A wading component in the origin of hominid bipedality?

Fundamental to the understanding of human evolution are plausible solutions to questions surrounding why, uniquely among the primates, we habitually move bipedally almost totally without the aid of the upper limbs. However, despite about 150 years of intensive application of Darwinist theory by scientists of various genres, there remains little confidence or consensus in the field of palaeoanthropology about solutions to such questions.
This thesis (in Chapter 2) comprehensively reviews 42 models that have been published to provide potential solutions to these problems. Then, in Chapter 3, for the first time, an evaluative framework is proposed for models of bipedalism which is then used to assess and compare those described. This evaluative framework is essentially a “marking rubric” for which an on-line tool (see www.tinyurl.com/BipedalModels) is provided, which the reader may use to criticise the author’s assessment and substitute his or her own. Among the strongest models, according to this evaluation, are ones suggesting that a key driver of early hominid bipedalism was regular wading through shallow water. The author’s own wading model “River Apes … Coastal People” is briefly outlined in chapter 2 and evaluated in chapter 3, for the sake of completion, but the reader should refer to the later part of the thesis, namely chapters 5, 6 and 7, (specifically sections 7.2 and 7.3) for a detailed examination.
Chapter 4 examines, in detail, the wading hypothesis, which has hitherto received comparatively little attention from paleoanthropologists and human anatomists. The historical context of the wading hypothesis is described and it is proposed that its association with (in my opinion) the unfortunately labelled “aquatic ape hypothesis” accounts for why this idea has received less respect than it might have deserved. According to the evaluative framework described here, all models, including ones involving wading, have weaknesses and the rest of the thesis is an attempt to address some of them by testing various predictions of the wading hypothesis in order to arrive at a better wading model.
Two identified weakness of published wading models are addressed in Chapter 5, namely that they make few scientifically testable predictions and that they are somewhat contradicted by arguments of energy efficiency. One such prediction is that wading makes bipedalism less energetically costly in the earliest hominins still lacking the anatomical adaptations for bipedal efficiency of modern humans. The cost differential between optimal (i.e. fully upright, extended hip, extended knee, EHEK, human-like) and non-optimal gaits (i.e. Bent Hip Bent Knee, BHBK, chimp-like), according to this test, should be reduced in water. An empirical study from experiments on human subjects is reported, which found that the cost differential was indeed reduced from approximately 55% on dry land, to approximately 18% for a BHBK gait with 50o knee flexion, at 0.6 m/s in water depth of approximately 1m. At greater depths, and/or using gaits with greater knee-flexion, the cost differential was reduced still further, or eliminated completely. This suggests that wading might well provide an optimal scenario for the earliest hominids to adopt bipedalism before they had evolved an anatomy to make it energetically efficient.
Another weakness of published wading models is addressed in Chapter 6. None of them offer any hard evidence from the fossil record of early hominin bipeds. This thesis reports a geometric morphometric study of the hominoid hip, comparing the shape of the australopithecine pelvis with extant great apes and humans. It tests the prediction that the shape differences between the australopithecine and human pelvis is consistent with adaptations for more efficient wading. Triangular sets of landmarks including hip muscle origin, the centre of rotation of the acetabulum, and a generated (fixed point) landmark modelling an insertion point on a fixed femur, were used to calculate the lever arm of the major muscle blocks involved with hip movement. For each specimen, each lever arm was expressed as a ratio of every other, yielding over 135,000 ratios in all. This data was then explored using a business analysis data summarising tool, the Pivot Table feature of Microsoft Excel. This allowed the production of succinct species summaries of broad lever arm groups, such as those pertaining to abduction compared to those pertaining to extension, or for rotation, compared to flexion. The pivot table feature is designed so that these broad summary data can be “drilled down” to get to ever increasing levels of detail, ultimately to the individual level arm ratio pairs. Although the scope of this study necessarily limited how “deep” this “drill through” process could be done, the method clearly shows promise for studies of this type in the future and produced results that were consistent with the prediction that australopithecine pelvic shape appears to be adapted to generating greater lever arms for abduction/adduction and rotation, as a ratio to extension/flexion than modern humans. This is what might be expected if regular wading was a part of their locomotor repertoire.
The thesis concludes, in chapter 7, by putting wading hypotheses of the origin of hominin bipedal origins in the broader context of waterside hypotheses of human evolution generally. An assessment of the mislabelled “aquatic ape hypothesis” is summarised and a new model of human evolution (called “River Apes… Coastal People”) is offered which draws upon the strengths of the still current mainstream “savannah-based” paradigm and those that propose many human traits that may have resulted from some selection from moving and procuring food from water. This hybridization of seemingly incompatible ideas is possible, it is argued, simply by scaling back the degree of selection for wading, swimming and diving being proposed, and acknowledging that even very slight levels of selection can, in evolutionary terms, rapidly overcome the effects of drift.