Fig. 14.2 Temporal distribution of the hominid taxa, showing general adaptive patterns in terms of the categories used in the text to describe the different evolutionary strategies of the hominids.
infer that the last common ancestor lived in relatively large communities, were male kin bonded, with female dispersal, and hostile to other communities (Wrangham, 1987; Foley, 1987, 1989).
The shift to a hominid and australopithecine grade is initially associated with greater levels of bipedalism. Bipedalism is energetically efficient as a means of moving around the ground, and is likely to be associated with greater terrestriality. Time budget-based models have suggested that where approximately 60% of the feeding and travelling occurs on the ground, the advantages of bipedalism will exceed the costs (Foley, 1992; Foley and Elton, 1998). This implies that bipedalism is likely to evolve under conditions in which resources are either in trees that are far apart, or else in which the food can be reached on the ground. Bipedalism may, therefore, evolve in relatively wooded environments, although not in forests, and in particular where animals are forced into ranging more widely. The foraging ecology of the earliest hominids would thus be one in which the key determinant is extended day and home ranges, and this in turn is likely to alter other energetic parameters such as thermoregulation (Wheeler, 1985)
The primary trend among these bipedal apes over the period 5.0 to 1.5 Myr is that of megadonty, associated with some increase in body size, cranial robusticity, and perhaps high levels of sexual dimorphism. All of these suggest an essentially African ape lifestyle adapted to desiccating environments where food resources consisted of relatively hard, coarse fruits and nuts. Later australopithecines would have been more specialised forms, with an ability to survive in more open habitats, possibly even extreme grasslands. These savannah apes remained restricted to African biomes, and speciated and became extinct as local populations. The inferred socioecology of the African savannah bipeds would share many characteristics with that of the common ancestor. The move to more open environments with widely dispersed and scarce, high-quality plant foods may have led to smaller communities distributed over wider territories. Reference to living hominoids would suggest that these australopithecines could have retained male kin-bonded groups (Foley and Lee, 1989), but that a higher risk of predation would have reduced the degree of fission within communities. The nature of resource distribution is such that these hominids are likely to have had extensive day and home ranges, and this, in addition to increased heat stress, is probably the main factor underlying their key adaptive trait, bipedalism.
The diet of the megadontic australopithecines has generally been con sidered as a specialisation: to seeds, fibrous plant foods, and low-quality, coarse foods (Kay, 1985; Grine, 1981). While there must be an element of this in their adaptations, it should also be recognised that as large-bodied, highly intelligent apes, they would have been opportunistic frugivores, hunters and scavengers, and thus their diet would have included meat. In addition, a key to their ability to survive in these more arid environments may have been the ability to tap into underground plant resources, perhaps with the aid of simple technology.
In ecological terms, the primary trend in early hominid evolution appears to be a series of increasingly specialised adaptations (bipedalism, teeth) to survival in relatively arid and open terrestrial African environments (Vrba, 1985,1996; Reed, 1997), probably occurring in the context of an essentially African ape grade of cognitive capabilities and social strategies. The evolution of Homo after 2.0 million years constitutes a new trend, and one that is diametrically opposed to that of the australopithecines. Morphologically, the shift can be seen in dental and facial reduction, enlarge-ment of the brain, and loss of a more ape-like upper body as the hominid body form became more linear (Ruff, 1991). In addition, hominids became far more widespread across Africa and Eurasia, with indications of a significant technological dependence.
The socioecological basis for these trends starts with dietary change. In contrast to the australopithecines, there is less evidence for dependence upon (plant) foods requiring heavy mastication. This, in the context of archaeological evidence, may be interpreted as a shift to greater use of animal resources (Bunn and Kroll, 1986), either through hunting or scavenging. High-quality resources would have changed time budgets (higher search times, possibly reduced foraging times; e.g. Chapter 12), extended day and home ranges, increased habitat tolerance, and affected social organisation and group structure. Meat would have provided a higher quality resource, which would help fuel brain enlargement and also a reduction in gut size (thus the change in body shape) (Foley and Lee, 1991; Wheeler and Aiello, 1995). As more omnivorous opportunists, Homo were able to extend their species range, and although still largely confined to warmer and more open environments, they were no longer African endemics, but Old World cosmopolitans. The key social change that can be inferred from both the dietary shift and the correlated change to slower growth rates and delayed maturation is that more encephalised infants require greater parental investment. It has been argued (Foley and Lee, 1989) that it is at this point that more exclusive relationships between males and females became tacked on to an existing male kin-bonded community structure, although these are likely to have been polygynous rather than monogamous.
While early Homo did disperse beyond Africa, the relative stability of adaptive grades during the lower and middle Pleistocene, and the evidence for environmental limitations (e.g. exclusion from tropical forests, riverine specialisation, and inability to persist in northerly latitudes during colder phases) show that the phase of opportunistic omnivores was also evol-utionarily and ecologically static. Then, however, from around 300000 years ago, two major changes are apparent. The first of these is an acceleration in the rate of brain size evolution (Leigh, 1992), and the second is evidence for repeated dispersals and phases of population expansion (Lahr and Foley, 1994). These developments coincided with the evolution of neanderthals, the evolution of anatomically modern humans, and major changes in technology, especially the appearance of prepared core and blade production techniques of stone artefact manufacture (Foley and Lahr, 1997).
During this phase of human evolution, technology was both becoming more flexible and allowed far greater adaptive flexibility. Underlying this must almost certainly be a cognitive shift, perhaps linked to life history parameters identical to those found in modern humans. It is likely that at this stage (from around 250 Kyr) the socioecological elements would have been very much like those observable ethnographically - kin-based groups of hunter-gatherers, with cultural and linguistic traits used to mark out ethnic and population differences, elaborated culturally in many different ways. The extent to which these traits evolved over the period, and whether there were major differences between neanderthals and modern humans, is a matter of considerable dispute, but it is likely that the socioecology of this period is one in which there was great variability built around a few simple core strategies - male kin bonding, communities of culturally identifiable individuals, intergroup tensions, and high levels of parental care and within-group alliance structures - conditioned by demographic and environmental conditions. The result is the dynamic world of the later Pleistocene that runs relatively seamlessly into the ethnographic present.
It is tempting to use the technological colonisers' phase of human ecological evolution to account for all current human variation, especially as the amount of subsequent biological change is relatively small. However, from an ecological point of view, this would be misleading, for a major change has occurred in the last few thousand years.
The evidence points to modern humans having evolved in the last 200 Kyr, but having effectively dispersed across the planet in only the last 50Kyr. For all but the last 10 Kyr they lived as hunter-gatherers, at relatively low overall population size. During the last 10000 years, however, human population has grown by three orders of magnitude; socioecologically, community size has become much larger, and social structures highly variable, hierarchical and complex. These changes are the result of agriculture. Whereas there has been considerable controversy about the nature of domestication, there is consensus that it involves a major control over resources and has led to both population growth and environmental change. This is the world to which most people are accustomed and in all probability adapted. The basis for this adaptation is the ability to produce large quantities of easily digestible and energy-rich plant foods - root crops and cereals, but especially the latter. Plant cultivation lies at the heart of the current socioecology of humans, despite the fact that a number of animals are also domesticated and provide an important subsistence base (see Chapter 15). The primary food resource for most people is a cereal crop, supplemented to a greater or lesser extent by meat and dairy produce. In this sense, humans are the dominant herbivores of a global ecosystem. This contrasts with much of the ecological foundation for the earlier phases of hominid evolution, in which it was the ability to widen diet breadth to incorporate meat that was critical.
The socioecological correlates of domestic plants are many, but perhaps the key ones are: they are highly predictable in space and time, although abundance can fluctuate markedly; returns can be increased very significantly by increasing effort; they are easily digestible, and thus are an excellent weaning food; access to them can be relatively easily controlled by individuals rather than shared. As a result, agriculturally based communities can have very high reproductive rates and high population densities, although these can be subject to major local fluctuations; they are likely to be territorial and experience intercommunity aggression and conflict; differential control of access to resources combined with a greater potential to coerce less well-situated individuals and communities lead to very marked hierarchical differences or despotism. Male kin-based systems predominate, as do polygynous mating patterns. Above all, the simplified ecological structure can result in marked variation from area to area, with a consequential variability in social system and cultural pattern in which membership of the community is itself a highly significant element of the adaptive process (Aunger, 1996). The socioecology of humans today and in the recent past is characterised by high levels of community membership signalling (cultural variation, language). Biologically, there is also a trend towards reduced body size, gracility of skeletal form, and reduced sexual dimorphism. Finally, the nature of the adaptation is one in which it is possible for the human population, through agriculture, to modify in very radical ways the environment itself, and in particular to homogenise it as a source of agricultural productivity.
Socioecology and the evolution of human social behaviour
There is little doubt that humans have evolved in much the same way as any other species in terms of pattern (cladogenetic radiations, extinction and adaptive trends) and process (natural selection). The three available sources of information about the socioecology of humans - living humans, living non-human primates, and the fossil record - provide a relatively coherent picture, despite the very different nature of the type of data they yield.
In terms of resource structure shaping social behaviours, the key shifts appear to have been: (1) more dispersed and poor-quality foods in the drier habitats of Pliocene Africa, with a resulting shift in locomotion that was fundamental to what followed, and an extension of day ranges and probably increased time stress; (2) greater and more efficient use of scavenged and hunted meat, providing an alternative high-quality food, resulting in the energetic conditions for major life history modifications, and greater habitat tolerance; (3) a subsequent shift to energy-rich cereals, allowing massive increases in population densities and hierarchical community structures.
The phylogenetic context for these resource shifts is important, and it can be argued that it is large, male kin-bonded groups with dispersing females that provide the thread of historical continuity to hominid socioecological evolution (Foley and Lee, 1989; Wrangham and Peterson, 1996). Changes in resource distribution, availability and returns will affect the size and degree of substructuring and the rate of fission, but will not affect the fundamental structure. As such, it is the hominid clade's capacity to maintain large groupings that will have been critical to survival either in a predator-rich environment during the Pliocene, or else in the context of antagonistic intergroup encounters. A switch to dispersing males would have been individually lethal and likely to make such groups highly vulnerable. As such, male kin bonding may represent something of an irreversible strategy in social evolution, unless there is a complete loss of sociality or communities become so large that sex-specific dispersal/residence patterns become unnecessary.
If male kin bonding is the continuity element in human social evolution, male-female relationships and the nature of parental care are the novelties. Higher quality resources which can be shared and weaning foods are the key resource structures that are likely to have changed mating and parenting strategies within the group, and led to the cognitive shifts underlying close relationships between the sexes, in association with delayed life history strategies and high levels of parental care. Finally, the very recent past has seen massive growth and diversification of the human populations, leading to a diversity of social structures, but the ethnographic record perhaps demonstrates that these are all variations on a long-term theme.
In the context of comparative primate socioecology, the pattern outlined here is unusual in the extent to which it has been possible to integrate the observable patterns among extant species with the dynamics of change through time. This is only possible, perhaps, because of the amount of attention that has been focused on the hominid fossil record. However, the nature of the inferences drawn have implications for the comparative method in general. This emphasises the importance of taking phylogenetic history into account; thus, the amount of adaptive evolutionary change that has occurred is considered relative to a common ancestral node (Harvey and Pagel, 1991; Purvis, 1995; see also Chapter 3). For humans, that node would be the last common ancestor with Pan. An estimate of the common ancestral node for Pan-Homo brain size, for example, would be a midpoint between 350 and 1400 g - 875. However, the fossil record shows that this is in fact the cranial capacity of Homo ergaster at less than two million years, as opposed to the common ancestor at over five million years. The problem clearly arises when, of a pair of extant sister clades, all the evolutionary change in a certain character is occurring in one lineage only. This is not simply a case of selection in one lineage and phylogenetic inertia in the other. Stabilising selection is presumably the force acting in one lineage, and directional selection on the other; evolutionary forces are thus acting on each, but in different evolutionary directions. The key point about the fossil record is that it pinpoints the timing of switches in selection in a more accurate way than phylogenetic inference such as contrasts can. Returning to brain size as an example, inferred rates of encephalisation in the hominids based on phylogenetic comparison would be either 130 g/ Myr or 210 g/Myr, depending upon whether the Pan or midpoint ancestral estimate was used. A knowledge of the fossil record would show something very different: encephalisation rates among the australopithecines hominids over a three million-year period would maximally be 55 g/Myr; for Homo it would be 347 g/Myr; and in fact the evolution of Homo sapiens involved a rate of 800 g/Myr over the last half million years, while the Asian Homo erectus lineage had an encephalisation rate of 210g/Myr. While these figures should be corrected for body size, the basic message would be unchanged: evolutionary change is not distributed equally among lineages, but is highly variable. The variability of the nature and direction of change, or the balance between stabilising and directional selection, is the heart of evolutionary issues. Comparisons between species and higher taxa can provide some insight into that variability; adding the phylogeny of living taxa can refine that, but it is still incomplete without a knowledge of the actual distribution of events through time. To put this all another way, our understanding of primate socioecology is strongly influenced by the effects of differential extinction.
The key points relating to hominid socioecology can be summarised as follows. First, when a lineage such as the African ape/human clade is considered with the fossil record incorporated, and thus with a greater emphasis on time, the overall pattern becomes far more complex. The pruning of an evolutionary tree by extinction may well remove entire evolutionary trends, as is the case with the australopithecines. Second, the fossil record shows that human behavioural traits did not evolve as a package, but were accumulated during the course of separate transitions over a number of events. However, the full consequences of these adaptive shifts, as observable in the high-density living of post-neolithic populations, occurred only in the last 10000 years and subsequent to any major biological changes in the human population. Third, the scale and pattern of human evolution are consistent with microevolution, with a balance of stabilising and directional selection operating on fluctuating populations being a sufficient mechanism. Fourth, if there are key points in the evolution of humans and their ancestors, these are most likely to have been: during the terminal Miocene when novel locomotor features evolved; at the base of the Pleistocene when changes in foraging strategy (meat eating) allowed dispersal into multiple habitats and beyond Africa; around 300 000 years ago when it appears the basis for modern human life history, cognitive and behavioural traits were established; and during the last 20 000 years when, for the first time in hominid history, population densities really became a significant global factor, and the socioecology of the human species went beyond the normal expectations of the comparative method.
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