ideal precursor for striding bipedalism, but it is also my contention that is also works rather well as a precursor to knuckle-walking too, if one considers a kind of “ex-biped” whose locomotor repertoire now involves less wading than before but more locomotion through dense wooded vegetation.

There is so much overlap between the different published ideas on the origin of hominin bipedalism that it is difficult to say, with any accuracy, even how many models there are. However, an objective audit would reveal at least 20 distinct ideas and possible more than 40. Most of these ideas on bipedal origins have been published by reputable scientists who have considered the problem for years. It seems unlikely, then, that any single idea will prove to be absolutely correct rendering all the others absolutely wrong. Much more likely that whatever actually drove hominid bipedality included elements of all most, if not all, of the models. One criterion, then, that one might expect to be a key component of a good model of bipedal origins is how complimentary, or at least compatible, it is with the others.

An analysis of the models shows that climbing models and carrying ones appear to be a little contradictory as one cannot carry and climb at the same time. But it could be argued that wading models are complimentary to most.

Few of the published models of bipedal origins seem to be phrased in a strict, Popperian, scientific phraseology. Specifically, few make testable predictions and set out to test them. It was with this criticism in mind that I set out to make predictions of the wading hypothesis, as well as come up with a set of objective criteria to criticise them. I argue that the wading hypothesis, as defined and tested here, meets this criterion as well as any other.

A final attribute of a good model of bipedal origins is that it should also be consistent with, or complimentary to, ideas that explain the whole gamut of human phenotypic traits that differ from the great apes. It is not only in our mode of locomotion that we differ. We also have undergone a clear process of encephalisation and dental/masticatory reduction that seem to indicate a distinctive shift in diet. The body hair pattern of humans also differs significantly from chimpanzees and gorillas. Although we are not exactly “naked apes”―our follicle density is similar to that of the great apes―there is no question that the pattern of body hair―large areas covered with very fine vellus hairs as opposed to terminal hairs―differs greatly. Perhaps most remarkable of all is the human predisposition to complex grammatical language. Although there remains a great deal of controversy and criticism about the so-called “aquatic ape hypothesis” (Hardy, 1960; Morgan 1972, 1982, 1993, 1997) , there is no doubt that, at least in theory, waterside models do attempt to explain all of these elements by proposing a rather simple shift in adaptive pressure for moving through water and procuring food from aquatic habitats. In this regard, wading is just one of three modes of locomotion (swimming and diving being the other two) which may have had an effect on human evolution.

Triangular sets of three points were derived for each hip bone landmark associated with a muscle origin for each skeletal sample, including two generated landmarks. The following method was the basis for the calculation of lever arms. It should be noted that it describes a simplified, unspecified muscle. Known caveats and difficulties with the method are described afterwards.

1) A pseudo landmark (landmark 0―AcCenterGen) was generated to represent the centre of rotation of the acetabulum on each hip bone. As shown in Figure 15, it was calculated as the midpoint of four landmarks (2, 3, 4 and 5) on the rim of the acetabulum.

2) Another pseudo landmark was generated to represent a standard position of the greater trochanter of the femur relative to each hip bone. It was calculated, as shown in Figure 16, using a vector from the landmark at the centre of the acetabulum (landmark 1―AcCent) through the calculated landmark just described (landmark 0 AcCenterGen―Figure 16(A)), the centre of rotation of the acetabulum, through to a point away from the hip. The distance was calculated by using the actual length of the femoral head from the same individual (Figure 16(B)). In this way, a landmark on the hip bone was generated to represent a generalised muscle insertion point (Figure 16(C)).

3) On each hip bone, for each landmark representing a muscle origin, the lever arm was calculated using the following procedure:

a) A triangle of 3 3D co-ordinates was processed (Figure 17):

o = the centre of rotation about the centre of the femoral head.

f = the generated landmark representing the end of the femoral neck.

m = the landmark representing the muscle origin.

b) The angle (x) is calculated for .

c) The distance MF is calculated.

d) MF is multiplied by sine(x) to get the lever arm.