Isaac (1978)
Classification: Forelimb pre-emption (carrying models)
Mnemonic: "Freeing of the hands"
Specific Model: Carcass carrying back to gallery forest bases’ hypothesis.
Original Proponent(s): Issac (1978)    
Assessment: Popularity Ranking of classification: 1st out of 9 categories (86% of texts)
Detailed Evaluation: #19 /42 (55%)
Simple Evaluation: #35 /42 (49%)
Basic Summary: Through the scavenging of carcasses on the savannah, it became necessary to carry back pieces of meat back to the home base. This required the use of forearms to carry the food and therefore precluded them for use in locomotion, thus driving the adoption of bipedalism.    


His model goes further than merely explaining the adoption of bipedalism…


“I suggest therefore that one conveniently divide the continuum of human evolution into two phases. Probably the first involved shifts in the basic systems of locomotion and subsistence, plus two new ingredients - tools and food sharing. This led to a pattern of adaptation which in hindsight we see as proto-human, but which is probably better termed 'early hominid', since it was probably a nonhuman system that was effective in its own right." Isaac (1978:244)


"It is widely believed that the process involved extensive feedback amongst the several subsystems, thus: hunting facilitated food-sharing since meat was more readily carried than any other common food stuff; missiles, weapons and tools facilitated the killing and butchering of larger and larger animals; bipedalism facilitated weapon use, the carrying of food for sharing, and long range mobility; gathered vegetable foods remained as a staple and as an insurance policy against failure in the hunt, so that division of labour (and pair bonding?) gave stability to the system; bags and baskets facilitated food-sharing and division of labour etc." Isaac (1978:245)


Isaacs makes the point that he did not see this process as necessarily driven by any particular change in climate (Isaac 1978:245) although he makes extensive references to the savannah and gallery forest habitats which would have protruded into them. For example, he writes “it can thus be argued that savannah sensu lato constituted a vacant ecological niche for an animal of the hominoid grade, capable of basing its subsistence on a combination of hunting and foraging.” (p240) and “…hominids, whilst colonizing the savannah, may have preferred to keep their home bases in strips of woodland that extended out into more open country” (p239.)

 Strengths: Evidence is provided:

“Most of the early sites at Olduwai and the two excavated sites at the Lower Member of the Koobi Fora Formation consist of coincident patches of stone artefacts and scatters of broken bones. The conclusion seems inescapable that the same hominids who made the artefacts concentrated the bone. It also seems virtually certain that the bone was the residue discarded after the consumption of meat.” Isaac (1978:235.)

Weaknesses: Much of Isaac’s evidence is based on early stone tool assemblages dated to at most around 2 million years ago. This is problematic in that the earliest evidence for the use of such tools was, even in 1978, predated by fossil evidence of bipedalism (e.g. AL 288-1, Johanson 1974) by at least 1.5 ma    
1.1 Survival Value Fair 6 Like most carrying models, this model provides strong arguments for selection. Carrying food and/or weapons clearly has potentially strong selective advantages.    
1.2 Sexual Selection Fair 5 This model does not propose any specific behaviour to promote sexual selection and so was judged 'fair'.    
1.3 Not Teleological Poor 3 Modern humans today can carry things easily, mainly because their anatomy has become adapted to an efficient kind of bipedalism. It is not clear that this behaviour would have been so easy and therefore advantageous in intermediate forms carrying food back to bases.    
2.1 Improved Food Acquisition Good 9 This models is very strong in promoting food acquisition as the key driver for the change in behaviour and so is rated highly.
2.2 Accounts for Predation Poor 2 Placing already vulnerable hominins in more open habitats burdened with food was judged poor by this criteria.    
2.3 Why Apes are not Bipedal Fair 5 Unlike most of the ‘carrying models’ this one does at least postulate a reasonable scenario (a difference in habitats) why extant apes were not also subject to the same pressure.    
2.4 Extant Analogues Poor 4 The essential factor Isaacs describes in getting the process of bipedalism started is basically carrying scavenged meat back to home bases. There is very little evidence of extant apes doing this, although clearly they have been observed carrying food items generally.    
2.5 Applies to Both Sexes Good 9 Both sexes were proposed to participate in the proposed types of carrying.    
3.1 Hominid Anomalies Fair 4 Isaacs does not address some of the early hominin anatomical anomalies any more than most carrying models.    
3.2 Fits Paleoecological Record Fair 6 The notion that early hominids lived in fairly wooded habitats, perhaps around gallery forest projections into open savannah grasslands, appears to fit Isaac’s model.
3.3 Precursor to Strider and knuckle Walker Poor 4 This model was judged rather poor, like most carrying models, in proposing a strong precursor for both human bipedalism and knuckle-walking.    
4.1 Extended Explanatory Power Fair 6    
4.2 Complimentary Good 7 This is model that is complementary to any that rely on carrying tools and/or weapons and the general concepts behind Issac's ideas are fairly compatible with most other models.    
4.3 Falsifiable or Testable Poor 0 Issacs makes no predictions that could falsify his hypothesis.    
References Isaac GI. The archaeological evidence for the activities of early African hominids In: Jolly CJ, Ed. Early Hominids of Africa London: Duckworth 1978.