Sinclair et al (1986)
Classification: Forelimb pre-emption (carrying models)
Mnemonic: "Freeing of the hands"
Specific Model: Migration-Carrying Hypothesis
Original Proponent(s): Sinclair et al 1986;    
Assessment: Popularity Ranking: 1st out of 9 broad categories (86% of texts)
Simple Evaluation: #36 /42 (48%)
Detailed Evaluation: #21 (=3) /42 (54%)
Basic Summary:

In a short letter to Nature, Sinclair et al (1986:307) suggested a new and rather ingenious-sounding idea on bipedal origins: That specifically long-distance on-foot scavenging, alongside existing herds of ungulate migrating species, was the key driver.


The idea is rather theoretical, being based on the assumption that there was an "unfulfilled niche" for any mammalian scavenger that would be able to combine long-distance locomotor efficiency with carrying. Like most carrying models, it is open to criticisms of being teleological and also increases the risk of predation.


They suggest several arguments in favour of this hypothesis.

·         From an ecological point of view they suggest that this was an “unfulfilled niche for a mammalian scavenger” and that any species that could combine long-distant efficiency with the ability to carry their young along with them would be at a distinct selective advantage. This niche would offer an abundant and constant supply of carcasses (“at least 1 carcass per 20 km 2 per day” p370) much more than non-migratory systems.

·         They support this hypothesis by citing two species of vulture, which follow the migrating ungulates, which are considerably more numerous than sedentary species.

·         They suggest that bipedalism was “a necessary adaptation” (p307) to exploit this food supply because it relied upon carrying of infants and efficient long distant walking. Mammal predators and scavengers, they suggest, do not follow migrations “because their young are slow growing and cannot travel with the adults.” (p307)

·         This hypothesis, it is argued, helps to explain the early adoption of stone tools by early Homo, as carcasses would require fairly rapid butchering and “avoid competition with other stronger mammal predators” (p308.)


However their paper appears to have a number of weaknesses:

·         Firstly, there is a distinct paucity in consideration of other ideas. They suggest (p307) that “the current explanation” was Isaac’s (1978). But it is unlikely that, since Darwin, there has never been one explanation that has won primacy, and certainly not in 1986. Isaac’s chapter in ‘Early Hominids of Africa’ (1978) was not the only view on bipedal origins published in that volume. Later, they suggest that “The Alternative hypothesis” is that “bipedal hominids were plant gatherers in a savannah home range” (p308). The use of this term is unfortunate, as they are not proposing their hypothesis against any null, but suggesting it is the only real alternative to their own. Their characterisation of Isaac’s idea as being about ‘plant carriers’ is also unfortunate, because his (much more thorough) paper made it clear that he also saw early hominids as scavengers of carcasses. Much of the evidence he gave in support of this was the use of stone tools and that even very small flakes chipped off stones could be very useful in dismembering carcasses. (e.g. Isaacs 1978:234)

·         Models relying on early meat-eating as drivers appear to be contradicted by evidence which suggests that the earliest bipeds were not actually meat eaters (see, for example Andrews 1981) and certainly not hunters.

·         It is difficult to imagine how butchering a carcass and carrying it alongside the migrating herd would “avoid” competition with big savannah predators, as they suggest (p307). The smell of exposed meat would no doubt attract a great deal of attention. Indeed their model has nothing to say at all about how the migrating hominids would avoid becoming prey themselves. Compared to the ungulates next to them, they’d be relatively slow and vulnerable, especially the mothers with infants under arm, and whose presence forms a major part of this hypothesis.

·         Furthermore, the authors’ claim that infant carrying would be the key differential to encourage bipedalism appears tenuous. Carrying infants is easier for a quadrupedal primate, as the infant can simply climb on the mother’s back. Adopting bipedalism for this reason would seem to be counter-intuitive.

·         Although humans are undoubtedly efficient long-distant walkers today – due mainly to a rather specialised anatomy – it is not clear if the earliest bipeds could have been much more efficient from the beginning – before those specialised traits had evolved.

1.1 Survival Value Good 6:  The general carrying models all provide very strong arguments for selection. Carrying things (infants, food, or weapons) clearly has potentially strong selective advantages.    
1.2 Sexual Selection Fair 6: The migration-carrying model provides slightly more scope for sexual selection as it provides greater scope for 'division of labour' between ths sexes.    
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.    
2.1 Improved Food Acquisition Good: 8: This was rated highly because the whole model is based on improved food acquisiton.    
2.2 Accounts for Predation Poor 0 : Although this specific model does suggest, like many using the premption motive, that early use of weaponry would have helped to overcome savannah predators, the model’s insistence on placing women and children there, alongside the herds of ungulates and alongside fellow tribesmen and women carrying lumps of fresh meat does more than negate that argument.    
2.3 Why Apes are not Bipedal Fair 7: This was judged better than the carrying defalt as it proposes a clear demarcation between te two lineages.    
2.4 Extant Analogues Fair 2: Conversely, great apes have not been associated with migrating herds and so the model was judged poorly by this criterion.    
2.5 Applies to Both Sexes Good 9: Although many specific sub-models in this category are quite explicit in suggesting sex-based roles for the kind of carrying being proposed. It is suggested that overall ‘carrying’ as a general concept works well for both sexes.    
3.1 Hominid Anomalies Fair 4: As far as can be seen in the literature, none of the carrying models offer this model as any kind of explanation for the rather peculiar differences between Australopithecus and Homo in lower limb anatomy.    
3.2 Fits Paleoecological Record Fair 7: Because of the inclusion of gallery forests into the model, this evaluation was better than the carrying average.    
3.3 Precursor to Strider and knuckle Walker Fair 5: The long distance aspect of this model gave it a slight edge compared to the carrying average.    
4.1 Extended Explanatory Power Fair 6: Many of the carrying models were originally proposed merely as part of a bigger model of early human evolution. Lovejoy’s “Provisioning Model” in particular (see below) is particularly comprehensive in attempting to explain several of the ape-human differences at one fell swoop.    
4.2 Complimentary Good 7: Many of the models, if not all, assume that carrying played some part in fixing the switch to bipedal locomotion to some degree and therefore this can be said to be one of the most compatible models known.    
4.3 Falsifiable or Testable Poor 0 : Few, if any, of the proponents of carrying models have attempted to make testable predictions.    
References Sinclair, A R E; Leakey, Mary D; Norton-Griffiths, M (1986). Migration and hominid bipedalism. Nature Vol:324 Pages:307-308