Lovejoy (1981)
Classification: Forelimb pre-emption (carrying models)
Mnemonic: "Freeing of the hands"
Specific Model: Male Provisioning Model
Original Proponent(s): Lovejoy 1981    
Assessment: Popularity Ranking: 1st out of 9 categories (86% of texts) - 50% of texts mentioned this model specifically.
Simple Evaluation: #24 (=4) /42 (56%)
Detailed Evaluation: #17 (=3) /42 (56%)
Basic Summary: Complex model proposing that specifically male provisioning of food to females in monogamous pairs resulted in increased selective pressure for bipedalism through increased carrying. Based upon the assumption that for apes to compete with more r-selected old world monkeys, human ancestors would have become even more k-selected and that females would have maximised their reproductive efficiency by acuiring a novel resource (i.e. males) in order to achieve this.    
Discussion: Lovejoy’s thesis is based upon the observation that there seems to be a trend in human evolution towards ever greater ‘k selection’ (i.e. socio-sexual systems which maximise reproductive success by greater parental investment in the young and longer maturation times, inter-birth intervals, pregnancies and life expectancies) away from the typical primate pattern which is more ‘r selected’ (i.e. systems geared towards maximising reproductive success by reducing the parental investment, maturation times, inter-birth intervals and lengths of pregnancies.) Furthermore, he postulates that as relatively slow reproducing apes were being out-competed by faster reproducing old world monkeys in the Pliocene, that both inter-birth interval reduction and increased survivorship must have been achieved in successful hominid clades. (p344).

Lovejoy's balance
Life History Balance (After Lovejoy 1981:343)

His argument about bipedalism is that by gathering and collecting food items for their female partners, male hominids would be better able to improve their kin selection. Unlike Hewes and Isaacs, Lovejoy suggests that the food would comprise eggs, small amphibians and reptiles, nuts and fatty fruits as possible food sources rather than animal carcasses (pers. Comm. 2003.) Lovejoy asserts that in order for female hominids to maximise their reproductive potential, one of their few options open to them was to enlist a resource that few other primates use, but that is commonly used in other orders (e.g. canids and aves (p345)) – that is the utilisation of a male food provider. Following on from this is the logic that bipedal locomotion would maximise the transportation of such food items. The model assumes that sexual pairing was already prevalent at this time and Lovejoy is quite explicit, although speculative, in proposing that early bipedal hominids such as Australopithecus afarensis were basically monogamous. (p345)

However, the reliance of the provisioning model on monogamy is problematic as there is little evidence in the fossil record for it. Indeed the consensus view of A. afarensis has been that it exhibited a fairly large degree of sexual dimorphism indicating, if anything, a very different sexual social system (see, e.g. McHenry 1991). Analysis of sex ratio evidence suggests that australopithecine male-female numbers were unequal, another counter indicator of monogamy (Reno et al 2003). Carrying food as a model for early adopters of bipedalism has another general problem, however: It makes already terrestrially vulnerable hominids even more vulnerable to attack from predators. If walking alone through open woodland is dangerous, how much more dangerous would it be to do so with both arms loaded with food items?
 Strengths: The model is quite elegant in that it explains several strands of human evolution at the same time, namely bipedality, increased monogamy and altriciality. It also satisfyingly fits the evidence that an early radiation of apes, apparent in the Miocene, was suprceded by a nother radiation of old world monkeys.    
Weaknesses: The model heavily relies on the assumption that early hominins were mongamous for which there can be very little hard evidence.    
1.1 Survival Value 4 (Fair) Lovejoy does make a sophisticated case for selection on the basis of the resulting increased survivability from this shift in locomotion. In addition to the increased food procurement proposed by the division of labour resulting from monogamous pair bonding, he suggests that better mothering skills, e.g. resulting in fewer infant deaths from falling out of trees (p 343), as a consequence of safer carrying, would result in better reproductive success compared to old world monkeys, which were more ‘r selected’. Substituting known life history data from old world monkeys, apes and humans into the formula below, Lovejoy (1981:344:((4)) was able to demonstrate that with annual survivability figures of 98% or more, the typically human ‘k’ selection becomes advantageous over the typically ‘r selected’ old world monkey pattern in terms of reproductive success. Lovejoy's formula Lovejoy's formula Lovejoy's formula
However, Lovejoy appears to overlook a couple of key factors which would alter the results of this table significantly.

• The human menopause was not considered. Lovejoy’s equation assumed that females potentially could continue to give birth at the same inter-birth interval until the age of 60. If realistic figures for menopause were used instead, survivability levels even closer to 1.0 would be needed to give Homo the same advantage.

• The mothering ability was not offered as a factor in the 1981 paper. Lovejoy reports that even in apes the success of raising a first infant is much worse than later infants. The presence of elder females around, especially grandmothers who have passed into the menopause, would be likely to give humans an edge in this area over other species. Perhaps this was the main point Lovejoy should have been making.

Generally, even accepting the basic argument that a more ‘k selected’ species would be able to out-compete the old world monkeys, it is not clear that the provisioning model provides that selection, either through improved mothering (infant carrying) or through paternal provisioning. In terms of the survivability Lovejoy’s thesis relies upon, it seems unlikely that bipedal carrying of small infants is any safer, for a terrestrial primate, than quadrupedal clinging.
1.2 Sexual Selection 9 (Good) Although Lovejoy is perhaps controversial in suggesting a distinct division of labour (rather in the same way as dogs and birds) (p345) between guarding the infants and looking for food, he does at least specify an argument that would conducive to sexual selection of males by females.    
1.3 Not Teleological 2  (Poor) As Lovejoy's important assumption of monogamy has little evidence except in modern humans. The model was judged poor by this criterion.    
2.1 Improved Food Acquisition 7  (Good) Assuming that male provisioning would be more successful than the alternative (giving Lovejoy the benefit of the doubt) it was judged fairly well here.    
2.2 Accounts for Predation 2  (Poor) Lovejoy does not discuss the problem of predation very much, except to suggest that women and children would benefit from the safety of a home base. But he is not specific about this what kind of home base is being proposed. If it were arboreal then the risk of infants falling would be increased. As this is one of the risk factors Lovejoy cited as a potential driver to induce carrying behaviour in the mother, arboreal home bases would seem to run counter to his main thesis. If, on the other hand, the home bases were on the ground, it would appear to place both women and children at greater risk from predation. The resulting division of labour – with males foraging a greater range, whilst females stayed with infants in a smaller one seems to add to this problem. Females and infants appear more vulnerable to attack, and so do the male providers. As the model depends on an assumption of greater monogamy, any loss of males in the population would have a dramatic effect on the reproductive success of the proposed nuclear family unit.    
2.3 Why Apes are not Bipedal 7 (Good) Lovejoy’s key point is that in habitats that were less rich, male provisioning through greater ranges would allow them to improve their reproductive success through an even more ‘k’-like strategy than the apes. This is as good an attempt at explaining why humans, and not the great apes, became bipedal as any.    
2.4 Extant Analogues 2 (Poor) No ape analogues are proposed by Lovejoy, although he does cite examples of male provisioning from callitrichids (e.g. marmosets) (Lovejoy 1981:345).    
2.5 Applies to Both Sexes 3 (Poor) Lovejoy's is a rather assymetrical model of bipedal origins in terms of the roles of the two sexes. However, it must be assumed that whereas males were carrying provisions, females were carrying infants.
3.1 Hominid Anomalies 4 (Fair) Lovejoy does not go into any details about early hominin anatomical anomalies.    
3.2 Fits Paleoecological Record 6 (Fair) The provisioning hypothesis is generally based on the known radiations of apes in the Miocene and old world monkeys later.    
3.3 Precursor to Strider and knuckle Walker 4 (Fair) The Provisioning hypothesis does not provide a strong argument for a precursor to both human bipedalism and great ape knuckle-walking.    
4.1 Extended Explanatory Power 9 (Good) The main strength (and perhaps also a weakness) of Lovejoy’s model is that it does attempt to explain several major ape-human differences in one fell swoop. His 1981 paper starts by describing the five main characters that separate man from other hominoids: a large neocortex, reduced dentition, material culture, bipedality and unique sexual and reproductive behaviour. He goes on to suggest that there is little in the fossil evidence to suggest that the large brain and material culture were characteristics of early human evolution but, oddly as there is also very little evidence for it, suggests that the sexual and reproductive behaviour “may be the sin qua non of human origin” (Lovejoy 1981:341.)    
4.2 Complimentary 7 (Good) Lovejoy's ideas are compatible with most models of bipedalism and complementary to many, especially other carrying models. However, like other carrying models it was judged to be rather incompatible with climbing models.    
4.3 Falsifiable or Testable 5 (Fair) Lovejoy does not make a particularly robust scientific case for his hypothesis.    
References Lovejoy, C. Owen (1981). The Origin of Man. Science Vol:211 Pages:341-350
Lovejoy, C. Owen (1988). The Evolution of Human Walking. Scientific American Vol:259 Pages:118-125
Lovejoy, C. Owen (1993). Modelling human origins: Are we sexy because we're smart, or smart because we're sexy? In: The Origin and Evolution of Humans and Humanness. D. T. Rasmunsen, Ed. Jones & Bartlett (Boston)