Human Biology and the Origins of Homo : An Introduction to Supplement 6Author(s): Leslie C. Aiello and Susan C. AntónReviewed work(s):Source: Current Anthropology, Vol. 53, No. S6, Human Biology and the Origins of Homo(December 2012), pp. S269-S277Published by: The University of Chicago Press on behalf of Wenner-Gren Foundation for AnthropologicalResearchStable URL: http://www.jstor.org/stable/10.1086/667693 .

Accessed: 23/01/2013 13:36

Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at .http://www.jstor.org/page/info/about/policies/terms.jsp

.JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range ofcontent in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact support@jstor.org.

.

The University of Chicago Press and Wenner-Gren Foundation for Anthropological Research are collaboratingwith JSTOR to digitize, preserve and extend access to Current Anthropology.

http://www.jstor.org

This content downloaded on Wed, 23 Jan 2013 13:36:28 PMAll use subject to JSTOR Terms and Conditions

Current Anthropology Volume 53, Supplement 6, December 2012 S269

� 2012 by The Wenner-Gren Foundation for Anthropological Research. All rights reserved. 0011-3204/2012/53S6-0002$10.00. DOI: 10.1086/667693

Human Biology and the Origins of HomoAn Introduction to Supplement 6

by Leslie C. Aiello and Susan C. Anton

New fossil discoveries relevant to the origin of Homo have overturned conventional wisdom about the nature ofthe australopiths and early Homo, and particularly Homo erectus (including Homo ergaster). They have eroded priorassumptions about the differences between these genera and complicated interpretations for the origin and evolutionof Homo. This special issue surveys what is now known about the fossil evidence and the environmental contextof early Homo. It also moves beyond the hard evidence and sets the stage for integrated, multidisciplinary studiesto provide a framework for interpretation of the hard evidence. The underlying premise is that to understand theadaptive shifts at the origin of Homo, it is essential to have a solid understanding of how and why modern humansand other animals vary. Contributors to this issue include paleoanthropologists, human biologists, behavorialists,and modelers. We tasked each with bringing her or his special expertise to bear on the question of the origins andearly evolution of Homo. The papers in this collection are a product of a week-long Wenner-Gren symposium heldin March 2011, and this introduction integrates this work and its significance for Homo.

What We Once Knew . . .

The origin of Homo holds particular sway for us and has oftenbeen seen as the point in our evolution when the balance tipsfrom a more ape-like to a more human-like ancestor. By theturn of this century, a conventional wisdom had grown uparound the origin of Homo and particularly Homo erectus thatcast this species as the first hominin to take important bio-logical and behavioral steps in the direction of modern hu-mans (Anton 2003; Shipman and Walker 1989). Homo erectuswas envisioned as a large-brained, small-toothed, long-legged,narrow-hipped, and large-bodied hominin with relatively lowsexual dimorphism. By virtue of a higher-quality, perhapsanimal-based diet, H. erectus is said to have ranged farther,cooperated more, and quickly dispersed from Africa (Aielloand Key 2002; Anton, Leonard, and Robertson 2002; Mc-Henry and Coffing 2000; Walker and Leakey 1993). The pau-city of early Homo fossils of Homo habilis sensu lato (includingHomo rudolfensis) meant that comparisons of Australopithecus((Paranthropus) were made to H. erectus (including Homoergaster) rather than to other early Homo. And the distinctionsbetween Australopithecus and Homo were perhaps overem-

Leslie C. Aiello is President of the Wenner-Gren Foundation forAnthropological Research (470 Park Avenue South, New York, NewYork 10016, U.S.A. [laiello@wennergren.org]). Susan C. Anton is aProfessor in the Center for the Study of Human Origins, Departmentof Anthropology, New York University (Rufus D. Smith Hall, 25Waverly Place, New York, New York 10003, U.S.A. [susan.anton@nyu.edu]). This paper was submitted 12 XII 11, accepted 8VII 12, and electronically published 27 IX 12.

phasized by the diminutive size of the most complete Aus-tralopithecus skeleton (A.L. 288-1; Lucy), on the one hand,and the surprisingly large size of the most complete H. erectusskeleton (KNM-WT 15000; Nariokotome boy), on the other(e.g., Ruff 1993). The comparisons between H. erectus andHomo sapiens were so strongly drawn that the inclusion inthe genus of some of the earliest species, such as H. habilisand H. rudolfensis, was seriously questioned on the basis oftheir more australopith-like postcranial skeleton, among otherthings (Wood and Baker 2011; Wood and Collard 1999, 2007).

The fossil record never ceases to upset conventional wis-dom, and over the past 2 decades, new discoveries from Eastand South Africa, Georgia, and even Indonesia have chal-lenged these stark distinctions between Australopithecus andH. erectus and within non-erectus early Homo. In particular,new small-bodied and small-brained finds from the Republicof Georgia and Kenya call to question claims for universallylarge size in H. erectus (e.g., Gabunia et al. 2000; Potts et al.2004; Simpson et al. 2008; Spoor et al. 2007) and focus ourattention instead on the range of variation within that taxon.This variation in H. erectus has most often been referred toas sexual dimorphism and/or regional/climatic adaptations(Anton 2008; Spoor et al. 2007), although short-term accom-modations and phenotypic plasticity are likely to have playedan important role (see Anton 2013). And larger-sized, longer-legged Australopithecus have been found (Haile-Selassie et al.2010), as have members of that genus who may share somepostcranial characteristics with Homo (Asfaw et al. 1999; Ber-ger et al. 2010; Kibii et al. 2011; Kivell et al. 2011; Zipfel etal. 2011). Additionally, new fossil remains of non-erectusHomo and new work on previously known remains emphasize

This content downloaded on Wed, 23 Jan 2013 13:36:28 PMAll use subject to JSTOR Terms and Conditions

S270 Current Anthropology Volume 53, Supplement 6, December 2012

the diversity of the early members of the genus and the waysin which they differ from Australopithecus (Blumenschine etal. 2003; Spoor et al. 2007).

Yet despite this increased appreciation of variation in earlyfossil Homo, little time has been spent evaluating the rela-tionships between morphology and behaviors in extant taxa,especially modern humans, in different ecological circum-stances. We maintain that these are the data that are essentialto create a more nuanced understanding of the implicationsand expectations of anatomical changes at the origin of ourgenus—an understanding that goes beyond simple assump-tions of sexual or climatic variation.

Topic and Rationale

The increasing number of early Homo fossil finds and theirdiversity in size and shape suggest that this discussion sur-rounding the origin and evolution of early Homo is likely toform a major focus for the next decade of paleoanthropo-logical work. As such, our goal was to bring together humanbiologists, behaviorists, modelers, and fossil experts to inte-grate the rich extant data sets with the new details of the fossilrecord. Understanding the adaptive shifts at the origin ofHomo is dependent on a solid understanding of how and whymodern humans vary and particularly on the relationshipbetween human behavior, human morphology, and humanlifestyle and life history variation.

Among the highly variable features in living humans arefeatures such as body size and aspects of life history thatseparate us from other primates. In many cases, human var-iation in, for example, growth rates, fertility, and perhaps evenlifespan, can be traced to such environmental or behavioralfactors as nutritional sufficiency and unavoidable (extrinsic)mortality. Such phenotypic plasticity provides a more rapidresponse to environmental challenges than does geneticchange, but the fact that genetic change can follow has beenlong suggested in human biology (e.g., Kuzawa and Bragg2012). Thus, understanding the causes of human phenotypicplasticity can provide important clues to understanding bothwithin and between species variation in the morphology ofour hominin ancestors.

Unavoidably, the biology of early Homo will be unlike thatof ourselves, however—and therefore, primate and mam-malian trends are also important to understand. And becauseat some point cooperation in hunting or breeding becameimportant to survival, considering how both carnivory andcooperation influence life history, body size, and body shapeand whether they leave a detectable signal are important con-siderations as well.

We set out, then, to probe the meaning of the newly iden-tified ranges of variation in size and shape in early Homobased on empirical evidence of how extant humans, non-human primates, and social carnivores respond energetically,physiologically, and socially to changes in resource availabilityand to stress from climatic, environmental, and other factors.

We argue that understanding the response of extant organ-isms, especially humans, in shifting environments providesan ideal basis for understanding the integration of bioculturalresponses to environmental constraints. The application ofthese data in light of the known fossil record can help us tounderstand these past populations, their constraints and adap-tive strategies. By mining the rich data sets of our subspe-cialties, we sought to forge a stronger and more nuancedunderstanding of the adaptive shifts that can—or cannot—be inferred at the base of our genus and to set out a seriesof hypotheses and predictions to be tested against future fossiland archaeological data. The results of an intense 5-day Wen-ner-Gren Symposium in Sintra, Portugal, in March 2011 andour follow-up analyses are presented in this special issue.

Setting the Stage

We begin the volume, as we did the symposium, by reassessingthe fossil foundation of what we now know regarding genusHomo. Anton (2012) provides an overview of the genus andits species and the differences between Australopithecus andHomo. The first recognizable members of the genus Homoappear at approximately 2.3 Ma, suggesting that the genusevolved earlier, but substantial fossil evidence does not appearuntil about 2.0 Ma. Her paper focuses our attention on theimportance of individual fossil data points for understandingdiversity within and between groups, and she concludes thata strong case can be made for at least three different morphsbetween 2.0 and 1.5 Ma: an 1813-group, a 1470-group, andHomo erectus (including Homo ergaster). She avoids the useof taxonomic names for the 1813-group and 1470-group be-cause of uncertainty over group affiliation of type specimensfor early Homo species (e.g., Homo habilis and Homo rudol-fensis; Leakey et al. 2012). Her paper also provides an intro-duction to what is now known about the distinct featuresthat separate the three different morphs from each other andfrom Australopithecus (see Anton 2012, tables 1–8, and Antonand Snodgrass 2012, tables 1–6, for membership of thesemorphs and distinguishing morphological and inferred be-havioral features). Additionally, she suggests that, on average,early Homo is larger of body and brain than Australopithecus,and H. erectus is larger than other early Homo. That said, thesurprising facts, particularly to those who have been involvedin paleoanthropology for a considerable time, are the degreeof diversity within the morphs and that, in some ways, themorphs are more similar to each other than was previouslyimagined. For example, all early Homo, including H. erectus,may exhibit substantial amounts of sexual dimorphism, andH. erectus is less fully modern in body proportions than hasbeen previously claimed. These themes and their implicationsare further plumbed in the contributions by Holliday (2012),Pontzer (2012), and Plavcan (2012).

Fossils cannot be understood and interpreted without theircontext, and Potts (2012) provides an overview of the envi-ronmental and archaeological background for the evolution

This content downloaded on Wed, 23 Jan 2013 13:36:28 PMAll use subject to JSTOR Terms and Conditions

Aiello and Anton Origins of Homo S271

of Homo in eastern Africa between 3.0 and 1.5 Ma. He con-cludes that there were major episodes of moist-arid variabilityduring this period, superimposed on an overall drying trend.The first appearance of Homo at approximately 2.3 Ma (Kim-bel et al. 1996; and of the Oldowan at approximately 2.58Ma; Semaw et al. 2003; but see McPherron et al. 2010) aswell as the proliferation of the genus after 2.0 Ma coincidewith particularly high levels of climate variability, suggestingthat adaptive plasticity in its broadest developmental, physi-ological, and behavioral manifestations was integral to theevolution of Homo. For example, stone tools, which have astrong stratigraphic persistence in the archaeological recordafter 2.0 Ma, provide an efficient behavioral mechanism toenhance foraging ability, enabling predictable returns in achanging environment. But they also pose an energetic chal-lenge of material transport over distances as great as 1–13 kmby about 2.0 Ma (Braun et al. 2008).

Food, Morphology, and Locomotion

One means of offsetting the energetic cost of tool and rawmaterial transport as well as increased body and brain size isdietary expansion to higher-quality food resources, whichmight involve access to animal resources (as well as a widerrange of plant food; Aiello and Wells 2002; Aiello and Wheeler1995; Leonard and Robertson 1997). Such resources wouldalso serve to buffer environmental instability and resultingchanges of food resources across space and over time (Potts2012). Some direct evidence for a dietary shift in early Homo(including even more substantial changes in Homo erectus)relative to the diet of Australopithecus is provided by Ungar(2012), who reviews dental macro and micro anatomy andwear. In particular, all early Homo teeth are most similar toextant animals that do not use fracture-resistant foods. Inother words, the genus seems not to have used particularlyhard-brittle foods or especially tough foods. However, withinearly members of the genus, there are some differences thatsuggest a broader subsistence base for H. erectus that includedmore tough foods than other early Homo. This dental evidenceis consistent with increased meat eating (or eating other non-brittle foods) and tool use in food preparation (perhaps evencooking) over the condition in Australopithecus, with abroader range of foods eaten by H. erectus than other earlyHomo. These results could be a hard tissue signal of dietaryand behavioral plasticity to temper environmental vacillation.

Like the dietary results, other papers suggest that someadaptations once thought to appear with H. erectus arise atthe origin of the genus or even earlier. Holliday (2012) pro-vides new analyses and an overview of our current knowledgeof body size and body proportions. The unexpected outcomeis that our prior understanding that H. erectus was uniqueamong the early hominins in having long legs and a narrow,heat-adapted body is wrong. Leg length scales with body mass,and large-bodied australopiths have long, human-like legs andhuman-like thoraces, while new analyses of the H. erectus

pelvis demonstrate that its bi-illiac breadth was broad andaustralopith-like (Simpson et al. 2008; but see Ruff 2010).Because locomotor efficiency is primarily a function of relativelimb length (Pontzer 2012), and because limb length is al-lometrically related to body size in all hominins, larger-bodiedindividuals of any taxon would be more efficient in walkingand running, with faster optimal speeds and increased ab-solute speed. Long leg length and arm length also have ther-moregulatory advantages in hot climates, which would be adistinct advantage either during locomotion or at rest.

Given the similar scaling relationships between limb lengthsand body size across hominins, Holliday concludes that thereis little evidence of a major locomotor shift between Aus-tralopithecus and early Homo (including H. erectus), a pointthat is shown by the analyses presented by Pontzer (2012) aswell. However, they note that a significant difference remainsbetween these genera in terms of mean body size; early Homois approximately 33% larger than Australopithecus, and H.erectus is approximately 15% larger than other early Homo,even when the recently discovered small H. erectus fossils (e.g.,from Georgia, Kenya, Tanzania, and perhaps Ethiopia) andlarge Australopithecus are included in estimates.

Despite having similar proportions as earlier hominins, anumber of symposium contributions emphasize that largersize itself has important energetic, locomotor, and survivalconsequences for Homo. Holliday (2012) and Pontzer (2012)point out that across mammals, larger body size equates witha larger home range size, which would be exaggerated furtherif Homo was also more carnivorous (Anton, Leonard, andRobertson 2002). Pontzer (2012) develops the implicationsof this by demonstrating that across mammals there is noselection for greater locomotor efficiency (as proxied bychanges in limb proportions) in those species with largerhome range sizes. Instead, species that travel farther adopt ahigh-throughput strategy (increased daily energy expenditurein relation to body size and a correspondingly greater repro-ductive investment), resulting in greater lifetime reproductiveoutput. This suggests that in Homo, as in other far-rangingmammals, there must have been an increased energy budgetto provide for increased brain and body growth and repro-duction. Outside of a more calorie-rich diet at the carnivorousend of the omnivorous spectrum, Pontzer (2012) argues thatan increased daily energy expenditure would suggest greaterfood availability, perhaps implying the origins of food sharing.These results are consistent with the symposium papers onliving humans and extant carnivores by Migliano and Guillon(2012), Kuzawa and Bragg (2012), and Smith and colleagues(2012).

Body Size and Growth

While height in humans is influenced by a number of en-vironmental and idiosyncratic factors (see references in Ku-zawa and Bragg 2012; Migliano and Guillon 2012), it isachieved through a combination of speed and duration of

This content downloaded on Wed, 23 Jan 2013 13:36:28 PMAll use subject to JSTOR Terms and Conditions

S272 Current Anthropology Volume 53, Supplement 6, December 2012

growth, which is in turn dependent on resource availabilityand mortality probability (Kuzawa and Bragg 2012; Miglianoand Guillon 2012). This may provide a clue for possible in-terpretations of size variation in Homo. In human popula-tions, the greater the probability of mortality, the earlier isthe age of maturity (and the shorter the period of growth),to ensure maximum reproductive output. Based on the anal-ysis of small-scale human societies, Migliano and Guillon(2012) demonstrate that the main determinant in height var-iability in humans is the probability of mortality—the lowerthe mortality probability, the taller the populations. They alsoshow that environment has an important effect, although dietis not significant in their analyses. This probably results fromdata limitations in their large comparative analysis.

Kuzawa and Bragg (2012) emphasize this nutritional sideof the equation and argue that nutritional abundance is as-sociated with a faster growth rate, earlier maturity, and largeradult sizes. Because males are affected more than females,sexual size dimorphism is increased with greater nutritionalabundance. Nutritional stress has the opposite effects—slowergrowth, later maturity, and reduced size dimorphism. If mor-tality rates were high and precluded later maturity, smallerbody size would be expected.

Pfeiffer’s (2012) work on the bioarchaeological record ofthe small-bodied KhoeSan, however, reminds us of the mul-tifactorial effect on size and of the issue that small size maybe the default in the absence of selective factors for largersize. That is, bigger may not always be better. In her particularcase study she finds no evidence for the traditional drivers ofsmall size: nutritional insufficiency, early maturation, highextrinsic mortality, or climate change. In light of this, shesuggests instead that the long-term relaxation of selection forlarge size was allowed due to the relative isolation of theKhoeSan and therefore an absence of competition with large-sized human populations. This might favor increasingly largemale size and shape changes to the pelvis that accommodatedrelatively large infants in small mothers, which might oth-erwise favor large females. It is important, then, to considerthe multiple and sometimes conflicting causes for size changein light of their influence on developmental plasticity.

At present we lack the detailed data from the fossil recordto assess intraspecific differences in growth and developmentwith the aim of inferring the possible roles of nutrition, mor-tality probability, or other factors in shaping the observed sizedifferences in the hominins. However, because the size varia-tion, particularly in Homo erectus, is similar to that found inmodern humans (Migliano and Guillon 2012), there is everyreason to assume that similar factors were in play and that sizedifferences in the hominins also reflect an adaptive plasticitysimilar to that observed in modern humans. Migliano andGuillon (2012), Kuzawa and Bragg (2012), and Bribiescas andcolleagues (2012) also emphasize the important role of behav-ior, and particularly cooperation, in buffering both nutritionalsufficiency and mortality probability, thereby setting the stagefor body size increase. A series of alternative scenarios for con-

sidering especially the regional variation in H. erectus derivedfrom these principles are developed in detail in the concludingpaper of this issue (Anton and Snodgrass 2012).

Although more data are sorely needed, what we know aboutthe tempo and mode of growth in the early hominins, basedon rates of dental maturation, is summarized by Schwartz(2012). These data suggest that both Australopithecus and earlyHomo have more rapid maturation than Homo sapiens, whichcould reflect environments of higher extrinsic mortality. How-ever, there are probably differences between the genera as well.Dental eruption in Australopithecus and Paranthropus is com-parable to, or faster than, Gorilla gorilla beringei, the fastestof the living great apes. Dental eruption in H. erectus is equiv-alent to Pongo pygmaeus pygmaeus, the slowest of the livinggreat apes, and just below the large range of eruption ages inmodern humans. The somewhat extended developmentalschedule of H. erectus relative to Australopithecus is consistentwith mortality reduction and increased body size. There isstill much to learn, but one definite conclusion of Schwartz’ssynthesis is that the full suite of modern human life historywith extended periods of growth and development was notpresent in early Homo, including H. erectus, and possibly didnot appear in its modern form until much later in time.

While these papers address size differences between humanpopulations and what we know about the tempo of homininmaturation, Plavcan (2012) raises the important issue of sex-ual size dimorphism. He provides a detailed overview of whatis currently known (and knowable) about sexual size dimor-phism in hominins and living primates and provides a num-ber of caveats in relation to interpretation of the evidence. Ithas long been assumed that sexual dimorphism is a featureobservable in hominins that can be directly and causally re-lated to social behavior across primates, and particularly tomale competition over mates (Leigh 1992; Plavcan 2001; Plav-can and van Schaik 1997a, 1997b). However, Plavcan cautionsthat although all highly dimorphic primate species are po-lygynous, the inverse relationship is not straightforward, andany hominin inferences can only be made with extreme cau-tion.

A particularly important aspect of his research is the focuson female size. He argues that female size represents the op-timum for the particular environment, and male size repre-sents a trade-off between the costs of deviating from thisoptimum and the benefits of larger size in mate competition.Female size change alone does not result in marked changesin dimorphism across extant species. Although there is somevariation within species, without a change in mating com-petition, which drives male size increase or decrease, an in-crease in female size should simply be tracked by an equivalentincrease in male size. Beyond this, he notes that size variationwithin H. erectus is “unremarkable” relative to human levelsand that our understanding of sexual size dimorphisms re-quires better understanding of temporal changes in male andfemale size relative to each other in both humans and non-human primates. He makes a specific call for further system-

This content downloaded on Wed, 23 Jan 2013 13:36:28 PMAll use subject to JSTOR Terms and Conditions

Aiello and Anton Origins of Homo S273

atic research on intraspecific geographic and sexual variationin primates.

The evolution of male life history trade-offs has not beena major focus in hominin evolution, outside of the relativelysimple association between reduced dimorphism and a moveaway from polygynous social systems and strong size-drivenmating competition. However, Bribiescas and colleagues(2012) argue that it is central to the evolution of Homo.Consistent with Plavcan’s interspecific analyses demonstratingno correspondence between reduced dimorphism and anyparticular social system, Bribiescas and colleagues (2012)point to the fact that humans, with their relatively reduceddimorphism, are unique among the apes not only in thedegree of their paternal parenting behavior but also in itsvariation. There is variation in the amount of time and energyinvested and in the type of offspring care, provisioning, andother involvement. Paternal parenting behavior is dependenton context, including the availability of other caregivers suchas grandmothers or siblings.

The important point is the move away from energy in-vestment in large male size that permits not only energy al-location to parenting behavior but also to other aspects oflife history including fertility and longevity. One particularlynovel aspect of their work is the argument that increased malefertility at older ages may have contributed to the emergenceof female longevity and the evolution of the female postre-productive lifespan and increased female reproductive effortthrough grandmothering and child care. This provides analternative, or perhaps complementary, explanation to theGrandmothering Hypothesis (Hawkes et al. 1998, 2011; Ka-chel, Premo, and Hublin 2011a, 2011b; O’Connell, Hawkes,and Blurton Jones 1999).

It is now clear that we cannot be sure whether there wasany significant difference in dimorphism between Australo-pithecus and Homo (either early Homo or H. erectus ; Anton2012; Holliday 2012; Plavcan 2012; Pontzer 2012). However,if we could be sure that dimorphism was reduced in H. erectus,we would have direct evidence of a change in mating behaviorleading to a reduction of sexual selection acting on male size,and likely involving a major change in social organizationinvolving increased levels of cooperation and allocare, andperhaps increased longevity. Given the importance of under-standing the relationship between male and female size, itwould be useful to expend the effort to understand morecritically the relationship between skeletal and body mass di-morphism and how it varies among modern human popu-lations.

Models for Cooperation, Sociality, LifeHistory, Body Size, and Brain Size

Although we cannot be confident that there was a change insexual size dimorphism associated with the evolution ofHomo, the idea that cooperation and food sharing may havebeen major distinguishing factors between the Australopithe-

cus and early Homo was a common theme that developedfrom a variety of perspectives throughout the symposium.Many of the previously mentioned papers suggest that theabilities to maximize food and to limit predation are criticalto increasing body and brain size. Food sharing and conse-quent group cooperation are means of achieving this. In lightof the several lines of evidence pointing toward the impor-tance of sociality and cooperation, several symposium par-ticipants explored the correlates of such behavior in extantorganisms as well as the concept of cooperation as a meansof expanding an organism’s “capital.”

Nonhuman primates provide the logical starting point be-cause of their close phylogenetic relationship to humans. Theydemonstrate the roots of human evolutionary plasticity par-ticularly in dietary/niche expansion, extended life history, andincreasing social complexity with extensive cooperation andcommunication (Anton and Snodgrass 2012). These abilitiesprovide the basis for the elaborate niche construction ob-served in humans that involves accelerating biocultural com-plexity and an increasing reliance on cooperation in all aspectsof hominin life (Fuentes, Wyczalkowski, and MacKinnon2010; Odling-Smee, Laland, and Feldman 2003).

However, primates are not the only models for homininbehavior, and useful insights can be drawn from, for example,other large-brained animals such as dolphins, cooperativebreeders among all orders, and large-bodied mammal speciesthat inhabit woodland and savanna environments. In the late1960s, Schaller and Lowther (1969) wrote a now classic paperon the relevance of carnivore behavior to the study of earlyhominins. Their basic premise was that to understand socialityin hominins, it would be productive to draw inferences fromanimals that are ecologically similar, such as social carnivores,as well as animals that were closely related, such as the pri-mates. At the time, this work represented a major innovationin the interpretation of early hominin behavior. Smith andcolleagues (2012) take up where Schaller and Lowther left offmore than 40 years ago to demonstrate commonalities inbehavior and morphology between humans and the Carni-vora. Sociality among carnivores is the exception rather thanthe rule, but significant features related to sociality and co-operation in the Carnivora include cursorial hunting of largegame in open habitats, a relatively tall body build (shoulderheight in relation to body mass), reduced sexual dimorphism,larger brains, a high reproductive output (in this case largerlitters), allocare of infants, increased weaning age, and largerpopulation density. Many of these features (with the possibleexception of reduced sexual dimorphism) are reminiscent ofthe morphology of Homo and may help to infer behavior andlife history of early Homo (including Homo erectus). Indeed,many of these features in social carnivores help to reinforcethe inferred relationships between dietary change, morphol-ogy, and cooperation reached by other symposium partici-pants using other data sets.

Brain size expansion in Homo may provide another, in-dependent avenue for inferring the presence of cooperative

This content downloaded on Wed, 23 Jan 2013 13:36:28 PMAll use subject to JSTOR Terms and Conditions

S274 Current Anthropology Volume 53, Supplement 6, December 2012

breeding. Isler and van Schaik (2012) draw on their previouswork on the Expensive Brain Hypothesis (Isler and van Schaik2009) and on a large comparative mammalian database tounderscore the relationship between brain size increase andcooperation in the form of allocare. They argue that acrossmammals, large brain size is generally correlated with a re-duction in population growth rate. This correlation is drivenby the extended ontogenetic periods necessary for the growthof larger-bodied and larger-brained offspring and the corre-spondingly longer interbirth intervals required. Allocare pro-vides extra resources to the mother, resulting in early weaningof the infant and a shorter interbirth interval. An importantaspect of their work is the prediction based on extant primatesthat a mean endocranial capacity of 600–700 cc would be the“gray ceiling” beyond which cooperation in the form of allo-care would be essential if the population were to grow fastenough to replace itself and avoid extinction. The great apesseem to be at the very farthest extension of this relationship,just barely allowing population replacement without allocare.The fact that the mean brain size of the largest-bodied Aus-tralopithecus species (478 cc in Australopithecus afarensis; Hol-loway, Broadfield, and Yuan 2004) converges on that of mod-ern apes suggests that cooperative breeding probably had notyet appeared in these early hominins. However, by the timeof H. erectus, with brain sizes uniformly over 700 cc, repro-ductive cooperation would seem to have been a necessity.

Allocare is undoubtedly a key element that enabled hom-inins to break through the gray ceiling, but it is only oneelement of capital in hominin evolution (Kaplan, Lancaster,and Robson 2003; Kaplan et al. 2000). Wells (2012) sees capitalas a key and integrating concept that brings together manyof the themes of intraspecific and interspecific variation andplasticity that emerged from the symposium. He defines cap-ital as a generalized energy currency that can be expended ina variety of ways to increase adaptive flexibility. This resultsin the fact that humans are “uniquely under-committed” toany specific niche.

Wells (2012) talks about social capital as well as physicalcapital. Social capital is facilitated by larger brain sizes, storedin social relationships and variably expended to achieve pred-ator protection or to enable food security (particularlythrough sexual division of labor and allocare). Physical capitalis stored within the body as adipose tissue or extracorporeallyin food hoards and similarly used to avoid predators andprovide sufficient nutrition. Because it is difficult to assessfrom the fossil record, adipose tissue has received relativelylittle attention in human evolution. Yet it is a major featuredistinguishing us (and our sexual dimorphism; see Anton andSnodgrass 2012; Plavcan 2012) from nonhuman primates andone that buffers the costs of reproduction against food short-ages in fluctuating environments (Knott 1998; Kuzawa 1998).It also is the source of signaling molecules responsible forenergy trade-offs between competing biological functionssuch as growth, immune function, and reproduction (Wells2009). It may be one of the major factors responsible for

differences in life history strategies among human popula-tions, and it has been correlated with large brain sizes acrossmammals (Navarrete, van Schaik, and Isler 2011). It is thusimportant to develop creative ways to infer adiposity fromfossil record.

Storing energy in generalized currencies (social relation-ships and adipose tissue) means that various aspects of lifehistory (growth, reproduction, and immunity) can be“funded” according to the state of the environment and over-all energy availability (Wells 2012). If conditions demand it,one aspect may be prioritized at the expense of others, re-sulting in the life history variation and its outcomes in featuressuch as growth rates, adult body size, fertility, and possiblylifespan that are observed in modern humans (Bribiescas,Ellison, and Gray 2012; Kuzawa and Bragg 2012; Miglianoand Guillon 2012). The symposium provided a vehicle forbringing together the disparate data sets of our subdisciplinesinto a framework that suggests ways in which variation isproduced, organized, and interrelated in the extant world.

What We Know Now and What We Hope toKnow in the Future

The final paper of the volume takes up the challenge of usingthis framework to generate means of assessing the variationobserved in the fossil record and the biocultural relationshipbetween hominin morphology, hominin behavior, and thefluctuating environment of the African Pliocene and Pleis-tocene (Anton and Snodgrass 2012). What we know is thatearly Homo existed in a highly variable environment (a non-equilibrium ecosystem) that may have placed adaptive plas-ticity at a premium. The type of body size variation observedin early Homo is consistent with the range observed in modernhumans that is mediated by life history differences in growthand development that are dependent on energy availabilityand mortality probability (Kuzawa and Bragg 2012; Miglianoand Guillon 2012; Migliano, Vinicius, and Lahr 2007). Dietarydifferences, involving increased dietary breadth and a morecarnivorous diet, are also evident between these hominins andare consistent with greater adaptive plasticity than inferredfor the australopiths. Comparative studies suggest that larger-bodied hominins would have had to adopt a high energythroughput strategy, and this, together with the increase inbrain size, would presuppose increased cooperation in theform of allocare and sexual division of labor. The increasedcooperation and sociality would also be significant in groupprotection in a relatively dangerous terrestrial environmentand create a relatively safe “niche” that would be consistentwith later maturation (and perhaps increased longevity) thatbegins to be evident in the dental maturation evidence forHomo erectus in relation to the other early hominins.

These results provide the basis for a model for the evolutionof Homo involving an integrated feedback loop that drove lifehistory evolution and contributed to cultural change (Antonand Snodgrass 2012). The central elements of this model are

This content downloaded on Wed, 23 Jan 2013 13:36:28 PMAll use subject to JSTOR Terms and Conditions

Aiello and Anton Origins of Homo S275

cooperative behavior, diet, cognitive abilities, and extrinsicmortality risk. This model also generates a number of testablehypotheses (e.g., to explain size variation in the hominins),but each requires additional data from both the fossil and themodern records to test. Increased body size across genera maybe hypothesized to signal either decreased extrinsic mortality,increased nutritional sufficiency, or both. On the one hand,specific predictions are made about rates of growth, timingof weaning, and timing of growth cessation in each scenario—so additional growth data, especially from dental developmentand especially for the earliest Homo, are needed. But beyondsimply “finding more fossils,” additional means of assessing“hard tissue” growth in extant populations in ways that arecomparable to fossil samples are needed—and the need toconsider intrapopulational variation among extant primateand nonprimate mammals to the level that it is done in humanpopulations is also required. There are issues of scale andcomparability in our data sets that can only be remedied bylong-term analyses of extant populations.

The increase in brain and body size between other earlyHomo and H. erectus again suggests decreased extrinsic mor-tality and/or increased nutrition. These features may reflectadaptations to the higher mortality rates in terrestrial envi-ronments and perhaps cooperative hunting—as shown in thesocial carnivores. The extended developmental schedule of H.erectus is consistent with mortality reduction, possibly as theresult of behavioral changes involving some form of coop-eration. Here, along with additional fossil data and more com-parable data sets, a key endeavor will be mining the rich socialcarnivore data sets and creating new ones that consider aspectsof morphology, behavior, and variation. Regarding extrinsicmortality, new ways can and should be developed to inter-rogate the archaeological and extant records for predator loadand the degree to which this can be assessed for differenthominin species. Novel means should also be developed forassessing the ways in which population variation and physicalcharacter development (including size and dimorphism) areinfluenced by the high degree of climate variability that char-acterized this period of evolutionary history.

For example, it has been hypothesized for the small-sizedDmanisi sample that nutritional insufficiency and perhapsisolation resulted in short-term accommodations or adapta-tions (e.g., Anton 2003, 2013; see also Migliano and Guillon2012). If this is the case, one would expect lower growth ratesand a longer period of maturation in relation to larger-bodiedH. erectus. Alternatively, if the short stature was due to a highmortality environment, one would expect the smaller speci-mens to have more rapid growth rates and a shorter periodof maturation. Clearly this requires greater detail than wecurrently possess regarding H. erectus growth, but it providesa place to start.

While we end, as many such symposia do, with a decidedplea for more fossil remains in different localities, we hopeto move beyond that to an agenda of integrated and multi-disciplinary studies to provide a framework against which to

test these predictions. For example, to understand the factorssurrounding the evolution of Homo, it will be essential totarget intra- and interpopulational research on energetic andlife history variation in various climatic, nutritional, and mor-tality environments. We need to tease out the complex in-terrelationships among these and other variables to under-stand the fundamental correlates of body size, brain size, andsexual dimorphism (Kuzawa and Bragg 2012; Smith et al.2012). Among many other things, we want to know, for ex-ample, how skeletal dimorphism tracks body mass dimor-phism across populations, especially in humans (Plavcan2012), and how individual skeletal features are influenced inmales and females in differing circumstances (Bribiescas, El-lison, and Gray 2012; Kuzawa and Bragg 2012).

Our thinking about the origins of Homo has continued tochange since Homo habilis was announced (Leakey, Tobias,and Napier 1964), the almost complete Nariokotome H.erectus skeleton was discovered (Brown et al. 1985), andDmanisi and other material changed our ideas about variationin H. erectus (Gabunia et al. 2000; Potts et al. 2004; Simpsonet al. 2008; Spoor et al. 2007). New fossils will undoubtedlycontinue to be uncovered. However, this material cannot beinterpreted in a vacuum, and the more we know about intra-and interspecific variation in modern humans and other an-imals, the stronger the foundation we have for a rich under-standing of our evolutionary past.

Acknowledgments

We would like to thank the Wenner-Gren Foundation for theopportunity to hold this symposium and to publish the resultsas an open-access supplementary issue of Current Anthro-pology. We would also like to thank all of the participants forlively and stimulating discussion and debate over a 6-dayperiod in March 2011 at the Tivoli Palacio de Seteais Hotelin Sintra, Portugal. This experience will be fondly remem-bered for a long time to come. We would like to give specialthanks to Chris Rainwater (New York University), who servedas the rapporteur for the meeting; to Emily Middletown (NewYork University), who provided invaluable assistance in help-ing to edit and prepare all of the manuscripts for publication;and to Lisa McKamy and the editorial and production staffat the University of Chicago Press for their help in bringingthis issue to fruition. The meeting would not have been assuccessful as it was without the deft organizational skills ofLaurie Obbink, the Wenner-Gren Foundation Conference As-sociate, and for this we are most grateful.

References CitedAiello, Leslie C., and Catherine Key. 2002. Energetic consequences of being

a Homo erectus female. American Journal of Human Biology 14:551–565.Aiello, Leslie C., and Jonathan C. K. Wells. 2002. Energetics and the evolution

of the genus Homo. Annual Review of Anthropology 31:323–338.Aiello, Leslie C., and Peter Wheeler. 1995. The expensive-tissue hypothesis:

This content downloaded on Wed, 23 Jan 2013 13:36:28 PMAll use subject to JSTOR Terms and Conditions

S276 Current Anthropology Volume 53, Supplement 6, December 2012

the brain and the digestive system in human and primate evolution. CurrentAnthropology 36:199–221.

Anton, Susan C. 2003. Natural history of Homo erectus. Yearbook of PhysicalAnthropology 46:126–170.

———. 2008. Framing the question: diet and evolution in early Homo. InPrimate craniofacial function and biology: papers in honor of Bill Hylander.Christopher J. Vinyard, Christine E. Wall, and Matthew J. Ravosa, eds. Pp.443–482. New York: Springer.

———. 2012. Early Homo: who, when, and where. Current Anthropology53(suppl. 6):S278–S298.

———. 2013. Homo erectus and related taxa. In Companion to paleoanthro-pology. David Begun, ed. Hoboken, NJ: Wiley-Blackwell.

Anton, Susan C., William R. Leonard, and Marcia Robertson. 2002. An eco-morphological model of the initial hominid dispersal from Africa. Journalof Human Evolution 43:773–785.

Anton, Susan C., and J. Josh Snodgrass. 2012. Origins and evolution of genusHomo: new perspectives. Current Anthropology 53(suppl. 6):S479–S496.

Asfaw, Berhane, Tim White, Owen Lovejoy, Bruce Latimer, Scott Simpson,and Gen Suwa. 1999. Australopithecus garhi: a new species of early hominidfrom Ethiopia. Science 284:629–635.

Berger, Lee R., Darryl de Ruiter, Steven E. Churchill, Peter Schmid, KristianJ. Carlson, Paul H. G. M. Dirks, and Job M. Kibii. 2010. Australopithecussediba: a new species of Homo-like australopith from South Africa. Science328:195–204.

Blumenschine, Robert J., Charles C. Peters, Fidelis T. Masao, Ron J. Clarke,Alan L. Deino, Richard L. Hay, Carl C. Swisher, et al. 2003. Late PlioceneHomo and hominid land use from western Olduvai Gorge, Tanzania. Science299:1217–1221.

Braun, David R., Thomas Plummer, Peter Ditchfield, Joseph V. Ferraro, DavidMaina, Laura C. Bishop, and Richard Potts. 2008. Oldowan behavior andraw material transport: perspectives from the Kanjera Formation. Journalof Archaeological Science 35:2329–2345.

Bribiescas, Richard G., Peter T. Ellison, and Peter B. Gray. 2012. Male lifehistory, reproductive effort, and the evolution of the genus Homo. CurrentAnthropology 53(suppl. 6):S424–S435.

Brown, Frank, John Harris, Richard Leakey, and Alan Walker. 1985. EarlyHomo erectus skeleton from west Lake Turkana, Kenya. Nature 316:788–792.

Fuentes, Agustın, Matthew A. Wyczalkowski, and Katherine C. MacKinnon.2010. Niche construction through cooperation: a nonlinear dynamics con-tribution to modeling facets of the evolutionary history in the genus Homo.Current Anthropology 51:435–444.

Gabunia, Leo, Abesalom Vekua, David Lordkipanidze, Carl C. Swisher, ReidFerring, Antje Justus, Medea Nioradze, et al. 2000. Earliest Pleistocenecranial remains from Dmanisi, Republic of Georgia: taxonomy, geologicalsetting, and age. Science 288:1019–1025.

Haile-Selassie, Yohannes, Bruce M. Latimer, Mulugeta Alene, Alan L. Deino,Luis Gibert, Stephanie M. Melillo, Beverly Z. Saylor, Gary R. Scott, and C.Owen Lovejoy. 2010. An early Australopithecus afarensis postcranium fromWoranso-Mille, Ethiopia. Proceedings of the National Academy of Sciences ofthe USA 107:12121–12126.

Hawkes, Kristen, Peter S. Kim, Brett Kennedy, Ryan Bohlender, and JohnHawks. 2011. A reappraisal of grandmothering and natural selection. Pro-ceedings of the Royal Society B 278:1936–1938.

Hawkes, Kristen, James F. O’Connell, Nicholas G. Blurton Jones, Helen Al-varez, and Eric L. Charnov. 1998. Grandmothering, menopause, and theevolution of human life histories. Proceedings of the National Academy ofSciences of the USA 95:1336–1339.

Holliday, Trenton W. 2012. Body size, body shape, and the circumscriptionof the genus Homo. Current Anthropology 53(suppl. 6):S330–S345.

Holloway, Ralph L., Douglas C. Broadfield, and Michael S. Yuan. 2004. Thehuman fossil record: brain endocasts: the paleoneurological evidence. New York:Wiley.

Isler, Karin, and Carel P. van Schaik. 2009. The expensive brain: a frameworkfor explaining evolutionary changes in brain size. Journal of Human Evo-lution 57:392–400.

———. 2012. How our ancestors broke through the gray ceiling: comparativeevidence for cooperative breeding in early Homo. Current Anthropology53(suppl. 6):S453–S465.

Kachel, A. Friederike, Luke S. Premo, and Jean-Jacques Hublin. 2011a. Grand-mothering and natural selection. Proceedings of the Royal Society B 278:384–391.

———. 2011b. Invited reply: grandmothering and natural selection revisited.Proceedings of the Royal Society B 278:1939–1941.

Kaplan, Hillard, Kim Hill, Jane Lancaster, and Magdalena A. Hurtado. 2000.A theory of human life history evolution: diet, intelligence, and longevity.Evolutionary Anthropology 2000:156–185.

Kaplan, Hillard, Jane Lancaster, and Arthur Robson. 2003. Embodied capitaland the evolutionary economics of the human life span. In Life span: evo-lutionary, ecological, and demographic perspectives. James R. Carey and Shri-pad Tuljapurkar, eds. Pp. 152–182. New York: Population Council.

Kibii, Job M., Steven E. Churchill, Peter Schmid, Kristian J. Carlson, NichelleD. Reed, Darryl J. de Ruiter, and Lee R. Berger. 2011. A partial pelvis ofAustralopithecus sediba. Science 333:1407–1411.

Kimbel, William H., Robert C. Walter, Donald C. Johanson, Kaye E. Reed,James L. Aronson, Zelalem Assefa, Curtis W. Marean, et al. 1996. LatePliocene Homo and Oldowan tools from the Hadar Formation (Kada HadarMember), Ethiopia. Journal of Human Evolution 31:549–561.

Kivell, Tracy L., Job M. Kibii, Steven E. Churchill, Peter Schmid, and Lee R.Berger. 2011. Australopithecus sediba hand demonstrates mosaic evolutionof locomotor and manipulative abilities. Science 333:1411–1417.

Knott, Cheryl D. 1998. Changes in orangutan caloric intake, energy balance,and ketones in response to fluctuating fruit availability. International Journalof Primatology 19:1061–1079.

Kuzawa, Christopher W. 1998. Adipose tissue in human infancy and child-hood: an evolutionary perspective. Yearbook of Physical Anthropology 41:177–209.

Kuzawa, Christopher W., and Jared M. Bragg. 2012. Plasticity in human lifehistory strategy: implications for contemporary human variation and theevolution of genus Homo. Current Anthropology 53(suppl. 6):S369–S382.

Leakey, Louis S., Philip V. Tobias, and John R. Napier. 1964. A new speciesof the genus Homo from Olduvai Gorge. Nature 202:7–9.

Leakey, Meave G., Fred Spoor, M. Christopher Dean, Craig S. Feibel, SusanC. Anton, Christopher Kiarie, and Louise N. Leakey. 2012. New fossils fromKoobi Fora in northern Kenya confirm taxonomic diversity in early Homo.Nature 488:201–204, doi:10.1038/nature11322.

Leigh, Steven R. 1992. Patterns of variation in the ontogeny of primate bodysize dimorphism. Journal of Human Evolution 23:27–50.

Leonard, William R., and Marcia L. Robertson. 1997. Comparative primateenergetics and hominid evolution. American Journal of Physical Anthropology102:265–281.

McHenry, Henry M., and Katherine Coffing. 2000. Australopithecus to Homo:transformations in body and mind. Annual Review of Anthropology 29:125–146.

McPherron, Shannon P., Zeresenay Alemseged, Curtis W. Marean, JonathanG. Wynn, Denne Reed, Denis Geraads, Rene Bobe, and Hamdallah A.Bearat. 2010. Evidence for stone-tool-assisted consumption of animal tissuesbefore 3.39 million years ago at Dikika, Ethiopia. Nature 466:857–860.

Migliano, Andrea Bamberg, and Myrtille Guillon. 2012. The effects of mor-tality, subsistence, and ecology on human adult height and implications forHomo evolution. Current Anthropology 53(suppl. 6):S359–S368.

Migliano, Andrea B., Lucio Vinicius, and Marta Mirazon Lahr. 2007. Lifehistory trade-offs explain the evolution of human pygmies. Proceedings ofthe National Academy of Sciences of the USA 104:20216–20219.

Navarrete, Ana, Carel P. van Schaik, and Karin Isler. 2011. Energetics and theevolution of human brain size. Nature 480:91–93.

O’Connell, James F., Kristin Hawkes, and Nicolas B. Blurton Jones. 1999.Grandmothering and the evolution of Homo erectus. Journal of HumanEvolution 36:461–485.

Odling-Smee, F. John, Kevin N. Laland, and Marcus W. Feldman. 2003. Nicheconstruction: the neglected process in evolution. Monographs in PopulationBiology 37. Princeton, NJ: Princeton University Press.

Pfeiffer, Susan. 2012. Conditions for evolution of small adult body size insouthern Africa. Current Anthropology 53(suppl. 6):S383–S394.

Plavcan, J. Michael. 2001. Sexual dimorphism in primate evolution. AmericanJournal of Physical Anthropology 116:25–53.

———. 2012. Body size, size variation, and sexual size dimorphism. CurrentAnthropology 53(suppl. 6):S409–S423.

Plavcan, J. Michael, and Carel P. van Schaik. 1997a. Interpreting hominidbehavior on the basis of sexual dimorphism. Journal of Human Evolution32:345–374.

———. 1997b. Intrasexual competition and body weight dimorphism in an-thropoid primates. American Journal of Physical Anthropology 103:37–68.

Pontzer, Herman. 2012. Ecological energetics in early Homo. Current An-thropology 53(suppl. 6):S346–S358.

This content downloaded on Wed, 23 Jan 2013 13:36:28 PMAll use subject to JSTOR Terms and Conditions

Aiello and Anton Origins of Homo S277

Potts, Richard. 2012. Environmental and behavioral evidence pertaining tothe evolution of early Homo. Current Anthropology 53(suppl. 6):S299–S317.

Potts, Richard, Anna K. Behrensmeyer, Alan Deino, Peter Ditchfield, andJennifer Clark. 2004. Small mid-Pleistocene hominin associated with EastAfrican Acheulean technology. Science 305:75–78.

Ruff, Christopher B. 1993. Climatic adaptation and hominid evolution: thethermoregulatory imperative. Evolutionary Anthropology 2:53–60.

———. 2010. Body size and body shape in early Homo: implications of theGona pelvis. American Journal of Physical Anthropology 58:166–178.

Schaller, George B., and Gordon R. Lowther. 1969. Relevance of carnivorebehavior to study of early hominids. Southwestern Journal of Anthropology25:307–341.

Schwartz, Gary T. 2012. Growth, development, and life history throughoutthe evolution of Homo. Current Anthropology 53(suppl. 6):S395–S408.

Semaw, Sileshi, Michael J. Rogers, Jay Quade, Paul R. Renne, Robert F. Butler,Manuel Dominguez-Rodrigo, Dietrich Stout, William S. Hart, Travis Pick-ering, and Scott W. Simpson. 2003. 2.6-million-year-old stone tools andassociated bones from OGS-6 and OGS-7, Gona, Afar, Ethiopia. Journal ofHuman Evolution 45:169–177.

Shipman, Pat, and Alan Walker. 1989. The costs of becoming a predator.Journal of Human Evolution 18:373–392.

Simpson, Scott W., Jay Quade, Naomi E. Levin, Robert Butler, GuillaumeDupont-Nivet, Melanie Everett, and Sileshi Semaw. 2008. A female Homoerectus pelvis from Gona, Ethiopia. Science 322:1089–1092.

Smith, Jennifer E., Eli M. Swanson, Daphna Reed, and Kay E. Holekamp.

2012. Evolution of cooperation among mammalian carnivores and its rel-evance to hominid evolution. Current Anthropology 53(suppl. 6):S436–S452.

Spoor, Fred, Meave G. Leakey, Patrick N. Gathogo, Frank H. Brown, SusanC. Anton, Ian McDougall, Christopher Kiarie, F. Kyalo Manthi, and LouiseN. Leakey. 2007. Implications of new early Homo fossils from Ileret, eastof Lake Turkana, Kenya. Nature 448:688–691.

Ungar, Peter S. 2012. Dental evidence for the reconstruction of diet in Africanearly Homo. Current Anthropology 53(suppl. 6):S318–S329.

Walker, Alan, and Richard Leakey, eds. 1993. The Nariokotome Homo erectusskeleton. Cambridge, MA: Harvard University Press.

Wells, Jonathan C. K. 2009. The evolutionary biology of human body fat: thriftand control. Cambridge: Cambridge University Press.

———. 2012. The capital economy in hominin evolution: how adipose tissueand social relationships confer phenotypic flexibility and resilience in sto-chastic environments. Current Anthropology 53(suppl. 6):S466–S478.

Wood, Bernard, and Jennifer Baker. 2011. Evolution in the genus Homo.Annual Review of Ecology, Evolution, and Systematics 42:47–69.

Wood, Bernard A., and Mark C. Collard. 1999. The human genus. Science284:65–71.

———. 2007. Defining the genus Homo. In Handbook of paleoanthropology.Winfried Henke, Helen Rothe, and Ian Tattersall, eds. Pp. 1575–1610. Ber-lin: Springer.

Zipfel, Bernhard, Jeremy M. DeSilva, Robert S. Kidd, Kristian J. Carlson,Steven E. Churchill, and Lee R. Berger. 2011. The foot and ankle of Aus-tralopithecus sediba. Science 333:1417–1420.

This content downloaded on Wed, 23 Jan 2013 13:36:28 PMAll use subject to JSTOR Terms and Conditions