Classification: Habitat Compulsion
Mnemonic: Wading in Fallback Zones
Specific Model: Wetland Foraging
Original Proponent(s): Ellis (1991), Wrangham et al (2009)    
Basic Summary: In times or aridity, early hominins may have resorted to fallback foods in wetland habitats. This could have been significant enough in early human evolution that wading through them bipedally may have even been a factor in the adoption of bipedalissm.    
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. None referred to this idea specifically as it is so new.
Simple: #2 (=2) /42 (84%)
Detailed: #3 / 42 (80%)
Discussion: One of the earliest proponents of waterside hypotheses of human evolution, Derek Ellis, has long argrued that wading in wetland habitats may hav eplayed a significant role in human evolution. In a series of papers and chapters in the 1990s, the view was expounded that coastal or other wetlands could have been a key factor in human evolution.

Independently, and more recently, Richard Wrangham has been studying baboons in the Okavango Delta in South Africa  for several years and has been developing a view that such wetland habitats may have played a key role in human evolution. In 2005 he published his "Delta Hypothesis" which described how such habitats, with relatively biomass production could have acted as important refugia in times of aridity.

The idea was enhanced in a paper published in AJPA in 2009 "Shallow-Water Habitats as Sources of Fallback Foods for Hominins" where, in addition, Wrangham and collegues identify bipeedal wading through such habitats as a potential factor in the evolution of hominin bipedality.
 Strengths: These ideas have many strengths: They provide good survival value, extra food acqisition, have good analogues in extant primates and are consistent with the paleoecological evidence.    
Weaknesses: There are no discernable weaknesses in this model.    
1.1 Survival Value 9 (Good) This idea provides a very elegant and specific means of enhancing survival value by suggesting that fallback foods in wetland refugia in times of aridity could have literally made the difference between life and death for groups of early hominins.    
1.2 Sexual Selection 5 (Fair) This model wwas judged neutral by this criterion.    
1.3 Not Teleological 9 (Good) As the model is based on observations of extant baboons it was deemed not teleological at all.    
2.1 Improved Food Acquisition 9 (Good) The model provides a realistic means for improved food acquisition, particularly in times of aridity.    
2.2 Accounts for Predation 5 (Fair) This model was judged neutral by this criterion. Any 'gain' from avoiding land based predators would have to be weighd up against 'losses' incurred from increased risk of predation from aquatic predators.    
2.3 Why Apes are not Bipedal 8 (Good) Assuming Pan/Gorilla were not exposed to such times of aridity, perhaps living in equatorial rain forests, this model provides an elegant way of accounting for Pan-Homo-gorilla divergence.    
2.4 Extant Analogues 8 (Good) The model is based on good evidence in extant baboons. Unfortunately, however, extant apes were not observed in the Wrangham et al (2009) study..    
2.5 Applies to Both Sexes 9 (Good) This model applies equally to both sexes.    
3.1 Hominid Anomalies 5 (Fair) This model was judged neutral on this criterion.    
3.2 Fits Paleoecological Record 8 (Good) This model fits the growing consensus on the paleoecological context for early human evolution fairly well. However, being based on a specific and unique wetland habitat in South Africa detracted from it slightly.    
3.3 Precursor to Strider and knuckle Walker 7 (Good) The wading and gathering component provides a good precursor for both striding bipedalism and knuckle walking in chimpanzees although this area of discussion was not developed much in the Wrangham et al (2009) paper.    
4.1 Extended Explanatory Power 4 (Fair) This model was judged slightly worse than neutral on this criterion as no other aspect of human evolution was explained by it.    
4.2 Complimentary 7 (Good) This model was judged complimentary to most other models and compatible with the others except Wheeler's thermoregulatory model.    
4.3 Falsifiable or Testable 6 (Fair) This model was judged slightly better than neutral here.    
References Wrangham, R (2005). The Delta Hypothesis: Hominoid Ecology and Hominin Origins. In: Lieberman, Daniel E; Smith, Richard J; Kelley, Jay (eds.), (2005). Interpreting the Past: Essays on Human, Primate and Mammalian Evolution in Honour of David Pilbeam. Brill Academic Publishers (Boston) 
Wrangham, R., Cheney, D., Seyfarth, R., Sarmiento, E. (2009) Shallow-Water Habitats as Sources of Fallback Foods for Hominins. American Journal of Physical Anthropology 140:630-642.