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Human Evolution: Overview Introductory article

Bernard A Wood, The George Washington University, Washington, DC, USA Article Contents

. Evolutionary Context The fossil evidence for evolution can be traced from close to 4.5 Ma to the present. . The Earliest Hominins The new term for modern and the human is ‘hominin’, which replaces the . Discoveries in Southern Africa older name ‘’. . – A of Large-toothed Hominins . The Beginnings of , or Are They? Evolutionary Context . Homo Emerges in Africa . Homo Moves Out of Africa There is abundant evidence that the living most . Archaic Homo closely related to modern humans (Homo sapiens) are the . Modern Human Origins () and the (Gorilla). Both are . Peopling the Planet nonhuman in their appearance and behaviour and it was assumed that they were more closely related to each other than either was to modern humans. However, when their genetic identities are compared there is evidence that the The Earliest Hominins DNA in both the nucleus and the mitochondria ofthe cells The first creature to show rudimentary human specializa- ofmodern humans and the chimpanzee are very similar. tions is known as ramidus, the 4.5 Ma-old An increasing number ofresearchers are convinced that the remains ofwhich were recovered at a site called Aramis, in similarities between them suggest a shared common Ethiopia, in late 1992. The fossils share some features with ancestry to the exclusion ofthe gorilla, but others maintain living , others with the African in general that the relationships between Homo sapiens, Pan and but, crucially, several significant features of the teeth and Gorilla are so close that it is not possible to link two ofthem forearm bones are shared with later hominins. Thus, the to the exclusion ofthe third. discoverers suggested that these fossils belong to a hominin Differences in the DNA can be used to provide an species, and not an one, and although they initially estimate ofhow long lineages have been independent. The allocated it to another group ofextinct hominins, molecular differences between living people and the living , they have subsequently assigned it to a African apes suggest that the lineage which includes new genus, Ardipithecus. This genus, together with all the modern humans has been separate from the rest of the other non-Homo genera, are informally referred to as apes (or hominoids) for between 5 and 8 Ma. Animals in ‘australopithecins’. the fossil record that are judged to be more closely related Judging fromthe size ofone ofthe bones that make up to modern humans than to the apes have traditionally been the shoulder joint, at least one individual ofthis early called ‘hominids’, and those that are closer to the apes than hominin weighed about 40 kg. The chewing teeth were to modern humans have been called ‘pongids’. Now that relatively small. The opening through which the spinal there is good evidence that Homo and Pan are so closely cord passes was close to the centre ofthe skull, suggesting related, scientists are using ‘hominid’ to refer to the that the posture and gait of Ardipithecus was respectively , which includes Homo and Pan, and they use more upright and bipedal than is the case in the living apes. ‘hominin’ to refer to the Hominini, which includes The remains ofthe animals and the plants foundwith A. only modern humans and the human clade. Modern ramidus suggest that the bones had been buried in a humans and their ancestors are thus known as ‘hominins’. location that was close to, ifnot actually within, woodland. For many years was likened to a The thin enamel covering on the teeth suggests that the diet ladder, with earlier, more primitive, species being ‘re- of A. ramidus may have been similar to that ofthe placed’ by later, more advanced ones, with modern chimpanzee, including fruit, vegetation and some small humans at the top ofthe ladder ofascent. Recent evidence animals. Although evidence ofonly one species of suggests that the metaphor ofa ladder is no longer an Ardipithecus has been recovered, it is likely that it will appropriate one. Instead, the hominin evolutionary tree is prove to be just one variant ofthis group. much better likened to a bush that has multiple stems A little later in time there is fossil evidence of another leading off from close to the base, as well as closer to the australopithecin known as Australopithecus afarensis. This crown (Figure 1). All but one ofthe stems stop well short of name was given in 1978 to fossils recovered from , in the highest point ofthe bush; these are hominin lineages Tanzania, and from the site of Hadar in Ethiopia. Material that have no living descendants. allocated to A. afarensis has been dated to between 3 and 4Ma.

ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Publishing Group / www.els.net 1 Human Evolution: Overview

Later Homo 0 H. erectus

1 P. boisei

H. ergaster H. habilis P. robustus H. rudolfensis

2 A. garhi P. aethiopicus A. africanus BP ? A. afarensis 3

H. sapiens H. neanderthalensis 0

Millions of years A. bahrelghazali H. heidelbergensis A. anamensis P. verus 4 0 .25 ? ? 1 ? A. ramidus .5 P. troglodytes 5 2 ? ? ? P. paniscus .75 H. antecessor 3

Common ancestor of hominins and Pan

Figure 1 This diagram shows the approximate temporal ranges of the main hominid taxa. It assumes that modern humans and the chimpanzees shared a common ancestor; one interpretation of the of Pan is given in the right-hand box. The unnamed taxa, marked with a question mark, are based on the informed speculation that there is likely to be as much variety in the early phase of hominid evolution as there is between 3 and 1.5 Ma. Bold dashed lines represent likely evolutionary relationships; dotted lines are even more speculative statements about ancestor–descendant relationships. The left-hand box shows one interpretation of the taxonomy of later Homo.

The fossil record of A. afarensis includes substantial capable ofupright, bipedal, walking, it was not adapted for fragments of several skulls, many lower jaws and sufficient long-range . This indirect evidence for the limb bones to be able to estimate the likely body size of A. locomotion of A. afarensis is complemented by the afarensis. The collection also includes just less than halfof discovery, at Laetoli, ofseveral trails offossilfootprints. the skeleton ofan adult female.Its field number is AL-288, These were made more than three million years ago when but it is better known as ‘’. The picture of A. afarensis several individuals walked across a wet layer ofvolcanic that emerges is ofa species which ranged in body mass from ash and they provide very graphic and direct evidence that about 25 kg, for a small female, to more than 50 kg for a A. afarensis was capable ofbipedal locomotion. The size of large male. The brain volume of A. afarensis was between the footprints and the length of the stride provide 400 and 500 cm3. This is larger than the average corroboration for stature estimates between 1 m and ofa chimpanzee, but, ifthe estimates ofthe body mass of A. 1.5 m, which were based on the lengths ofthe limb bones afarensis are anything like correct, then relative to its of A. afarensis. estimated body mass the brain of A. afarensis is no larger Fossil hominin remains dating to between 4 and 4.5 Ma than that ofthe chimpanzee. However, there is little doubt have been found at Kanapoi in northern Kenya. In some that the chewing teeth – the premolars and molars – of A. ways these resemble A. afarensis, but in others they are afarensis are relatively larger than those ofthe living apes. more primitive and in yet other ways they show some A. afarensis apparently lived in a more ‘open’ woodland incipient Paranthropus features (see below). They have environment than A. ramidus. The shape ofthe pelvis and thus been placed in a separate species, Australopithecus the lower limb suggests that although A. afarensis was anamensis. Hominin fossils dating to c. 3.5 Ma have been

2 ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net Human Evolution: Overview recovered from the desert, in Chad, West Africa. These is known about the postcranial skeleton except that the differ from A. afarensis, yet they also differ from A. shape ofthe hip joint is much like that of Australopithecus. anamensis. The West African fossils have thus been placed In Paranthropus is represented by two in a new species, Australopithecus bahrelghazali. The species (Figure 1), both ofwhich have large jaws and discovery of suggests that cra- chewing teeth. The earlier and more primitive ofthe two nially-primitive hominins were still extant 2.5 Ma ago. species is referred to as Paranthropus aethiopicus. It has a smaller brain, around 400–450 cm3, a more ape-like face and larger anterior (i.e. incisor and canine) teeth than does the later species, which is called Paranthropus boisei. Discoveries in Southern Africa The earlier ofthese two East Africanspecies of Paranthropus spans the interval between 2.5 and 2.3 Ma, Nearly 50 years before the discovery of the Ethiopian and the later one between 2.3 and 1.4 Ma. The relatively fossils belonging to A. afarensis, an early hominin child’s large chewing teeth in the two species has prompted the skull had been found in 1924 in a cave at the Buxton suggestion that the East African forms were specialist Limeworks at Taung, in the northernmost part ofCape feeders on a diet that required heavy mastication. What Province, . The new hominin was described by little information exists about their postcranial skeleton and placed in a new genus and species, suggests that it was much like that of Australopithecus. Australopithecus africanus, which literally means the ‘southern ape’ ofAfrica. Since the discovery at Taung, fossils belonging to A. africanus have been found at other cave sites in southern The Beginnings of Homo,orAreThey? Africa. The vast majority of the additional evidence has come from Sterkfontein, with contributions from Maka- Long before Louis and ’s patient fieldwork at pansgat and, more recently, from Gladysvale. The age of had been rewarded with the discovery of the A. africanus-bearing breccias in the southern African the OH (Olduvai Hominid) 5 cranium (initially attributed caves has been estimated to be between 2.4 and 3 Ma. to Zinjanthropus boisei, then to Australopithecus boisei, but Males and females of A. africanus differed in body size, later to Paranthropus boisei (see above)), their efforts had but probably not to the degree they did in A. afarensis. The resulted in the discovery ofstone chopping and flake picture that has developed of A. africanus suggests that its belonging to what was to become known as the physique was much like that of A. afarensis except that its industry. This stone industry was more primitive than chewing teeth were larger and the skull was not as ape-like. any ofthe handaxes that had been recovered in the Its brain was larger than that of A. afarensis, but not previous century from European sites and from younger substantially so, and the postcranial skeleton suggests that strata at Olduvai Gorge. It was perhaps natural to assume although A. africanus was capable ofwalking bipedally, that OH 5 represented the hominin species that made the such a gait was probably energetically relatively inefficient. stone tools, but discoveries made in the 1960s were to The remains ofother animals foundwith A. africanus challenge this assumption. suggest that the habitat was a combination ofgrassland The new material included the remains ofthe vault, or and trees. roof,ofa skull and a piece ofa lower jaw, which was given the designation OH 7. Subsequently the remains ofa hand, foot and lower leg were found. Although these specimens were frustratingly incomplete, the Leakeys and their colleagues were convinced that the larger cranial capacity Paranthropus – A Genus of Large- and smaller tooth size ofthe new material, together with toothed Hominins what were then regarded as the advanced features of the limb bones, made it a stronger contender for the role of Just as there were East African and southern African toolmaker than Zinjanthropus. The new material was regional variants of Australopithecus, there are two included as a new species, within the genus Homo. The regional variants ofanother hominin genus called Para- name given to it, , literally means ‘handy nthropus. They are often referred to as ‘robust’ australo- man’, or ‘maker ofthe tools’. pithecines because oftheir relatively massive facesand Fossils that resemble the Olduvai H. habilis remains lower jaws. were subsequently unearthed from the site of Remains ofthe southern Africanvariety, Paranthropus in northern Kenya. As this evidence accumulated it became robustus, come from caves at , Kromdraai, apparent that variation within H. habilis was beginning to and Gondolin, and are dated to between 2 and exceed that which could reasonably be expected in a single 1.5 Ma. The brain, face and chewing teeth are larger than hominin species. In particular, the differences between two those of A. africanus, yet the incisor teeth are smaller. Little crania from Koobi Fora, such as KNM-ER 1470 and

ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net 3 Human Evolution: Overview

KNM-ER 1813, and between mandibles such as OH 13 dysplasia in the WT 15000 skeleton and it would be unwise and KNM-ER 1802, were cited as being too extreme to be to assume that all the individuals in this species have the subsumed within H. habilis. This led to the suggestion that same proportions. Turning to the teeth and jaws of H. specimens like KNM-ER 1470 and 1802 represented a ergaster, when they are related to body size they are no second species of‘early Homo’, and this was given the name larger than those ofAfricanand Australian samples of . modern humans. Likewise, the timing and pattern oftooth Present evidence suggests the H. habilis sensu stricto (i.e. development in H. ergaster is closer to that in modern a subset of‘early Homo’) dates from just less than 2 Ma to humans than is the case for any of the other early hominin around 1.6 Ma, and H. rudolfensis from perhaps as old as species for which we have evidence. When we turn to 2.5 Ma to 1.8 Ma. Compared to H. rudolfensis the face of evidence about the brain, the affinities with modern H. habilis sensu stricto is reduced in width and the opening humans are not so strong. While H. ergaster has a cranial ofthe nose is more sharply defined than it is in H. capacity ofbetween 800 and 900 cm 3 – greater than that of rudolfensis. The brain volume of H. habilis sensu stricto is H. rudolfensis – when this capacity is related to the larger around 500–700 cm3, whereas the estimated brain volume estimated adult body mass of H. ergaster, then the relative for H. rudolfensis is in the order of700–800 cm 3, with an brain size of H. ergaster is similar to that of H. rudolfensis average of750 cm 3. Contrary to initial interpretations, the and H. habilis sensu stricto. body shape of H. habilis sensu stricto more closely resembled that ofits forerunnersthan later species of Homo. This suggests that the posture and locomotion of H. habilis sensu stricto were much like that of Homo Moves Out of Africa Australopithecus and Paranthropus. Unfortunately, there are no postcranial bones definitely associated with any of More than a century ago, in 1891, Eugene Dubois reported the cranial remains of H. rudolfensis. the discovery ofa skull and femurthat were collected on the Overall, the case for including H. habilis sensu stricto and bank ofthe Solo River at Trinil in what was then called H. rudolfensis within our own genus, Homo, is weaker than Java, now Indonesia. Since then many more similar- it was in the 1960s and 70s and some scientists have looking remains have been located in Indonesia, mainly suggested that they should either be included in Australo- from the Sangiran region where the Solo River cuts pithecus, or referred to a new genus. However, as we shall through and rocks that have been see in the next section, by around 1.9 Ma ago there is good thrown up into a dome. evidence from East Africa of an early hominin that has Much ofmodern Indonesia is intensively farmedand stronger claims for inclusion in our own genus. little soil remains undisturbed. This makes the exact location ofmany ofthe finds difficult to pinpoint, and the paucity ofvolcanic ash and the weakness ofthe magnetic signals in the sediments means that the rocks are Homo Emerges in Africa difficult to date with any accuracy. There is, however, reasonably sound evidence that some ofthe finds at The discovery in 1984 ofthe remains ofa virtually complete Sangiran may be around 1 Ma, and there have been recent juvenile male skeleton, known as KNM-WT 15000 at West claims that other sites may be in the order of1.8 Ma old. Turkana confirmed the presence of Homo in East Africa. This would mean that the first hominins must have left Over the previous decade similar-looking skulls, jaws and Africa some time before this, and the first species to do so limb bones had been recovered from Koobi Fora. was likely to have been a H. ergaster-like hominin, or a Although many ofthese had been recognized as resembling more primitive precursor ofit. remains from Asia and elsewhere in Africa, it Archaeological evidence suggests that hominins had was not until the discovery ofKNM-WT 15000 that their occupied and the Near East by around less than 1 affinities could be confirmed. Although the West Turkana Ma and perhaps even earlier, and at the southern African skeleton was dated at around 1.5 Ma, other remains site ofSwartkrans there is evidence ofan ‘early Homo’-like attributed to the same species are as old as 1.9 Ma. Some hominin around 1.5 Ma. Remains of H. erectus persist in workers have given the species a new name H. ergaster, the Far East, at the Chinese site ofZhoukoudian, formerly while others refer to it as ‘early African H. erectus’. Choukoutien, near to Beijing, as late as 200 ka before the What makes the inclusion ofthese remains in the genus present. This means that H. erectus was present in at least Homo more soundly based than is the case for H. habilis one region while hominin species that are closer in sensu stricto or H. rudolfensis? The lengths ofthe long appearance to modern humans – – bones ofskeleton fromWest Turkana confirm that this were making their appearance. hominin species has limb proportions much like those of modern humans; its trunk, however, is relatively short. However, there are signs ofa pathology called skeletal

4 ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net Human Evolution: Overview

Archaic Homo The earliest fossil evidence of hominins with skulls, jaws, teeth and limb bones equivalent to those ofcontemporary Before we turn to the origin of modern humans we must humans comes from Koobi Fora, in northern Kenya; first consider a substantial collection offossilsthat are Klasies River Mouth, southern Africa; from the Omo, in more modern human-like than H. erectus, yet which are Ethiopia; and from the Skhul and Qafzeh caves in the Near not fully modern in appearance. The best known material East. All ofthese remains date fromas early as 250 ka to in this ‘archaic Homo’ category are the remains that have around 100 ka. Evidence about modern human origins also been attributed to Homo neanderthalensis. The character- comes fromthe pattern ofvariation ofthe modern human istics ofthis species include a large, globular-shaped genotype. The prediction ofthe ‘out ofAfrica’hypothesis cranium, jaws and teeth that are set well forward in the is that any adaptively neutral variation in the genotype face, and particularly robust limb bones with large joint would be greatest in the regional modern human popula- surfaces. The earliest come from and tion that had existed for the longest time, namely that from France and they are dated to 300–400 ka, with some Africa. Analysis of the variation in mitochondrial and evidence being as old as 700–800 ka. The youngest nuclear DNA suggest that this was the case, but antiquity fossils are dated to around 30 ka. The ofthe gene pool is not the only explanation forthere being precursors of H. neanderthalensis belong to Homo heidel- less admixture in Africa than in other regions. A similar bergensis, and fossils referred to this taxon are known from pattern would result ifthe Africanpopulation had been Europe, Africa and Asia. larger than that in other parts ofthe world. The case foran Hominins that show the characteristic Neanderthal African origin for early modern humans is a strong one, morphology are confined to Western Europe, the Near but for the moment it must be judged to be probable, but East and adjacent parts ofAsia. For much ofthe period ‘not proven’. when Neanderthals are present the climate was oscillating between cold, glacial, phases and shorter, warmer, inter- glacial, periods. The short stature and robust limb bones of the Neanderthals are thus just the body-build that would Peopling the Planet be expected under the influence ofAllen’s rule. Nean- derthals are often portrayed as primitive hominins who Modern humans were dispersed across the Old World by lacked the sophisticated technology and artistic sophisti- 25 to 35 ka. By that time, and perhaps as early as 150 ka and cation ofthe populations that replaced them in Europe. certainly by 59–60 ka, they had also managed to cross the This interpretation belies the fact that the tools that are water barriers between the Asian mainland and Australa- associated with many, but not all, Neanderthal sites are a sia. Reductions in sea level associated with the major good deal more varied in design than the handaxes that glaciations would have shortened any sea crossings; characterize the Lower Palaeolithic. There is also evidence nonetheless the colonization ofAustralia must have that they buried their dead and used grave goods. involved the ability to make a raft, or an equivalent sea- Populations of‘archaic Homo sapiens’ peoples from going craft. other regions are not so characteristic in their appearance, Present archaeological evidence suggests that the but all have some distinguishing features. It is the extent to occupation ofthe New World did not take place until which those characteristics are continued within the 10–12 ka. Modern humans could have entered the New regional populations that succeeded them that lies at the World across the landbridge that was established between root ofongoing debates about the origin ofanatomically Siberia and Alaska between around 7 and 20 ka, but the modern humans. earliest archaeological evidence ofmodern human occupa- tion ofSiberia dates from20–30 ka. Artefacts and the remains of animals found in associa- tion with anatomically modern human remains in the Old World provide compelling evidence that by 40 ka, ifnot Modern Human Origins before, organized hunting was part of the survival strategy ofthese early populations. Some archaeologists place the Two hypotheses for the origins of modern humans have origin ofhunting much earlier, but in many cases it is been put forward. One, called the ‘out of Africa’ or ‘Noah’s possible to point to other explanations for the association Ark’ hypothesis, suggests that the genetic modifications ofanimal bones with evidence ofhuman occupation. The that were responsible for the shift to an anatomically first evidence ofsea shell middens occurs in coastal modern human morphology only occurred once, and in archaeological sites by around 15–20 ka. Africa. The rival ‘multiregional’ hypothesis proposes that Farming and permanent settlement do not become the shift to an anatomically modern human morphology evident in the archaeological record until about 12 ka occurred several times, but only once in each ofthe major before the present. While the best known evidence comes population centres. from the ‘fertile crescent’ in the Near East, the discovery of

ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net 5 Human Evolution: Overview drainage ditches in New Guinea, and ofherding in East Further Reading Africa, around the same time as the evidence for grass domestication along the Nile, demonstrates that changes Howells WW (1997) Getting Here: The Story of Human Evolution,2nd edn. Washington, DC: The Compass Press. from a hunter–gatherer lifestyle were occurring across the Jones S, Martin R and Pilbeam D (eds) (1992) The Cambridge globe. Thereafter, the balance between physical and Encyclopedia of Human Evolution. Cambridge: Cambridge University cultural evolution has shifted so far towards the latter that Press. evidence ofmorphological change all but disappears. Klein RG (1999) The Human Career: Human Biological and Cultural There are changes in the nature ofthe inhabitants in some Origins, 2nd edn. Chicago, IL: Chicago University Press. parts ofthe world, but these are predominantly the result of Tattersall I (1995) The Fossil Trail: How We Know What We Think We migrations and not ofevolution within a local region. Know About Human Evolution. Oxford: Oxford University Press. Wood BA and Collard MC (1999) The Human Genus. Science 284:65– 71.

6 ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net