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Home , Archaic humans, Australopithecine, Kenyanthropus, Neanderthal

EDITORIAL

Darwin and the recent African origin of modern

Richard G. Klein1 Program in Biology, Stanford University, Stanford, CA 94305

n this 200th anniversary of When Darwin and Huxley were ac- The Course of ’s birth and tive, many respected scientists sub- In the absence of , Darwin could the 150th anniversary of the scribed to the now discredited idea that not have predicted the fundamental pat- publication of his monumen- human races represented variably tern of human evolution, but his evolu- Otal The Origin of (1859) (1), it evolved populations of sapiens. tionary theory readily accommodates seems fitting to summarize Darwin’s The original skull had a the pattern we now recognize. Probably views on human evolution and to show conspicuous browridge, and compared the most fundamental finding is that the how far we have come since. Darwin with the skulls of modern humans, it , who existed from at famously neglected the subject in The was decidedly long and low. At the same least 4.5 million to 2 million ago, Origin, except near the end where he time, it had a large braincase, and Hux- were distinguished from primarily noted only that ‘‘light would be thrown ley regarded it as ‘‘the extreme term of by anatomical specializations for habit- on the origin of man and his history’’ by a series leading gradually from it to the ual , and it was only after 2 the massive evidence he had compiled highest and best developed of [modern] million years ago that people began to for evolution by means of natural selec- human crania.’’ It was only in 1891 that acquire the other traits, including our tion. In The Descent of Man (1871) (2), Euge`neDubois (4) found the first hu- unusually large , that readily dis- he said that addressing human evolution man that could not be similarly tinguish us from the living apes. in 1859 would ‘‘only add to the preju- characterized. The specimen was a skull- The greatly expanded fossil record dices against my views.’’ Satisfied now shows that the australopithecines com- that those prejudices had significantly cap from Trinil, Java, and it had a sig- nificantly smaller braincase and more prised multiple species, and it suggests receded, he deployed an array of com- Homo primitive features than its Neanderthal that our own , , descended parative anatomical, embryological, and from one of these about 2.5 million counterpart. We recognize it today as behavioral observations to argue that years ago. The oldest flaked artifacts the first specimen of the archaic human people had evolved in the same manner date from about the same time, and it as other species. He emphasized the species, . Thirty-four years seems reasonable to assume that Homo comparative anatomical details in later, in 1925, Raymond (5) de- and co-evolved. Thomas Huxley’s monograph Evidence scribed an even more primitive fossil—a Darwin was no stranger to scientific as to Man’s Place in (1863) (3) to child’s skull from Taung, South , controversy, and he would surely not be substantiate the particularly close evolu- that was the first known specimen of an surprised that despite all we have tionary relationship between people and . Although scientific learned, specialists vigorously debate the the ‘‘anthropomorphous’’ apes. He also recognition of the Neanderthal, Trinil, precise evolutionary relationships among reiterated Huxley’s prescient inference, and Taung fossils was not immediate, the australopithecines and Homo. Fig. 1, grounded in the distribution of the espe- they illustrated the basic phases in hu- however, presents a phylogeny that most cially humanlike African apes, that the man evolution that we recognize today. authorities would probably accept as a last shared ancestor of people and apes Darwin no doubt would be immensely reasonable working hypothesis. It lists lived in tropical Africa. pleased to see how they have now the earliest species of Homo as Homo joined thousands of other fossils that habilis, and it implies that by 1.7 million Development of the Human Fossil unequivocally document the fundamen- years ago, had evolved Record tal course of human evolution. into the more advanced species known The fossil record now confirms that Modern geology was born in Darwin’s variously as or African Darwin and Huxley were right to place time, and with it came indications that Homo erectus. Sometime between 2 and human origins in Africa, but when they the earth must be many millions of roughly 1.6 million years ago, Homo er- were writing, fossil support for human years old. Darwin paid special attention gaster became the first human species to evolution was almost absent. The most to this point, because he knew that expand from Africa to . meaningful exception was the Neander- great antiquity was required to accom- Following the initial Out-of-Africa thal skullcap and associated limb modate the evolution of species. How- event, and random ge- recovered by quarry workers from a ever, even after the discovery of Homo netic drift began to drive populations in near Du¨sseldorf, Ger- erectus and the australopithecines, the Africa, , and eastern Asia in dif- many in 1856. Unfortunately, the antiq- time span of human evolution remained ferent morphological directions. Mor- uity of the bones was unclear and there phological differentiation was particu- seemed to be a reasonable possibility uncertain, and many specialists assumed that the last shared ancestor of humans larly clear by 500,000–400,000 years ago, that the skull came from a pathological and from this time onwards, there were and apes existed no more than one mil- modern human. Similar skulls and limb at least three evolving human lineages. bones from other sites, excavated from lion years ago. The application of potas- layers with ancient stone tools and the sium/argon dating at in bones of extinct , eventually 1961 first pushed the date back to at Throughout 2009 PNAS will publish several collections of showed that Neanderthal morphology least 1.8 million years ago (6), and po- articles examining various aspects of evolution and evolu- was not pathological and that the Nean- tassium/argon and other numeric dating tionary theory. These collections include In Light of Evolu- tion III: Two Centuries of Darwin; Biogeography, Changing derthals had inhabited Europe before methods applied to new African sites Climate, and Niche Evolution; Out of Africa: Modern Hu- modern humans. However, the addi- now place it firmly before 4.4 million man Origins; Plant and Insect Biodiversity; and Evolution in tional fossils appeared only after Dar- years ago (7). Sparse fossils imply that it Health and Medicine. win’s death in 1882 and Huxley’s retire- will ultimately fall between 7 and 6 mil- The author declares no conflict of interest. ment in 1885. lion years ago (8). 1E-mail: [email protected].

www.pnas.org͞cgi͞doi͞10.1073͞pnas.0908719106 PNAS ͉ September 22, 2009 ͉ vol. 106 ͉ no. 38 ͉ 16007–16009 Downloaded by guest on September 25, 2021 Millions of Homo Homo Homo Millions of Artifact Traditions neanderthalensis sapiens erectus years ago years ago Later & 0 0 Upper 0.05 0.25 & Homo heidelbergensis boisei 1 1 Homo (or ) Paranthropus rudolfensis Homo 1.65 habilis robustus Homo 2 ergaster 2 ? 2.5 garhi ? ? Paranthropus 3 Australopithecus aethiopicus 3 africanus ? Australopithecus Kenyanthropus afarensis platyops (flaked stone 4 Australopithecus 4 artifacts ? anamensis unknown) ? ? ramidus

5 5 Ardipithecus kadabba

Orrorin ? tugensis tchadensis 6 6

Fig. 1. A working phylogeny of the australopithecines and Homo (after ref. 19). Flaked stone artifacts appeared at about the same time as the earliest species of Homo. The initial expansion of humans from Africa coincided roughly with the shift from the Oldowan to the Acheulean (handaxe) traditions. The subsequent expansion about 50,000 years ago coincided with the shift from the Middle Stone Age/Middle Paleolithic to the Later Stone Age/ traditions.

These may always have been able to ex- skeleton, actually represent a distinct niously extracted from Neanderthal change genes, but distance and small species as opposed to a small-bodied bones (11). The genes of living humans population size probably limited gene modern human afflicted by a growth imply that the source population for the flow, and the composite fossil and ar- disorder (10). The issue can probably be expansion was probably located in east- cheological records indicate that the Af- resolved only by the recovery of addi- ern or possibly southern Africa (12), rican lineage spread to replace or tional fossils and by greater clarity on and humans outside of Africa descend swamp the others beginning roughly their temporal and spatial relationship from a small number of migrants. This 50,000 years ago. It is thus reasonable to to contemporaneous Homo sapiens. Out-of-Africa founder event was actu- supply the lineages with biological spe- ally just the first in a series of similar cies labels: Homo sapiens in Africa, Out-Of-Africa (Again) events that eventually resulted in the Homo neanderthalensis in Europe, and The expansion of Homo sapiens from peopling of the world (13, 14). Darwin Homo erectus in the eastern Asia. Some Africa to Eurasia about 50,000 years ago could never have anticipated the molec- specialists would add a fourth lineage is now known colloquially as Out-of- ular support for Out-of-Africa, but in for , a highly distinctive Africa, although it might better be fact that might be true even if he had human form that is thought to have ex- called Out-of-Africa 2, in recognition of died 100 years later, because the semi- isted on the island of , , the much earlier human dispersal from nal molecular study—a kind of tipping between perhaps 95,000 and 13,000 Africa shortly after 2 million years ago. point—was published only in 1987 (15). years ago (9). Homo floresiensis is not The expansion at 50,000 years ago is Out-of-Africa is surely one of the great considered further here, because author- documented by an extraordinary conflu- scientific discoveries of the past 25 ities disagree on whether the principal ence of fossil and molecular genetic ob- years, and its acceptance terminated a fossils, a tiny skull and associated partial servations, including from DNA inge- 100- debate on the place of the Ne-

16008 ͉ www.pnas.org͞cgi͞doi͞10.1073͞pnas.0908719106 Klein Downloaded by guest on September 25, 2021 anderthals in human evolution. They are promoted the final development of the much of southern and northern Africa now known to have been an extinct side modern human with its seemingly in the millennia immediately before branch of humanity that contributed few infinite capacity for innovation (16). A 50,000 years ago, probably because of if any genes to their modern human larger number ascribe the behavioral adverse climatic conditions in the mid- successors. change to social, economic, or demo- dle of the . To the Many details of Out-of-Africa remain graphic change, perhaps above all to extent that population growth can be to be worked out, and disagreement population growth that increased the detected in Africa, it came after, not persists, for example, on the extent to frequency and density of transformative before, Out-of-Africa (16). Continuing which dispersing modern Africans and interactions among individuals and archaeological research promises to pro- archaic Eurasians may have interbred groups (17). vide additional crucial demographic and especially on what promoted the At the moment, the genetic explana- details. relatively sudden Out-of-Africa expan- tion suffers from the absence of direct The articles assembled for the Special sion. Most authorities attribute the ex- evidence for mutational change, al- pansion to behavioral changes that con- though such evidence may emerge when on Out-of-Africa cannot resolve ferred a substantial Darwinian fitness the modern human and Neanderthal the continuing uncertainties, but they advantage, that is, that allowed modern can be compared. A draft of summarize much of the anatomical, ar- humans of African descent to survive the complete Neanderthal is chaeological, and genetic evidence that and reproduce at a significantly higher expected within months (18). The demo- led to its discovery, and they help lay rate than the they en- graphic explanation faces the objection the groundwork for research that should countered in Eurasia. However, special- that there is no evidence for population one day explain why it occurred. The ists disagree sharply on how to explain growth in Africa before 50,000 years answer should intrigue all modern hu- the underlying behavioral shift. Some ago. In fact, human populations became mans, because it will illuminate what it attribute it to a genetic that all but archeologically invisible over is that makes our species so special.

1. Darwin C (1859) by Means of 8. Lebetard A-E, et al. (2008) Cosmogenic nuclide dating 13. Jakobsson M, et al. (2008) Genotype, haplotype and Natural Selection (John Murray, London). of Sahelanthropus tchadensis and Australopithecus copy-number variation in worldwide human popula- 2. Darwin C (1871) The Descent of Man and Selection in bahrelghazali: Mio- hominids from Chad. Proc tions. Nature 451:998–1003. Relation to Sex (John Murray, London). Natl Acad Sci USA 105:3226–3231. 14. Li JZ, et al. (2008) Worldwide human relationships in- 3. Huxley TH (1863) Evidence as to Man’s Place in Nature 9. Morwood MJ, et al. (2004) and age of a ferred from genome-wide patterns of variation. Sci- (Williams and Norgate, London). new hominin from Flores in eastern Indonesia. Nature ence 319:1100–1104. 4. Dubois E (1994 (1892)) Paleontological investigations 431:1087–1091. 15. Cann RL, Stoneking M, Wilson AC (1987) Mitochondrial 10. Culotta E (2006) Skeptics seek to slay the ‘‘hobbit,’’ on Java (translation of ‘‘Palaeontologische onder- DNA and human evolution. Nature 329:31–36. calling Flores skeleton a modern human. Science zoekingen op Java’’). Naming our Ancestors: an An- 16. Klein RG (2008) Out of Africa and the evolution of 313:1028–1029. thology of Hominid , eds Meikle WE, Parker human behavior. Evol Anthropol 17:267–281. 11. Serre D, Pa¨a¨ bo S (2006) The fate of European Neander- ST (Waveland, Prospect Heights, IL), pp 37–40. 17. Mace R (2009) On becoming modern. Science thals: results and perspectives from ancient DNA anal- 5. Dart RA (1925) Australopithecus africanus: the man- yses. Revisited: New Approaches and 324:1280–1281. of South Africa. Nature 115:195–199. Perspectives, eds Harvati K, Harrison T (Springer, Dor- 18. Pennisi E (2009) Tales of a prehistoric . 6. Leakey LSB, Evernden JF, Curtis GH (1961) Age of Bed 1, drecht, The ), pp 211–220. Science 323:866–871. Olduvai Gorge, Tanganyika. Nature 191:478–479. 12. Tishkoff SA, et al. (2009) The genetic structure and 19. Klein RG (2009) The Human Career: Human Biological 7. WoldeGabriel G, et al. (1995) Age of early hominids. history of Africans and African Americans. Science and Cultural Origins (Univ Chicago Press, Chicago), 3rd Nature 376:559. 324:1035–1044. Ed.

Klein PNAS ͉ September 22, 2009 ͉ vol. 106 ͉ no. 38 ͉ 16009 Downloaded by guest on September 25, 2021