Cartmill (1983), Carrier (1984)
Classification: Feeding Models
Mnemonic: Hand to Mouth
Specific Model: Terrestrial Hunting through Endurance Running
Original Proponent(s): Cartmill (1983) Carrier (1984)    
Basic Summary: The only kind of food worth carrying long distances is meat therefore hunting meat must have played a key role in the adoption of bipedalism. Endurance running provides a means for bipedalism to be coupled with carrying to this end. Overlaps with several models, notably, Lieberman's ER Model.    
Assessment: Popularity: Feeding Models were ranked 2nd (out of 9) most popular in the texts reviewed. 8% of texts referred to this idea specifically.
Simple: #28 (=4) / 42 (49%)
Detailed: #30  (=4) / 42 (50%)
Discussion: In an interesting paper "Four Legs Good, two legs bad: Man's Place (if any) in Nature" published in 1983, Matt Cartmill reassessed the now largely  discredited "killer ape" hypothesis.

The paper began with an interesting speculation from Charles Darwin. Catrmill  noted about early human ancestors that "for such weak creatures, social or not, the tradition to human status would have been a dicey business, so Darwin suggested that they might have inhabited a large tropical island free of predators or competitors, "such as Australia, New Guinea, or Borneo, which is now the home of the orang." Cartmill plotted a quick history on this question of survival, noting that a weapon-wielding "killer ape" had been a popular idea, promoted by Raymond Dart, untill work by Vrba and Brain discredited the idea largely because most of the fossils Dart had attributed to the prey of australopithecines had almost certainly been that of big cats instead, including australopithecines themselves.

Cartmill revived the 'killer ape" idea, suggesting that abence of stone tools do not necessarily indicate an absence of meat eating. Cartmill claimed that "meat could have been important in their diet and they could have brought down and butchered game of various sorts in ways that didn't involve chipping pebbles" Cartmill (1983:77.) 

He outlined 5 major points of the 'classic' hunting hypothesis of th 1960s thus:

1) The australopithecines were slow running animals, living in open country full of large predators. They lacked natural defences and so (no matter what you thought of Dart's bone "tools") they must have depended on weapons of some sort. Only a weapon-dependent animal that needed its hands more for striking than for running could have evolved the australopithecines' two footed-gait. And their small canines showed that they had long since abandonned teeth for weapons in threatening and fighting predators and each other.

2) Whatever the resource had been that tempted the australopithecines' ancestors out of the trees onto the savanna, their weapons allowed them to exploit the flesh of other animals - at first small helpless game and scavenged carrion; then larger and larger game as they became more skillfull killers..

3) Hunting activities lead to greater intelligence, but due to birth restrictions, infant head had to grow mainly after birth - so the infant had to survive a long period with an embryonic, rapid growing, but poorly co-ordinated nervous system.

4) Bipedality made this possible because it freed the hands for carrying things. The helpless baby could be carried by its mother instead of having to cling to her fur. With males able to kill game and carry it to a safe place, the mother needn't have to foragefor herself with her hands full of offspring.

5) To ensure that males continued to help their female and brought home the bacon, the females became progressively sexually receptive. The result was the creation of the family, providing a stable social unit for their long, long childhood - which, added the Freudians, set the Oedipus complex in motion for the first time.

The importance of meat was emphasised by a point cited from Kim Hill. Cartmill reminded us that she "has pointed out, meat is the only common food that comes in sufficiently large and nutritious chunks to be worth carrying far narehanded. Without a big shopping bag, you can't transport enough nuts, carrots and birds' eggs to feed three people when you get home. (And if shopping bags were present from the beginning, bipedalism was unecessary, since you can carry a bag in your teeth and still run on all fours." (Catrmill 1983:77.)

David Carrier’s piece in Current Anthropology offered further arguments for terrestrial predation being the driver for hominid bipedalism. His argument was based on observations surrounding human locomotor efficiency. Noting that typical cursorial quadrupeds are at least twice as efficient (Oxygen consumed per unit mass per distance travelled) Carrier reminded us that another parameter of locomotion, in addition to speed and efficiency, is endurance. And it is in this area that humans, appear to have rather an edge over other types of mammals. Citing mainly anecdotal evidence of hunting behaviours of indigenous people from four different continents, a good case was made that humans are actually able to out-run many species specifically by exhibiting greater stamina, in the long run.

From here, Carrier suggests that this ability was due to several anatomical and physiological traits in the hominid line that must have evolved, and could have been adaptive for this very life style. Carrier suggests that our near naked bodies have allowed us to sweat independently of breathing rhythms, unlike the majority of mammals which use panting as their main method of dissipating body heat, which is tightly synchronised with, and hence limited by, the breathing cycle. This has allowed us, rather uniquely in mammals, to avoid over heating whilst running long distances, particularly in high temperatures. Furthermore, being bipedal, these respiratory cycles have also been ‘de-coupled’ from rigid gait patterns inherent in quadrupeds. Namely, we have the ability to breath more rapidly, out of synchrony to the strides we are taking whilst running, whereas quadrupeds are forced to take breaths only during hind limb propulsion phases.

Carrier also suggests that humans have evolved more efficient ways of storing carbohydrate energy locally in muscles for long distance running. From here, the proposal is that these traits would allow humans, almost uniquely, to exploit a diurnal (and specifically during a midday time zone) hunting niche which allowed them to run down prey that, although being much faster over short distances, were susceptible to overheating during prolonged periods of exertion. Even big cats on the savannah could not compete with humans in this regard, it is claimed, and it is argued that this endurance running niche would explain not only the evolution of the striding bipedalism of modern humans but also the evolution of our loss of body hair too.

Current Anthropology often publish papers along with comments from other authors giving their feedback on the paper. The main author is then invited to have the last word in reply to this feedback. Although most of the comments were supportive a number of interesting counter-points were raised. For example, Scott (1978:490) cited Newman (1970) in suggesting that sweat cooling required a great deal of drinking water to replenish that lost and noted that “man can sweat like a horse but not drink like one” suggesting that sweat cooling may have been used on the savannah once cultural responses to carrying water around had been invented but was unlikely to have evolved there. Others (e.g. So 1978:491) pointed out that this ability for endurance running was likely to have followed the evolution of bipedalism and not preceded it, suggesting that it really should not, therefore, be used as an argument for bipedal origins.
 Strengths: Carrying and gathering models appear strongest when high energy foods such as meat are postulated as the driving force. Coupling this with human propensity to endurance running is potentially quite satisfying. It also explains loss of human functional body hair.
Weaknesses: Placing relatively small, weak hominins out on the open plains chasing prey over long distances apperas to put them at great risk of predation themselves. Endurance Running is clearly a pst hoc modern human phenomenon. It is not so clear that it could argued to have driven that bipedality in the first place.    
1.1 Survival Value 4 (Fair) Assuming that ER running was as advantageous as the author ascribes, this would suggest quite strong selection for traits allowing long distance running. However, it is not clear that this selection would also translate into obligate terrestrial bipedalism – i.e. walking    
1.2 Sexual Selection 6 (Fair) This model was judged slightly higher than neutral because "bringing back the bacon" (as Cartmill put it) has been seen by many as a key aspect of sexual selection.    
1.3 Not Teleological 1 (Poor) This is, perhaps, where the model is at its weakest. Carrier’s main evidence is from modern human hunter-gatherers. It is falling into the teleological trap to suggest that this ability, relying on traits that are clearly now fully evolved, could have actually driven the evolution of those traits. Carrier assumes that “early australopithecines were fully bipedal” Carrier (1978:489) but even this is some distance from safely assuming that they too were glabrous and had an efficient striding gait whilst running    
2.1 Improved Food Acquisition 6 (Fair) Assuming that endurance running was a succesful means of food procurement, and there is plenty reason to doubt it would have been, this model is judged better than neutral on this criterion.    
2.2 Accounts for Predation 2 (Poor) Carrier’s model places human out on the savannah running, relatively slowly, after prey which are getting progressively heat stressed and tired. There is very little discussion about how this vulnerability to large predators would have been dealt with other than some suggestions that being during the midday period most big cats would probably not be looking for food and the notion that increased intelligence and sociability would, somehow, have been able to cope with the threat.
2.3 Why Apes are not Bipedal 5 (Fair) If one assumes, as do many of these models, that the main causal event in the Pan-Homo divergence was a switch in habitat, with hominids moving into more arid, open habitats and other great apes staying in more densely forests ones, then this model holds quite well. However much of the evidence of the paleohabitats of the early hominins seems to contradict this view.    
2.4 Extant Analogues 1 (Poor) 6) Is observed in extant Pan/Gorilla (or at least some other large primates.) ‘Poor’. Very few mammals, and none among the primates, are known to use ER as a method for hunting. Carrier suggests that some of the sweat cooling features of humans appear to be inherited from Old World monkeys but fails to demonstrate how this is related to his proposed model. Much more likely is that cholinergic sweat glands evolved in OWM for some other, quite different, reason.    
2.5 Applies to Both Sexes 3 (Poor) Carrier, in his reply to comments, suggests that ER hunting was just as applicable to females as to males, as this is the case for other mammals, notably African Hunting Dogs. However this might be seen as somewhat convenient as the modern human hunter gather primary evidence on which his model is based clearly shows that only males regularly participate in such ER hunting methods.    
3.1 Hominid Anomalies 4 (Fair) Although both Catrmill and Carrier base their ideas on the need to explain how a small, relatively weak hominin such as an australopithecine might survive on the open plains, they dot not mention the peculiar postcranial anatomy of australopithecines and provide no explanation for them.    
3.2 Fits Paleoecological Record 5 (Fair) Again, the general consensus that the African climate did become more arid and open is not really in dispute and so the Cartmill/Carrier model does fit in quite well with that general view. However, as we are specifically considering the early adoption of bipedalism here, as opposed to its later optimisation, it must be remembered that early bipeds appear to have lived in generally wet and wooded habitats.    
3.3 Precursor to Strider and knuckle Walker 7 (Good) Running clearly overlaps significantly with human-like bipedalism so this model was judged good here.    
4.1 Extended Explanatory Power 7 (Good) In that his model is very much tied to explaining the relative hairlessness of humans compared to other primates, the model does go beyond bipedal origins but not much further than that.    
4.2 Complimentary 7 (Good) This model was judged complimentary to most carrying and behavioural models and any that worked in open habitats. It was judged compatible with the others.    
4.3 Falsifiable or Testable 6 (Fair) Carrier does propose three tests which would serve as some kind of validation for his hypothesis. Unfortunately one of them – that australopithecines were also glabrous – is unlikely to ever be shown one way or the other.    
References Carrier, D R (1984). The energetic paradox of human running and hominid evolution. Current Anthropology Vol:25 Pages:483-494
Cartmill, M (1983). Four Legs Good, two legs bad: Man's Place (if any) in Nature. Natural History Vol:92(11) Pages:64-79