Primates As Comparative Models For Human Evolution

For obvious reasons, we are not able to examine living examples of our hominid and hominin ancestors. Extrapolation from the fossil record is also limited due to the rarity of fossilization and the difficulty of accurate inference. The study of extant primates therefore offers us priceless comparative models, which we can use to gain insights into tool use, behavioural ecology, phylogenetic relationships, diet, hunting, dispersal and cognition; all crucial aspects of human evolution.

The level of technology expressed by chimpanzees is roughly analogous to the australopithecine/ homo habilis Oldowan industry. It must be noted that the Oldowan was more sophisticated in the flaking of stones to produce sharp edges (Wynn et.al 2011). Whilst evidence of chimpanzees using modified lithics is scant, there are numerous examples of their proficiency with designing wooden tools. Sanz et al. (2004) document chimpanzees in the Congo Basin using stout sticks to penetrate subterranean termite nests, and then inserting a fishing 'probe' created by fraying the tips of sticks to create a brush-like tool for maximizing termite-catching efficiency. Oldowan hominins were known to have used and discarded tools in situ as the need for them arose, similar to the observed chimpanzees; and further, were very likely to have used wooden tools as well as lithics. One of the primary hypotheses for the absence of stone hand-axes in H.erectus assemblages east of the Movius line (China, South-East Asia) is a reliance on widely-dispersed bamboo forests as the basic resource for creating tools (Lycett & Bae 2010). The poor preservatory qualities of tropical and equatorial climates and the rapid degradation of 'soft' organic material mean that these tools could not have survived to the present day. The examination of primate technology is thus an invaluable and unique window into the tool-making abilities of our ancestors; in turn a corollary of cognitive evolution. The use of tools is a central derivative in theories of social development; van Schaik et al. (1999) assert that "strong mutual tolerance was the key factor in the explosive increase in technology among hominids", following from extensive study of orang-utan and chimpanzee tool use. The paper argues that social tolerance is a prerequisite of innovating and transmitting complex ideas, and casts light onto the links between brain size, group size and cognitive evolution expounded in the Social Brain hypothesis. Namely, that an increase in group size preceded the increase in brain size, and that the increase in group size was itself due to ecological pressures, perhaps increased predation risks associated with the shift from the semi-arboreal to savannah environment (Dunbar 2003). The specific means by which the cognitive demands of structuring and maintaining large social configurations are explored by Dunbar with regards to the evolution of uniquely 'human' phenomena, such as language. He proposes that language allows for an incredibly efficient means of bonding due to its multilateral deployment, conductivity to multi-tasking, and ability to convey information that occurred in absentia; but crucially notes that standard grooming techniques would have been more than sufficient to maintain social cohesion in early hominid societies (as demonstrated in Swedell and Plummer's study of hamadryas baboon social organization (2012)). Dunbar maps neocortex ratio to mean group size in extant primate taxa (neocortex-body size ratio being a primary indicator of cognitive ability and group size), uses this regression equation to predict group size for our hominid ancestors (neocortex size here being estimated as a proportion of the cranial capacity), and in turn uses group size to predict grooming time amongst these ancestors (again using extant primate taxa (in this case catarrhines) as a model). The conclusion drawn is that language would have had to evolve at grooming time ?30% (Dunbar 2003: 175), and that by tracking back through the mapping of group size - Homo erectus would have lacked language, which would have fully developed with the appearance of Homo sapiens (with Homo neanderthalis possibly possessing a rudimentary form of non-grammatical language). The statistical analysis demonstrates the usefulness of extant primates as the comparative basis for complex models exploring the social/cognitive evolution of the hominid lineage.

Dietary reconstructions of extinct hominids can be refined through observation of extant primate species. Wood & Schroer (2012) describe the main methods used for analysing fossil evidence with regards to diet; the functional morphology of the cranium, mandible and teeth; 'finite element analysis' (to determine the level of stress that could have been endured); and macrowear/microwear, dental topographic and stable isotope analyses of the teeth. These analyses are then compared to living primates in order to determine what kinds of food the extinct hominin was eating on a regular basis. Using extant primates as comparative models is a key stage of this process - otherwise, one is liable to draw inaccurate conclusions. For example, classic macrowear/microwear analyses of Paranthropus boisei teeth have tended to lead to the conclusion that it was a 'nut specialist'. However, dental comparison between Lophocebus albigena (grey-cheeked mangabey) and Cercocebus atys (sooty mangabey) demonstrate similar levels of wear, despite the fact that the former only consumes hard foods in periods of 'resource scarcity' (Lambert 2010) and the latter consumes hard foods as a regular part of its diet. The 'overprinting' of microwear surfaces by macrowear caused by hard foods can cause the drastic over-estimation of hard food consumption; what Teaford & Oyen (1989) refer to as the "last supper effect". Thus, a closer examination of P. boisei masticatory anatomy and dentition, leading to Wood & Schroer's revised conclusion that the species was a "consumer of grass, sedges, or tubers that supplements its preferred diet with nuts or other hard-objects". Our inferences of extinct hominin diet are to a large extent based on using our data of extant species as a reference frame. In turn, this has important ramifications - major dietary changes and divergences acting as "key milestones" in the study of our evolutionary history (Ungar & Sponheimer 2011).

The evolution of meat-eating and hunting strategies amongst our ancestors can also be modelled with reference to extant primate species. Extensive fieldwork amongst chimpanzees has revealed a widespread hunting culture; at Gombe, Tanzania (Teleki 1973, Goodall 1986), Mahale, Tanzania (Uehara et al. 1992) and Tai, Ivory Coast (Boesch & Boesch 1989). At Gombe, chimpanzees may hunt, kill and eat more than 150 small-to-medium-sized animals per annum, with their most common prey being the red colobus monkey (comprising 80% of the total kills). A few other points are of note - hunting is almost exclusively an adult/adolescent male pursuit, is conducted as a social activity in large co-operative groups, and is seasonal, with 40% of kills occurring in the harsher August/September dry period (Wrangham 1990). These chimpanzees, whilst inhabiting a semi-arboreal environment quite different from the African savannah roamed by H. erectus, can still be used as a model for answering the question - "why did our hominid ancestors begin to hunt?" Most theories focus on the high nutritional value of meat and the efficiency of this to a diminished digestive tract and rapidly expanding "[energy] expensive brain tissue" (Aiello & Wheeler 1995). Amongst chimpanzees, whilst the nutritional argument is certainly important, an additional social angle has been brought to light by more detailed studies. Kortlandt (1972) proposes that hunting by chimpanzees could be a form of social display, by which males display their prowess to the wider community at Gombe. Nisihida et al. (1991) further document the alpha male of the Mahale group taking control of the carcasses at the end of a hunt, and withholding/apportioning it to other chimpanzees according to their status as rivals/allies; thereby using hunting as a means of 'political' control. These are insights that shed light onto the group mechanics of hunting and meat-eating amongst early hominids that we simply cannot glean from the fossil record of the Homo lineage. The social aspect of hunting and meat-eating; the co-operation required for the hunt, the status and bonding amongst males enabled by the activity; the hunting 'binge' in the drier months; and the power-plays that ensue in the dividing of the spoils contribute significantly to our understanding of cognitive and group implications of diet in the context of human evolution. It is important to note that extant primate behaviour is a model and not an exact reconstruction, but it is nevertheless invaluable in exploring our hominid ancestry.

Genetic studies of human and extant non-human primates are also key to our understanding of human evolution. For example, the controversial Homo/ Pan/ Gorilla phylogeny can be resolved to some extent by resorting to genetics. Anatomical/ morphological studies suggest that Pan and Gorilla are more closely related to each other than to Homo, but more recent DNA studies seem to uphold the view that Pan and Homo share the most recent common ancestry. Rogers (1993) posits that this inconsistency can be explained by the hypothesis that the LCA of the "three lineages was polymorphic at a number of loci" and therefore the "diversification was effectively a trichotomy". The model is based on the study of living primates and the observation of significant DNA polymorphism in these species, e.g. Ferris et al.'s (1981) analysis of mtDNA nucleotides in chimpanzees (nucleotide divergence of up to 2%), orang-utans (Sumatran/Bornean conspecific divergence at an average of 5%) and gorillas (as high as 9%). This challenges the standard notion that there were two very separate and definitive diversification events. This remains one of many hypotheses, but it is clear that genetic evidence has much to offer for the reconstruction of our phylogenetic heritage and in the search to map the precise details of human evolution.

MtDNA possesses several qualities which makes it immensely useful in the study of human evolution. The rate of substitution in mitochondrial DNA is, at a rough approximation, five to ten times faster than the average rate of substitution in the nuclear genome (Brown et al. 1979). One region in particular, the D (displacement) loop, and specifically the HV1 (base pairs 16024-16365) and HV2 (base pairs 73-340), comprising 610bp in total, are deemed to be 'hypervariable', with mutations occurring at a much faster than normal rate (Repiská et al. 2010). Furthermore, because mitochondria in the male gamete are located in the tail of the sperm and do not enter the egg during fertilization, mtDNA is nearly exclusively inherited from the matrilineal lineage. These two traits were used by Cann et al. (1987) as the theoretical underpinning for a restriction-mapping analysis of 147 human mtDNA samples to demonstrate that all human beings (alive today) are descended from Mitochondrial Eve; "one woman who is postulated to have lived about 200,000 years ago, probably in Africa." Prior to this, the generally accepted model of human evolution was the 'multiregional evolution hypothesis' - that all hominids from 2.5mya onwards comprised a single species, and had evolved continuously and globally into modern H. sapiens from regional populations. This view was dealt a severe blow by "Mitochondrial Eve"; which, by demonstrating a recent maternal common ancestor in Africa, gave implicit support to the 'Out of Africa' hypothesis; that all modern humans are descended from an African population that migrated across the world over the last 200,000 years, displacing previous hominids (Meredith 2011). The picture of human evolution is further fleshed out by recent genetic analyses of modern humans and ancient human populations, such as the Denisovans and Neanderthals. 4-6% of the 'Melanesian' genome (extrapolated from Papuan and Bougainville Islander samples) is derived from the Denisovan population (Reich et al. 2010) and 4% of non-African human DNA is Neanderthal in origin. 'Out of Africa' remains the most plausible theory, but a large degree of admixture with local populations also seems to have occurred during the last 200,000 years.

There are many facets of primatology related to human evolution, that I have not covered in detail above; for example, the use of hamadryas baboons as a model of early hominid multilevel sociality. Nevertheless, it is clearly demonstrable that through genetic and behavioural studies, extant primate species provide a wide range of invaluable insights into the how, what, when, where and why of our evolutionary history.

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