Human Evolution: Overview Introductory article

Bernard A Wood, The George Washington University, Washington, DC, USA Article Contents
. Evolutionary Context
The fossil evidence for human evolution can be traced from close to 4.5 Ma to the present. . The Earliest Hominins
The new term for modern humans and the human clade is ‘hominin’, which replaces the . Discoveries in Southern Africa
older name ‘hominid’. . Paranthropus – A Genus of Large-toothed Hominins
. The Beginnings of Homo, or Are They?

Evolutionary Context . Homo Emerges in Africa

. Homo Moves Out of Africa

There is abundant evidence that the living animals most . Archaic Homo

closely related to modern humans (Homo sapiens) are the . Modern Human Origins

chimpanzee (Pan) and the gorilla (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 of the cells
The first creature to show rudimentary human specializa-
of modern humans and the chimpanzee are very similar.
tions is known as Ardipithecus ramidus, the 4.5 Ma-old
An increasing number of researchers are convinced that the
remains of which 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 of the gorilla, but others maintain
living chimpanzees, others with the African apes 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 of them
forearm bones are shared with later hominins. Thus, the
to the exclusion of the third.
discoverers suggested that these fossils belong to a hominin
Differences in the DNA can be used to provide an
species, and not an ape one, and although they initially
estimate of how long lineages have been independent. The
allocated it to another group of extinct hominins,
molecular differences between living people and the living
Australopithecus, 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 from the size of one of the bones that make up
to modern humans than to the apes have traditionally been
the shoulder joint, at least one individual of this 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 of the skull, suggesting
related, scientists are using ‘hominid’ to refer to the family
that the posture and gait of Ardipithecus was respectively
Hominidae, 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 Tribe Hominini, which includes
The remains of the animals and the plants found with 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, if not actually within, woodland.
For many years human evolution 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 of the
placed’ by later, more advanced ones, with modern
chimpanzee, including fruit, vegetation and some small
humans at the top of the ladder of ascent. Recent evidence
animals. Although evidence of only one species of
suggests that the metaphor of a 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 of this 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 of the stems stop well short of
name was given in 1978 to fossils recovered from Laetoli, in
the highest point of the 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
4 Ma.

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 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
Millions of years BP

?
A. afarensis
3

H. sapiens H. neanderthalensis
0 A. bahrelghazali
H. heidelbergensis A. anamensis
P. verus
4 0
.25 ?
?
1
? A. ramidus

.5
2 P. troglodytes
5
? ?
? 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 taxonomy 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 of upright, bipedal, walking, it was not adapted for
fragments of several skulls, many lower jaws and sufficient long-range bipedalism. 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 half of discovery, at Laetoli, of several trails of fossil footprints.
the skeleton of an adult female. Its field number is AL-288, These were made more than three million years ago when
but it is better known as ‘Lucy’. The picture of A. afarensis several individuals walked across a wet layer of volcanic
that emerges is of a 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 of bipedal 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 brain size corroboration for stature estimates between 1 m and
of a chimpanzee, but, if the estimates of the body mass of A. 1.5 m, which were based on the lengths of the 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 of the 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 of the 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 of the 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

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 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 of the hip joint is much like that of Australopithecus.
anamensis. The West African fossils have thus been placed In East Africa Paranthropus is represented by two
in a new species, Australopithecus bahrelghazali. The species (Figure 1), both of which have large jaws and
discovery of Australopithecus garhi suggests that cra- chewing teeth. The earlier and more primitive of the 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 of these two East African species 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 of Cape feeders on a diet that required heavy mastication. What
Province, South Africa. The new hominin was described by little information exists about their postcranial skeleton
Raymond Dart 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’ of Africa.
Since the discovery at Taung, fossils belonging to A.
africanus have been found at other cave sites in southern The Beginnings of Homo, or Are They?
Africa. The vast majority of the additional evidence has
come from Sterkfontein, with contributions from Maka- Long before Louis and Mary Leakey’s patient fieldwork at
pansgat and, more recently, from Gladysvale. The age of Olduvai Gorge 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 of stone chopping and flake tools
picture that has developed of A. africanus suggests that its belonging to what was to become known as the Oldowan
physique was much like that of A. afarensis except that its industry. This stone tool industry was more primitive than
chewing teeth were larger and the skull was not as ape-like. any of the 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 of walking 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 of other animals found with A. africanus challenge this assumption.
suggest that the habitat was a combination of grassland The new material included the remains of the vault, or
and trees. roof, of a skull and a piece of a lower jaw, which was given
the designation OH 7. Subsequently the remains of a 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 of the 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 of another hominin genus called Para- name given to it, Homo habilis, literally means ‘handy
nthropus. They are often referred to as ‘robust’ australo- man’, or ‘maker of the tools’.
pithecines because of their relatively massive faces and Fossils that resemble the Olduvai H. habilis remains
lower jaws. were subsequently unearthed from the site of Koobi Fora
Remains of the southern African variety, Paranthropus in northern Kenya. As this evidence accumulated it became
robustus, come from caves at Swartkrans, Kromdraai, apparent that variation within H. habilis was beginning to
Drimolen 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

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 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 of African and Australian samples of
Homo rudolfensis. modern humans. Likewise, the timing and pattern of tooth
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
of the nose is more sharply defined than it is in H. capacity of between 800 and 900 cm3 – 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 of 700–800 cm3, with an brain size of H. ergaster is similar to that of H. rudolfensis
average of 750 cm3. Contrary to initial interpretations, the and H. habilis sensu stricto.
body shape of H. habilis sensu stricto more closely
resembled that of its australopithecine forerunners than
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 of a skull and femur that were collected on the
Overall, the case for including H. habilis sensu stricto and bank of the 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 Pliocene and Pleistocene 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 of modern Indonesia is intensively farmed and
stronger claims for inclusion in our own genus. little soil remains undisturbed. This makes the exact
location of many of the finds difficult to pinpoint, and
the paucity of volcanic ash and the weakness of the
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 of the finds at
The discovery in 1984 of the remains of a 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 of 1.8 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 of these had been recognized as resembling more primitive precursor of it.
Homo erectus remains from Asia and elsewhere in Africa, it Archaeological evidence suggests that hominins had
was not until the discovery of KNM-WT 15000 that their occupied Europe 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 of Swartkrans there is evidence of an ‘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 of Zhoukoudian, 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 of these 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 of the long appearance to modern humans – Homo heidelbergensis –
bones of skeleton from West 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 of a pathology called skeletal

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 Human Evolution: Overview

Archaic Homo The earliest fossil evidence of hominins with skulls, jaws,
teeth and limb bones equivalent to those of contemporary
Before we turn to the origin of modern humans we must humans comes from Koobi Fora, in northern Kenya;
first consider a substantial collection of fossils that 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 of these remains date from as 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 from the pattern of variation of the modern human
istics of this species include a large, globular-shaped genotype. The prediction of the ‘out of Africa’ 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 Neanderthals come from Spain 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
Neanderthal fossils are dated to around 30 ka. The of the gene pool is not the only explanation for there 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 if the African population had been
Europe, Africa and Asia. larger than that in other parts of the world. The case for an
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 of Asia. For much of the 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 of Allen’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 of the 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 of Australia 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 of the 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 of ongoing debates about the origin of anatomically Siberia and Alaska between around 7 and 20 ka, but the
modern humans. earliest archaeological evidence of modern human occupa-
tion of Siberia dates from 20–30 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, if not
Modern Human Origins before, organized hunting was part of the survival strategy
of these early populations. Some archaeologists place the
Two hypotheses for the origins of modern humans have origin of hunting 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 of animal bones with evidence of human occupation. The
that were responsible for the shift to an anatomically first evidence of sea 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 of the major before the present. While the best known evidence comes
population centres. from the ‘fertile crescent’ in the Near East, the discovery of

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 Human Evolution: Overview

drainage ditches in New Guinea, and of herding in East Further Reading

Africa, around the same time as the evidence for grass
Howells WW (1997) Getting Here: The Story of Human Evolution, 2nd
domestication along the Nile, demonstrates that changes
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 of morphological change all but disappears. Klein RG (1999) The Human Career: Human Biological and Cultural
There are changes in the nature of the inhabitants in some Origins, 2nd edn. Chicago, IL: Chicago University Press.
parts of the world, but these are predominantly the result of Tattersall I (1995) The Fossil Trail: How We Know What We Think We
Know About Human Evolution. Oxford: Oxford University Press.
migrations and not of evolution within a local region.
Wood BA and Collard MC (1999) The Human Genus. Science 284: 65–
71.

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