Totem: The University of Western Ontario Journal ofAnthropology

Volume 13 | Issue 1 Article 11

6-21-2011

The Robust Australopithecines: Evidence for thegenus ParanthropusAmy RotmanThe University of Western Ontario

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Recommended CitationRotman, Amy (2005) "The Robust Australopithecines: Evidence for the genus Paranthropus," Totem: The University of Western OntarioJournal of Anthropology: Vol. 13: Iss. 1, Article 11.Available at: http://ir.lib.uwo.ca/totem/vol13/iss1/11

The Robust Australopithecines: Evidence for the genus Paranthropus

Keywordsrobust australopithecines, Paranthropus, morphological, trait analysis

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The Robust Australopithecines:Evidence for the genus Paranthropus

Of great debate among anthropologistsover the past few decades is the identity of the"robust" australopithecines and if this groupwarrants a separate genus name, Paranthropus,to distinguish them from the "gracile"australopithecines. The first robustaustralopithecine to be discovered was found byRobert Broom in 1949 in South Africa and wasnamed Paranthropus robustus. Since thenhowever, debate has raged on concerning thevalidity of the species name Paranthropus. Avariety of models have been presented to accountfor various interpretations of the fossil materialsof the robust australopithecines. This paper willfocus on two polar extreme views of taxonomy:one that consolidates all hominid species into asingle lineage, and another that allows for thesplitting of hominid branches into two distinctgenera. A critical examination will beundertaken in this paper, dealing specifically

with the fossil KNM- WT 17000 as the initialspecimen through whom the Paranthropinesevolved. Through a trait analysis, a paradigmanalysis, and an ecological and taxonomicdiscussion I will be able to better interpret thefossil record of the robust australopithecines.Through the examination of the concepts oftraits, paradigm, and ecology and taxonomy, itwill be argued that the differences between therobust australopithecines and the gracileaustralopithecines is great enough to warrant theuse of the genus name Paranthropus in dealingwith the robust species, specifying that they arein fact a separate clade from theAustralopithecines and later Homo.

Trait AnalysisThe chronology of early hominid

phylogeny is wrought with differinginterpretations as to the exact sequence of humanevolution. Of great concern is the determinationof which species is considered the last commonancestor of the Paranthropus lineage and of theHomo lineage, if we do concede to the genericsplitting of the two forms. The position taken inthis paper is that the Paranthropus and Homoclades diverged approximately 3-2.5 millionyears (MY) ago from a common ancestor ofAustralopithecus afarensis (Chamberlain1991:141).

A trait analysis is required in order togain an understanding of general morphologicaldifferences and similarities among the earlyhominids. Broadly speaking, the cranial anddental traits of the Paranthropines suggestspecializations that relate to heavy chewing-most likely of coarse, tough, fibrous plant foods(Foley 1995:84). The earliest evidence of thedivergence into the Paranthropine lineage wasmarked by the appearance of the fossil KNM-WT 17000, otherwise known at the "BlackSkull." Found in deposits in West Turkana,Kenya, KNM- WT 17000 (from here on namedParanthropus aethiopicus), was initiallyassigned to the species Paranthropus boisei,based on several of its robust features, such asthe extremely large size of the palate and theteeth (namely the molars and premolars), thebuild of the infraorbital and nasal areas, and itsdish-shaped midface with forwardly positionedzygomatic bones (Klein 1999:213). It will laterbe demonstrated that this initial taxonomicnaming is misleading, and therefore thetaxonomic name Paranthropus aethiopicus isused for this specimen. It is widelyacknowledged that amongst the robust group of

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early hominids, robusticity of traits increasedthrough chronological history. Therefore, it isnoted that Paranthropus robustus andParanthropus boisei were markedly more robustthan the earlier Australopithecines in many oftheir features. Whether these two specimenswere simply geographic variants of one anotheror they reflected a greater taxonomic distinctionis currently under debate. However, thesecontemporaneous species did share anextraordinary number of characteristics. It is intheir masticatory apparatus where the description'robust' applies. While their incisors and canineswere relatively small, their cheek teeth weregreatly expanded -- as noted in the fact that theirdeciduous and permanent premolars were almostfully molarized (Klein 1999:215). As comparedto the more gracile australopithecines, whoseteeth do not show such extreme changes in size,the robusts' teeth are said to be indicative of amore specialized dietary adaptation.

In terms of skull morphology, the robustgroup of early hominids is in direct contrast withboth the earlier, gracile australopithecines, aswell as with the larger-brained early hominids,beginning with Homo habilis, who existedcontemporaneously with them. The robustspecies' dish-shaped faces were characterized byvery powerfully built zygomatic arches whicharise far forward on the maxilla (Klein1999:215). Their mandibles were very thick anddeep, with tall and broad ascending rami whicharise much more laterally and inferiorly thanthose found in Australopithecus afarensis (Klein1999:215). These traits, along with theiranteriorly placed sagittal crests, reflect a muchgreater emphasis on the temporalis muscle andother bones, which suggests an increasedspecialization in mastication. Homo specimensare seen to be lacking in the dentalspecializations noted in the paranthropines, asthey have much smaller and more parabolic jaws.They have more even sized (generalized) teethand much longer and larger brain cases whichstand at around 600-700 cc (Foley 1995:84). Inaddition to this, Homo faces are small and muchless prognathous than those found in theparanthropines (Foley 1995:84).

When Walker initially assigned KNM-WT 17000 to Paranthropus boisei, the fortyfeatures used to show their close affmity actuallyprovide us with misleading interpretations. In amore recent analysis of the same materialconducted by Kimbel, White and Johanson(1988), it was shown that sixteen of the fortycharacter states that were displayed by KNM-

WT 17000 (that were previously believed tospecifically tie it to Paranthropus boisei) wereactually chronologically primitive traits that areshared with the last common ancestor,Australopithecus afarensis. As well, thirteen outof sixteen derived characteristics that werecommon between KNM-WT 17000 andParanthropus boisei were revealed to be presentin other Australopithecine fossils (Kimbel et at.1988:260). Based on these re-analyses, Kimbelet al. (1988) show that there were only threecharacters used in Walker's analysis whichdirectly link KNM- WT 17000 withParanthropus boisei. With this in mind, I canargue that the fossil KNM- WT 17000 ismorphologically dissimilar to Paranthropusboisei and therefore, it must be understood as aseparate species, one that was intermediatebetween the early australopithecines and thelater, more robust paranthropines. Kimbel etat.'s (1988) re-analysis of the character traits ofKNM- WT 17000 deals with thirty-two traits. Ofthese, twelve are primitive to Australopithecusafarensis, six are derived traits shown to beshared with all later hominids (includingAustralopithecus africanus, Paranthropusrobustus, Paranthropus boisei, and to a smallextent early Homo), an additional twelve werederived and shared with only the paranthropines,and only two were shown to be derived andshared exclusively with Paranthropus boisei(Kimbel et al. 1988:261).

In the above re-analysis, thecharacteristics analyzed demonstrated theaffiliation of KNM -WT 17000 to the other earlyhominids. The primitive traits demonstratedKNM-WT 17000's derivation from the earlierAustralopithecus afarensis. These features alsoruled out Australopithecus africanus as the lastcommon ancestor for the Paranthropines and theearly Homos. The primitive characters shared byKNM- WT 17000 and Australopithecus afarensiswere not demonstrated in the Australopithecusafricanus fossils, therefore to include them at thebeginning of the lineage of the Paranthropineswould imply an extraordinary amount ofcharacter reversals. The remaining derivedfeatures shared in common with KNM-WT17000 and the Paranthropines implies that theyare all part of the same phylogeny, evolving fromAustralopithecus afarensis (in whom thesefeatures were not demonstrated). Those derivedtraits also exhibited in Australopithecusafricanus are understood to be parallelisms.

A phylogenetic hypothesis to bestdemonstrate the affmity of these early hominids,

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taking into account the suite of character traitspreviously discussed, is that Australopithecusafricanus is the exclusive ancestor of Homo.There are a number of derived characteristicsused to confIrm the validity of this hypothesis,including an overall reduction in pneumatizationof the mastoid process of the temporal bone, anincrease in steepness of the frontal bone, and amean increase in the relative length of the upperoccipital (Kimbel et al. 1988:263). Thishypothesis requires the least amount ofparallelisms and is therefore the mostparsimonious in examining early hominidphylogeny. Therefore, through the analysis ofcharacter traits perfonned by Kimbel et al.(1988), a clear tree of phylogenetic relationshipscan be drawn to demonstrate that KNM- WT17000 (Paranthropus aethiopicus) andAustralopithecus africanus diverged fromAustralopithecus afarensis several million yearsago. The robust paranthropines evolved from thefossil KNM-WT 17000, while the early homosevolved from Australopithecus africanus in aseparate branch of evolution.

Paradigm AnalysisIn understanding the phylogenetic

relationship that is accepted in this paper, it isimportant to be aware of how a particularparadigm can greatly affect one's analysis andinterpretation of the fossil record. Phylogeny isconcerned primarily with the course of evolutionand the branching or diverging events which takeplace. Dating and morphology of the fossilrecord suggests that the hominid fossil recordconstituted a number of distinctive evolutionarybranching events as opposed to comprising of asingle evolutionary lineage (Foley 1995:72).The phylogeny of the hominid lineage that Ipropose is that between 3 and 2.5 MY ago,Australopithecus afarensis was the last commonancestor to lead to two diverged lineages, themegadontic specialists (the Paranthropines), andthe larger-brained generalists (Homo). However,despite evidence that is continually debatedamongst anthropologists, there is still noconsensus as to which phylogenetic framework isthe best representative of human evolution. Thisis the problem of paradigm.

Each anthropologist brings his or herown biases and worldviews into an analysis ofthe fossil record, and these are consequentlyreflected within the hypotheses, methodology,and conclusions at which they arrive. AsWolpoff (1976) discusses in a paper on thetheory behind paleoanthropological

controversies, all data exists within a theoreticalframework. Wolpoff explained that the fIeld ofpaleoanthropology is almost subjective in thesense that "there are neither refutation norproofs ... only probability statements for whichany and all observations are appropriate"(Wolpoff 1976:95). In this sense, Wolpoffconceded that the field of anthropology does relyupon differing paradigms and that eachanthropologist's paradigm will bring a newinterpretation on the old material.

A difference in opinions on trait list biasis shown in the papers written by Skelton andMcHenry (1998) and in Strait and Grine (1998),which discussed early hominid phylogeny.Skelton and McHenry criticize Strait and Grine'sgrouping of Paranthropus aethiopicus withParanthropus robustus and boisei to create amonophyletic clade. Skelton and McHenry's(1998: 109) analysis placed Paranthropusaethiopicus within the sister clade of P.africanus, P. robustus, P. boisei, and Homo.These varied phylogenies were arrived atbecause of different assumptions and procedureswhich relate to the effect of trait list bias. Thedistinctions that are noted are due to differentassumptions of correlations among traits, as wellas an unequal representation of the functionalcomplexes of traits (Skelton and McHenry1998: 109). Although this trait list bias is agreedupon as causing problems in both analyses, thereis no consensus between these groups ofanthropologists as to how to compensate for thebias.

Strait and Grine (1998: 115) believe thatthe best chance for arriving at an agreement overearly hominid phylogeny requires an opendiscussion about characteristics andmethodology. They evaluate Skelton andMcHenry's (1998) analysis, and in particulartheir use of functional groupings of traits. Theyexplain that Skelton and McHenry's separationof the anterior dentition from the traits relating toheavy chewing does not qualify as a validfunctional grouping. Diet is responsible fortrends seen in both anterior and posteriordentition, as well as in other masticatory featuresof the skull, and therefore, anterior dentitioncannot be separated from this grouping (Straitand Grine 1998: 115). In this sense, we can seehow each group's paradigm affects the ways inwhich they group character traits.

Strait and Grine (1998) believe incharacter trait lists that take into accountcharacteristics that are not only functionallyrelated, but also phylogenetically compatible

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with one another -- this is the nature of cladistictheory. They state that Skelton and McHenry(1998) use complexes which are not reasonablewithin cladistics. The phylogenetic relationshipaccepted in this paper is in agreement with thatof Strait and Grine (1998). Paranthropusaethiopicus is shown as being differentiated fromAustralopithecus afarensis, and early Homothrough its megadontic characteristics. Anteriordentition morphology is directly related to heavymasticatory action and is thus considered part ofa similar functional complex as the rest of themasticatory apparatus which functions to sustainheavy chewing.

An Ecological and Taxonomic DiscussionApproximately 3-2.5 million years ago,

conditions of cooling and drying arose on theAfrican landscape; this was the fIrst widespreadNorth Polar glaciation. As Vrba (1988:407)explains, physical, environmental changes thathave evolutionary repercussions can arise fromtwo sources: tectonic plate movement which isrelatively localized, and global climatic changeswhich are widespread. The global event whichoccurred 2.5 million years ago initiated bothclimatic and biotic changes within Africa. Theresult of these changing conditions was thecreation of more extensive open habitats and achange in the form of vegetation to a moreresistant arid-tolerant variety (Bromage andSchrenk 1995:110). The amount of woodlandavailable decreased greatly, and early hominidswere forced to adapt to living in an open savannahabitat. The selective pressures that arose inconjunction with the changing environmentallandscape resulted in the phyletic splitting ofAustralopithecus afarensis into two separatelineages, Paranthropus and Homo (Bromage andSchrenk 1995: 110).

Adaptations to the environment and tocoarse, fIbrous food items are noted in the fIrstparanthropine, Paranthropus aethiopicus.Throughout time, the Paranthropus lineageadapted itself more fully to this new openhabitat. As can be noted in the laterParanthropines, facial robusticity and megadontyprevailed throughout the existence of the genus.The robust lineage became more specialized inresponse to resources that were prevalent in thesemore open environments. Vrba proposes afunctional link between the robust cranialmorphology of the Paranthropines and the foodresources that were prevalent within the open,arid environments that they occupied. Based onan analogy of the functional morphologies of

other mammals, Vrba (1988) explains that therobust dental and cranial morphology directlyreflects an adaptation for feeding on tough foods.This is best seen in the robust cranialcharacteristics: greatly thickened tooth enamel,the expansion of cheek teeth (the molars andpremolars) and a reduction of the anterior(canines and incisors) teeth, as well aspronounced and expanded areas of insertion forthe immense masticatory muscles (Vrba1988:418). Through an analysis of these traits, itseems clear that the robust features were anadaptation to the changing ecology of theenvironments occupied in Africa and the foodseaten by the paranthropines.

An examination of another earlyhominid species living contemporaneously withParanthropus, Homo habilis/ rudolfensis,illustrates a parallel evolution in some of thefeatures associated with this adaptation to a moreopen, savanna environment (Bromage andSchrenk 1995: 110). Many of the masticatoryand dental adaptations seen in the cranium ofearly Homo demonstrate a similar adaptation totougher fruit and savanna foods. However, theHomo lineage is considered much moregeneralized in terms of its dental traits than theparanthropines. The evolutionary trajectory thatHomo followed turned out to be very differentfrom that of Paranthropus.

It can be argued that the key variable inthe endurance and survival of theaustralopithecines and Homo was 'culture' andthe paranthropines were lacking in this ability forculture. Although the environmental pressuresresulted in extreme adaptations, the Homolineage was able to further adapt to changingenvironments because of their use of 'culture' interms of the development of more sophisticatedtools. The environment provided these separategenera with a series of selective challenges towhich they adapted through similar means.However, approximately 2 MY ago, theenvironment once again underwent great changesand returned to dryer and more humidconditions.

Fossil evidence suggests thatParanthropus boisei dispersed southward inAfrica, and under these more moderateconditions, evolved into Paranthropus robustus(Bromage and Schrenk 1995: 112). This isargued because fossil evidence suggests thatParanthropus robustus was a less extremeversion of the "hyper-robust" Paranthropusboisei. It can be argued that while the earlyParanthropines adapted to the cool, dry climate

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and open savanna by developing more robustfeatures in order to survive on the tough foodsavailable, Paranthropus robustus existed in amuch more temperate environment and thus aslight reversal in the robusticity of the skulldeveloped.

This biogeographic perspective helps usto further understand the forces that acted uponthe phylogenetic splitting of the early hominidsinto these two very distinct groups, withParanthropus aethiopicus leading the way forthe more megadont Paranthropines. I argue thatecological forces were the main pressures thatacted in producing these separate lineages ofearly hominid evolution.

Concluding NotesA great problem plaguing modem

anthropologists concerns the placement of therobust group of early hominids within theevolutionary line. Although many people arguefor a single lineage for the evolution of the earlyhominids, it can be demonstrated that this is notnecessarily a logical argument. When evaluatingwhether the robust group constitutes genetic-level differences from the Australopithecines andearly Homo, one must take into account generalmorphological differences between fossils, thebias of paradigm and its influence on one'sprofessional work, as well as ecologicalconcerns.

In an analysis of the traits of theParanthropus lineage in comparison to theAustralopithecines and early Homo, it has beendemonstrated that the primitive and derived traitsallow for the genetic-level splitting of theParanthropines, using Australopithecus afarensisas the last common ancestor. This was primarilyvisible in the Paranthropines' unique dental andcranial specializations that relate to mastication.In an examination of the issue of paradigm, it isnoted that every individual brings his or her ownbiases and world-views into an experiment, andtherefore, we must examine all sides of anargument and draw our own conclusionsregarding their validity. In terms of ecologicalvariables, it is demonstrated that changingenvironmental and climatic changes have a greatinfluence on the adaptation and survival of earlyhominids. The shifting ecology of the Plio-Pleistocene required that the early hominidsadapt to their new environments and habitats.The results of these adaptations werespecializations so unique to the Paranthropines asto make a genetic-level split from theAustralopithecines and Homo logical. Therefore,

as outlined in the previous arguments, there issufficient evidence to argue for the genusParanthropus, deriving from a last commonancestor of Australopithecus afarensis.

Bromage, T.G., and Schrenk, F. 1995.Biogeographic and climatic basis for a narrativeof early hominid evolution. Journal of HumanEvolution. 28:109-114.

Chamberlain, A.T. 1991. A ChronologicalFramework for Human Origins. WorldArchaeology. 23:137-146.

Foley, Robert 1995. Humans Before Humanity.Oxford: Blackwell Publishers Ltd.

Kimbel, W.H., White, T.D., and Johanson, D.C.1988. "Implications of KNM-WT 17000 for theEvolution of "Robust" Australopithecines". InEvolutionary History of the "Robust"Australopithecines. Frederick E. Grine, (editor).pp. 259-268. New York: Aldine de Gruyter.

Klein, Richard G. 1999. The Human Career:Human Biological and Cultural Origins: 2nd

Edition. Chicago: The University of ChicagoPress.

Skelton, R.R., and McHenry, H. 1998. Trait listbias and a reappraisal of early hominidphylogeny. Journal of Human Evolution 34: 109-113

Strait, D.S., and Grine, F.E. 1998. Trait list bias?A reply to Skelton and McHenry. Journal ofHuman Evolution. 34:115-118.

Vrba, Elisabeth. S. 1988. "Late PlioceneClimatic Events and Hominid Evolution". InEvolutionary History of the "Robust"Australopithecines. Frederick E. Grine, (editor).pp 405-426. New York: Aldine de Gruyter.

Wolpoff, M.H. 1976. Data and Theory inPaleoanthropological Controversies. AmericanAnthropologist. 78:94-96.

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