Bartholomew & Birdsell (1953)
Classification: Forelimb pre-emption (carrying models)
Mnemonic: "Freeing of the hands"
Specific Model: Tool/Weapon Carriage
Original Proponent(s): Bartholomew & Birdsell 1953 (17); Washburn 1960 (18) ; Marzke 1986 (19)      
Assessment Popularity Ranking of classification: 1st out of 9 categories (86% of texts)
Simple Evaluation: #19 / 41 (54%)
Detailed Evaluation: #12 (=2) / 41 (59%)
Basic Summary: The regular carriage of tools and weapons, specifically, was a key factor in the adoption of bipedal posture and locomotion.    
Discussion: The importance of tool making has long been recognised as a significant measure of ‘progress’ in human evolution. It is backed with good evidence from the fossil and archaeological record. The degree of sophistication of stone tools found associated with hominids has been seen as a key indicator of this and, therefore, tool carriage has been viewed as a key motivating factor for our bipedalism. Although the idea that forelimb pre-emption generally was a key factor in the evolution of bipedalism originated with Darwin (and later Dart) Bartholomew and Birdsell (1953) suggested that the real advantage of freeing of the hands was in using tools: “Only man has his locomotion essentially unimpeded whilst carrying or using a tool” (p 482.) Washburn (1960:69) and Hocket (1960:96) made similar arguments.  
 Strengths: Offers a realistic reason for increased bipedality backed with good evidence from the fossil record.    
Weaknesses: There are a number of counter arguments...
• Many early tools would have been relatively small and easy to carry without the need to change the mode of locomotion. Hewes (1961:693) points out that “most apes or monkeys can carry a stick tripodally for fairly long distances without fatigue or in open country, cross-wise in the mouth just as domesticated dogs can be trained to carry sticks.”
 • Tool use, generally, does not require bipedal locomotion at all. Much tool use is done whilst sitting down, squatting or kneeling positions. This is as true in apes as it is in humans. Hewes (1961:694).
• Although it is of course possible that the earliest hominids used wooden clubs, parts of branches or bones, there is little evidence for hominid tool use until long after bipedalism had become well established and bipedal posture and locomotion without weapons is surely more prone to predator attack than moving on all fours would be.
• There is a great deal of evidence in chimpanzees and, more recently, in wild western lowland gorillas, that shows that tool use is not an exclusively human trait. In chimpanzees much of this tool use is whilst sitting down whilst Gorilla gorilla gorilla were recently observed using a stick to gauge the depth of water, and as a balancing aide, whilst wading in shallow pools (or beis) in the Congo (Breuer et al 2005.)  
1.1 Survival Value 6 (Fair) This model was rated slightly higher than neutral on this criterion because it was judged that some tools might increase the likelihood of survival.    
1.2 Sexual Selection 6  (Fair) This model was rated slightly higher than neutral on this criterion because it was judged that holding tools might improve the likelihood of sexual selection.    
1.3 Not Teleological 2 (Poor) The ubiquitous adoption of technology by modern humans makes this a popular idea, however it does also tend to imply a teleological factor to it.    
2.1 Improved Food Acquisition 6 (Fair) This model was rated slightly higher than neutral on this criterion because it was judged that holding tools might improve the likelihood of food acquisition.    
2.2 Accounts for Predation 7 (Good) Assuming that at least some of these tools are interpreted as weapons then it is possible that tool use itself may have given a degree of protection from predators.    
2.3 Why Apes are not Bipedal 3 (Poor) As most extant apes use and carry tools without adopting bipedality this model was judged poor on this criterion.    
2.4 Extant Analogues 6 (Fair) Although many primates, and especially chimpanzees, have been observed making tools, as well as using them, this has not been associated with any increased tendency towards bipedality.    
2.5 Applies to Both Sexes 7 (Good) Although weapon use/carriage has mostly been associated with males, tool use in females is also plausible.    
3.1 Hominid Anomalies 3 (Poor) Proponents of this model have not used the anatomy of early hominins as part of their argument.    
3.2 Fits Paleoecological Record 7 (Good) One of the key points in favour of this kind of model is the very strong evidence of stone tools associated with early hominid sites. Although this evidence is not really quite early enough to be clearly linked as a factor that might have actually begun the trend towards bipedality, it is, nonetheless, a key indicator.    
3.3 Precursor to Strider and knuckle Walker 3 (Poor) Proponents of this model have not proposed any particular arguments based on the precursive mode of locomotion.    
4.1 Extended Explanatory Power 6 (Fair) Marzke (1986) has argued that this factor also explains the evolution of human-like hands. Promotion a greater intelligence and social factors are sometimes also explained by some proponents.    
4.2 Complimentary 7 (Good) Like most carrying models, this was jdged complimentary to most others and contradictory to climbing models.    
4.3 Falsifiable or Testable 5 (Fair) This model was judged neutral on this criterion.    
References Bartholemew, G A; Birdsell, Joseph B (1953). Ecology and the Protohominids. American Anthopologist Vol:55 Pages:481-498.
Breuer, Thomas; Ndoundou-Hockemba, Mireille; Fishlock, Vicki (2005). First Observation of Tool Use in Wild Gorillas. Plos Biology Vol:3(11)e380 Pages:1-3
Hewes, Gordon W (1961). Food Transport and the Origin of Hominid Bipedalism. American Anthropologist Vol:63 Pages:687-710
Marzke MW. Tool use and the evolution of hominid hands and bipedality. In: Else JG, Lee PC, eds. Primate evolution. London: Cambridge University Press, 1986:203–209.
Washburn, Sherwood L (1960). Tools and Human Evolution. Scientific American Vol:203 Pages:63-75.