EDITORIAL Darwin and the recent African origin of modern humans Richard G. Klein1 Program in Human Biology, Stanford University, Stanford, CA 94305 n this 200th anniversary of When Darwin and Huxley were ac- The Course of Human Evolution Charles Darwin’s birth and tive, many respected scientists sub- In the absence of fossils, 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 Species (1859) (1), it evolved populations of Homo sapiens. tionary theory readily accommodates seems fitting to summarize Darwin’s The original Neanderthal 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 australopithecines, 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 years ago, Origin, except near the end where he time, it had a large braincase, and Hux- were distinguished from apes 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 bipedalism, 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 brains, that readily dis- he said that addressing human evolution man fossil 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 genus, , 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 Homo erectus as other species. He emphasized the species, . Thirty-four years seems reasonable to assume that Homo comparative anatomical details in later, in 1925, Raymond Dart (5) de- and stone tool technology 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 Nature (1863) (3) to child’s skull from Taung, South Africa, 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 australopithecine. 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, Homo habilis 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 Homo ergaster 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 Eurasia. meaningful exception was the Neander- great antiquity was required to accom- Following the initial Out-of-Africa thal skullcap and associated limb bones modate the evolution of species. How- event, natural selection and random ge- recovered by quarry workers from a ever, even after the discovery of Homo netic drift began to drive populations in limestone cave near Du¨sseldorf, Ger- erectus and the australopithecines, the Africa, Europe, 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 Olduvai Gorge in bones of extinct animals, 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 October 1, 2021 Millions of Homo Homo Homo Millions of Artifact Traditions neanderthalensis sapiens erectus years ago years ago Later Stone Age & 0 0 Upper Paleolithic 0.05 0.25 Middle Stone Age & Homo Middle Paleolithic heidelbergensis Paranthropus boisei 1 1 Acheulean Homo (or Kenyanthropus) Paranthropus rudolfensis Homo 1.65 habilis robustus Homo 2 ergaster 2 Oldowan ? Australopithecus 2.5 garhi ? ? Paranthropus 3 Australopithecus aethiopicus 3 africanus ? Australopithecus Kenyanthropus afarensis platyops (flaked stone 4 Australopithecus 4 artifacts ? anamensis unknown) ? Ardipithecus ? ramidus 5 5 Ardipithecus kadabba Orrorin ? Sahelanthropus 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/Upper Paleolithic 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 Homo floresiensis, 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 Flores, Indonesia, 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.