3.1.1: SEED EATING HYPOTHESIS
Classification: Feeding Models
Mnemonic: Hand to Mouth
|Specific Model:||Seed Eating Hypothesis|
|Original Proponent(s):||Jolly 1970|
Popularity: Feeding Models were ranked 2nd (out of 9) most popular in the texts
reviewed. 31% referred to this idea specifically.
Simple: #32 (=2) / 42 (48%)
Detailed: #30 (=3) / 42 (50%)
|Basic Summary:||Foraging and eating seeds from savannah grasses led to bipedalism|
The Seed-Eaters: A New Model of Hominoid Differentiation Based on a Baboon Analogy (Jolly 1970)
One of the most cited papers in paleoanthropology is Clifford Jolly’s ‘Seed-eating hypothesis’. Jolly began his thesis by attacking those models which, in his view, did not give satisfactory causal factors for the origin of hominid bipedalism and only offered arguments which would reinforce the behaviour once started. These ‘feedback models,’ as he referred to them, promote scenarios that both result in and result from more bipedalism. For example, carrying models suggest scenarios that would both encourage more bipedalism and result from more bipedalism. He criticises the circularity of these models which, he claims, renders them unable to explain their own beginnings: “In fact the more closely the elements of the hominid complex are shown to interlock, the more difficult it becomes to say what was responsible for setting the feedback spiral in motion, and for accumulating the elements in the cycle in the first place.” Jolly (1970:5)
Specifically, Jolly critiques two main groups of models of hominin divergence, especially those broadly grouped under ‘hunting’ or ‘tool/weapon use’ and then the threat posture model proposed by Livingston (1962) and Wescott (1967). Many of his criticisms surround the question of whether dental reduction would logically follow from increased tool use and the contradictions in the evidence used to support that view. This extract is typical:
If the theory of artefactual determinism is to be applied consistently, regular tool and weapon-making has to be extended back into the Miocene, and also attributed to Hominoidea other than the direct ancestor of the Hominidae, whether one considers this to be Ramapithecus, Oreopithecus, or neither. Simons (1965) regards Ramapithecus is as too early to be a tool-maker, but he and Pilbeam (1965) suggest that it was a regular tool-user, like the savannah chimpanzee' (Goodhall-I964; Kortlandt 1967.) This is eminently likely, but is no explanation for anterior dental reduction since the chimpanzee has relatively the largest canines and incisors of any pongid, much larger than those of the gorilla, which has never been observed to use artefacts in the Wild. To explain hominid dental reduction on these grounds, therefore we presumably have to postulate that the basal hominids were much more dependent upon artefacts than the chimpanzee, without any obvious explanation of why this should be so. One would also expect signs of regular tool making to appear in the fossil record at least as early as the first signs of dental reduction, rather than twelve million years later. The more artefactually sophisticated the wild chimpanzee is shown to be, of course, the weaker the logic of the tool/weapon determinant theory becomes, rather than the other way about, as its proponents seem to feel. Jolly (1970:7)
Jolly’s earlier criticism of ‘feedback models’ is also invoked to counter this kind of model, noting that “… the more proficient a hunter the non-bipedal, large-canined, large-incisored chimpanzee is found to be, the less plausible it becomes to attribute the origin of converse hominid traits to hunting.” Jolly (1970:8)
On the threat display idea, Jolly makes two objections. The first one “… that it is ilIogical to invoke the behaviour of living apes to explain the origin of something that they themselves have not developed; if upright display leads to habitual bipedalism, why are gorillas still walking on their knuckles?” Jolly (1970:9), could be countered by an argument suggesting that the ancestors of the living apes have just not performed this behaviour sufficiently to have caused them to follow the same pathway as our ancestors.
The second “… if hominid bipedalism were initially used solely in display, why should they have take to standing erect between episodes? Even if we grant that the savannah is more predator-ridden than the forest (a view often stated but seldom substantiated, even for the recent, let alone the Tertiary), it is difficult to believe that attacks were so frequent as to make defensive display a way of life” Jolly (1970:9), appears to be more difficult to counter.
Jolly’s point about ‘feedback models’ is apt, and it is easy to see how some benefits of bipedalism could get confused with potential causal factors. It is a type of thinking that has been labelled Lamarckism, where beneficial behaviour (if practiced enough in one generation) is proposed to be passed onto the future generations through unspecified means. Darwinism, on the other hand, requires that this behaviour be translated into the currency of benefit in terms of selection of the genes responsible for whatever traits favour that behaviour. It would seem however that such beneficial behaviours are likely to incur greater selective fitness upon the individual, thus serving the genes which made them possible and therefore rendering them compatible with Darwinism after all. This would make Jolly’s criticism of them less valid. Indeed one of the criteria proposed for an ideal bipedal model proposed here is that it should confer benefit throughout its evolution and not just as an ‘end result’. This usually invokes a high degree of positive feedback in the model.
Having criticised Jolly’s initial objections of some of the common aspects of other models, we may now turn to his own ideas specifically. The evidential basis of them is a systematic comparison between two species of baboon; one more adapted to open habitats than the other , and how the differences are mirrored in comparisons between hominids and chimpanzees. In a nutshell, his basic argument is that Theropithecus are more adapted to open habitats and ‘seed eating’ than Papio. Before looking at his findings in detail it is worth pointing out that there is already a difficulty here because Jolly does not make it clear as to which species of Papio he is referring to. He merely writes “Table 1 summarises characters by which either early Pleistocene Hominidae differ from Pan, or Theropithecus from Papio and Mandrillus, listed without regard to their function interrelationships or significance, Jolly (1970:9.) Indeed, later in the paper he uses one such Papio species (P. hamadryas) along with Theropithecus as examples of savannah-woodland species.
According to Rowe (1996) there are eight species of baboons and their habitats and diets are described as in the table below.
It should be noted that, according to these reports, the diets of all of these species include seeds (although these reports suggest that geladas rely on them more) and so it is reasonable that Jolly’s premise should be at least questioned.
It is likely (Oxnard, personal comment) that Jolly was referring to P. anubis in his list but even if that is assumed, there would not appear to be as large a distinction in habitat between the species as Jolly implies.
Assuming his premise is correct, however, there are other criticisms that can be made of his approach. Below is a subsection of his table which listed characters where hominids and Pan differed (A) compared to where Theropithecus and Papio differed. Here only behavioural and postcranial structures are shown as they are the ones most relevant to this study.
A) refers to traits distinguishing hominins from Pan and other great apes. B), characters distinguishing from Papio and Mandrillus.(C) features of the hominid complex not seen in Theropithecus. D) Features of Thereopithecus not seen in Hominidae.
One criticism that can be made is that the list of characters chosen for comparison is apparently arbitrary. Jolly finds 22 out of 48 character sets which show parallelisms but one is tempted to ask how many character sets could have been listed that would have shown far fewer.
He suggests that “This hypothesis can be tested by checking the elements of the complexes for cross-occurrence in Papio & Pan. If the high number of common characters were simply due to chance, rather than to parallelism, we should not expect significantly fewer of the Hominid characters to appear in Papio (as opposed to Theropithecus), or significantly fewer of the Theropithecus complex characters to occur in Pan.” Jolly (1970:12)
But this argument appears flawed because the list of characters selected for each comparison (hominid with Theropithecus or Papio with Pan) is still itself not a random sample. On top of this, some of the parallels Jolly reports can also be questioned. For example, hominids, like Theropithecus, are reported as having moved to more open habitats. On this, Jolly writes: “Of these, only one certain one appears in the behaviour category, largely because of the impossibility of observing the behaviour of fossil forms. Inferences of behaviour from structure are, of course, not permissible at this stage of analysis. The single common character is the basic one of true open-country habitat, inferred largely from the death-assemblages in which early Theropithecus and Hominidae are found, as well as the habitat of . Jolly (1970:12).
This is far from certain, however. Hominids are not proved as having lived in open habitats merely because their fossil sites share faunal assemblages that include species adapted to such niches, any more than they are indicated as being semi-aquatic merely because crocodilians and hippotami are present. There is also a body of evidence which suggests that early hominid actually lived in relatively wet and wooded, as opposed to arid and open, habitats (See, for example WoldeGabriel et al 2001 and Trauth et al 2005.)
There do appear, nevertheless, to be a few interesting parallels which give his model some weight. It is beyond the scope of this study to consider cranial features in too much detail but a major aspect of Jolly’s argument does follow from some parallels in this area. Canine reduction along with molar increase is a phenomenon of hominid-ape divergence in need of an explanation and Jolly may well have stumbled across an interesting parallel in baboons. If early hominids did begin to procure a greater percentage of their food from seeds, it is logical that dental reorganisation would proceed along the lines Jolly suggests. And, if seed eating became a major part of the diet is further likely that a large primate would procure much of this whilst sitting down. As Jolly puts it: “thus, truncal erectness is more habitual than in any non-bipedal catarrhine, and the mastoid process becomes explicable.” Jolly (1970:13). Whilst this is hardly bipedalism it does, at least, place them in a more upright posture, in a terrestrial setting, more often. This is an idea favoured, as we shall see, by another author, John Kingdon.
The rest of Jolly’s paper outlines a two phase model of hominid-ape divergence which suggests that a seed-eating first phase came before a later, meat-eating, phase. It is proposed that the seed-eating phase resulted in a kind of stable evolutionary platform that existed for several million years, characterised by some early form of bipedalism. It is claimed that this accounts for several evidential aspects of the fossil record, again mainly concerning the large disparity between the appearance of evidence for dental reduction and the appearance of the use of stone tools in the fossil record.
Jolly makes a strong case that something other than meat-eating through increased tool use probably accounts for dental reduction in early hominids, and possibly a number of other ape-hominid differences (e.g. sexual differences in hair covering). However, when it comes to an explanation for a shift towards bipedalism, there is a distinct paucity of specific arguments for why putative ancestors that moved into grassy habitats generally, or a seed eating lifestyle specifically, would have switched to this form of locomotion. Indeed makes several points himself which increase scepticism about this. For example: “Most of the postcranial elements of the hominid complex are absent in Theropithecus, being related to upright bipedalism (Clark 1964)” Jolly (1970:13.) And: “In Theropithecus, this behavioural trait (and its associated adaptive features) are superimposed upon a thorough ongoing, cercopithecoid quadrupedalism, producing a locomotor repertoire in which the animal abandons' bipedal' bottom-shuffling for quadrupedal locomotion when it moves fast, or for more than a few paces” Jolly (1970:18-19.)
However, undeterred, Jolly manages to convince himself that despite this, it was the increased truncal erectness from “sitting while foraging (hands free)” (Jolly 1970:20) seeds in open habitats that was the essential missing ingredient to bipedal origins. “This combination of heritage and adaptation may have been the elusive determinant of terrestrial bipedalism, a gait that is inherently 'unlikely', and which would thus have begun as a gelada-like shuffle. Locomotion of any kind is infrequent during gelada-like foraging, so that (unlike hunting!) it is an ideal apprenticeship for an adapting biped” (Jolly 1970:19.) So, when it comes to postulating a model to explain bipedal origins, the closest Jolly comes is to provide a scenario for more truncal erectness. It might be suggested that, as it is assumed that hominids evolved from large climbing apes, we already had that.
Jolly’s own arguments against other models can easily be directed against his. Of the tool-weapon holding idea, for example, Jolly suggests: “The objection to this notion is again that is iIIogical to invoke the behaviour of living apes to explain the origin of something that they themselves have not developed; 'if upright display leads to habitual bipedalism, why are gorillas still walking on their knuckles?” Jolly (1970:9) One might ask if seed-eating lead to bipedalism, why are gelada baboons amongst the most committed quadrupeds?
One interesting point Jolly makes, as evidence for a grassy habitat being a key factor, is that many early hominid habitats are associated with edaphic grasslands, prone to flooding. This view seems to have been strengthened by subsequent fossil discoveries which increasingly appear to place early hominids in gallery forest habitats which are both close to grasslands and yet relatively wet and wooded at the same time. It was the shift in climate, Jolly points out, towards a generally arid but seasonally wet zone east of the rift valley that specifically promoted the savannah grassland habitat as a climax ecosystem. This is an aspect we shall return to in other models.
Szalay (1975) “Hunting-Scavenging Protohominids: A Model for Hominid Origins”
Szalay’s paper is largely a critique of Jolly’s (1970) ‘seed-eating hypothesis, based largely on contesting the assumption that the enlarged cheek dentition, as seen in robust australopithecines, was not the ancestral form but the derived form. Szalay suggests that the lack of sexual dimorphism but general increase in size of the canine dentition of early hominids suggests a functional cause and that cause, he postulates, was likely to have been increased meat-eating generally and, as canine reduction is also a prominent feature, specifically scraping meat from dead carcasses (Szalay 1975:428.)
|Strengths:||Jolly's seed eating hypothesis is strongest in that it offers a theoretical mode of selection that might have caused divergence in the great ape lineage and sets out to explain cranio-dental changes and some others. If the human lineage acted in the way he postulates, and if the other great apes did not, it might explain why we alone became bipedal. It also offers a plausible way of increased food procurement.|
|Weaknesses:||However, even these strengths must be questioned as they are very firmly reliant on an "open plains" model for human evolution that has increasingly become discredited and does not meet the current concensus about the paleoecological record. Putting this major objection to one side, the model is already very weak on predator avoidance.|
|1.1 Survival Value||3 (Poor) Jolly makes no specific recommendation as to how eating seeds in more open habitats might favour bipedalism|
|1.2 Sexual Selection||5 (Fair) This model was judged neutral on this criterion.|
|1.3 Not Teleological||3 (Poor) Jolly derides such ‘feedback’ models as being unable to explain their own origins but by avoiding this aspect himself, he makes his model vulnerable to criticisms which see this kind of feedback loop as a requirement|
|2.1 Improved Food Acquisition||7 (Good) As with all food gathering models, Jolly's seed eating model does provide a scenario for increased food procurement.|
|2.2 Accounts for Predation||2 (Poor) Moving out to more open habitats, shared presumably by bovids and other grass-eating fauna would make early hominids more prone to predation. Jolly does not provide any good arguments as to why this would suceed.|
|2.3 Why Apes are not Bipedal||7 (Good) In the sense that Pan/Gorilla are proposed not to have moved into a more open, seed-eating niche, this aspect is covered by Jolly’s model. However, the criticism might instead be applied: Why are not gelada baboons bipedal?|
|2.4 Extant Analogues||5 (Fair) Jolly offers no evidence of any analogues in extant apes, but his whole model is based on proposed parallels seen in differences between species of extant primates (baboons) and those between apes and hominids.|
|2.5 Applies to Both Sexes||9 (Good) Jolly’s seed eating hypothesis applies equally to both sexes.|
|3.1 Hominid Anomalies||6 (Fair) Jolly’s model does not consider the detailed postcranial aspects of Australopithecus afarensis, as it predates its discovery. However his most detailed anatomical analysis of australopithecines considers their cranial and dental anatomy and this he does very thoroughly|
|3.2 Fits Paleoecological Record||2 (Poor) Evidence since jolly's paper has increasingly suggested that habitats were generally more wet and wooded than could have plausibly supported his model.|
|3.3 Precursor to Strider and knuckle Walker||5 (Fair) As with other postural feeding models, the ‘seed eating hypothesis’ does include behavioural elements which overlap with human bipedalism.|
|4.1 Extended Explanatory Power||7 (Good) Perhaps the main strength of Jolly’s paper is in offering a good explanation for the dental and other cranial features of early hominids. This is, in my opinion, where his baboon analogies are most convincing. He also suggests that his two-phase model explains several other aspects of early hominin evolution|
|4.2 Complimentary||5 (Fair) Jolly's seed eating model is complimentary to most carrying and food gathering models but is contradictory to those based on arboreal settings.|
|4.3 Falsifiable or Testable||4 (Fair) One of Jolly’s complaints about contemporaneous models at the time of his writing was that they few made any testable predictions. His work, on the other hand, may at least be tested against further fossil discoveries.|
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