Classification: Combination
Mnemonic: Not one thing or another
Specific Model: Combination of Multiple Factors
Original Proponent(s): Napier 1964, Sigmon 1971, Rose, 1985, Day 1986    
Basic Summary: A specific combination of factors was responsible for the evolution of bipedalism.    
Assessment: Models in this category were the 8th most popular (of 9) in the texts surveyed with only 14% referring to them.
Simple #22 (=2) / 42 (53%)
Detailed #27 (=3) / 42 (53%)
Discussion: “The evolution of bipedal walking in hominids” Napier 1964
Napier’s classic paper in the early 1960s used an anatomical analysis of Proconsul as a basis for considering the precursive form of locomotion to hominid bipedalism. His conclusion was largely that their forearms were used very much as extant ‘semibrachiators’ do today, particularly those among the New World monkeys. Later in evolution, Napier argues, the widespread, although quite rare, facultative bipedalism of primates suggests that truncal erectness was also part of the precursive form of locomotion.

Generally, Napier rejects arguments that the immediate predecessor to the earliest hominid bipeds were in any way specialised.

“… O[o]ne must look for the antecedent stage of hominid bipedalism not among the fully specialised brachiators, (as is, of course, now almost generally agreed) nor among specialised quadrupeds adapted to ground-living life, but among agile forms that employed their forelimbs in suspending the body and their hind limbs in supporting and propelling it in such a manner as to promote mobility of the hips without incurring the specialisations associated with extreme prehensility of the feet.”
Napier (1964:685)

Furthermore whatever factors drove it, Napier argues that they were not singular.
“It seems unlikely that any single factor was responsible for such a dramatic change in behaviour. In addition to the advantages of accruing from ability to carry objects - food or otherwise - the improvement of the visual range and the freeing of the hands for purposes of defence and offence must equally have played their part as catalysts.”
Napier (1964:687)

The model is that by a slow, incremental, process unspecialised apes, pre-adapted to upright posture and movement through a climbing ancestry gradually adopted more bipedalism for a variety of reasons.
“Bipedal Behaviour and the Emergence of Erect Posture in Man” Sigmon 1971
Sigmon begins with a review of literature for behavioural bipedalism in great apes which, as evidence was more limited at the time, was largely centred on the chimpanzee. She contrasted evidence that was taken from Goodall’s initial observations in the Gombe published in 1965 (her later paper, in 1968, was not cited) and Kortland (1961) with that of Reynolds and Reynolds (1965) and Nissen (1931).

The former pair of studies reported significant bipedal behaviour on the ground whereas the later pair did not, only exhibiting bipedal behaviours often in trees. It was proposed was that this data implied that chimpanzees living in a more wooded habitat were less prone to bipedalism than those living in more open ones. As Sigmon put it:
“Thus the nature of the habitat occupied by the four groups of chimpanzees reported in the above studies appears to have been a major factor in determining the amount of bipedal behaviour that was observed. A semi-open environment, as in Goodall's and Kortland's studies, appears to be a stimulus for increased use of bipedal locomotion, while a dense rain forest environment, as in Nissen's and Reynolds' and Reynolds' study, does little to encourage a two-legged gait.”
Sigmon (1971:56)

The second major point of the paper is that as chimpanzees exhibit bipedalism in several behavioural contexts it is unlikely that a single specific behaviour should be proposed as the major preadaptation for erect bipedalism. It is this aspect of Sigmon’s thesis that qualifies her under the category ‘combination of factors’. As she puts it, bipedalism might be expected to be used “whenever it improves the chances of survival of the animal.” Sigmon (1971:59) Her thesis is therefore that a change from closed to more open habitats caused various behavioural behaviours to be practiced more and that this translated, eventually, into selection for anatomical traits that made that bipedalism more efficient. The main criticisms of the model are that its basic premise, that a move to more open habitats favours slightly more bipedalism, isn’t strongly supported by the evidence.

The four studies cited by Sigmon did not give unequivocal support to the premise and others since could be argued to directly contradict it. Also, those primates that may be characterised as most bipedal (e.g. Hylobates, Ateles) live in amongst the most closed woodland habitats. Sigmon’s model will now be considered in the light of our framework of features for an ‘optimal’ model.

“Bipedalism, Pressures, Origins and Modes”, Day 1986
Day summarised the view of bipedal origins, current in the mid 1980s, very succinctly and provided a useful if brief review of the literature on the subject to that date.

As far as this section is concerned, Day outlined three key pressures for the evolution of bipedalism:
• Improved food acquisition
• Improved predator avoidance
• Improved reproductive success

He then reviewed the published work in the literature that may shed light on which specific factors may have worked in these three areas and appeared to support almost all of them.

Specifically, in terms of food acquisition, he cited carrying ideas (Hewes 1961), the importance of woodland and vertical climbing in primates (Napier 1964), positional behaviour in food acquisition (Ripley 1979, Prost 1965, Rose 1984) and models surrounding the scavenging of food (Shipman 1983, 1984.)

In favour of predator avoidance, Day cited, without strong support, Kortland’s (1980) suggestion that the use of thorny bushes could have been used to keep large predators at bay but argued that “upright stance and tree climbing ability both combine to increase the visual horizon for early warning of danger so that an early retreat to a safe place becomes a viable strategy” (Day 1986:189.)

Finally, Day cites Lovejoy’s (1981) provisioning model in support of his claim that reproductive success would have been enhanced by bipedalism. In conclusion he wrote
“I believe the pressures for the evolution of upright stance and bipedal gait can be explained in terms of improved survival advantage through enhanced food acquisition (new options in positional behaviour, food carriage and hunting), predator avoidance (running and climbing, early warning of danger and freed hands for threat displays and defence) and improved reproductive success (two freed hands for safer infant carriage and for nursing at pectoral mammae in the light of a prolonged infant dependency)”
Day (1986:190).

The rest of the paper considered ‘origins’ and ‘modes’. He gives a brief history of how the views of the precursive form of locomotion changed as evidence accumulated throughout the last century but concluded that “even this brief survey of the literature shows that we really have no clear idea of what form of locomotion, in what creature, preceded, or was immediately pre-adaptive for upright posture and bipedal gait” (Day 1986:191). The biggest part of the paper discussed the ‘modes’ of bipedalism of two classes of hominid: the australopithecines and the Homo erectus class. He concludes “nobody now seriously disputes the upright stance and bipedalism of the australopithecines, or of Homo erectus” Day (1986:197.) s="style11">
 Strengths: The relatively vague nature of these models accounts for both their strengths and their weaknesses. Compatibility with other models is the most unsurprising strength of these models.    
Weaknesses: Explaining why apes are not bipedal is one of the weaknesses. The more vague the idea being proposed to account for human bipedalism the less clear it is as to why it could equally have applied to other great apes. Similarly, accounting for early hominin anaomalies and providing tests that are falsifiable are more difficult for models that are less precise.    
1.1 Survival Value 4 (Fair) These models were judged slightly worse than neutral here because, it is argued, that more specific models provide more clear cut arguments for survival advantage.    
1.2 Sexual Selection 5 (Fair) These models were judged neutral on this criterion.    
1.3 Not Teleological 5 (Fair) These models were judged neutral on this criterion.    
2.1 Improved Food Acquisition 5 (Fair) These models were judged neutral on this criterion.    
2.2 Accounts for Predation 5 (Fair) These models were judged neutral on this criterion.    
2.3 Why Apes are not Bipedal 2 (Poor) The vague nature of these models prevented them being judged well on this criterion.    
2.4 Extant Analogues 5 (Fair) These models were judged neutral on this criterion.    
2.5 Applies to Both Sexes 9 (Good) That multiple factors were proposed in these models meant that they were judged 'good' by this criterion.    
3.1 Hominid Anomalies 2 (Poor) The vague nature of these models prevented them being judged well on this criterion.    
3.2 Fits Paleoecological Record 5 (Fair) These models were judged neutral on this criterion.    
3.3 Precursor to Strider and knuckle Walker 5 (Fair) These models were judged neutral on this criterion.    
4.1 Extended Explanatory Power 7 (Good) These models were judged better than neutral on this criterion because humans are generalists and these models speak of a shift towards greater generalisation.    
4.2 Complimentary 9 (Good) As these moels include many of the individual aspects of other models it is not surprising that they are the most compatible with the others.    
4.3 Falsifiable or Testable 3 (Poor) The vague nature of these models prevented them being judged well on this criterion.    
References Day, M. (1986). Bipedalism: Pressures, Origins and Modes. In: Wood, Bernard; Martin, Lawrence; Andrews, Peter (eds.), (1986). Major topics in primate and human evolution. Cambridge University Press (New York)
Napier, J. The evolution of bipedal walking in hominids. Archives of Biology 75:673-708, (1964).
Rose, M D (1985). Locomotor Anatomy of Miocene Hominids. In: Delson, Eric (eds.), (1985). Ancestors: The Hard Evidence. Alan Liss Inc (New York)
Sigmon, B. Bipedal Behaviour and the Emergence of Erect Posture in Man. American Journal of Physical Anthropology 34:55-58, (1971).