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.

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.