On the origin of modern humans: Asian perspectives
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The peopling of Asia
In
recent years, there has been increasing focus on the paleoanthropology
of Asia, particularly the migration patterns of early modern humans as
they spread out of Africa. Bae et al. review the current state
of the Late Pleistocene Asian human evolutionary record from
archaeology, hominin paleontology, geochronology, genetics, and
paleoclimatology. They evaluate single versus multiple dispersal models
and southern versus the northern dispersal routes across the Asian
continent. They also review behavioral and environmental variability and
how these may have affected modern human dispersals and interactions
with indigenous populations.
Science, this issue p. eaai9067
Nos últimos anos, tem havido um foco crescente na paleoantropologia da Ásia, particularmente nos padrões de migração dos primeiros humanos modernos à medida que eles se espalharam para fora da África. Bae et al. revisar o estado atual do registro evolucionário humano asiático do Pleistoceno Superior da arqueologia, paleontologia hominínea, geocronologia, genética e paleoclimatologia. Eles avaliam modelos de dispersão simples versus múltiplos e rotas de dispersão do sul versus norte em todo o continente asiático. Eles também revisam a variabilidade comportamental e ambiental e como isso pode ter afetado a dispersão humana moderna e as interações com as populações indígenas.
Structured Abstract
BACKGROUND
The earliest fossils of Homo sapiens
are located in Africa and dated to the late Middle Pleistocene. At some
point later, modern humans dispersed into Asia and reached the far-away
locales of Europe, Australia, and eventually the Americas. Given that
Neandertals, Denisovans, mid-Pleistocene Homo, and H. floresiensis
were present in Asia before the appearance of modern humans, the timing
and nature of the spread of modern humans across Eurasia continue to be
subjects of intense debate. For instance, did modern humans replace the
indigenous populations when moving into new regions? Alternatively, did
population contact and interbreeding occur regularly? In terms of
behavior, did technological innovations and symbolism facilitate
dispersals of modern humans? For example, it is often assumed that only
modern humans were capable of using watercraft and navigating to distant
locations such as Australia and the Japanese archipelago—destinations
that would not have been visible to the naked eye from the departure
points, even during glacial stages when sea levels would have been much
lower. Moreover, what role did major climatic fluctuations and
environmental events (e.g., the Toba volcanic super-eruption) play in
the dispersal of modern humans across Asia? Did extirpations of groups
occur regularly, and did extinctions of populations take place?
Questions such as these are paramount in understanding hominin evolution
and Late Pleistocene Asian paleoanthropology.
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Os primeiros fósseis de Homo sapiens estão localizados na África e datam do final do Pleistoceno Médio. Em algum ponto depois, os humanos modernos se dispersaram pela Ásia e alcançaram locais distantes da Europa, Austrália e, por fim, as Américas. Dado que neandertais, denisovanos, homo do Pleistoceno médio e H. floresiensis estavam presentes na Ásia antes do aparecimento dos humanos modernos, o momento e a natureza da propagação dos humanos modernos pela Eurásia continuam a ser temas de intenso debate. Por exemplo, os humanos modernos substituíram as populações indígenas quando se mudaram para novas regiões? Alternativamente, o contato da população e o cruzamento ocorreram regularmente? Em termos de comportamento, as inovações tecnológicas e o simbolismo facilitaram a dispersão dos humanos modernos? Por exemplo, costuma-se supor que apenas os humanos modernos eram capazes de usar embarcações e navegar para locais distantes, como a Austrália e o arquipélago japonês - destinos que não seriam visíveis a olho nu desde os pontos de partida, mesmo durante estágios glaciais quando os níveis do mar teriam sido muito mais baixos. Além disso, qual o papel das principais flutuações climáticas e eventos ambientais (por exemplo, a supererupção do vulcão Toba) na dispersão dos humanos modernos pela Ásia? Extirpações de grupos ocorreram regularmente, e extinções de populações aconteceram? Perguntas como essas são fundamentais para a compreensão da evolução dos hominídeos e da paleoantropologia asiática do Pleistoceno Superior.
ADVANCES
An
increasing number of multidisciplinary field and laboratory projects
focused on archaeological sites and fossil localities from different
areas of Asia are producing important findings, allowing researchers to
address key evolutionary questions that have long perplexed the field.
For instance, technological advances have increased our ability to
successfully collect ancient DNA from hominin fossils, providing proof
that interbreeding occurred on a somewhat regular basis. New finds of H. sapiens
fossils, with increasingly secure dating associations, are emerging in
different areas of Asia, some seemingly from the first half of the Late
Pleistocene. Cultural variability discerned from archaeological studies
indicates that modern human behaviors did not simply spread across Asia
in a time-transgressive pattern. This regional variation, which is
particularly distinct in Southeast Asia, could be related at least in
part to environmental and ecological variation (e.g., Palearctic versus
Oriental biogeographic zones).
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Um número crescente de projetos de campo e laboratório multidisciplinares focados em sítios arqueológicos e localidades fósseis de diferentes áreas da Ásia estão produzindo descobertas importantes, permitindo que os pesquisadores abordem questões evolutivas fundamentais que há muito tempo deixam o campo perplexo. Por exemplo, os avanços tecnológicos aumentaram nossa capacidade de coletar com sucesso DNA antigo de fósseis de hominíneos, fornecendo provas de que o cruzamento ocorria de forma regular. Novas descobertas de fósseis de H. sapiens, com associações de datação cada vez mais seguras, estão surgindo em diferentes áreas da Ásia, algumas aparentemente da primeira metade do Pleistoceno Superior. A variabilidade cultural discernida em estudos arqueológicos indica que os comportamentos humanos modernos não se espalharam simplesmente pela Ásia em um padrão transgressivo no tempo. Esta variação regional, que é particularmente distinta no Sudeste Asiático, pode estar relacionada, pelo menos em parte, à variação ambiental e ecológica (por exemplo, zonas biogeográficas paleárticas versus orientais).
OUTLOOK
Recent
findings from archaeology, hominin paleontology, geochronology, and
genetics indicate that the strict “out of Africa” model, which posits
that there was only a single dispersal into Eurasia at ~60,000 years
ago, is in need of revision. In particular, a multiple-dispersal model,
perhaps beginning at the advent of the Late Pleistocene, needs to be
examined more closely. An increasingly robust record from Late
Pleistocene Asian paleoanthropology is helping to build and establish
new views about the origin and dispersal of modern humans.
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PERSPECTIVA
Descobertas recentes de arqueologia, paleontologia hominínea, geocronologia e genética indicam que o modelo estrito "fora da África", que postula que houve apenas uma única dispersão na Eurásia há aproximadamente 60.000 anos, precisa de revisão. Em particular, um modelo de dispersão múltipla, talvez começando com o advento do Pleistoceno Superior, precisa ser examinado mais de perto. Um registro cada vez mais robusto da paleoantropologia asiática do Pleistoceno Superior está ajudando a construir e estabelecer novas visões sobre a origem e a dispersão dos humanos modernos.
Descobertas recentes de arqueologia, paleontologia hominínea, geocronologia e genética indicam que o modelo estrito "fora da África", que postula que houve apenas uma única dispersão na Eurásia há aproximadamente 60.000 anos, precisa de revisão. Em particular, um modelo de dispersão múltipla, talvez começando com o advento do Pleistoceno Superior, precisa ser examinado mais de perto. Um registro cada vez mais robusto da paleoantropologia asiática do Pleistoceno Superior está ajudando a construir e estabelecer novas visões sobre a origem e a dispersão dos humanos modernos.
Regions of assumed genetic admixture are also shown. ka, thousand years ago.
Abstract
The traditional “out of Africa” model, which posits a dispersal of modern Homo sapiens
across Eurasia as a single wave at ~60,000 years ago and the subsequent
replacement of all indigenous populations, is in need of revision.
Recent discoveries from archaeology, hominin paleontology,
geochronology, genetics, and paleoenvironmental studies have contributed
to a better understanding of the Late Pleistocene record in Asia.
Important findings highlighted here include growing evidence for
multiple dispersals predating 60,000 years ago in regions such as
southern and eastern Asia. Modern humans moving into Asia met
Neandertals, Denisovans, mid-Pleistocene Homo, and possibly H. floresiensis,
with some degree of interbreeding occurring. These early human
dispersals, which left at least some genetic traces in modern
populations, indicate that later replacements were not wholesale.
The
origin of modern humans has long perplexed us. As the well-known
paleoanthropologist William Howells remarked more than four decades ago
[(1),
p. 477], “That part of human history covering the emergence of modern
man and his regional differentiation continues to be surprisingly
obscure. Locations of some elements of agreement or controversy…have
long been clear, but the dimensions of the whole problem are far from
obvious. The trees are familiar, but the forest is not.”
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O modelo tradicional “fora da África”, que postula uma dispersão do moderno Homo sapiens pela Eurásia como uma única onda há aproximadamente 60.000 anos e a substituição subsequente de todas as populações indígenas, precisa de revisão. Descobertas recentes de arqueologia, paleontologia hominínea, geocronologia, genética e estudos paleoambientais contribuíram para uma melhor compreensão do registro do Pleistoceno Superior na Ásia. Descobertas importantes destacadas aqui incluem evidências crescentes de dispersões múltiplas anteriores a 60.000 anos atrás em regiões como o sul e o leste da Ásia. Os humanos modernos que se mudaram para a Ásia encontraram neandertais, denisovanos, Homo no Pleistoceno médio e possivelmente H. floresiensis, com algum grau de cruzamento ocorrendo. Essas primeiras dispersões humanas, que deixaram pelo menos alguns traços genéticos nas populações modernas, indicam que as substituições posteriores não foram no atacado.
A origem dos humanos modernos há muito nos deixa perplexos. Como o conhecido paleoantropólogo William Howells observou há mais de quatro décadas [(1), p. 477], “Aquela parte da história humana que cobre o surgimento do homem moderno e sua diferenciação regional continua a ser surpreendentemente obscura. A localização de alguns elementos de acordo ou controvérsia ... há muito tempo é clara, mas as dimensões de todo o problema estão longe de ser óbvias. As árvores são familiares, mas a floresta não. ”
A origem dos humanos modernos há muito nos deixa perplexos. Como o conhecido paleoantropólogo William Howells observou há mais de quatro décadas [(1), p. 477], “Aquela parte da história humana que cobre o surgimento do homem moderno e sua diferenciação regional continua a ser surpreendentemente obscura. A localização de alguns elementos de acordo ou controvérsia ... há muito tempo é clara, mas as dimensões de todo o problema estão longe de ser óbvias. As árvores são familiares, mas a floresta não. ”
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Perhaps
the primary reason for this is that there has been no universal
consensus on the traits that make us human. Arguments for a large
cranial capacity defining our species went by the wayside with the
realization that Neandertals, whose cranial capacity is slightly larger
than ours on average, were not actually Cossack soldiers or pathological
anomalies, but rather were of a much greater geological age than
previously thought and in fact were penecontemporaneous with the
earliest modern humans (2, 3). Further, we once assumed that “culture” clearly distinguished us from all other life forms. However, when Jane Goodall (4)
returned from her field studies in the 1960s with the discovery that
chimpanzees made and used tools, it raised questions about whether we
could continue to use a broad variable such as culture to define humans.
The few generally agreed-upon autapomorphies specific to modern humans
include such phenotypic traits as the presence of a mental eminence and a
globular braincase. However, given the absence of the latter trait
among the recently reported early modern human fossils from Jebel
Irhoud, Morocco (5, 6), it may be possible that it developed much more recently, only within the past 130,000 years.
Some
of the earliest morphologically modern humans are reported from the
sites Omo Kibish, dating to ~195 thousand years ago (ka) (7), and Herto, dating to ~160 ka (8), both located in the Horn of Africa (Fig. 1A).
These fossils have been used for the past several decades to support an
East African origin for modern humans, corroborating similar findings
from genetic studies (9–11). Yet the recent discovery of modern human fossils from Jebel Irhoud dating to ~310 ka (5, 12)
raises important questions about the singular role of East Africa in
the genesis of modern human morphology. Further, questions exist about
whether so-called modern human behaviors appeared around the same time
as modern human morphology, with most studies supporting a slower
behavioral transition that occurred over the span of several hundred
thousand years in Africa (13).
Archaic human distributions are also shown. (A) Initial dispersal(s) of Homo sapiens, based on sites predating 60 ka. (B) Subsequent migration pathways of H. sapiens,
based on evidence from the most important sites dated to between 60 and
30 ka. Yellow stripes and translucent orange represent tentative ranges
for Neandertals and early Homo sapiens, respectively.
Despite the general acceptance that Homo sapiens
arose in Africa, the initial arrival and survival of modern humans in
different areas of the world continue to be strongly debated. Over the
past several decades, Asia has been receiving increasing attention in
these discussions, particularly because it is considered the conduit
through which H. sapiens arrived in distant locales such as
Western Europe, Australia, and eventually the Americas. Importantly, the
Asian continent, bounded roughly by the Pacific, Indian, and Arctic
Oceans on three sides and Europe to the west, includes a wide range of
latitudinal, longitudinal, and even altitudinal variation, which has
major implications for human evolution (14).
Because the questions of what, where, how, and especially why with
regard to our becoming “human” continue to be of great interest, we
evaluate the debate on modern human origins and, specifically, how the
Asian record contributes to addressing such questions.
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Apesar da aceitação geral de que o Homo sapiens surgiu na África, a chegada inicial e a sobrevivência dos humanos modernos em diferentes áreas do mundo continuam a ser fortemente debatidas. Nas últimas décadas, a Ásia tem recebido atenção cada vez maior nessas discussões, principalmente por ser considerada o canal pelo qual o H. sapiens chegou a locais distantes como a Europa Ocidental, Austrália e, eventualmente, as Américas. É importante ressaltar que o continente asiático, limitado aproximadamente pelos oceanos Pacífico, Índico e Ártico em três lados e pela Europa a oeste, inclui uma ampla gama de variações latitudinais, longitudinais e até altitudinais, que têm implicações importantes para a evolução humana (14) . Porque as questões sobre o quê, onde, como e especialmente por que no que diz respeito a nos tornarmos “humanos” continuam a ser de grande interesse, avaliamos o debate sobre as origens do homem moderno e, especificamente, como o registro asiático contribui para abordar tais questões.
The big questions
Findings
from archaeology, hominin paleontology, geochronology, genetics, and
paleoclimatology have all been contributing to a better understanding of
the Late Pleistocene human evolutionary record in Asia. Here we discuss
some of the big questions that paleoanthropologists are investigating
across Asia: Can modern human dispersal out of Africa be considered a
single event occurring only after 60 ka, or is the picture more
complicated? By which route(s) did modern humans disperse across Asia?
What was the nature of the interactions between modern humans and
hominin groups already present in Asia? What role did geographic and/or
paleoenvironmental variations play in modern human dispersals?
Can the modern human dispersal be considered a single event occurring only after 60 ka?
Variations
of the “out of Africa” (OoA) model may be broadly categorized as
follows: (i) a single dispersal occurring during marine isotope stage
(MIS) 5; (ii) multiple dispersals beginning during MIS 5; (iii) a single
dispersal occurring during MIS 3; and (iv) multiple dispersals
beginning during MIS 3. We detail each of these broadly defined models
below, but we begin with the single-dispersal MIS 3 iteration because it
has received the greatest attention, followed by the multiple-dispersal
MIS 5 model.
Single dispersal during MIS 3
The
traditional OoA model proposes that a single major dispersal event of
modern humans out of Africa and into Eurasia occurred some time after 60
to 50 ka (15).
In this model, earlier dispersals by anatomically modern humans into
the Levant (e.g., Qafzeh and Skhul) were minor in scale and, for all
intents and purposes, evolutionary dead ends. This OoA model was largely
supported by early genetic studies and to various extents by
archaeology, geochronology, and hominin paleontology (16–21).
The 60-ka dispersal continues to be based primarily on genetic studies
of present-day human population variation across the world. Given the
degree of variability in the genetic clock (because of varying mutation
rates and other uncertainties), it may be possible that this event
occurred earlier or later than the 60-ka marker (18, 22). More recent whole-genome studies (23–25)
suggest that a single major dispersal event occurred during which “all
contemporary non-Africans branched off from a single ancestral
population” [(26), p. 179] some time during the Late Pleistocene (27).
Archaeologists have also set the boundary between 60 and 50 ka on the
basis of the timing of the behavioral and technological transition from
the Middle to the Upper Paleolithic (20, 21, 28).
Multiple dispersals during MIS 5
Another
dispersal scenario that is receiving increasing attention is the
possibility that multiple dispersals began by the beginning of the Late
Pleistocene (14, 29–31).
Given the increasing number of paleontological and archaeological
reports of hominins from sites in different areas of Asia that
supposedly predate 60 ka, it would appear that earlier dispersals out of
Africa made it to the Levant (Qafzeh and Skhul) and probably also to
South, East, and Southeast Asia (Fig. 1A and Table 1) (32–42).
At least some traces of these earlier dispersals would be expected to
be present in the modern record. A recent genetic study seems to support
the fossil and archaeological records in that ~2% of the genome of
modern Papua New Guineans derives from an ancestry that predates the
postulated 60-ka expansion by modern humans out of Africa (43).
Table 1 Background data for all Asian Late Pleistocene sites associated with early modern humans that appear in Fig. 1A.
Triple
plus signs indicate high confidence (hominin fossils and archaeology
found in clear association with solid dates); double plus signs, medium
confidence (hominin fossils and/or archaeology found in association or
questionable association with dates); single plus sign, weak confidence
(hominin fossils and/or archaeology found in questionable association
with dates).
View this table:
Single dispersal during MIS 5 and multiple dispersals during MIS 3
These
two models receive the least support from the various scientific
records. The model of a single dispersal during MIS 5 only works if
modern humans suddenly appeared all across Asia in large numbers and
remained so continuously throughout the Late Pleistocene. Although a
growing number of sites have been dated to MIS 5 and 4, and a number of
these are considered to be associated with modern humans, the data do
not seem to reflect a major dispersal event at this early date (compare Fig. 1, A and B).
No genetic studies of which we are aware support such a model. The
model of multiple dispersals during MIS 3 only works if there is no
evidence for modern humans before MIS 3 (compare with Fig. 1A).
By which route(s) did modern humans disperse across Asia?
The
most widely discussed OoA routes have East Africa as the departure
point into Asia. Researchers have hypothesized two distinct paths: The
first involves crossing over from northern Egypt to the Sinai Peninsula,
whereas the second involves crossing the Bab al Mandab Strait to Yemen
in the southernmost part of the Arabian Peninsula (30, 44–46) (Fig. 1A).
The Bab al Mandab Strait is usually about 20 km wide, and during
glacial periods, it may have only been about 5 to 15 km wide. Given the
short distances involved, the other continent would have been visible
from the points of both departure and arrival (47),
as is the case today. Yet this route requires a water crossing that
would have made rafting a necessity. The Sinai Peninsula, on the other
hand, is the only land corridor that has persisted since the permanent
closing of the Tethys Seaway during the Miocene. This land route remains
the strongest, and, for some, the most parsimonious candidate for the
key dispersal pathway beyond Africa (48, 49).
There
is little consensus as to the route(s) taken once outside of Africa.
Two major directional dispersal models have been proposed: northern and
southern. The northern dispersal model posits that humans moved north of
the Sinai and through the Negev region to reach the eastern
Mediterranean Levant (48).
Evidence to support this includes the early modern human sites of Skhul
and Qafzeh in Israel that date to between 120 and 90 ka and proposed
technological similarities between artifact assemblages in northeastern
Africa and the Levant (44, 48, 50, 51).
After this period, there is no evidence for the presence of modern
humans in the Levant until about 55 ka or later, based on the recent
findings from Manot Cave (52).
Because of this paucity of evidence, the early presence of modern
humans in the Levant is considered by many to be a failed dispersal
event.
The basic tenet of the southern dispersal
hypothesis is an eastward expansion from the Horn of Africa via the
Strait of Bab al Mandab, across the southern part of the Arabian
Peninsula, along the Yemen-Oman littorals to the Persian Gulf, and then
along the coast of the Indian subcontinent (18).
Upon arrival at the Sunda Shelf, and after a short hiatus, humans
dispersed to Sahul, eventually reaching Australasia soon after 60 ka (53).
The southern coastal dispersal route out of Africa and around the
Indian Ocean continues to receive substantial attention, in part because
a number of genetic studies seem to support it [e.g., (18, 19, 25)].
However, for modern humans to have been able to traverse the southern
coastal route, regular access to fresh water and utilization of marine
resources would have been necessary (54, 55).
Unfortunately, clear evidence for a coastal dispersal around the Indian
Ocean is not present for regions outside Sunda, and instead, dispersal
from the Sinai across terrestrial regions of Arabia and southern Asia is
surmised as the main route of population movements (29–31).
It is possible that although coastlines provided favorable habitats, at
least occasionally, reliance on such resources should be seen as only
one component of the whole human subsistence package (31).
FADs and TAQs
An
important point in developing a more concrete understanding of the
timing and direction of human movements outside Africa is that the first
appearance datum (FAD) of modern humans can serve as terminus ante quem
(TAQ) for when these dispersals began. In this regard, with the
emphasis on the supposed “end points,” the key arrival areas are Europe,
Australasia, the Japanese archipelago, and eventually the New World.
Europe has long been at the forefront of these discussions because it is
a heavily studied region and an area that was occupied by Neandertals
for several hundred thousand years, with only about a 5000- to 3000-year
overlap with modern humans after initial colonization by the latter
group (56). Although migration corridors between Europe and the Levant may have been present during MIS 5 (57),
evidence for an unequivocal FAD by modern humans before 44 ka is
absent. The earliest example of fully modern human anatomy in Europe is
the mandibular fragment from Kent’s Cavern in England and the Cavallo
teeth from southern Italy (58, 59), dating to between 41 and 44 ka (Fig. 1B).
However, given the geographic location of both sites, and assuming that
dispersals into Europe originated from the Levant or elsewhere in
northern Asia and traveled westward, this opens the possibility that
earlier evidence for modern humans in central or eastern Europe may be
present.
It has long been argued that modern humans were
the only hominin taxon capable of peopling Australasia, particularly
because it would have involved the ability to build sturdy watercraft
and navigate the open seas (60).
The peopling of Australasia took on greater importance once it was
realized that it likely occurred some time between 60 and 40 ka (61, 62). The recent reanalysis of the Madjedbebe (Malakunanja II) site in northern Australia pushes the initial peopling of the region back to at least 59 ka and possibly 65 ka (63),
although no fossils have been recovered from the site. With early
modern human fossils being reported in mainland Southeast Asia (36, 39, 40), Indonesia (42), and the Philippines (38) (Fig. 1A) that predate the findings from Madjedbebe (65 ka) and New Guinea [49 ka (64)], this is perhaps not all that surprising.
A
major colonization event that has not received as much attention is the
peopling of the Japanese archipelago; it has been suggested that this
could have occurred during MIS 6, when a land connection was likely
present, or some time during MIS 3, when no land connection existed (28, 65–67).
Given that only a few sites in Japan predate 40 ka, and that there are
questions about the context and/or artifactual nature of the materials
at those localities, the earliest presence of modern humans in Japan is
considered to be about 40 ka (67).
Assuming that this date is correct, the initial colonization would have
had to involve some type of watercraft and a high degree of seafaring
skill (28, 34, 65, 66).
The timing and route by which humans arrived in the Americas have been the subjects of a long and often heated debate (68, 69).
Most data appear to support colonization through Beringia either via
the ice corridor or the coastal route some time during or right after
the last glacial maximum (~20 to 15 ka) (68).
However, to reach North America, human foragers had to have first
arrived in Siberia. The earliest peopling of Siberia north of 50°N
appears to have only occurred some time between 50 and 45 ka, as
indicated by the human femur from Ust’-Ishim in western Siberia (70), and the earliest peopling north of 60°N occurred perhaps only as recently as 32 ka (68, 71).
Neandertals and Denisovans were clearly in southern Siberia during the
first half of the Late Pleistocene, but more northward dispersals were
likely by modern humans (Fig. 1B).
How did modern humans interact with hominin groups already present in Asia?
When
humans arrive in a new territory, one of several things may happen,
generally ranging from admixture and the sharing of culture to
competition between and possibly the extinction of one of the
populations (45).
Data from genetics and archaeology have directly contributed to
increasing our understanding of what may have happened when Late
Pleistocene humans moved into new regions of Asia.
Introgression
Over
the past decade, technological advances in the field of ancient DNA
analysis have allowed scientists to obtain uncontaminated genome-wide
data from Pleistocene hominin fossils. This has shown that a fair degree
of introgression occurred between Neandertals and modern humans (27, 72–74). In fact, estimates of Neanderthal DNA present in non-African modern humans generally range between 1 and 4% (72).
One estimate from the Romanian Pestera çu Oase 1 fossil is as high as
9%, suggesting that this particular modern human with a minimum age of
40,000 years may have had a Neandertal ancestor as recently as four to
six generations back (75).
The recent genetic identification of penecontemporaneous Denisovans in
southern Siberia further complicates the Late Pleistocene human
evolutionary picture in Asia (76–79). Reich and colleagues (80)
estimated that about 5% of modern-day Melanesian DNA originates from
this ancestral Denisovan population, although more recent estimates are
between 1 and 3% (27, 81). Further, Prüfer and colleagues (73)
found that Neandertals, who were also present at Denisova Cave,
interbred with Denisovans; similarly, gene flow occurred between
Denisovans and a yet-to-be identified hominin population. In the latter
case, Prüfer and colleagues (73) postulated that the gene flow could be from an ancestral population, such as H. erectus.
Thus, a growing number of studies indicate multiple admixture events
between modern humans, Neandertals, Denisovans, and a nonidentified
population—events assumed to have occurred in Asia (11, 73, 82).
Substantial overlap in time ranges in these same areas also lends
support to likely interactions between these different populations (Fig. 2).
The
dotted line represents the proposed 60-ka exodus out of Africa
supported by various studies. If the 60-ka model is correct, modern
humans should not appear in Asia earlier than that. NGRIP, North
Greenland Ice Core Project.
Genetic studies have shown that the admixture
between non-African modern humans and Neandertals could have occurred as
recently as 40 to 86 ka (70, 75).
The younger date would tend to support the 60-ka OoA model, whereas the
older admixture date could be interpreted as one of the early
dispersals thought to have occurred during MIS 5. Malaspinas and
colleagues (23)
presented an even more complex admixture model and suggested a series
of events that occurred between 72 and 42 ka, which included a “ghost”
lineage. To further complicate the matter, Posth and colleagues (74) recently suggested that the beginning of H. sapiens–Neandertal introgression could date to as far back as ~270 ka.
Although
indications of introgression now commonly appear in the genetic
literature, we should attempt to determine whether it is possible to
identify how genetic interchange appears phenotypically. A range of
studies of hybrids among closely related nonhuman primates identified
examples of dysgenesis (“hybrid weakness”) and/or heterosis (“hybrid
vigor”) when evaluating a diversity of size and shape variables (83). A number of fossils have been proposed as possible hybrids in the Late Pleistocene human fossil record (84), including from Pestera çu Oase in Romania (85) and Zhirendong in China (36). Martinón-Torres and colleagues (86) even recently suggested that H. floresiensis
may be a hybrid. The key to phenotypically identifying a hybrid may be
to observe, with some regularity, unusual traits (e.g., supernumerary
teeth) that are not present in the proposed parent population but
suddenly appear in the supposed offspring.
Cultural diffusion
In
a straightforward scenario, modern humans would have moved out of
Africa and into Asia carrying with them a set of standard “modern”
behaviors (e.g., the use of blades, microblades, art, and symbolism).
This is commonly referred to as the “human revolution” model, thought to
reflect the Middle to Late Paleolithic transition (21, 28, 55).
Numerous studies, however, have questioned whether a behavioral
revolution actually took place and whether a model, originally developed
for the western European record, is suitable to other regions of the
Old World [e.g., (13, 34, 87–90)].
Further, the human behavioral and skeletal records do not line up
neatly: Modern human behaviors often appear in areas where modern human
fossils do not. Behaviors traditionally considered to be representative
of modern humans now have been reported in association with Neandertals,
Denisovans, and possibly other hominin taxa (91–93).
Blade
technology, long considered one of the core components of the Upper
Paleolithic in Asia, appeared in western Asia after 50 ka and arrived in
South and North Asia sometime afterward. Microblades appeared during
early MIS 3 in South Asia and late MIS 3 in North Asia, becoming more
prominent after 35 and 25 ka, respectively (31, 33, 34, 94, 95).
Interestingly, early blade and microblade technologies have yet to be
identified in Southeast Asia, including southern China. Because
Southeast Asia represents a different biogeographic zone (Oriental) than
the other regions (Palearctic), this suggests that different ecological
demands required the development of a different behavioral toolkit to
survive (14, 34, 96–98). Early evidence of rock art (99) and deep-sea pelagic fishing (100) in Southeast Asia are clear signs of other forms of modern human adaptation.
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Em um cenário simples, os humanos modernos teriam saído da África e entrado na Ásia carregando consigo um conjunto de comportamentos "modernos" padrão (por exemplo, o uso de lâminas, microlâminas, arte e simbolismo). Isso é comumente referido como o modelo da “revolução humana”, pensado para refletir a transição do Paleolítico Médio para o Tardio (21, 28, 55). Numerosos estudos, no entanto, questionaram se uma revolução comportamental realmente ocorreu e se um modelo, originalmente desenvolvido para o registro europeu ocidental, é adequado para outras regiões do Velho Mundo [por exemplo, (13, 34, 87-90)]. Além disso, os registros do esqueleto e do comportamento humano não se alinham perfeitamente: os comportamentos humanos modernos freqüentemente aparecem em áreas onde os fósseis humanos modernos não aparecem. Comportamentos tradicionalmente considerados representativos dos humanos modernos agora foram relatados em associação com neandertais, denisovanos e possivelmente outros taxa hominíneos (91-93).
A tecnologia de lâmina, por muito tempo considerada um dos componentes principais do Paleolítico Superior na Ásia, apareceu no oeste da Ásia após 50 ka e chegou ao Sul e ao Norte da Ásia algum tempo depois. As microlâminas apareceram durante o início do MIS 3 no Sul da Ásia e no final do MIS 3 no Norte da Ásia, tornando-se mais proeminentes após 35 e 25 ka, respectivamente (31, 33, 34, 94, 95). Curiosamente, as primeiras tecnologias de lâmina e micro-lâmina ainda não foram identificadas no sudeste da Ásia, incluindo o sul da China. Como o Sudeste Asiático representa uma zona biogeográfica (oriental) diferente das outras regiões (Paleártica), isso sugere que diferentes demandas ecológicas exigiam o desenvolvimento de um kit de ferramentas comportamentais diferente para sobreviver (14, 34, 96-98). As primeiras evidências da arte rupestre (99) e da pesca pelágica em alto mar (100) no sudeste da Ásia são sinais claros de outras formas de adaptação humana moderna.
A tecnologia de lâmina, por muito tempo considerada um dos componentes principais do Paleolítico Superior na Ásia, apareceu no oeste da Ásia após 50 ka e chegou ao Sul e ao Norte da Ásia algum tempo depois. As microlâminas apareceram durante o início do MIS 3 no Sul da Ásia e no final do MIS 3 no Norte da Ásia, tornando-se mais proeminentes após 35 e 25 ka, respectivamente (31, 33, 34, 94, 95). Curiosamente, as primeiras tecnologias de lâmina e micro-lâmina ainda não foram identificadas no sudeste da Ásia, incluindo o sul da China. Como o Sudeste Asiático representa uma zona biogeográfica (oriental) diferente das outras regiões (Paleártica), isso sugere que diferentes demandas ecológicas exigiam o desenvolvimento de um kit de ferramentas comportamentais diferente para sobreviver (14, 34, 96-98). As primeiras evidências da arte rupestre (99) e da pesca pelágica em alto mar (100) no sudeste da Ásia são sinais claros de outras formas de adaptação humana moderna.
---------------------------------------------------------------------------------------------------------------
It
has long been thought that modern human foraging groups moving over the
northern route through Asia carried with them a modern behavioral
package (28, 34, 101),
sometimes equated with the Initial Upper Paleolithic technocomplex.
Interestingly, however, the recent identifications of Denisovans and
Neandertals in Denisova Cave—a site that has traditionally been known
for the presence of a diverse Upper Paleolithic industry (e.g., blades,
bone tools, and bone ornaments) (102)—has complicated this dispersal model, particularly given the apparent absence of H. sapiens fossils at the site (Box 1).
A set of perforated teeth and ostrich eggshell and bone pendants were
excavated from layer 11 of Denisova Cave, the same layer that is
assigned to Neandertals (sublayer 11.4) and Denisovans (sublayer 11.2) (73).
This may add to the growing evidence that, at least on a small scale,
Neandertals and perhaps other hominins were capable of symbolic behavior
(91, 92).
Alternatively, we may be witnessing a series of local extinction and/or
replacement events at Denisova Cave that involved all three hominin
populations, with modern humans solely responsible for the Upper
Paleolithic industries. Another site with evidence of very early
symbolic behavior (~45 ka) is Kara Bom, which is located, like Denisova
Cave, in the Russian Altai Mountains (102).
Box 1
Who were the Denisovans?
On
the basis of DNA sequencing of a juvenile finger bone found almost a
decade ago, a previously unknown Late Pleistocene hominin population was
identified from Denisova Cave in the Altai Mountains in southern
Siberia, now referred to as the Denisovans (76, 77).
The hominin fossil assemblage from Denisova Cave includes this distal
manual phalanx from layer 11.2 (Denisova 3) and an upper left M3 or M2
tooth from layer 11.1 (Denisova 4), found in association with various
archaeological materials typical of both the late Middle Paleolithic
(e.g., Levallois flakes) and the Upper Paleolithic (e.g., microblades
and ornaments made from ground stone) (113).
In addition, an upper molar (Denisova 8) was found at the interface of
layers 11.4 and 12 in 2010, tentatively also identified as an M3 or M2 (78), and a lower left molar (Denisova 2) was recently added to the small number of fossils from the site (79).
The archaeological sequence begins at about 250 ka; Denisova 8 has been
tentatively dated to >50 ka, whereas Denisova 3 and 4 have been
dated to between 50 and 30 ka (76–78), although it is likely that these dates will be revised after further geochronological study.
Despite
the fact that genetic analyses indicated that the Denisovans were
statistically significantly different from both modern humans and
Neandertals, comparative morphological studies of the phalanx and molars
were inconclusive (11, 77, 78).
Thus, these remains were referred to simply as Denisovans and not
assigned a new species name. Since these initial studies, however,
suggestions have arisen that Denisovans may be present in the fossils of
Chinese mid-Pleistocene Homo (or even Penghu 1 from Taiwan) (14, 86, 114).
Initial comparative studies have shown that the Denisovan dentition
displays similarities with those of the Chinese Xujiayao hominin fossils
and Teshik-Tash and Oase 2 fossils from western Asia (115, 116). To further complicate matters, at least three Neandertal fossils were also identified at the site (Denisova 5, 9, and 11) (73, 117).
--------------------------------------------------------------------------------------------------
Quem foram os denisovanos?
Com base no sequenciamento de DNA de um osso de dedo juvenil encontrado quase uma década atrás, uma população de hominídeos do Pleistoceno Superior anteriormente desconhecida foi identificada na caverna Denisova nas montanhas Altai no sul da Sibéria, agora conhecida como Denisovans (76, 77). A assembléia fóssil de hominídeo da caverna de Denisova inclui esta falange manual distal da camada 11.2 (Denisova 3) e um dente superior esquerdo M3 ou M2 da camada 11.1 (Denisova 4), encontrado em associação com vários materiais arqueológicos típicos do Paleolítico Médio tardio ( por exemplo, flocos de Levallois) e o Paleolítico Superior (por exemplo, microlâminas e ornamentos feitos de pedra fundamental) (113). Além disso, um molar superior (Denisova 8) foi encontrado na interface das camadas 11.4 e 12 em 2010, provisoriamente também identificado como um M3 ou M2 (78), e um molar inferior esquerdo (Denisova 2) foi recentemente adicionado ao pequeno número de fósseis do local (79). A seqüência arqueológica começa em cerca de 250 ka; Denisova 8 foi datado provisoriamente como> 50 ka, enquanto Denisova 3 e 4 foram datados entre 50 e 30 ka (76-78), embora seja provável que essas datas sejam revisadas após um estudo geocronológico adicional.
Apesar do fato de que as análises genéticas indicaram que os denisovanos eram estatisticamente significativamente diferentes dos humanos modernos e dos neandertais, os estudos morfológicos comparativos da falange e dos molares foram inconclusivos (11, 77, 78). Assim, esses restos foram referidos simplesmente como Denisovanos e não receberam um novo nome de espécie. Desde esses estudos iniciais, entretanto, surgiram sugestões de que os denisovanos podem estar presentes nos fósseis do Homo chinês do meio do Pleistoceno (ou mesmo Penghu 1 de Taiwan) (14, 86, 114). Estudos comparativos iniciais mostraram que a dentição denisovana exibe semelhanças com as dos fósseis hominíneos chineses Xujiayao e dos fósseis Teshik-Tash e Oase 2 da Ásia ocidental (115, 116). Para complicar ainda mais as coisas, pelo menos três fósseis de Neandertal também foram identificados no local (Denisova 5, 9 e 11) (73, 117).
Com base no sequenciamento de DNA de um osso de dedo juvenil encontrado quase uma década atrás, uma população de hominídeos do Pleistoceno Superior anteriormente desconhecida foi identificada na caverna Denisova nas montanhas Altai no sul da Sibéria, agora conhecida como Denisovans (76, 77). A assembléia fóssil de hominídeo da caverna de Denisova inclui esta falange manual distal da camada 11.2 (Denisova 3) e um dente superior esquerdo M3 ou M2 da camada 11.1 (Denisova 4), encontrado em associação com vários materiais arqueológicos típicos do Paleolítico Médio tardio ( por exemplo, flocos de Levallois) e o Paleolítico Superior (por exemplo, microlâminas e ornamentos feitos de pedra fundamental) (113). Além disso, um molar superior (Denisova 8) foi encontrado na interface das camadas 11.4 e 12 em 2010, provisoriamente também identificado como um M3 ou M2 (78), e um molar inferior esquerdo (Denisova 2) foi recentemente adicionado ao pequeno número de fósseis do local (79). A seqüência arqueológica começa em cerca de 250 ka; Denisova 8 foi datado provisoriamente como> 50 ka, enquanto Denisova 3 e 4 foram datados entre 50 e 30 ka (76-78), embora seja provável que essas datas sejam revisadas após um estudo geocronológico adicional.
Apesar do fato de que as análises genéticas indicaram que os denisovanos eram estatisticamente significativamente diferentes dos humanos modernos e dos neandertais, os estudos morfológicos comparativos da falange e dos molares foram inconclusivos (11, 77, 78). Assim, esses restos foram referidos simplesmente como Denisovanos e não receberam um novo nome de espécie. Desde esses estudos iniciais, entretanto, surgiram sugestões de que os denisovanos podem estar presentes nos fósseis do Homo chinês do meio do Pleistoceno (ou mesmo Penghu 1 de Taiwan) (14, 86, 114). Estudos comparativos iniciais mostraram que a dentição denisovana exibe semelhanças com as dos fósseis hominíneos chineses Xujiayao e dos fósseis Teshik-Tash e Oase 2 da Ásia ocidental (115, 116). Para complicar ainda mais as coisas, pelo menos três fósseis de Neandertal também foram identificados no local (Denisova 5, 9 e 11) (73, 117).
------------------------------------------------------------------------------------------------------------------
What role did geographic and/or paleoenvironmental variations play?
Modern
humans dispersing out of Africa and into Asia were able to adjust to a
diversity of new environments and often would have faced a variety of
geographic barriers (e.g., mountain ranges, major riverways, deserts,
and seas) (30, 31, 34, 45, 46).
Biological adaptations would have facilitated dispersals during the
Late Pleistocene to high-altitude regions such as the Qinghai-Tibetan
Plateau by <30 a="" class="xref-bibr" href="https://science.sciencemag.org/content/358/6368/eaai9067#ref-103" id="xref-ref-103-1" ka="">103
Major natural events
such as the Toba volcanic super-eruption at 74 ka in Sumatra, Indonesia,
are surmised to have contributed to global cooling, substantial
landscape resculpting, and the die-off of floras and faunas, leading to
mammalian bottlenecks and extinctions (105, 106). Ambrose (107) proposed that the Toba super-eruption led to the extinction of archaic humans and a population crash in modern H. sapiens. However, on-the-ground research indicated that terrestrial environments were reshaped more subtly in India (108) and Africa (109) and that Middle Paleolithic populations in the Indian subcontinent survived the super-eruption (33, 108).
The
present-day submergence of coastlines containing traces of hominin
occupation will hinder reconstruction of dispersal routes, particularly
during major glacial periods when sea levels were ~100 m below their
current levels (54).
A good example of this is the eastern China seaboard: During glacial
periods, the coastal plain would have extended outward 400 to 600 km and
connected areas such as the Shandong Peninsula in eastern China with
the Korean Peninsula, facilitating movement of a variety of animals,
including humans, across the dry Yellow Sea (94).
In the Korea-China case, there is good reason to assume that a number
of archaeological sites and Pleistocene faunas are present in this
submerged former coastal plain. It is not clear that the same argument
can be made for all coastal regions in Asia. For instance, proponents of
the southern coastal dispersal model argue that the reason that there
is a paucity of evidence of coastal occupation is that the sites are now
submerged (17, 55).
However, the coastline along the Indian Ocean rim and the western side
of the Sunda Shelf has areas of relatively steep drop-off, so that it is
unlikely that many coastal sites would be now submerged; moreover,
archaeological surveys in these steep coastal shelves and in some
uplifted shoreline areas have failed to find Late Pleistocene coastal
sites (31).
So what happened then?
Growing evidence indicates that modern human dispersals out of Africa into Asia occurred by 60 ka and afterward (Fig. 1B). Such dispersal events across Eurasia are supported by a diversity of studies from genetics and archaeology (11, 20, 28).
However, increasing findings indicate that multiple dispersals out of
Africa by early modern humans began during MIS 5, resulting in their
earlier arrival in distant localities in the Levant, South Asia,
Southeast Asia, and China (33, 35–37, 39–42, 50) (Fig. 1A).
The
initial dispersals out of Africa during MIS 5 were likely by small
groups of foragers, and these appear to have moved along both a southern
and a northern route. The initial dispersal by modern humans northward
reached at least Qafzeh and Skhul. The southern route may have followed
the coast around the Indian Ocean (18–20),
but archaeological evidence dominates for inland dispersal corridors,
where a diversity of habitats occur and where reliable freshwater
rivers, lakes, and animal resources were present (29–31).
Sites dating to early MIS 5 to MIS 4 that appear in places such as
India, southern China, Laos, Sumatra, and the Philippines are all
located a fair distance from any paleocoasts. At least some of these
early dispersals left low-level genetic traces in modern human
populations (43).
A
later, major OoA event most likely occurred some time around 60 ka and
thereafter. This later dispersal by larger and more demographically
successful human populations masked genetic traces of the earlier
dispersals (73). These dispersals across Asia occurred in both northerly and southerly directions (28). In the move north, modern human foragers skirted the Qinghai-Tibetan Plateau to its south to eventually reach Siberia (70).
At around the same time or soon after, populations reached Europe.
These northward-traveling human populations carried an advanced toolkit
comprising microliths, blades, composite tools, and symbolic objects
(beads and colorants), which eventually facilitated their dispersal and
successful establishment in higher latitudes and altitudes. These
foraging groups carrying a specialized microblade toolkit eventually
made their way to the Americas through Beringia (69).
The southern route includes possible movement pathways through the
Indian subcontinent, mainland Southeast Asia, and eventually as far
north as central China and southward into the Southeast Asian islands
and on to Australasia. Questions remain as to why modern human foragers
arriving in Southeast Asia discarded blade and microblade stone tool
industries, although evidence indicates that communities had ground
stone technologies, which represented adaptations to new environments (63).
Further, there are questions concerning what happened when different
foraging groups, originating in areas to the north and south, met in
central China (34, 89).
Moving forward: Future directions of Late Pleistocene Asian paleoanthropology
A
rigid definition of the OoA model positing that modern humans dispersed
from Africa only after 60 ka and simply replaced all indigenous
populations (e.g., mid-Pleistocene Homo, Neandertals, Denisovans, and H. floresiensis)
with no interbreeding can no longer be considered valid. What is needed
now is to develop more detailed models that include the growing
evidence for early dispersals, contractions, extirpations, and
extinctions of human groups and lineages, while recognizing that not all
pre–60 ka dispersals were evolutionary dead ends. Further, there are
still large swathes of territory in Asia that remain largely unexplored,
and previously identified sites and materials are in need of renewed
study. Fortunately, the increasing number of multidisciplinary research
programs launched in Asia over the past few decades has resulted not
only in the reporting of many important sites and findings, but also the
accumulation of information that fills in gaps in the evolutionary
record, thereby facilitating broader interregional comparisons.
As
attested to by the great interest generated by the Central Asiatic
Expeditions in the early 20th century, Asia was once considered to be
the cradle of mankind (14).
Although important finds from Europe and Africa over the course of the
past century have diverted attention, Asia is a continent that has much
to offer to research on modern human origins, thereby shaping what we
think we know about our evolution and history.
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References and Notes
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- Acknowledgments: We thank the participants of the Wenner-Gren Symposium “Human Colonization of Asia in the Late Pleistocene” for stimulating and lively discussions in Sintra, Portugal. We acknowledge the support over the years from the Wenner-Gren Foundation for Anthropological Research, the National Geographic Society, the U.S. National Science Foundation, the Academy of Korean Studies, the College of Social Sciences and School of Pacific and Asian Studies at the University of Hawaiʻi at Manoa (C.J.B.), the Max Planck Institute for the Science of Human History (K.D. and M.D.P,) and the European Research Council [ERC-2012-AdG-324139 PALAEOCHRON and ERC-2016-STG-715069 FINDER (K.D.) and ERC-2011-AdG-295719 PALAEODESERTS (M.D.P.)]. We thank L. Aiello and the anonymous reviewers for their comments and recommendations.
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