Reprint
An Asian perspective on early human dispersal
from Africa
by
Robin
Dennell1
and Wil Roebroeks2
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This article is reprinted from Nature,
438:1099-1004, 22/29 Dec 2005
1 R. Dennell, Department of Archaeology, University of
Sheffield, Sheffield S1 4ET, United Kingdom,
E-mail r.dennell@sheffield.ac.uk
2. W. Roebroeks, Department of Archaeology, Leiden
University, PO Box 9515, 2300RA Leiden, The Netherlands
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1. Introduction
The past decade has seen the Pliocene and Pleistocene fossil
hominin record enriched by the addition of at least ten new taxa,
including the Early Pleistocene, small-brained hominins of Dmanisi,
Georgia, and the diminutive Late Pleistocene Homo floresiensis from
Flores, Indonesia. At the same time, Asia's earliest hominin presence
has been extended up to 1.8 million years (Myr) ago, hundreds of
thousands of years earlier than previously envisaged. Nevertheless,
the preferred explanation for the first appearance of hominins
outside Africa has remained virtually unchanged. We show here that it
is time to develop alternatives to one of palaeoanthropology's most
basic paradigms: "Out of Africa1."
A key assumption in accounts of early hominin evolution is that
the genus Homo originated in Africa, and an early form,
classified either as Homo ergaster or H. erectus sensu
lato (see Box 1), was the first to leave about
1.7-1.9 million years (Myr) ago (depending upon one's choice of dates
and specimens), and then colonized southern Asia as far as
400o N. The identification of east Africa as the 'core'
area for the genus Homo (including H. ergaster) as well
as tool-making seems secure to most palaeoanthropologists, and the
most recent attempts at modelling early hominin dispersals start
implicitly from the assumption that H. ergaster originated in
east Africa and then dispersed across Asia (ref. 1, 2). In fact, the
evidence that H. ergaster originated in east Africa is less
convincing than it seems. H. ergaster marks such a radical
departure from previous forms of Homo (such as H.
habilis) in its height, reduced sexual dimorphism, long limbs and
modern body proportions (ref. 3) that it is hard at present to
identify its immediate ancestry in east Africa (ref. 4). Not for
nothing has it been described as a hominin "without an ancestor,
without a clear past" (ref. 5).
2. Who were the first
Asians?
At present, we have very little information on where, when and
which hominins first appeared in Asia, and the expansion of H.
ergaster across Asia in the Early Pleistocene remains a massive
assumption, even if it is routinely treated as a historical fact. It
is assumed that it migrated out of Africa along the Nile Valley or
across the southern end of the Red Sea, but there is no
archaeological or fossil hominin evidence that hominins were in the
Nile Valley in the Lower Pleistocene; and there are no Oldowan sites
in the Sinai, southern Negev, or in southwest Arabia at the alleged
point of entry to Asia. The only Asian Early Pleistocene fossil
hominin evidence comprises three incisors from 'Ubeidiya, Israel
(1.4-1.0 Myr ago), attributed to H. erectus sensu lato
(s.l.) by default (ref. 6); the 1.7-Myr-old specimens from
Dmanisi, Georgia (ref. 7), which have recently been classified as a
very early type of H. ergaster(ref. 8) and/or a new taxon,
H. georgicus (ref. 9); and the specimens attributed to H.
erectus sensu stricto (s.s.) in Java (ref.10), 5,300 miles
away and regarded by many (ref. 11-16) but not all (ref. 10, 17-20)
as different from the east African H. ergaster, The earliest
of these is the Mojokerto cranium (ref. 2), which now seems to have
been found in context, despite previous misgivings (ref. 22), and is
dated to 1.81 +/-0,04 Myr ago (ref. 23); the key specimens from
Sangiran have been dated to about 1.6-1.7 Myr ago (ref. 23, 24). (The
Early Pleistocene mandible and teeth attributed to Homo from
Longuppo (ref. 25), southern China, probably belonged to an ape (ref.
26, 27)). This meagre list of sites is supplemented by archaeological
instances of Early Pleistocene artefacts in Asia that are attributed
to H. erectus s.l.; examples are Erq el-Ahmar, Israel,
claimed to date to the Olduvai Event (ref. 28), and the Nihewan
basin, north Chinam (ref. 29). The only reason why the earliest tool
assemblages in Asia are attributed to H. erectus s.l.,
is that palaeoanthropologists have already decided that, in effect,
it was the only hominin capable of migration out of Africa, and with
sufficient Wanderlust (ref.13) to do so.
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Box
1: Homo erectus and Homo ergaster
H. erectus - or more properly, Pithecanthropus
erectus - was first discovered at Trinil in Java by
Eugene Dubois in 1891. In the 1930s, further discoveries of
hominin remains elsewhere in Indonesia and at Chou-kou-tien
(now Zhoukoudian) in China were seen as broadly similar,
even if initially given their own generic names (such as
Meganthropus and Sinanthropus). In 1950, Ernst
Mayr reclassified all this material as H. erectus,
with the Trinil specimens as the type fossils. Subsequent
African specimens were also called H. erectus, as
were much later specimens from Europe. H. erectus
thus became for a while the earliest hominin that was
thought to have lived in Asia, Africa and Europe.
In the past few years, some have doubted that the east
African specimens should be placed within the same
palaeospecies as the Asian ones, In the light of discoveries
at Koobi Fora, it has been suggested that the earliest
African examples should be called H. ergaster, after
the specimens found at Koobi Fora, including WT15000, the
magnificent 1.6 Myr-old skeleton of a young boy from
Nariokotome that was initially published as H.
erectus. Consequently, it is the African H.
ergaster that is now seen by some as the hominin that
first colonized Asia and formed the founding population of
what later became H. erectus in China and southeast
Asia. European specimens once regarded as late examples of
H. erectus or 'archaic Homo sapiens' are now
increasingly classified under the revived taxon of H.
heidelbergensis, a term first used to classify the
mandible from Mauer, Germany, found in 1907, To avoid
ambiguity, the term H. erectus is used here sensu
stricto to denote only those specimens from eastern
Asia, and H. ergaster is used to denote early east
African specimens of H. erectus sensu lato.
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3. Homo ergaster: wanderer
or stay-at-home?
The reason why H. ergaster is assumed to have been uniquely
capable of migrating out of Africa about 1.7-1.9 Myr ago into the
Asian grasslands is because of its long limbs, human-like body
proportions, probable efficient thermoregulatory mechanisms for
remaining cool in hot conditions, the ability to ingest large amounts
of meat in an environment rich in fauna but poor in plant foods for a
hungry primate, and a sufficiently large brain to deal with the
challenges of a more carnivorous niche (ref. 3) . This argument is
persuasive, except for the point that australopithecines had probably
colonized all the African savannah grasslands by 3.0-3.5 Myr ago
(ref. 30), and Australopithecus garhi was living in a similar
environment in northeast Africa by 2.5 Myr ago (ref. 31). As savannah
grasslands were extensive across southern Asia by 3.0 Myr ago (ref.
32), there are no reasons a priori why australopithecines
could not also have expanded into the Asian grasslands before H.
ergaster.
The fossil evidence that H. ergaster was in Asia in the
Early Pleistocene is not only weak, but extremely ambiguous. The
long-running debate (see Box 1) over whether the
Early Pleistocene Javan hominins should be classified as H.
erectus s.s (and thus different from their east African
counterparts) or incorporated with them as H. erectus s.l.,
and/or seen as composite (ref. 33) is still unresolved. Neither those
who regard them as an integral part of H. erectus s.l. (ref.
10) nor those who view them as derived from the African H.
ergaster question that the core area was east Africa. Neither
position is strengthened by the recent suggestion that the Mojokerto
child had an ape-like pattern of postnatal development (ref. 34)
unlike H. ergaster. Another 'big unknown' is the ancestral
form of the Late Pleistocene H. floresiensis (ref. 35,36),
which may prove to have roots deep in the Asian Pleistocene. The
Dmanisi hominins are harder still to assimilate within a simple model
of African origin and Asian dispersal by H. ergaster or H.
erectus s.l. The first discovery, of the mandible, could be
classified as H. erectus s.l. (ref. 37). The first
crania were regarded as a very early form of H. ergaster (ref.
8), and the latest, very small-brained one (D2700) as H.
ergaster but also closely related to H. habilis s.s. (ref.
38). Confusingly, one mandible (D2600) has been assigned to a new
taxon, H. georgicus (ref. 9). These recent assessments imply
that hominins dispersed from Africa earlier than the emergence of
large-bodied hominins such as the Nariokotome individual.
4. When could hominins first have
left Africa?
But how much earlier could this dispersal have been? If a hominin
at the same grade as H. habilis was able to exist outside
Africa, why not others? Why not follow the logic of Wood and
Collard's reasoning (ref. 39), that H. habilis is better
classified as A. habilis and suggest that the earliest Asians
were in fact australopithecine, with A. georgicus as their
first known representative outside Africa? This suggestion would open
a Pandora's box: if a hominin as small-brained (and probably as
short) as those at Dmanisi could colonize southwest Asia by 1.7 Myr
ago (and with no obvious African antecedents), why not at 2.6 Myr
ago, shortly after stone tool making became part of the hominin
repertoire (ref. 40)? Or why not even earlier, by, for example,
3.0-3.5 Myr ago, when the Saharan-Arabian desert barriers did not yet
exist (ref. 32)? If A. bahrelghazali, 2,500 km west of the
Rift Valley, implies that by 3.5 Myr ago "hominids were distributed
throughout the woodland and savannah belt from the Atlantic Ocean
across the Sahel through eastern Africa to the Cape of Good Hope"
(ref. 30), why could they not have done the same across the
grasslands of western, southern and central Asia?
5 Absence of evidence and
evidence of absence
The obvious retort to these questions is that there is no evidence
that australopithecines did migrate out of Africa. However, absence
of evidence is not enough; we need convincing evidence (so far not
forthcoming) that the absence is not the result of taphonomic
circumstance or lack of fieldwork, especially in a continent as large
as Asia. There are only a limited number of vertebrate fossil
assemblages for the Late Pliocene and Early Pleistocene of southwest
Asia (a region larger than Kenya, Ethiopia and Tanzania combined).
The Late Pliocene assemblage from Bethlehem (ref. 41) (Israel) is
very small, with only 11 taxa and dominated by animals with an adult
body weight of more than 60 kg and thus larger than hominins. These
fossils were found in coarse gravel (with clasts up to 0.5 m long) in
a clay matrix, in which small and fragile remains were most unlikely
to be preserved. At Kvabebi (ref. 42, 43), Georgia, dated to more
than 2.6 Myr ago (that is, earlier than the earliest stone tools in
Africa (ref. 40)), there are 21 mammalian taxa indicative of a
riverine and marshy environment. Two other Georgian localities,
Kocachuri and Calka, are slightly earlier than Dmanisi and yielded
small assemblages dominated by large taxa (ref.42). There are 21 taxa
represented at Dmanisi, and 33 (excluding microfauna) at 'Ubeidiya.
The point here is that small assemblages, with only a few taxa that
are mainly from large animals, are most unlikely to contain hominin
remains: in southwest Asia, two of the three large and fine-grained
assemblages did yield skeletal evidence of hominins (although very
little at 'Ubeidiya (ref. 6)). In central Asia, the Late Pliocene
record is poor because fossils are often found in coarse sediments
(ref. 44), although one surprising find is of the baboon (Papio
suschkini (ref. 45)) that is often regarded as a commensal of
Homo (ref. 46). Late Pliocene faunal assemblages from
northern Pakistan and India do not contain any hominins, and these
are also unknown for the entire Early Pleistocene: the earliest
fossil hominin evidence we have is Middle Pleistocene, from the
Narmada Valley (ref. 47), long after hominins are first in evidence
to both the west and east of the Indian subcontinent. Although many
vertebrate taxa are represented in the Upper Siwaliks of India (30 in
the Tatrot and 49 in the succeeding Pinjor Stage (ref. 48), most are
larger than hominins. It is also likely that the full range of taxa
is incomplete for the Indian subcontinent, because Megantereon
and Pachycrocuta are not recorded in India but are present in
Pakistan; in Pakistan, there is no evidence of Camelus and
small primates, and in neither country is Homotherium recorded
(ref. 49), although this is present to the west at Dmanisi, to the
north at Kuruksay, central Asia (ref. 44) and to the east at Longuppo
(ref. 25), south China. In mainland southeast Asia, there is no Late
Pliocene or Early Pleistocene fossil evidence. One of the few
instances for which we can be reasonably sure that H. erectus
s.l. (and other hominins) were absent is Longuppo (ref. 25),
south China, where four primates (Gigantopithecus,
Lufengpithecus, Macaca and Procyncocephalus) are
recorded among the 68 taxa present. The faunal assemblages from the
Yushe Basin (ref. 50) and the "Hipparion fauna" of north China (ref.
50) are of comparable quality to those from India, and the absence of
hominins is equivocal.
6. Ways forward: alternatives to
Out of Africa 1
If the above taphonomic review suggests that we cannot show the
absence of hominins from areas in Asia at a time before the little
evidence we have indicates their presence, we need to consider
alternatives to the current Out of Africa model. There are three
issues here. The first is when hominin(s) first left Africa - might
they, for example, have left shortly after they acquired the ability
to make stone tools, the earliest of which are currently 2.6 Myr old?
Or could they have left even earlier, about 3.0-3.5 Myr ago, when
some australopithecines were already living in the African
grasslands? The second issue is whether we yet know the full range of
hominins that inhabited both Africa and Asia in the Late Pliocene and
Early Pleistocene. Even in east Africa, several new taxa have been
claimed in the past decade (for example, A. anamensis (ref.
51), A. garhi (ref. 52), Ardipithecus ramidus
(ref. 53) and Kenyanthropus platyops (ref. 54)) and
doubtless more will be found. (An indication of how little we know
about Pleistocene east Africa is that only recently has the first
fossil evidence for chimpanzee been found (ref. 55)). In Asia, the
recent discoveries of H. georgicus and H. floresiensis
should make us very wary of assuming that H. erectus s.l. was
the only player on the Asian stage in the Early Pleistocene. Third,
Asia might not have been the passive recipient of whatever migrated
out of Africa but might have been a major donor to speciation events,
as well as dispersals back into Africa. Such two-way traffic is well
documented for other mammals in the Pliocene and Early Pleistocene,
such as Equus (ref. 56) and bovids ref. 57), with more
taxa migrating into than out of Africa. There is no reason why
hominin migrations were always from Africa into Asia, and movements
in the opposite direction might also have occurred, as has been
suggested for the Olduvai OH9 (refs 13, 58) and Daka (ref. 4)
specimens. We should even allow for the possibility that H.
ergaster originated in Asia (refs 23,59) and perhaps explain its
lack of an obvious east African ancestry as the result of immigration
rather than a short (and undocumented) process of anagenetic (in
situ) evolution. Although Darwin's suggestion (ref. 60) that "it
is somewhat more probable that our early progenitors lived on the
African continent than elsewhere" is widely quoted, it is worth
noting his next sentence: "But it is useless to speculate on this
subject... since so remote a period the earth has certainly undergone
many great revolutions, and there has been ample time for migration
on the largest scale".
We obviously need ways of testing these alternatives. The
overriding need is for data sets from Asia that are of comparable
quality to those from Africa. Although absence can never be
'demonstrated', we can at least put some constraints on its
probability by comparing the quality of the fossil record from the
inferred core and its 'peripheries'. Because the species we are
interested in were not very abundant in their world and hence in the
fossil record, knowledge of biasing factors is a prerequisite for the
study of their past distributions. The higher the trophic level of a
species, the smaller is its abundance in the real world and hence in
the fossil record (for the whole of the Asian Late Pliocene, we have
only two records of a puma, for example, separated by more than 3,000
miles (ref. 43)). Open, mesic-to-arid environments tend to preserve
fossils better than do forested and wetter environments (which is
probably why we have no fossil record for the gorilla and only one
observation - from open woodland - for the chimpanzee (ref. 55)).
Should faunal remains get covered in a sedimentary matrix, that
matrix obviously has to survive and be accessible, a condition that
is rarely met for Pliocene and Pleistocene sediments. The Rift Valley
constitutes a unique exception by its sheer size and the exposure of
fine-grained sediments of the relevant age, and includes many of the
key African sites, as well as Erq el-Ahmar, 'Ubeidiya and Gesher
Benot Ya'aqov in Israel.
7. Regional imbalances and future
challenges
Current large-scale imbalances between regional records are often
the result of differences in research history and intensity. To some
degree the Javan and Levantine records result from research initiated
during colonial times, and the east African record similarly owes a
great deal of its incipient (and prolific) research to the
consequences of its colonial history. In contrast, most parts of Asia
have experienced only a very limited survey of Neogene exposures, in
comparison with the heavy palaeoanthropological investments made in
east Africa during the past four decades. These regional imbalances
are crucial. After all, in the early twentieth century, east Asia was
thought to have been the centre of human origins because it had the
oldest fossils, and one of the marginal areas, Africa, had not yet
seen any significant fieldwork (ref. 61). Despite the imbalance in
research intensity (and hence number of sites), Asia has produced
major surprises in recent years, a testimony to its
palaeoanthropological potential. As an example, recent research in
China has extended its earliest hominin presence up to 1.66 Myr ago
(ref. 29), half a million years older than envisaged only a few years
ago, with the lowest levels of the fossiliferous sequence in the
Nihewan basin not having been reached yet. The increasing evidence
for Early Pleistocene hominins in China and Java stretches the limits
of current thinking on hominin evolution, as do the finds of H.
floresiensis and the Dmanisi hominin assemblage, the latter
recovered from an area where few would have expected Early
Pleistocene tiny-brained hominins two decades ago. These discoveries
underscore our poor ability to discern, let alone predict, the design
on the picture we try to piece together from the few pieces of the
jigsaw that we have. Again, absence of evidence is not enough: if we
postulate that species A migrated into area B, we need comparable
data sets to infer legitimately that it was absent before that date -
we need not just the FAD (first appearance date) of a taxon in a new
area, but also its LPA (last probable absence) (see Fig. 1). We will
never have certainty about LPAs, of course: the recently reported
finds from Pakefield, southern England, show that two centuries of
intensive research of the Cromer Forest beds failed to recover the
(indeed ephemeral) traces of an early Middle Pleistocene hominin
presence there (ref. 62). That such a surprise can turn up in one of
the best-researched areas of the Old World shows that we can never be
sure about LPAs and, more importantly, underlines the necessity of
working with data sets of comparable quality in both alleged donor
and alleged recipient regions (see also Box 2). For
Africa and Asia, comparability is still many generations of research
grants away. Nevertheless, we could do much more to reduce the level
of uncertainty over when hominins were last absent in Asia by
increasing the number and quality of fossil assemblages immediately
before their first alleged appearance.
8. "Africa" and "Asia", or
"Savannahstan"?
We also need different spatial units for investigating extinct
hominin populations. Since the time of the Greeks and the Romans, we
habitually refer to 'Africa' and 'Asia' as separate continents, each
somehow homogeneous and distinct from the other. Plants and animals
(and extinct hominins) are less respectful of our GraecoRoman
heritage. The landmasses we now call Africa and Asia are of course
enormously diverse, but they also have many plants, animals and
environments in common. In recent decades, palaeoanthropologists have
rightly emphasized the importance of savannah grasslands in hominin
evolution, both as a place where many types (including H.
ergaster) lived in the Late Pliocene and Early Pleistocene and as
having been important in influencing hominin brain size, post-cranial
anatomy, and diet. As noted earlier, Pliocene grasslands extended all
the way from west Africa to north China, and 'Savannahstan' might
prove a more useful spatial unit for modelling early hominin
adaptations and dispersals within them than simply an
undifferentiated 'Africa' or 'Asia'. For example, the African
hominins 1.9-1.7 Myr ago at Koobi Fora (Kenya) and Ain Hanech
(Algeria), and their slightly later counterparts in Asia at 'Ubeidiya
(Israel), and Majuangou (north China) were all living in broadly
comparable grassland environments, and it makes sense to place them
within the same frame of reference. This might also highlight
significant variation that is sometimes buried under a blanket term
such as 'Asia'. For example, the hominins in the Nihewan basin, north
China, and those in Java are both clearly in east Asia, but those in
Java inhabited a region that was considerably more densely wooded. It
is not the continent that matters in studying human origins so much
as the type(s) of environment with which early hominins were
associated.
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Figure 1. Dispersals, cores and peripheries.
The dangers of over-reliance on first appearance dates
(FADs) of when a taxon migrated from its core area, A, into
a new territory, B. Filled
green squares indicate the
first appearance of a taxon, open
green squares the presence of a taxon and
red circles fossil assemblages
without this taxon.
a. A hypothetical situation in which a taxon
originated in area A, and then migrated into territory
B.
b. The reliability of these FADs is considerably
strengthened by the numerous well-dated instances when their
last probable absences (LPAs) can be documented. Without
these, future discoveries might indicate (as shown in
c) that previous estimates of when a taxon first
appeared were too recent, as happened when the earliest
Javan hominins were redated from about 1.0 Myr old to 1.8
Myr old.
d. Even more alarmingly, future discoveries might
even show that the taxon probably originated in the area
that it was supposed to have colonized-as happened when the
centre of hominin origins was relocated from Asia to Africa
in the 1960s.
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Box
2: Neanderthals and moderns - where were
they?
Highlighting the imperfections of the fossil record
obviously has implications for studies of the distribution
of other species too. The Neanderthals are by far the
best-studied extinct hominins, with a rich fossil record
sampling hundreds of individuals, heavily biased towards the
western part of their range, western Europe. However, the
northern, eastern and southern limits to their distribution
are poorly documented, again because of an imbalance in
research intensity. The juvenile from Teshik-Task,
Uzbekistan, is the easternmost one known, at roughly 1,300
and 2,000 miles from its nearest fossil neighbours, Shanidar
in Iraq and Kiik-Koba in the Black Sea area, respectively.
The southern limit of their distribution is unknown, and may
have extended over the whole of Arabia and the Indian
subcontinent-we cannot be certain until these regions
produce the necessary fossil evidence. Distribution maps of
Neanderthals are palimpsests of range expansion and
contraction, probably hide many shifts of ranges in the
rhythm of climatic oscillations and most probably also
understate their full range. Chronological resolution is
significantly better than in the earlier Pleistocene,
however, and enables us to see the occasional
'interfingering' of their range with that of anatomically
modern humans in the Near East. If the Tabun C1 hominin does
indeed date to Marine Isotope Stage (MIS) 6, Neanderthals
were in the northern parts of the Near East before
anatomically modern humans were there in MIS 5, with
Neanderthals again present during MIS 4. As with Out of
Africa 1, the poor state of sampling of major parts of
western and central Asia should force us to be very humble
in our inferences on the core and peripheries in the
Neanderthal world. Comparable points can be made with regard
to Out of Africa 2 (the model according to which our own
species, H. sapiens (or 'anatomically modern humans')
evolved in Africa by 150-200 kyr ago and then migrated
outwards into Asia and Europe and eventually replaced all
indigenous populations in these regions). Current evidence
indicates that anatomically modern humans appeared in east
Africa by about 200,000 years ago (the new dates for
Omo-Kibish), whereas the earliest outside Africa are those
from Israel at about 115,000 years ago. There are no fossil
hominins of the same age as Omo-Kibish from southwest, south
or central Asia, so it is an open question as to when they
first appeared in those regions: possibly at 125,000 years
ago, perhaps 200,000 years ago, or even earlier. (The only
potential evidence is the specimen from Zuttiyeh Cave,
Israel, which is not particularly diagnostic and is almost
certainly considerably older than the associated Th-U date
of 154,000 years ago.) All we know about southwest Asian
hominins between 300,000 and 125,000 years ago is that
Neanderthals were present during part of MIS 6. As with Out
of Africa 1, if we cannot show when modern humans were last
absent in a region, we have no secure means of knowing from
the skeletal record whether that region was core or
peripheral. Although genetic studies of modern humans
strongly imply that they originated in Africa, these studies
are notoriously vague as to when they first dispersed, or
even whether the modern genotype dispersed without any
population expansion from Africa. Current fossil evidence
from Asia is clearly incapable of testing these suggestions
at present.
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9. Hominins, not just Homo,
outside Africa
We also need to focus more on hominins and not just Homo
when studying early hominins outside Africa. Archaeological
approaches to early lithic assemblages in Asia are a good case here.
Any stone tool assemblage in Asia dated as older than 1.9 Myr ago
(the earliest date that Homo is supposed to have left Africa)
is either dismissed or (more usually) ignored (ref. 63); undated
Oldowan tools are assumed to date from after 1.9 Myr ago and not from
2.6 Myr ago (the date of their first appearance in east Africa); and
stone tool assemblages in Asia dated to the Olduvai Event (1.77-1.95
Myr ago) and not associated with hominin remains are automatically
attributed to Homo erectus s.l. However, there is no
reason why Oldowan assemblages in Arabia cannot be older than 1.9 Myr
old, or why the tools from Ain Hanech (ref. 64) in Algeria or Erq el
Ahmar in Israel were made by H. erectus s.l., not least
because similar assemblages were made in east Africa at that time
(and earlier in some cases) by H. habilis, H. ergaster
and probably H. rudolfensis, A. garhi and
Paranthropus. We may be due for some big surprises in
discovering that H. ergaster was not the only, or even the
first, African tool-making hominin to leave home.
10. Human evolutions writ
new?
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Figure 2. The hominin world about 1.7 Myr ago.
The circles denote radii of 1,000 miles. Blue circles
indicate known populations of Homo at this time:
H. ergaster and various australopithecines (including
A. (H.) habilis in east Africa, H. georgicus
in Georgia, H. erectus in Java, and
Paranthropus and Homo in southern Africa).
The ancestries of H. ergaster, H. georgicus and
H. erectus are unclear, as are their spatial extents
and relationships to each other. The yellow circles indicate
areas with no fossil hominin evidence but where stone tools
were being made: north China, Algeria and Pakistan. As
shown, there is ample 'ecological space' in Asia for more
hominins than currently recorded.
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As readers othis journal (Nature) will be aware,
Asia has produced some surprising discoveries in the past decade,
including two new palaeospecies of Homo. Recent African
discoveries in Chad are also highly pertinent to those interested in
early Asian hominins. The discovery of Sahelanthropus tchadensis
(ref. 65) shows that hominins were already well beyond the east
African Rift in the late Miocene, and A. bahrelghazali
indicates that the African grasslands were probably colonized by
3.0-3.5 Myr ago. The latter discovery raises obvious implications for
when the Asian grasslands were first colonized, and whether large
brains, modern body size and proportions, and obligate bipedalism
were essential for that process to occur. These recent finds are not
easily reconciled with the notion that hominins originated in the
Rift Valley and that H. ergaster was the first and only
hominin to migrate out of Africa in the Late Pliocene/Early
Pleistocene. Most probably, we are on the threshold of a profound
transformation of our understanding of early hominin evolution that
might prove as far-reaching as the demise of the notion of Man the
Hunter (ref. 66) in the early 1960s. The process was often painful
and accompanied by heated debate, but our understanding of early
hominin subsistence improved enormously (and, indeed, some parts of
the original model were strengthened by it (ref. 67). Although there
will doubtless be an understandable reluctance to abandon Out of
Africa 1, in its present form, as a model that is widely accepted as
adequate for explaining a very small amount of data from Asia, there
are benefits to be gained by widening our range of possible
hypotheses. The present model is stifling a rigorous evaluation of
how we can interpret the sparse, but recently much improved, data
upon which it rests. Useful initial steps would be for us to be more
explicit about just how few reliable observations we have of Early
Pleistocene hominins across the Old World. Other steps would be to
pay more attention to the comparability of data sets when evaluating
whether or not the absence of hominins is more than the outcome of
taphonomic circumstance or the history of fieldwork; additionally, an
emphasis on different spatial units and on hominins other than H.
ergaster (including earlier ones) might prove fruitful.
Meanwhile, if we cannot demonstrate the probable absence of a hominin
(including H. erectus) in a region, we should reserve
judgement as to when it first appeared there. Another useful step
would be to dispense with most of the arrows indicating movement from
alleged (but often unproven) core territories into (alleged but often
unproven) peripheral ones. It might be more profitable instead to
focus on populations (ref. 68) as the basic unit of study (see Fig.
2), each of which might have its own local origins and history, and
to accept that the boundaries of each, and the relations between
them, are at present unknown, because all we have are isolated
sampling points. Although these changes would radically alter the way
in which we view human evolution outside Africa, they might prove
more fruitful than continuing to envisage the earliest evidence for
hominins in Asia as the outcome of a conjectural migration, from an
unproven centre of origin and along hypothetical routes of
dispersal.
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Acknowledgments
We thank various colleagues for comments on earlier drafts of this
paper. RD thanks the British Academy for a three-year research
professorship for his research into Asian prehistory. This work was
supported by an internationalization grant of the Netherlands
Organisation for Scientific Research.
Last changed 30 March 2006