tattersall 2007 speciation

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JASs ReportsJournal of Anthropological Sciences

Vol. 85 (2007), pp. 139-146

the JASs is published by the I s t i t u to I t a l i ano d i An t ropo log ia www.isita-org.com

Neanderthals, Homo sapiens, and the question of speciesin paleoanthropology

Ian Tattersall

Division of Anthropology, American Museum of Natural History, New York NY 10024, USAe-mail: [email protected]

Summary - Species boundaries in the fossil record are frustratingly elusive to recognize, largely becauseof the untidy way in which biological diversity is packaged. Accepting that species are most fundamentallyindividuals, with origins, births, and extinctions, rather than essentialist collections of traits, means alsoaccepting that neither qualitative nor quantitative assessments of morphology will provide an infallible

guide to species status. Beyond a certain level of differentiation the problem ceases, however, and this levelis comfortably exceeded byHomo neanderthalensis. What is more, the Neanderthals appear not simplyto constitute a separate species lineage, but to form part of a larger endemic European clade to which suchdistinctive forms as the Sima de los Huesos and Steinheim fossils also belong. Tis clade has a contemporaryoccurrence in Europe with the larger and more cosmopolitan speciesHomoheidelbergensis (exemplified byspecimens such as those from Arago, Petralona, Kabwe, and Bodo, and probably also Dali and Jinniushan),showing that hominid history in this region is more complex than simply that of a sincle lineage evolvingtoward the terminal speciesHomo neanderthalensis.

Keywords - Neanderthals, European hominids, Homo neanderthalensis, Homo heidelbergensis,Human evolution, Species recognition.

Introduction

No problem in paleoanthropology is morefraught than that of recognizing species inthe hominid fossil record. How to apportionthe large mass of hominid fossils now knowninto biologically meaningful units has beendebated endlessly, and seems set to splinter

paleoanthropology for years to come. Te negativeconsequences of this lack of consensus are severe,not least because this failure to agree on our basicevolutionary units deprives our discipline of acommon terminology. Tis is bad enough amongcolleagues who know more or less what theirprofessional counterparts believe; but it is nothingshort of disastrous when it comes to communicatingour science to the public that supports us.

Tere are many reasons for this discord on

the matter of species and their recognition in thehominid record; but perhaps the unfortunatestate of affairs just described is above all a matterof unrealistic expectations. Te reductionisthuman mind is most comfortable operating inan environment where boundaries are sharp andcan be taken for granted; gray areas are alwaysproblematic. But while species are the basic units

into which groups of individuals are packagedamid the rampant diversity of Nature, suchpackaging is not necessarily neat. And it is preciselythe lack of tidiness so evident in the packagingof Nature that is reflected in the huge variety ofspecies definitions currently on offer. Indeed,there are currently so many definitions of speciesto choose from (at least 25 according to Coyne &Orr, 2004) that to make them usable they have inturn been subdivided into categories: inclusionary,


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140 The question of species in paleoanthropology

exclusionary, evolutionary, and so forth. All ofthese definitions have their attractions, all havetheir failings; but not one of them usefully appliesto all cases in the real world. Indeed it appearsthat, in our ongoing search for a better way ofdefining species in the abstract, we end up pleasingnobody in the attempt to please everybody.

Perhaps, then, it is time to step back and viewthe problem from another angle. Species are theend products of speciation, and almost everyspecies definition available takes for granted that

the processes of speciation, by which speciesoriginate, must produce a basically uniformoutcome. Yet the reality is that species, whichare best viewed as the products of speciationrather than as a definable category in themselves,are far from consistent in their characteristics.For speciation is nota unitary mechanism, andspecies are best viewed as a result that is onlyvisible in retrospect and that may come aboutthrough a whole raft of causes. Speciation maybe associated with major developmental changesin a population; or it may be associated with tinyexternal morphological cues, or even with subtleshifts in behavioral preferences. Te only thingthat all potential agents of speciation (at levelsranging from point mutations to chromosomalrearrangements to developmental cascades)need have in common is that they result in theestablishment of a population as an effectivelyindependent historical entity. Michael Ghiselin(1974) pinpointed the crux of the matter when hedescribed species as individuals; and he was wisenot to tout this characterization as a definition.

Te notion of species as individuals placesspecies in the same category as pornography, atleast insofar as it acknowledges that the entityconcerned defies absolute definition, and thusposes problems essentially of recognition. Italso means that the kind of problem that alphataxonomists face is not unique. Judges share it. And judges, who claim to know pornographywhen they see it even if they cannot satisfactorilydefine it, have long decided such cases on thepreponderance of the evidence. Perhaps this ishow paleoanthropologists should also proceed,and cease worrying about whether long-extinct

populations, now known only from fossils, couldpotentially have exchanged genes with others ornot something that is essentially unknowable.And it is also, to a large extent, irrelevant. Speciesare not like absolutely watertight containers: theyare dynamic and more or less cohesive entities which, to misappropriate with apologies a termfrom Niles Eldredge (2003), resemble a sloshingbucket, in which gene frequencies swill back andforth within the containing walls, the occasionaloverflow being effectively lost (attersall, in press).

Te key attribute of species is that they maintaintheir historical identity over sustained periods oftime, regardless of any incidental gene inflow oroutflow at the edges. In which case, the essentialquestion that has to be decided on the balance ofthe evidence is not whether a morphologically orotherwise diagnosable population can and/or didexchange genes with close relatives; it is whetherthis population is or was a fully individuatedhistorical entity, with a unique history. If we canshow reasonable cause to believe that such anentity is or was beyond the point of reticulation, we are justified in naming it a distinct species,

and in regarding it as a fully individuated actor inthe evolutionary play. Lines of evidence availableto enter into this evaluation are numerous, andall are embedded in one or another of the manyspecies definitions to hand. But it is important toappreciate that, short of the total inability to formzygotes, all of these lines of evidence, whether theyinvolve morphology, hybridization, behavior, orwhatever, are indirect. Among fossil forms andamong living ones with any degree whatever ofinterbreeding capacity there is, quite simply,no silver bullet that will infallibly tell you thatyou are dealing with an historically individuatedentity. And this is why we are obliged to lookat the preponderance of the evidence, limitedin this contribution to the strictly biological.

Homo neander t ha lens is:

morphological distinctiveness

From the very beginning there have beentwo opposing views on how to classify the large-


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brained fossil hominids we know informallyas Neanderthals. In the tradition establishedby Schaaffhausen (1859) and Huxley (1863)shortly after the discovery of the Feldhofertype fossil, many paleoanthropologists todaycontinue to regard the Neanderthals as a variantof Homo sapiens, H. s. neanderthalensis (e.g.,rinkaus, 1983; Wolpoff et al., 2004). On theother hand, many incline to the almost equallyvenerable view, first articulated by William Kingin 1864, that the Neanderthals constitute their

own species, Homo neanderthalensis (attersall,1986; Stringer & Gamble, 1993, Rak, 1998). Itis important to note that this is not an issue thatwill ever be settled by compromise between theseextremes: either the Neanderthals constitutedan individuated historic entity, or they did not,although there is no way to knowa priorihow suchindividuation might be expressed in the record.

Interestingly, there has never been anyreally significant debate over whether or notthe Neanderthals form a diagnosible entity. Inrecent decades suggestions have occasionally beenmade that one or another fossil reflects some

degree of hybridization between Neanderthaland modern populations (e.g. Wolpoff, 1980;Duarte et al., 1999); but none of them haswithstood close scrutiny, and none impinges oncore Neanderthal distinctiveness as reflected bythese hominids well-entrenched informal name.Indeed, Jeffrey Schwartz and I have argued thatin terms of morphology alone the Neanderthalsconstitute the most clearly demarcated extinctgroup of extinct hominids known (attersall &Schwartz, 2000, 2006; Schwartz & attersall,2002, 2005). Whether a particular fossil is or isnot a Neanderthal has rather rarely been an issue;and indeed, the boundaries of Neanderthal-nesshave tended to become somewhat sacrosanct, aswitness the allocation by Arsuaga et al. (1997) ofthe Atapuerca/Sima hominids to the species Homoheidelbergensis, despite the fact that they exhibit ahost of apomorphies in common with Neanderthals and few, if any, with the other fossils moreconventionally ascribed to Homo heidelbergensis.

Among the features traditionally cited asNeanderthal apomorphies (see, for example,

Hublin, 1978, 1988; Santa Luca, 1978;Vandermeersch, 1981; Stringer et al., 1984;Schwartz and attersall, 1996a,b, 2005; Rak &Hylander, 2003) are the rounded and double-arched supraorbital tori; inferomedially truncatedorbits; narrow lower face with sharply retreatingzygomatics; medial projections anteriorly in thelarge nasal fossa with a clearly delineated prenasalfossa below; puffi ness of the midface reflectingthe presence of large maxillary sinuses; an anglingalong the anterior squamosal suture that divides

the temporal fossa into clearly demarcatedanterior and posterior portions; the ovoid (enbombe) coronal cranial profile; the pittedsuprainiac fossa that lies above an occipitaltorus that is really only properly demarcatedbelow; highly pneumatized petrosal; long, narrowand ovoid foramen magnum; sigmoid notches ofmandible deepest in front of a low-set condyle;obliquely truncated gonial angles; sigmoid notchcrests that terminate close to the lateral endsof the condyles; relatively thin bone across thebase of the symphysis. Te molars have relativelycomplex but constricted occlusal surfaces, withcentroconids/centrocones and inwardly slopingsides. Additional dental apomorphies are noted byBailey (2002, 2004) and Bailey & Lynch (2005);and C studies by Spoor et al. (2003), amongothers, have suggested that the labyrinth of themiddle ear of Neanderthals is so distinct from thatof modern humans as to suggest differences inlocomotor behaviour and the kinematic propertiesof the head and neck (Spoor et al., 2003: 141).

Cranially, then, there is an unmistakableNeanderthal Gestalt: one that contrastsdramatically with that of Homo sapiens, which represents another theme entirely.

Tis observation holds equally for thepostcranial skeleton. Sawyer and Maley (2005)not long ago described a complete compositeNeanderthal skeleton confected from theremains of six incomplete skeletons from sites infour different countries. It has long been knownthat Neanderthals exhibited a whole variety ofdistinctive characteristics of the bones of thepostcranial skeleton, including such featuresas thick-walled long bones with restricted


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medullary cavities and expanded articularsurfaces; long scapulae with expanded rotator-cuff attachment areas and long and narrowglenoid fossae; long claviculae with relativelyflattened shafts; pelvises with flaring iliacblades and long, attenuated pubic rami; andthe large carpal tunnels, the expanded pollicaland ulnar distal phalangeal tuberosities, andthe generally accentuated muscle attachmentareas in the hand skeleton (see descriptions inrinkaus, 1983). What is remarkable about

the new reconstructed Neanderthal skeleton,however, is the degree to which it departsfrom Homo sapiens in the overall proportionsof the thoracic and pelvic regions (see Fig. 1).In contrast to the barrel-shaped human ribcage, which tapers slightly inwards not onlyat the top, but also at the bottom to matchthe narow pelvis, the Neanderthal rib cage isextremely narrow at the top despite broadshoulders defined by the long claviculae andflares dramatically out and down to matchthe very wide pelvic basin. Additionally,the mobile vertebral column is set against

a rather more inferiorly positioned sacrum,substantially reducing the effective verticallength of the waist (Sawyer and Maley, 2005).

It is important to note that the Neanderthalswere not unique in many of those features thatdistinguish their skeletons from that of Homosapiens. Both cranially and postcranially, manyof them are shared with certain earlier extincthominids from Europe. Foremost among suchfossils are those from Steinheim in Germany, andthe Sima de los Huesos at Atapuerca, in Spain.Te Steinheim cranium, uncertainly dated butprobably marginally earlier in time than anything

claimed with any certainty to be a Neanderthal,shares with Neanderthals certain features ofthe face (for example, supraorbital and orbitalmorphology, presence of a prenasal fossa) and ofthe cranial rear (e.g. suprainiac fossa, horizontaloccipital torus that is only fully defined below).Yet it is is distinctive in its tall coronal profile,broad zygomas, and a variety of other features(Schwartz & attersall, 2005; attersall &Schwartz, 2006). Few have cared to call this

specimen a Neanderthal, and indeed there islittle evident reason to do so; instead, it seemsmost rational to allocate the Steinheim specimento a sister taxon, allied to the Neanderthalsbut nonetheless distinct from them. A similarconclusion applies to the 500,000 year-old fossilsfrom the Sima de los Huesos. Tese, too, sharefeatures of the cranium with the Neanderthals, butfewer than in the case of Steinheim. Postcranially,the Sima remains are equally interesting, in thatthey also show the robustness of the postcranial

skeleton and the flaring of the pelvis that is sowell documented in the Neanderthals (attersall& Schwartz, 2006). aken together, these fossilssuggest the existence in the Middle Pleistocene ofEurope of an endemic hominid clade, consistingon present evidence of Neanderthals+Steinheim, with Sima as the immediate outgroup, to which all of these hominids belonged. Onchronological grounds it might just be arguedthat the Sima (500 kyr), Steinheim (ca. 225kyr) and Neanderthal (


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Fig . 1 - F ron t v iew o f t he compos i te Neander tha l ske le ton ( Sawyer and Ma ley , 2005) compared w i t h

a ske leton o f a modern Homo sapiens male o f s im i l a r s ta tu re . Pho to cou r tesy o f Ken Mowbray .

pneumatization that clearly excludes it frommembership in any pre-Neanderthal lineage.Evidently, more than one lineage of hominidexisted concurrently in Middle Pleistocene

Europe, and specimens such as those fromBilzingsleben and Vrteszlls hint tantalizinglyat more systematic complexity yet (Schwartz andattersall, 2005; attersall& Schwartz, 2006).


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that the two kinds of hominid would have sharedany elements of a specific mate recognitionsystem, and that any biologically significantlevel of gene exchange ever occurred betweenthem. Tis conclusion of genetic separatenesshas recently also been bolstered by molecularevidence (e.g., Krings et al., 1997; Caramelli etal., 2003; Lalueza-Fox et al., 2005). Tere is someheterogeneity among the samples of NeanderthalmtDNA that have been isolated just as thereis morphologically among Neanderthals from

different sites and times but all analyses showthe Neanderthal population to be a substantialoutlier to all modern human populations; andCurrat and Excoffi er (2004) have calculated thateven on optimistic assumptions the maximuminterbreeding rate between the populationsfollowing the Cro-Magnon incursion intoEurope would have been below 0.1%, indicatingeffective intersterility. Once again, the evidenceis strongly in favor of the inference that thetwo kinds of hominid were fully individuated.

Homo neander t ha lens is:

a fully individuated species?

Against the background just sketched, Homosapiens appears highly distinctive. Indeed, withits gracile and retracted face, tall, short braincase,bipartite brow ridges, unique chin structure,and a host of other features, anatomic Homosapiensdoes not pair comfortably with anythingelse yet known (Schwartz & attersall, 2005),and appears as the outlier among all known late

Middle and Late Pleistocene species of Homo.Te Neanderthals, on the other hand, fall securely within a wider hominid clade. Tus, by thesimple geometry of systematic relationships, itis barely credible that Neanderthals and modernhumans could fall within the same species which would necessarily have to be expanded toinclude such forms as the Sima hominids andeven possibly the Mauer/Arago group. Whatis more, the sheer scale of the morphologicaldifferences between the Neanderthals and Homosapiensmakes it vanishingly implausible that bothcould fall within the envelope of morphologicalvariation within a single species. I have pointedout elsewhere (attersall, 1986, 1993) thatosteodental differences between primate speciesclassified within the same genus are typicallysubtle, and that among mammals in general bonydistinctions on the order of those that separateNeanderthals and modern humans are commonlyassociated with different genera. It is fairly fruitlessto argue about exactly where in quantifiable termsit is reasonable to draw the morphological lineamong hominid species, but it is clear that in thiscase that limit has been abundantly exceeded.

Interestingly, the new Neanderthal skeletalreconstruction, as well as the studies of theNeanderthal pelvis by Rak (1990) and of inner earmorphology by Spoor et al.(2003), suggest thatdifferences in gait existed between Neanderthalsand modern humans. In particular, the verybroad and short waist would have imparted astiffness to Neanderthal movement that wouldhave made them cut a very distinctive figureon the landscape. Te consequent distinctivebehavioral signal further reduces the probability

Conclusion

In recognizing species in the fossil recordthere is, as pointed out above, no silver bulletthat will ever demonstrate absolutely definitivelythat two closely related lineages are historicallyindividuated, as species must be. In principle,then, no claim of this kind or indeed, to thecontrary can ever be proven. But science isfortunately not about proof (attersall, 2002);and the preponderance of the evidenceapproach advocated here makes ineluctiblethe conclusion that Homo sapiens and Homo

neanderthalensisare properly regarded as separatespecies. Much more diffi cult will be the task ofproperly distinguishing species within the widerNeanderthal clade. Since in the case of modernhumans vs Neanderthals we are facing an either/ordichotomy (one species, or two?), the conclusionthat the two kinds of hominid represent entitiesfully individuated from each other is extremelyrobust. Tis is because the evidence in favor ofseparate specific status is overwhelming, yet all


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that is required is to move beyond the point ofbalance. For the moment, at least, we can beconfident in dignifying Homo neanderthalensiswith its own individual identity: an identity that iswell deserved, but that has too often been denied.

Acknowledgments

Tis paper was presented on the occasion of theconference Nostro fratello Neanderthal ...when wewere not alone, held at the Parco Nazionale delCirceo (Sabaudia, 21-22 October 2006). I thankGiorgio Manzi for inviting me to participate in thisSymposium, and particularly for this opportunityto express my deep appreciation of the life andachievements of our friend and colleague AmilcareBietti, to whose fond memory it is dedicated.

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