Kingdon (2003)
Classification: Feeding Models
Mnemonic: Hand to Mouth
Specific Model: Terrestrial 'squat feeding' on gallery forest floors
Original Proponent(s): Jonathon Kingdon (2003)    
assessment: Popularity: Feeding Models were ranked 2nd (out of 9) most popular in the texts reviewed. Although this is a very new idea, one text referred to it explictly.
Simple: #7 (=2) / 42 (63%)
Detailed: #15 / 42 (58%)
Basic Summary: Bipedalism arose through adaaptations in 'ground apes' whilst feeding from fallen foods on the floor of gallery forests.    
Discussion: In 2003 the African zoologist Jonathan Kingdon wrote a whole book specifically on the evolution of human bipedalism called “Lowly Origins”, the title of which was taken from the very last words in Darwin’s Descent of Man. According to Kingdon, this might have been the first book to be specifically written about human bipedal origins. He writes: “although there are many scientific papers and single chapters of books that discuss bipedalism, this is probably the first to be devoted to it as a single dominant there - the central condition on which human evolution is predicated.” Kingdon (2003:2)

Kingdon begins his work by making an interesting observation: “One of the most striking and surprising peculiarities of equatorial African fauna and flora is the frequency with which forest and non-forest species form pairs. Amongst plants, amphibians, birds and mammals there are forest species whose closest relative is not another forest-adapted species but a non-forest sibling.” Kingdon (2003:10) He goes on to specify that by ‘non-forest’ he does not mean ‘savannah’ and that, in any case, carbon isotope evidence suggests that savannah habitats did not become widespread until 1-2 mya (Cerling, 1992.) The specific habitat he has in mind becomes clearer as he describes the process of climate change which seems to have characterised ape-human divergence.

One challenge for species adapting to new or different habitats has been the repetitive drying out and retreat of extensive forest into a network of narrow galleries and riverine strips. During the Plio-Pleistocene, this tended to coincide with each global glaciation and gave a special importance to rivers as focal areas or refuges. With the return of humid climates, forests could expand from their riverine cores and swallow up the intervening country. In the pages that follow, both minor and major rivers and their basins play a central role in my understanding of the relationship between forest and non-forest biota.” Kingdon (2003:11.)

Kingdon builds his case using evolutionary chronology. Starting with ‘On Being a Primate’ each chapter builds upon what probably came before it. ‘On Being an Ape’ leads onto ‘On Being a Ground Ape’ and then ‘On Becoming a Biped’ and ‘On Being a Manipulative Man-ape’ and so on. Kingdon’s thinking is clearly outlined in these chapter headings, especially the one which highlights the key new idea brought to the debate: The Ground Ape. Even before the discovery, in 1994, of Ardipithecus ramidus (lit. “stem ground ape”) Kingdon had been thinking about scenarios involving foraging for fallen foods on the ground and that the resulting types of locomotion could have been a significant part of the behavioural repertoire of early hominins. He writes: “The main reason why a quadrupedal stance becomes inappropriate for systematic and sustained ground-searching is that only one hand is available at a time. As Jolly was the first to point out, the solution is to drop onto the haunches and use long arms and agile hands to pick and choose.” Kingdon (2003:126)

Kingdon’s theory is quite unique in that it offers a rather simple explanation for the peculiar postcranial anatomy in general, and the shape of the pelvis in particular, of early hominin bipeds such as Australopithecus afarensis. “

To achieve this result [pelvis and legs often flexed with balance for ground feeding] four major modifications to the skeleton would have been necessary:
1. Long iliac wings fanning out to the rib cage would have had to disengage from the upper trunk and retract to the point where they ceased to be . . . [a hindrance to squatting],
2. Sacrum compression.
3. Back straightened to aid predator scanning,
4. Head orientation more orthograde,
5. Teeth,
6. Talus, heel bone adaptations,
7. Mobile shoulders,” Kingdon (2003:128.)

Kingdon takes this further and argues that ‘squat-feeding’ is a natural precursive form locomotion to human-like bipedalism:

A locomotory dimension of 'squat-feeding' is that wherever resources were densely scattered, the squatting could have been punctuated by extremely frequent but very abbreviated "advances" in which the forager shuffled forward, without rising completely, or briefly stood up on its hind legs. Whatever the frequency of such standing, I contend that it was only after the vertebral column had become fully vertical that routine bipedal walking could evolve. The energy savings of a fully upright body posture are sufficient to have persuaded Ishida that postural uprightness must have preceded bipedal locomotion. Using trained and untrained Japanese monkeys as models, he calculated a 30 percent saving in energy costs between upright and 'bent' bipedalism.” Kingdon (2003:134)

Kingdon backs this up this anatomical evidence with some convincing paleoecological and paleogeographical arguments. Going back to his initial observation, that African forest species tend to have closely related non-forest cousins, and having already suggested that it was in the East African littoral forest zone where the process of hominization occurred, he considers “…what might have been so odd about the east to trigger the development of such an aberrant ape. Kingdon (2003:121)

He lists out forty features of eastern coastal gallery forest ecosystems which would have differed from those of more tropical rainforests to the west. They include: Isolation by Somali arid corridor; small overall range, but long north-south; Gaps in the north-south distribution; Forest with lower canopies and shorter trunks; Lower annual rainfall with a drier microclimate but more seasonal and more erratic seasonality; More seasonally adapted plants; Fewer superabundant fruiters; More ground resources; Greater diversity of terrestrial animals; Many small, scattered food items; More nutritious and concentrated animal foods; Ecologically degraded borders to ranges; More consistent resources in narrow confined ranges; More competition on the ground; Larger primates and pigs; Smaller rodents and reptiles; Predators more common and diverse; Mammalian encounters more common. Kingdon (2003:121-122.)

The clear implication of these factors is that hominin ancestors living there would have to spend more time on the ground foraging for foods and be smarter at dealing with the more complex environment. Kingdon’s main point is the assumption that ‘squat-feeding’ was an essential pre-requisite for a re-organisation of the upper body which, itself, was a pre-requisite to bipedalism. As Kingdon put it: “In the sequence of events that is suggested here, rising up onto two legs must have been totally conditional on the anatomical, mechanical and behavioural innovations that accompanied ‘squat foraging.’ The new balance of the upper body left no other option but to straighten the legs. In the real sense this ‘unbending of the legs’ can be seen as subsidiary to the more revolutionary reorientation of the upper body. The development can be caricatured as ‘jack yourself up and do the same thing faster and higher’” Kingdon (2003:135.) Perhaps this is the main weakness of the model because there is very little evidence that such squat-feeding leads to a greater propensity to bipedalism. Jolly’s model, based on similar behaviours in gelada, had a similar weakness as geladas are among the most committed terrestrial quadrupeds. Furthermore, as with the postural feeding behaviour which is the basis of Hunt’s (1994) hypothesis, there does not seem to be much in common between the proposed initial behaviour and the form of locomotion that is proposed to have evolved from it.

As most authors assume a more arboreal ancestry for hominids, and as large arboreal primates have a tendency for upright posture through vertical climbing and/or brachiation, the squat-feeding hypothesis, like the postural feeding hypothesis, does not appear to contribute much to the problem. Ape ancestors would have already had a propensity to upright posture. The real problem, I suggest, is what factor, building on that legacy, would have made them more likely to actually move bipedally?
 Strengths: Kingdon, notably rare in this review of bipedalism models, goes into some detail about the post cranial anatomy of australopithecines and argues, quite convincingly that 'squat feeding' may well indeed explain the peculiarly braod platypelloid pelves that are very short in the anterior-posterior direction. In my opinion, he also makes a very compelling case for the gallery forest habitat being a key factor in the evolution of hominin bipedalism.    
Weaknesses: Where Kingdon is least convincing is in promoting 'squat feeding' as a viable form of food gathering. Fallen nuts and items from trees would hardly seem a sufficient cause to adopt a whole new mode of locomotion and, after all, these hominins were probably very adept climbers. Adept enough, indeed to pick as maany such fruits from the trees before they fell to the ground.    
1.1 Survival Value 2 (Poor)  It is not particularly clear why squat feeding would provide selection for bipedalism. Kingdon argues that the increased upright orientation of the spine and the improved balance resulting from this would be likely and a necessary precursor to upright bipedalism but this claim is not well backed up with evidence. It was judged poor by this criterion.    
1.2 Sexual Selection 5 (Fair) This model was judged neutral by this criterion.    
1.3 Not Teleological 5  (Fair) The squat-feeding idea does not seem to lead to bipedalism very easily. Kingdon does not offer any intermediate steps between it and increased bipedalism, other than Wrangham’s notion that it would be more efficient to remain in one posture rather than repeatedly switching between postures. On this point, however, the argument appears to be self defeating: Why switch from squat feeding to more upright bipedalism, only to squat back down again to feed in a few seconds? However Kingdon does at least propose a rather inovative way of getting early hominins to move to a more upright posture as a precursor to bipedalism. In this sense it is fair here.    
2.1 Improved Food Acquisition 7 (Good) Although 'squat feeding' hardly seems to provide much survival advantage, Kingson at least posits hominins in relatively food rich gallery forests where food acquisition should not have been too much of a difficulty.    
2.2 Accounts for Predation 7 (Good) Kingdon’s model proposes a ground ape living in habitats where there was a greater number and variety of predators but his suggestion is that the earliest bipeds were relatively good arborealists (Kingdon 2003:135) is a reasonable counter to that.    
2.3 Why Apes are not Bipedal 4 (Fair) Kingdon's model was judged slightly weaker than neutral on this criteria as 'squat feeding' in gallery forest habitats was not judged as a compelling reason for humans to diverge from their ape cousins in terms of locomotion.    
2.4 Extant Analogues 3 (Poor) Although plausible, Kingdon does not provide good evidence for squat feeding in extant chimpanzees and bonobos.    
2.5 Applies to Both Sexes 9 (Good) Both sexes could perform the squat feeding behaviour equally well.    
3.1 Hominid Anomalies 8 (Good) Kingdon is unsually thorough in attempting to account for the post cranial anatomy of australopithecines in his model. It was judged a plausible attempt at an explanation.    
3.2 Fits Paleoecological Record 8 (Good) This model appears to fit the currently known theoretical paleoecological record rather well although few hominins have yet been found in the precise geographical locations proposed by Kingdon.    
3.3 Precursor to Strider and knuckle Walker 4 (Fair) Although squat feeding does provide a few plausible exaptations for bipedalism it was judged here to be too far removed from human walking to be given a good rating.    
4.1 Extended Explanatory Power 4 (Fair) As with Hunt and Jolly, Kingdon’s model also attempts to explain dental reduction and other hominin characteristics such as reduced sexual dimorphism. This was judged slightly lower than neutral    
4.2 Complimentary 5 (Fair) This model was judged complementary with most models based in a forest context and with many feding and carrying models and incompatible with those relaint on movement through open habitats or using weapons.    
4.3 Falsifiable or Testable 4 (Fair) Kingdon's model does not appear to provide a series of falsiable predictions by which it may be tested but it is, nonetheless more evidence based than most.    
References Kingdon, J. (2003). Lowly Origins. Princteton University Press (Woodstock)