Human origins: Out of AfricaIan Tattersall1Division of Anthropology, American Museum of Natural History, New York NY 10024Edited by Richard G. Klein, Stanford University, Stanford, CA, and approved July 16, 2009 (received for review March 23, 2009)Our species, Homo sapiens, is highly autapomorphic (uniquely derived) among hominids in the structure of its skull and postcranialskeleton. It is also sharply distinguished from other organisms by its unique symbolic mode of cognition. The fossil and archaeologi-cal records combine to show fairly clearly that our physical and cognitive attributes both first appeared in Africa, but at differenttimes. Essentially modern bony conformation was established in that continent by the 200–150 Ka range (a dating in good agree-ment with dates for the origin of H. sapiens derived from modern molecular diversity). The event concerned was apparently short-term because it is essentially unanticipated in the fossil record. In contrast, the first convincing stirrings of symbolic behavior are notcurrently detectable until (possibly well) after 100 Ka. The radical reorganization of gene expression that underwrote the distinctivephysical appearance of H. sapiens was probably also responsible for the neural substrate that permits symbolic cognition. This exap-tively acquired potential lay unexploited until it was ‘‘discovered’’ via a cultural stimulus, plausibly the invention of language. Mod-ern humans appear to have definitively exited Africa to populate the rest of the globe only after both their physical and cognitivepeculiarities had been acquired within that continent.Homo sapiens 兩 evolution 兩 fossil record 兩 symbolic cognitionAf rica is in a profound sense thefount of human evolution. Notonly did our zoological familyHomin idae (Homo sapiens plusits extinct close relatives, often nowa-days restricted to the subfamily Homin i-nae; for the purposes of this article thedif ference is merely notional) originatethere ca. 7 Ma (1), but over the past 2Ma the continent has regularly pumpedout new kinds of hominid into otherareas of the Old World (2). The genusHomo evolved in Africa at some timeca. 2 Ma [all older contenders to Homost atus are debatable (3, 4)], then rapidlyspread out of its natal continent to pop-ulate Eurasia for the first time (5, 6).The first truly cosmopolitan species ofHomo, Homo heidelbergensis, is firstk nown from Africa at ca. 600 Ka (7),before appearing at sites in Europe andeastern Asia from ca. 500 Ka onward.The now-ubiquitous species H. sapiens,to which all living human beings belong,is initially documented in Africa as,somewhat later, is the first material evi-dence of the symbolic cognitive systemthat appears to be unique to humans.ModernH. sapiensIs Highly Derived inIts OsteologyMorphologically, our living species H.sapiens is extremely distinctive. It is notunique among hominids in having a largebrain [averaging ⬇1350–1400 mL in vol-ume (8)]; but it is unique in the propor-tions of the skull in which that brain ishoused, and in numerous smaller-scalecranial characteristics (4, 9). Among otherfeatures not found elsewhere, H. sapienspossesse s a short, tall and more or lessglobular braincase, beneath the front ofwhich a small, anteroposteriorly short anddelicately built face is distinctly retracted(10). The orbits are surmounted by indi-vidual supraciliary ridges; although notinvariably tiny, these are bipartite, with acentral portion separated from a lateralplate by an oblique crease (4, 11). In thelower jaw, the H. sapiens chin is not sim-ply a swelling at the external base of thesymphysis [which can be found elsewhereamong hominids (4, 12)]. Instead, it is acomplex structure in the form of an in-verted ‘‘T,’’ in which a vertical keelbounded by lateral depressions meets abasal transverse bar running between lat-eral tubercles (4, 12).H. sapiens is equally derived in thestr ucture of its postcranial skeleton. Forexample, in sharp contrast to the re-cently reconstructed skeleton of Homoneander thalensis (13), that of modernhumans is slender and delicately built;and although the Neanderthal rib cageis conical, tapering distinctly upwardsf rom a broad base that matches themarkedly flaring iliac blades of the pel-vis, in H. sapiens the thorax is barrel-shaped. It is relatively narrow and tapersinward at the bottom and at the top,while the relatively delicate pelvis belowit lacks lateral flare and has notablymore vertical iliac blades. The dissimi-larit y between the two species is strik-ing, and may have affected gait andexternal appearance (14). Nonetheless,until recently there was room for uncer-t ainty over which thoracic/pelvic condi-tion was derived within the genusHomo.The two best pelvic specimens re-ported for species of Australopithecus(15, 16) made it clear that a broad, flar-ing pelvis is primitive for Hominidae;but whether pelvic flare is also primitivefor the genus Homo [defined as thosehomin ids possessing essentially modernbody proportions (3)] was less evident.In contrast, the best skeleton of an earlyHomo (KNM-WT 15000 from WestTurk ana in Kenya) shows a weakly c oni-cal thorax and, as reconstructed, only amodestly wide pelvis (17). Still, it be-longed to an immature and thus incom-pletely developed individual. At thesame time, the excellently preserved andwidely flaring adult pelvis (SH Pelvis 1)f rom the Sima de los Huesos at At apu-erca in Spain (18) is that of a Neander-thal relative, and does no more thanc onfirm that this pelvic confor mation isprimitive for the Neanderthal clade.However, a recently reported adult pel-vis from Gona in Ethiopia (19), dated tobet ween 1.4 and 0.9 Ma and attributedby its describers to the same species asthe Turkana specimen, joins more lim-ited materials described earlier (20, 21)in showing great robusticity and thebroadly flaring c onformation. Availableevidence thus now strongly suggests thatthe wide, flat, heavy pelvic morphologyis indeed primitive for the genus Homo,in which case, the basic body form of H.sapiens, as well as that of its skull, ishighly derived.The same can also be said for theun ique mode of cognition possessed byall living H. sapiens. Alone among or-gan isms, as far as can be told, our spe-cies exhibits symbolic mental processes.That is to say, its members deconstr uctthe world around them into a huge vo-cabulary of ment al symbols. These theyc ombine and recombine in imaginationto describe alternate worlds and situa-tions,
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