The Use of Ecological Hypotheses in Australopithecine Taxonomy

Transcription

1 The Use of Ecological Hypotheses in Australopithecine Taxonomy ALAN C. SWEDLUND University of Massachusetts, Amherst Ecological concepts have been applied to lower Pleistocene hominid evolution for several years (e.g., Mayr 1951; Robinson 1963; Weiss 1972; Wolpoff 1971). In this paper the major ecological hypotheses are reviewed and their rationale considered in light of prevalent theories on speciation. This review suggests that the hypotheses are central to an understanding of taxonomy and that greater precision in their application is warranted. Allopatric and sympatric distributions of australopithecine populations are discussed. It is concluded that a single species taxonomy will fit the available evidence. THE ECOLOGY of early hominid populations has been an important topic in the anthropological literature for many years (e.g., Bartholomew and Birdsell 1953; Washburn 1960; DeVore and Washburn 1963; Hockett and Ascher 1964). These papers have been primarily concerned with the emergence of the human pattern with its attendant social organization, dietary habits, and behavioral modes, and less concerned with classification per se. Some past studies (especially Robinson 1954, 1961, 1963; Wolpoff 1968, 1971; Jolly 1970) have dealt with the classification of taxa in relation to ecology, while many other studies have postulated classifications almost exclusively on morphological grounds. The purpose of this paper is to briefly review current trends in early hominid classification, and to analyze the rationale of those trends in relation to ecological concepts. Although conclusions regarding classification will be made in this paper, my main purpose is not to defend any existing points of view; rather, I am attempting to show the ecological implications of alternative classifications and their supporting arguments. For the purpose of this paper, the early hominids are those specimens normally referred to as australopithecines. A further distinction will be made into those forms that are gracile and those that are robust. The gracile individuals include specimens referred to as Homo transvaalensis (Robinson 1967), Homo africanus (Mayr 1951), Australopithecus africanus (Dart 1925), Homo habilis (Leakey, Tobias, and Napier 1964), and their alternatives and synonyms. The robust forms include those referred to as Paranthropus robustus (Broom 1938), Zinjanthropus boisei (Leakey 1959), Australopithecus robustus (Tobias 1967), and their alternatives and synonyms. This dichotomy into two basic forms will be considered an oversimplification by some, but it represents a distinction which is now widely accepted (e.g., see Birdsell 1972; Lasker 1973; Buettner-Janusch 1973). The robust and gracile terms are most frequently attributed to the south African australopithecines, but the dichotomy can be extended to include several of the east African finds as well. For example, Homo habilis (Olduvai Gorge) is equated with africanus (Robinson 1965; Brace 1973), and Zinjanthropus boisei (Olduvai Gorge) is equated with south African robust forms (Robinson 1960). Specimens not specifically referred to as gracile and robust have, nevertheless, been Submitted for publication February Revision accepted for publication March 28,

2 516 AMERICAN ANTHROPOLOGIST [ 76,1974 placed together in respective lineages by some authorities (e.g., Pilbeam 1972; Pilbeam and Zwell 1973). Finds from East Rudolph, Kenya (Leakey, Mungai, and Walker 1972; Leakey and Walker 1973) and from the Omo Basin, Ethiopia (Howell 1969) have been assigned gracile and robust categories (e.g., Buettner-Janusch 1973), but it may be premature to equate all australopithecine specimens with south African samples at this time. However, the distinction does appear to be a generally recognized and valid one, and the two varieties seem to be widely distributed in Africa. Since the following discussion will rely heavily on the concepts of habitat and ecological niche, it is essential to define these terms with reference to their intended meaning in this paper. The term habitat refers to the place where a group of organisms lives (Odum 1971:234). It is assumed that the sub-populations (demes) of a single species might be found in widely separated geographic locations, and that their habitats might vary from one region to another, but their trophic level in a given habitat will not vary significantly. Although niche can certainly have a wide variety of meanings (e.g., see Odum 1971; Emlen 1973), it most often refers to the relationship between a species and the resources used by that species. As Odum (1971:234) states, the habitat is the organism s address and the niche is its profession. In this paper I use the term niche to refer to the population s function in the energy system of a community. This is analogous to Odum s (1971:234) trophic niche. MAJOR TAXONOMIC THEMES The history of taxonomy of the australopithecines is, to a great extent, a history of nominal proliferation (for recent reviews see Robinson 1967, 1972; Le Gros Clark 1967; Napier 1970; Pilbeam 1972; Brace 1973; Tobias 1973), but basic themes have resulted, and most classifications can now be placed into one of two current points of view: There are those who argue for (1) splitting of the australopithecines at the species level or above, and (2) those that would argue that the differences between the two varieties represent only intraspecific variation. The major morphological differences that have been noted between the gracile and robust forms have to do with dentition and related jaw musculature. Post-cranially, there appear to be only a few differences beyond those of gross size. According to Lovejoy and Heiple (1970), the gracile forms probably ranged from approximately three and one-half to four feet tall and weighed between 40 and 70 pounds. The robust forms were slightly taller and weighed between 80 and 140 pounds. Wolpoff (1973a) states that both forms were probably somewhat taller and heavier than Heiple and Lovejoy s estimates suggest. Although Robinson (1972) and Napier (1967) have suggested that australopithecine forms may not have had a fully human gait, recent analysis of the lower limb anatomy indicates it was probably the same, efficient, human locomotor apparatus for both forms (Lovejoy and Heiple 1970; Heiple and Lovejoy 1971; Lovejoy, Heiple, and Burstein 1973). The dentition and supporting musculature of the two forms do show marked differences according to Robinson (1963). The anterior teeth, including the canines, are reduced in the robust forms and the cheek teeth are quite large in comparison to those of thegracile. The robust forms tend to exhibit very large jaws and are very often characterized by an obvious sagittal cresting when the parietals are present. By contrast, the gracile forms have smaller jaws and little or no cresting. The differences in dental structure are acknowledged by authorities with widely divergent opinions (e.g., Robinson 1972; Brace 1973); it is the significance of the differences that has led to controversy. The major proponent of generic differences in the australopithecines, J. T. Robinson (1972), has concluded that the gracile forms fit well into the phylogeny of man, should be

3 Swedlund] AUSTRALOPITHECINE TAXONOMY 517 assigned to the genus Homo, and the species africanus. He believes the robust forms represent an evolutionary sideline that may be assigned to the genus Paranthropus. These two forms presumably were able to exist simultaneously because africanus was occupying a grassland habitat and utilized tools (had culture) in partaking of an omnivorous diet. Paranthropus, in contrast, lived in a marginal woodland habitat, was an herbivore, and possibly had tools but definitely not culture to the extent africanus did. The authorities that are more inclined to accept a taxonomic distinction at the species level (e.g., Le Gros Clark 1967; Howell 1967, 1969; et al.) still attribute differences as responses to differing dietary patterns, but do not regard the morphological differences to be great enough to warrant generic separation. Robinson considers Paranthropus ancestral to Homo africanus and believes that Paranthropus eventually became extinct. Others consider africanus the ancestral form. The proponents of a single species classification (e.g., Mayr 1951 [reversed his position in ; Wolpoff 1968, 1971; Brace 1971) regard the morphological differences as not being sufficiently great to warrant specific separation, and argue that the acquisition of culture provided man with a niche that inhibits functional specializations in morphology to the extent that a species separation implies. Mayr (1951) and Brace (1971, 1973) have suggested that the observable differences between gracile and robust forms might be explained as sexual dimorphism in a single species and that dimorphism in present-day pongids would support this position. A very important issue in this taxonomic controversy is, of course, time. The absolute dates and periods to which we can assign the various fossil hominids in man s phylogeny are a constant concern for human paleontologists, but of more immediate concern here are the relative periods of possible coexistence and/or succession of the two or more forms. We know that they existed for long periods of time. We assume that they were contemporaneous for at least part of that time, but did two australopithecine populations coexist? If so, what are the implications? Are they both dead ends? Did one survive and the other become extinct? Or, as members of the same species, did they simply evolve into more modern forms? INTERSPECIFIC DIFFERENCES Regardless of whether one accepts distinction at the generic or specific level, the kinds of interaction that will result are inter-specific. The primary differences between the gracile and robust forms, if one assumes inter-specific differences, are presumably those associated with an omnivore as opposed to an herbivore, and perhaps one form with a greater capacity for culture than the other. Ecological niches are being recognized which vary in at least one major way. Wolpoff (1968, 1971), amplifying the observations of Washburn (1951, 1960), has argued that culture in itself provides an ecological niche that transcends other niches and that acts against tendencies toward specialized morphology and acts to multiply, rather than to restrict, the number of usable environmental resources (1968:479). Hardesty (1972) notes that modern man s optimum niche is very general as a result of cultural adaptation, but often becomes quite specific in practice. Robinson (1963) in effect, argues that the usable resources themselves were limited for the australopithecines and that the exploitation of these limited resources led to certain adaptive responses in at least the gracile forms. The most significant of these responses was the development of culture. In attributing culture to the australopithecines, Robinson (1972) not only includes the physical evidence manifested in the form of early stone tools, but also considers the gracile (africanus) forms capable of a very high level of communicative ability (1972:246). Whether or not culture is assigned to only the gracile forms, or to this whole stage of hominid evolution is still

4 518 AMERICAN ANTHROPOLOGIST [ 76,1974 debated, but the consensus now appears to be that these hominids possessed culture, and that tools were only the obvious part of a larger behavioral complex (Brace 1962; Tobias 1971). Furthermore, according to Tobias (1971) and Robinson (1972), this cultural behavior is considered to be significantly advanced over the complex behavior recently observed in living nonhuman primates by Goodall (1965) and others. In this paper, the term culture is most often used in reference to the existence of stone tools; however, I extend the meaning of the term to include non-technical components of culture as well (White 1969). The possession of culture by australopithecines is only part of the issue; the cumulative effects of culture in human evolution also need to be considered. Most anthropologists readily acknowledge the pervasiveness of culture. Culture is, indeed, a very powerful unifying mechanism as can be seen by the fact that the genus Homo comprises only one species today. However, do we ascribe the same power to it then as now? With an awareness of the kinds of tools that have been discovered with or attributed to australopithecines, might we not take a conservative view of the influence of tools in early hominid evolution? Granted, tools probably were the means by which a slow, bipedal, hominid might survive during the Villafranchian, as Brace (1962) has so aptly discussed, but are these simple stone, bone, and wooden tools sufficient environmental screens to insure that man will not make dramatic morphological responses to the rigors of food acquisition? Odum (1971:147,196) has observed that the theories of ecologists are often very highly and positively correlated with the environment studied by the theorist; that is, the organism or aspect of the ecosystem that the ecologist studies most tends to become the most significant in his mind. The possible over-emphasis on culture as an explanatory device in early human prehistory might possibly be related to our recognition of its tremendous effect in later prehistory and today. Brace (1973) has posited a reduced effect of culture in the lower Pleistocene to account for the kind of dimorphism observed, and this argument can equally apply to a variety of other observations. The ecological and taxonomic observations referred to above have traditionally been explained by mechanisms of speciation and the subsequent interaction between two evolved forms. In order to understand these mechanisms more clearly it is necessary to consider the two processes separately. One possible reason for differentiation of the australopithecines is that they originated as allopatric species. This simply means that their geographic areas of dispersal were different and essentially non-overlapping. This possibility has not been very seriously considered because gracile forms and robust forms have been found in the same sites and even the same approximate levels (e.g., Homo habilis and Zinjanthropus boisei in Bed I of Olduvai Gorge). However, a closer look at the allopatric hypothesis may be warranted. Ecological succession (Odum 1969) is a process of change in habitat in a given geographical area. Succession results in a change or replacement of some species by others and is actually brought about by the modification of the environment by the biotic community. There are other disruptive factors which can alter habitats in a specific area, such as climatic changes and geological events. All of these processes could have been influential in the environment of early man and we are aware of climatic events during the late Tertiary that have been suggested as the primary reason for the appearance of the hominids (i.e., the increase in the number of broken forest and savanna habitats). A hypothetical situation can be reconstructed in which a generalized australopithecine species evolves during the late Pliocene, during a period of rapid transition in the habitats of its geographical range. Differentiation begins to occur as some populations remain in a woodland or broken forest habitat, while others tend to occupy grassland environments (size differences in the two emerging forms might even be explained by exclusion of smaller members of the species from favored occupation areas, or by new conditions in the savanna

5 Swedlund] AUSTRALOPITHECINE TAXONOMY 519 which selected. for smaller biomass of the savanna occupants). Subsequently, these two forms occupy each other s ranges as climatic changes, or ecological succession permits, but do not engage in direct competition with each other due to differences in temporal occupation. Any locale where the two forms are found to occur conjointly could be dismissed as a case where stratigraphic controls are not adequate to control for habitat and minor temporal differentiation. Wolpoff (1968, 1970) has provided a very convincing argument for a lack of adequate controls and lack of evidence for contemporaneity of forms in the case of Swartkrans. Is Bed I at Olduvai so tightly controllable that we can say Homo habilis and Zinjanthropus boisei (gracile and robust respectively in this classification) actually coexisted? We know that significant habitat changes occur in relatively short periods of time, and many of the African fossil bearing beds would not seem to provide the sensitive floral and faunal information necessary to identify these changes. A second possibility for the existence of the two forms in the same locale is that the sites are much later ones, after allopatric speciation had occurred. In this case, the two forms would have originated allopatrically, but would then be suitably distinctive in niche specialization so that they could occupy the same habitat without competing. This kind of interaction will be discussed in greater detail below. Another possibility is, of course, the argument that one temporally followed the other (see Eckhardt 1972). This position is not at all easily defended and, until we have much better dating and controls for the various specimens, this position is difficult to maintain. Some authorities considering the general problem have based their hypotheses on the assumption of sympatric speciation. Sympatric species are those originating or maintained through genetic and ecological isolation in common areas of dispersal. Whether or not speciation can occur under sympatric conditions (with no geographic isolation) is still debated (e.g., Mayr 1963; Smith 1966; Ehrlich and Raven 1969). The mechanism by which this kind of speciation might take place is disruptive selection. Disruptive selection occurs when the homozygous genotypes of a polymorphic trait are selectively favored. Consider a two allele locus with the possible genotypes AA, Aa, and aa. If genotype aa has an advantage in one niche of an habitat, while genotype AA has advantage in another, then disruptive selection could occur (see Smith 1966). However, the conditions under which it might occur are very severe (Smith 1966) and could probably not be met by hominid populations. In a recent review Emlen (1973: ) discusses the evidence for sympatric speciation in populations at various trophic levels. He suggests that the effects of disruptive selection would probably be most often experienced by non-mobile species with small niche breadths. Neither of these characteristics is true of hominids. Wolpoff (1971) has recently argued that the necessary genetic isolation would be extremely improbable for australopithecines. Thus, it would appear that if the australopithecines do represent two species, their origins most likely occurred under allopatric conditions. Once the origin of two (or more) species has been accounted for, the possible types of interaction must similarly be considered. In this particular case, although one might assume allopatric origins, there is still the question of allopatric or sympatric distributions. Virtually all of the authorities recognizing inter-specific differences believe that the robust forms eventually became extinct. The extinction of one of two co-evolving forms is a very basic part of the multiple species hypothesis. The most recent syntheses (e.g., Tobias 1973) giving dates for the two forms indicate they lived contemporaneously for at least two million years. During this relatively long period of time there are several types of interaction that could occur. In the allopatric model the two species would theoretically never come into direct contact with each other; therefore, no competition or interaction between the two would occur. Robinson (1963) has suggested two different habitats and a different ecology

6 520 AMERICAN ANTHROPOLOGIST [76,1974 (1963:394) for the two; others agree with this basic premise. If this was the case, then the robust forms would not have competed with the gracile. Robinson (1963:412) further comments that Paranthropus was probably unable to hold its own and became extinct. If one interprets this to mean that Paranthropus was unable to hold its own with Homo africanus or Homo erectus, then one is acknowledging their sympatry. If, however, one assumes that robust forms were forced to extinction by some other species, or biotic community, or nonbiotic event, then the model is kept consistent with allopatry. If the robust forms are accepted as a separate, allopatric species, the alternatives include: (1) Inability to compete with other herbivores (including anthropoids?). (2) Change of the environment sufficient to select against herbivores, which could occur in conjunction with (1). The resulting conditions might be: (a) Extinction through (1) and/or (2). (b) Evolution into something else as a result of pressures from (1) and/or (2). Thus, if allopatric species are recognized, the robust forms either become extinct or evolve into something else, but neither alternative would appear to be directly attributable to the existence of the gracile. In the sympatric model, however, the gracile have a larger role. Robust and gracile australopithecines are represented in several sites in South and East Africa (Tobias 1973). This, coupled with the fact that their known age of contemporaneity now spans some two million years, leads to serious doubts about their being totally allopatric throughout this time. Assuming two species existed, sympatric interaction would seem to be much more likely. Two widely distributed hominid species, living in a heterogeneous environment, might be expected to come together on infrequent occasions, or they might be expected to spend much of their time sympatrically. The latter would depend primarily on how distinctive their respective niches actually were and on the frequency of habitats that afforded exploitation of both niches. Odum (1971: ) provides a detailed discussion on the various types of species interaction that might occur in sympatric situations. His classification (see Table 1) gives nine types of interaction: three negative, three positive, two that have both positive and negative results, and one neutral. Of these various types, it appears that only 2, 3, and perhaps 6, have relevance to the australopithecine models. Although Type 2 competition may have occurred between the australopithecines, the dietary hypothesis seems to be most consistent with Type 3 competition. Furthermore, the postulated extinction of robust forms would also seem to be more consistent with Type 3. Extinction might occur as an end result of the process known as competitive exclusion. Mathematical modeling of competition between two sympatric species has led to the development of the competitive exclusion principle (Gause 1934; Hardin 1960; Rescigno and Richardson 1965). The principle states that the likelihood of stable coexistence between species varies inversely with their degree of ecological similarity. Experimental observation of two species competing for the same resource has led to the conclusion that the effects will be ecological separation of the two species or local extinction of the less adaptive one. Recent research (e.g., Ayala 1969, 1970) has questioned these alternative outcomes. Stewart and Levin (1973) have shown that very subtle differences in modes of resource utilization can, in fact, allow for two species to coexist even though mutually dependent on the same resource. Conversely, it should be emphasized that exclusion can occur when there does not appear to be a great deal of niche overlap between two species. This could be the case when one resource (or a very small number of resources) is extremely critical to both species.

7 Swedlund J A USTR A L OPI THEC INE TAXONOMY 521 TABLE I. ANALYSIS OF TWO-SPECIES POPULATION INTERACTION Species Type of Interaction* 1 2 General Nature of Interaction Neutralism 0 0 Neither population affects the other Competition : Direct interference type - - Direct inhibition of each species by the other Competition: Resource use type - - Indirect inhibition when common resource is in short supply Amensalism - 0 Population 1 inhibited, 2 not affected Parasitism + - Population 1, the parasite, generally smaller than 2, the host Predation + - Population 1, the predator, generally larger than 2, the prey Commensalism + 0 Population 1, the commensal, benefits while 2, the host, is not affected Protocooperation + + Interaction favorable to both but not Mutualism + + obligatory Interaction favorable to both and obligatory Indicates no significant interaction Indicates growth, survival, or other population attribute benefited (positive term added to growth equation) Indicates population growth or other attribute inhibited (negative term added to growth equation) *Types 2 through 4 can be classed as negative interactions. types 7 through 9 as Positive interae tions and 5 and 6 as both (from Odum 1971:211, with permission of author and publisher). The results of competition between sympatric populations of australopithecines could include the following: (1) exclusion in the form of total extinction of the robust forms; (2) exclusion in the form of reinforcement and maintenance of allopatry between the two forms; or (3) sufficient selection and subsequent adaptation so that character displacement (Brown and Wilson 1956) occurs and niche differences become great enough to allow continued sympatry. Competitive exclusion resulting in total extinction of the robust forms implies quite severe competition. In fact, it would seem that if competition between the two species was this severe, they would not have coexisted for two million years. Competitive exclusion resulting in local separation and the maintenance of allopatry seems possible. If population densities were low and the number of suitable habitats for both forms high, then this kind of exclusion could be operative for very long periods of time. Character displacement refers to the tendency for two similar species to diverge when they occur sympatrically. The divergences may be morphological, ecological, behavioral, or physiological. The mechanism by which divergence occurs, disruptive selection, is the same process responsible for sympatric speciation. If character displacement were operative, the effect would be to increase the differences between the two forms as the period of sympatry increased. This increasing divergence has been claimed by some observers (e.g., Tobias 1973;

8 522 AMERICAN ANTHROPOLOGIST [ 76,1974 addressed if this option is seriously considered. It relates to the adaptive pattern of the robust forms. If one acknowledges similar niches for the two forms, one is also acknowledging similar functional modes for the observed morphological differences. This tends to undermine the proposed origin of the morphological differences, and requires that some ecologically viable alternative be offered in lieu of the dietary hypothesis to account for allopatric speciation. INTRASPECIFIC DIFFERENCES Those arguing for a single species classification (e.g., Mayr 1951; Wolpoff 1968, 1971, 1973b; Brace 1973) assert that the morphological differences are not significant enough to warrant separation at the species level or above. They have presented two basic hypotheses to account for the morphological variation in australopithecine specimens: (1) The differences represent sexual dimorphism in the species Homo africanus, or (2) the differences represent subspecific, polytypic variation in addition to sexual dimorphism (Wolpoff 1971, 1973b). Mayr (1951:112) and Brace (1973) have observed that the differences could be the result of sexual dimorphism. Both have suggested that the differences observed in australopithecines are no greater than those found in males and females of the gorilla. The general size differences in the australopithecine forms can be looked at in terms of sexual dimorphism. Presumably the robust, larger individuals represent males and the gracile, smaller individuals, represent females. This is consistent with morphological attributes of other mammalian species. Generally, the males are larger than the females. DeVore and Washburn (1963) have reported an example from baboons (Papio) that accounts for this dimorphism. Among the baboons they studied they found that the average weight of males was approximately 75 pounds, while the average weight of females was a little over 30 pounds. These differences are reflected in the general size and robusticity of the cranial and post-cranial skeleton, as well as soft tissues. The effect of this difference is that the total biomass of a baboon troop is effectively reduced by approximately one-fourth what it would be if the females weighed the same as males. Male and female roles in the group are sufficiently distinct to permit this differentiation. This same pattern might also apply to the australopithecines. The single species theorists have also suggested that culture, as an adaptive mechanism, provides a niche in and of itself that would inhibit resource specialization and niche diversification. Mayr (1951) and Wolpoff (1971) suggest that the nature of culture and the tenets of competitive exclusion mitigate against both the origin and maintenance of two contemporaneous, hominid species. This idea has merit, but, as mentioned above, it also has some limitations as an explanatory device. On behalf of the argument, one can note that today man occupies a tremendous number of different habitats and overlaps into many different niches without significant taxonomic diversification. However, this same overlapping variation that exists in present-day human breeding populations is also very apparent between the sexes in these populations. Contrary to this, the australopithecines would seem to exhibit quite extreme sexual dimorphism if one accepts robust and gracile forms as two sexes. If culture is used to explain the lack of diversity at the specific level, but not applied at the sexual level, then the use of this as a heuristic device seems somewhat arbitrary and perhaps inconsistent with data on present-day hominids. Also, as discussed, one has to make assumptions about the effect of culture that may be slightly exaggerated for this early stage in cultural evolution. Differences in the size of teeth have been a primary source of evidence for both the single species theorists and the multiple species theorists. Brace (1971, 1973) has argued that the

9 Swedlund] AUSTRALOPITHECINE TAXONOMY 523 Campbell 1972; Pilbeam 1972). Weighing these possibilities collectively, it appears that while competition between gracile and robust forms might have been possible, it does not seem to be the most likely reason for the total extinction of the robust forms. The only other interaction type referred to by Odum (1971) that could possibly be invoked to account for the disappearance of the robust forms is 6, Predation! If the gracile are omnivores, and if they engage heavily in meat-eating, it is conceivable that they might engage in eating\ the robust forms. This hypothesis has been made before (e.g., Dart 1959). While it is certainly true that heavy predation can lead to the extinction of a prey species, this does not seem to be a likely result of predator-prey interaction between an omnivore and a closely related herbivore. Much more typical would be a state of fluctuating equilibrium between the predator and prey populations (May 1972). If the gracile were omnivores, it would be unlikely that they were totally, or even heavily, dependent on a single species of prey. Faunal remains found in association with australopithecines (e.g., Leakey 1965) tend to support this notion. For these reasons it does not seem likely that predation can be invoked as a cause for the extinction of robust australopithecines. One related possibility that has been suggested by others is aggression in the defense of territory (Ardrey 1961, 1966). This is a complex issue. If the gracile and robust australopithecines are two different species living in two basically different habitats, then the *probability of their coming together aggressively is low. If the two represent species living in the same habitat, but different niches, then there would seem to be little reason for aggression, except perhaps for occasional territorial maintenance. If their niches are the same, in the same habitat, then aggression is more of a possibility, although in most animal forms territorial defense and niche maintenance are accomplished through noncorporal means. The concept of warfare is probably not appropriate here, since evidence from hunting and gathering societies and nonhuman primates suggests that such large-scale violence would not likely occur, especially between species. Weiss (1972) has provided an illustrative model concerning interspecific competition between australopithecines. His equation includes both a trophic factor and a killing factor. The conclusion reached is that, based on the typical population density patterns of herbivores as opposed to omnivores, the competitive disparity is far too great to reflect a real situation, and predation does not provide the solution. A review of the multiple species theories reveals the following options: (1) If the two species were occupying very different niches, then there was probably not much direct competition. If this is true, and the extinction of robust australopithecines is accepted, then competition or selection against the robust forms must have occurred without the direct affect of the gracile. (2) If, on the other hand, the robust and gracile forms did occupy sufficiently similar niches and habitats, then competitive exclusion (possibly including aggression), could have resulted in the exclusion of the robust forms. To explain extinction while accepting the first option a model should be devised that has other herbivores (perhaps pongids or terrestrial cercopithecoids) being more successful than the robust, ultimately causing their extinction. Another model might invoke some kind of environmental change or catastrophe that would have eliminated suitable habitats for the robust, or driven them back into habitats where other herbivores already successfully occupied their niche. To explain extinction while accepting the second option a model should be devised that demonstrates exclusion whenever sympatry between the two forms occurs. Furthermore, the model must demonstrate that there is not suitable territory and niche space for the robust specimens to occupy in allopatric situations. There is another problem that must be

10 524 AMERICAN ANTHROPOLOGIST [ 76,1974 dental variation found in gracile and robust forms is no greater than that observed in representatives of present-day pongids. Robinson (1972), in contrast, has stated that the differences are sufficiently great to warrant generic separation. Recent studies (e.g., Frisch 1963; Schaffer 1968; Brace 1973; Pilbeam and Zwell 1973; Greene 1973) of hominid and pongid samples provide somewhat equivocal results if their conclusions are taken together. However, the two most statistically rigorous studies (Pilbeam and Zwell; Greene) reject the sexual dimorphic hypothesis. One recent study (Wolpoff 1973b) on sexual dimorphism in australopithecines was done by partitioning the gracile and robust populations into a South African sample and an East African sample. In this study, measurable sexual dimorphism was observed in canine measurements but the other teeth did not provide diagnostic data. One alternative for single species variation that has not been very seriously considered is that the variation observed in australopithecines might be the product of a highly polymorphic, continuously varying species. Assuming the emergence of a new species with wide geographic distribution, frequent isolation, and new adaptive mutations, the australopithecines might be expected to show considerable diversity arising from many random and non-random factors. Another alternative is that the differences observed relate to a polytypic species comprising two (or more) subspecies. In the rejection of multiple species Wolpoff (1971, 1973c) finds reasonable support for polytypic variation in australopithecines. One advantage of the continuously (polymorphic) varying and the racially (polytypic) varying models is that they can be utilized both synchronically and diachronically with relative ease. Another is that they allow for diversification above the sexual level without resorting to classification at the specific level. The potential inadequacies of the racial approach can be alleviated by using a deme approach to variation without resorting to traditional typological categorizations. The single species theories reveal the following ecological options: (1) If the two forms represent the two sexes of a single species, then the evidence is elusive. Evidence on total body size supports the contention in a logical way, while evidence on dental metric variability does not necessarily. Furthermore, if culture is used as an explanation for econiche uniformity in defense of the single species hypothesis, then it can also be used as an explanation for sexual uniformity. (2) If the two forms represent polymorphic or polytypic subspecies variation, then one still faces the problem of interaction types as stated above, but one also avoids some of the major pitfalls of both the sexual dimorphism and the multiple species hypotheses. For example, if the differences are racial, then one can accept robust forms in our ancestry, and thus explain their disappearance, without resorting to sexual differences. To explain single species variation while accepting the first option a model must be constructed that would use the concept of culture niche in a more consistently reinforcing way, and in a way that accounts for the greater degree and variant type of sexual dimorphism revealed in the australopithecine dentition. The second option seems the most acceptable of all, but it is not without problems. The morphological differences and their ecological causes still beg for an explanation. Range, habitat, and niche diversification still have to be considered in terms of subspecies interaction, but the consequences of such interaction are not analogous to those occurring above the specific level (see Table 1). Ambiguities concerning the dental evidence are still present, although they are not irreconcilable. CONCLUSIONS It appears that many of the existing classifications are beset with conceptual problems when ecological arguments are developed. Some of these problems can be attributed to a

11 Swedlund J AUSTRALOPITHECINE TAXONOMY 525 lack of adequate communication and valid concept borrowing from one discipline to the other. Current research (e.g., Weiss 1972; Wolpoff 1971) suggests that increasing ecological sophistication will characterize future anthropological studies. Many of the problems relate to the lack of good paleontological evidence, but it is a credit to all those who have postulated ecological hypotheses that they have done so within this limitation. The alternative would be a somewhat sterile, descriptive classification of early Pleistocene hominids. The ecological arguments provide depth that the fossils cannot. It is my opinion that the robust and gracile australopithecines fit most logically into a single species. I have chosen this option because it seems the most reasonable given certain ecological considerations, but there are many ecological and morphological issues to be addressed by future researchers. The question of extinction remains as one of these important issues. The major responsibility for the multiple species theorists appears to be the development of a credible ecological framework to explain the extinction of the robust forms. This framework must be consistent with existing knowledge of primate evolution and ecological theory. What are the major difficulties? We have a population of hominids that lasted for at least two million years. This population was probably subdivided into numerous small groups and these groups were distributed widely throughout the subtropical regions of the Old World. The distribution of the robust forms would suggest the ability to cross diverse habitats if not to occupy them. Paleoclimatological evidence (e.g., Butzer 1971; Isaac 1969) suggests that there were not sufficient climatic changes in Africa to severely restrict habitats for robust forms. Weiss (1972) study, in discussing aspects of population density, indirectly raises an issue that is very relevant to the question of extinction. We do not know much about population densities in the lower Pleistocene. In any given microgeographic situation the densities can be assumed critical enough for competition without reference to the macrogeographic situation, but if the total extinction of a form is being considered, then the total range and density of that form and its competitors must also be considered. While it may be reasonable to accept the local extinction of robust forms as a result of resource competition, it is more difficult to accept the total extinction of such a widely distributed form by this cause. If the dietary hypothesis is rejected in favor of other hypotheses, then these alternative hypotheses must be consistent with ecological parameters. If the dietary hypothesis is postulated and non-hominid species are suggested as competitors, then the theorists must reasonably demonstrate why a large population of herbivorous, bipedal hominids, with average cranial capacities of 500 cc., are incapable of adapting alternative subsistence modes or otherwise successfully competing with other organisms. The only alternative hypothesis that has been offered to explain the extinction of robust australopithecines is the aggression theory (e.g., Ardrey 1961, 1966). This hypothesis has been criticized on several levels (e.g., Montagu 1973; Weiss 1972; Brain 1972), and requires one to accept the openly aggressive attacks of one form against another closely related form. It requires that this activity not only takes place in limited situations in specific areas, but throughout the subtropical regions of the Old World. And this is as difficult to envision as resource competition.2 The hypotheses of the single species theorists seem to be more consistent with current knowledge of primate evolution and ecological processes. There is still the need to address the existence of real morphological differences in lower Pleistocene hominids, but this seems to me to be a reasonable task. This article is meant to be a critical review, but is is also intended to be an introduction. Research in the near future will hopefully elucidate, or eliminate, many of the problems discussed here. Richard Leakey s (1973) recent discoveries in the Lake Rudolf region have provided evidence for a Pliocene-Pleistocene hominid distinctive from the specimens referred

12 526 AMERICAN ANTHROPOLOGIST [ 76,1974 to in this paper. These finds may be the basis for a totally revised lower Pleistocene hominid classification. New finds are, of course, inevitable and desirable. Thus, the important conclusion is not that the australopithecines represent a single species, but that hominid taxonomy should reflect sound ecological principles. The only infallible criterion for the species taxon is whether or not the individuals comprising a study population are interbreeding successfully. This is a question that cannot be easily addressed with fossil evidence; the ecological evidence does make the issue more meaningful. Finally, it should be emphasized that the study of early human ecology is not dependent upon classification for its justification. The understanding of how early man lived, where he lived, what he ate, and who ate him is exciting. It is the domain of paleoanthropology. Whatever genus and species the australopithecines belong to, they were hominids-our subjects, and their niche(s) is relevant to ours. NOTES I would like to thank David Greene, J. S. Rhine, George Armelagos, Robert Euler, George Gumerman, Sheryl Green, David Carlson, Kenneth Weiss, Milford Wolpoff, and Bruce Levin for reading and commenting on various drafts of this paper. I would also like to thank Benita Weinstein and Lorraine Wysk for aid in preparation of this manuscript. Any errors or misinterpretations are of my own making. *Several investigators (e g., Robinson 1972; Tobias 1973; Campbell 1972) consider the most recent robust forms to be contemporaneous with the earliest Homo erectus forms. In the context of this paper, this could result in the substitution of Homo erectus for the term gracile when referring to the most recent interactions between the two forms. Campbell (1972) and Buettner-Janusch (1973: ) have discussed the great difficulties involved in distinguishing the latest forms of the australopithecine stage with the earliest forms of the Homo erectus stage. Evolution makes such distinctions very difficult because discrete categories can only partially reflect continuous processes. If future evidence confirms this coexistence, however, the same ecological parameters and their possible outcomes are applicable. If Homo erectus is defined by a commonly agreed upon date for divergence as Campbell (1972) suggests, then the distinction may not be ecologically significant. If the distinction is based upon ecologically significant criteria, then the issue must be addressed in light of the niche(s) attributed to Homo erectus. Assuming that Homo erectus represents a significant shift to large-scale hunting and a more cultural way of life could conceivably lead us to conclude that there was not significant competition for resources between the two forms. REFERENCES CITED Ardrey, Robert 1961 African Genesis. New York: Delta Books The Territorial Imperative. New York: Delta Books. Ayala, F. J Experimental Invalidation of the Principle of Competitive Exclusion. Nature 224 ~ Invalidation of Principle of Competitive Exclusion Defended. Nature 227 : Bartholomew, G. A., and J. B. Birdsell 1953 Ecology and the Protohominids. American Anthropologist 55: Birdsell, J. B Human Evolution. Chicago : Rand-McNally. Brace, C. L Cultural Factors in the Evolution of the Human Dentition. In Culture and the Evolution of Man. M. F. Ashley Montagu, Ed. New York: Oxford University Press Sex, Inadequacy and the Australopithecine Identity Conflicts. Proceedings of the 40th Annual Meeting of the American Association of Physical Anthropologists. American Journal of Physical Anthropology 35 : Sexual Dimorphism in Human Evolution. American Association of Physical Anthropologists, Yearbook of Physical Anthropology, Vol. 16.

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