Human origins Out of Africa Ian Tattersall1 Division of Anthropology American Museum of Natural History New York NY 10024 Edited 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 postcranial skeleton It is also sharply distinguished from other organisms by its unique symbolic mode of cognition The fossil and archaeological records combine to show fairly clearly that our physical and cognitive attributes both first appeared in Africa but at different times Essentially modern bony conformation was established in that continent by the 200 150 Ka range a dating in good agreement with dates for the origin of H sapiens derived from modern molecular diversity The event concerned was apparently shortterm because it is essentially unanticipated in the fossil record In contrast the first convincing stirrings of symbolic behavior are not currently detectable until possibly well after 100 Ka The radical reorganization of gene expression that underwrote the distinctive physical appearance of H sapiens was probably also responsible for the neural substrate that permits symbolic cognition This exaptively acquired potential lay unexploited until it was discovered via a cultural stimulus plausibly the invention of language Modern humans appear to have definitively exited Africa to populate the rest of the globe only after both their physical and cognitive peculiarities had been acquired within that continent Homo sapiens evolution fossil record symbolic cognition A frica is in a profound sense the fount of human evolution Not only did our zoological family Hominidae Homo sapiens plus its extinct close relatives often nowadays restricted to the subfamily Homininae for the purposes of this article the difference is merely notional originate there ca 7 Ma 1 but over the past 2 Ma the continent has regularly pumped out new kinds of hominid into other areas of the Old World 2 The genus Homo evolved in Africa at some time ca 2 Ma all older contenders to Homo status are debatable 3 4 then rapidly spread out of its natal continent to populate Eurasia for the first time 5 6 The first truly cosmopolitan species of Homo Homo heidelbergensis is first known from Africa at ca 600 Ka 7 before appearing at sites in Europe and eastern 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 evidence of the symbolic cognitive system that appears to be unique to humans Modern H sapiens Is Highly Derived in Its Osteology Morphologically our living species H sapiens is extremely distinctive It is not unique among hominids in having a large brain averaging 1350 1400 mL in volume 8 but it is unique in the proportions of the skull in which that brain is housed and in numerous smaller scale cranial characteristics 4 9 Among other features not found elsewhere H sapiens possesses a short tall and more or less globular braincase beneath the front of which a small anteroposteriorly short and delicately built face is distinctly retracted 10 The orbits are surmounted by indi vidual supraciliary ridges although not invariably tiny these are bipartite with a central portion separated from a lateral plate by an oblique crease 4 11 In the lower jaw the H sapiens chin is not simply a swelling at the external base of the symphysis which can be found elsewhere among hominids 4 12 Instead it is a complex structure in the form of an inverted T in which a vertical keel bounded by lateral depressions meets a basal transverse bar running between lateral tubercles 4 12 H sapiens is equally derived in the structure of its postcranial skeleton For example in sharp contrast to the recently reconstructed skeleton of Homo neanderthalensis 13 that of modern humans is slender and delicately built and although the Neanderthal rib cage is conical tapering distinctly upwards from a broad base that matches the markedly flaring iliac blades of the pelvis in H sapiens the thorax is barrelshaped It is relatively narrow and tapers inward at the bottom and at the top while the relatively delicate pelvis below it lacks lateral flare and has notably more vertical iliac blades The dissimilarity between the two species is striking and may have affected gait and external appearance 14 Nonetheless until recently there was room for uncertainty over which thoracic pelvic condition was derived within the genus Homo The two best pelvic specimens reported for species of Australopithecus 15 16 made it clear that a broad flaring pelvis is primitive for Hominidae but whether pelvic flare is also primitive for the genus Homo defined as those hominids possessing essentially modern body proportions 3 was less evident In contrast the best skeleton of an early 16018 16021 PNAS September 22 2009 vol 106 no 38 Homo KNM WT 15000 from West Turkana in Kenya shows a weakly conical thorax and as reconstructed only a modestly wide pelvis 17 Still it belonged to an immature and thus incompletely developed individual At the same time the excellently preserved and widely flaring adult pelvis SH Pelvis 1 from the Sima de los Huesos at Atapuerca in Spain 18 is that of a Neanderthal relative and does no more than confirm that this pelvic conformation is primitive for the Neanderthal clade However a recently reported adult pelvis from Gona in Ethiopia 19 dated to between 1 4 and 0 9 Ma and attributed by its describers to the same species as the Turkana specimen joins more limited materials described earlier 20 21 in showing great robusticity and the broadly flaring conformation Available evidence thus now strongly suggests that the wide flat heavy pelvic morphology is indeed primitive for the genus Homo in which case the basic body form of H sapiens as well as that of its skull is highly derived The same can also be said for the unique mode of cognition possessed by all living H sapiens Alone among organisms as far as can be told our species exhibits symbolic mental processes That is to say its members deconstruct the world around them into a huge vocabulary of mental symbols These they combine and recombine in imagination to describe alternate worlds and situations based on a capacity for generating a potentially infinite array of meanings Author contributions I T designed research performed
View Full Document
Unlocking...