Verhaegen, Munro and Pueuch (2003)
Classification: Habitat Compulsion
Mnemonic: Wading
Specific Model: Aquarboreal ("wading-climbing") Model
Original Proponent(s): Verhaegen, Munro and Puech (2003)    
Basic Summary: The earliest hominin bipeds were wading-climbing apes    
Assessment: Popularity: This model was classified under the subcategory, "wading habitat compulsion", and was ranked 7th most popular out of 9 categories  in the texts reviewed here. 14% referred to this idea specifically.
Simple: #4 /42 (80%)
Detailed: #2 / 42 (82%)
Discussion: ‘Aquarboreal Ancestors?’ (Verhaegen et al 2002)
Following the Hardy/Morgan idea and problems inherent in it, other proponents of the so-called ‘aquatic ape’ hypothesis arrived at models of bipedal origins which are more consistent with the fossil record and timescale as we currently understand it. Perhaps the best example of this thinking is that described by Verhaegen et al (2002).

The authors of this model take a quite different view to the timing of the so-called ‘aquatic phase’, stretching it both earlier and later than that postulated by Hardy and Morgan. According to Verhaegen et al. (2002) all great apes evolved from a last common ancestor that was already fairly well adapted to moving through water. It is important not to misinterpret this as being an “aquatic ape” or even a ‘semi-aquatic’ one. The term coined (originally by Marcel Williams,1997) to describe this life-style is “aquarboreal” (or climbing-wading), suggesting that they were adept both at climbing trees as well as moving through swamps and inundated terrain, rather like proboscis monkeys are today. Much of this bipedalism, clearly, would have been associated with wading through relatively shallow water for food.

According to this model Pan, Gorilla and Homo diverged from this common starting point. Gorilla and, especially, Pan became more adapted to dry ground – and hence reverted to quadrupedalism - whereas, according to the authors, Homo became still more aquatically adapted having migrated to the coasts where their life style included a significant amount of swimming and diving. Important in this later stage is the evolution of the ‘linear build’ associated with modern humans. According to the Verhaegen this evolved as an adaptation to more streamlined swimming and diving, rather than as a result of more efficient bipedalism. Indeed they argue that modern human obligate bipedalism would not have evolved without this swimming and diving ‘exaptation’.
Strengths: This model includes most of the strengths of the Hardy/Morgan ideas but also answers some of the objections to it and may be considered an improvement on the wading hypothesis.    
Weaknesses: In my opinion, the insistence by the authors that linear build was an adaptation to swimming and diving and a necessary pre-requisite fo bipedalism is a weakness. The model could also be better in addressing the problem of explaining early hominin postcranial anatomy and in making falsifiable predictions.    
1.1 Survival Value 9 (Good) In addition to keeping the head above water, the ‘aquarboreal’ model also suggests that vertical climbing, and possibly some brachiation too, was a significant part of the locomotor repertoire. This would only add more weight to the selection.  
1.2 Sexual Selection 5 (Fair) This model was judged neutral by this criterion.  
1.3 Not Teleological 9 (Good) This model is based on non-human behaviour.  
2.1 Improved Food Acquisition 7 (Good) This model was judged good but not as good as the Hardy/Morgan's coastal model.  
2.2 Accounts for Predation 9 (Good) As the proposed hominids in question here are very much arborealists, the question of predation is answered adequately  
2.3 Why Apes are not Bipedal 6 (Fair) This model was judged weaker than the Hardy/Morgan version because it is not as clear cut in explain Pan-Homo-Gorilla divergence.    
2.4 Extant Analogues 8 (Good) All great apes exhibit both vertical climbing and bipedal wading, both key behaviours of the postulated behavioural repertoire of these hominids.  
2.5 Applies to Both Sexes 9 (Good) This model applies to both sexes.  
3.1 Hominid Anomalies 6 (Fair) The authors make no mention of the specific postcranial anatomy of australopithecines, except to remind us that they appear to be adept arboreally.  
3.2 Fits Paleoecological Record 9 (Good) Although this model improves upon the wading ideas of Hardy/Morgan in suggesting a plausible model for the early phase of hominid evolution, it appears less solid in the later stages (post-Homo) where there is little fossil evidence to support a distinct swimming/diving phase for early members of the Homo genus.  
3.3 Precursor to Strider and knuckle Walker 7 (Good) All wading models meet this requirement very well.  
4.1 Extended Explanatory Power 7 (Good) Verhaegen et al explain more than just bipedalism but the aquarboreal part of their model is largely focused on that aspect only.  
4.2 Complimentary 5 (Fair) This model was judged complimentary to other arboreal and wading models but contradictory to those espousing open habitats.  
4.3 Falsifiable or Testable 4 (Fair) The authors’ paleogeographical basis for their model does, at least, provide some means of being tested. Their predictions would include that early hominids should be found in coastal mangrove niches is potentially open to future fossil discoveries although these are always open to interpretation due to taphonomic biases.    
References Verhaegen, M., Puech, P., Munro, S. Aquarboreal Ancestors? Trends in Ecology and Evolution 17:212-217, (2002).