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Opinion
Why we are not all multiregionalists
now
Chris Stringer
Department of Earth Sciences, The Natural History Museum, London, SW7 5BD, UK
Recent revelations that human genomes contain DNA
introgressed through interbreeding with archaic populations outside of Africa have led to reassessments of
models for the origins of our species. The fact that small
portions of the DNA of recent Homo sapiens derive from
ancient populations in more than one region of the world
makes our origins ‘multiregional’, but does that mean
that the multiregional model of modern human origins
has been proved correct? The extent of archaic assimilation in living humans remains modest, and fossil evidence outside of Africa shows little sign of the long-term
morphological continuity through to recent humans
expected from the multiregional model. Thus, rather
than multiregionalism, a recent African origin (RAO)
model for modern humans is still supported by the data.
Multiregional origins?
Recent revelations that human genomes contain DNA
introgressed through interbreeding with archaic populations outside of Africa have led to reassessments of models
for the origins of our species. The fact that small portions of
the DNA of recent Homo sapiens derive from ancient
populations in more than one region of the world makes
our origins ‘multiregional’, but does that mean that the
multiregional model of modern human origins has been
proved correct? Is it true, as one well-known blogger put it,
that ‘we are all multiregionalists now’ [1]? Should we,
according to another commentator, stop talking about
evolutionary trees and replace them with ‘braids’ [2]? More
remarkably, were RAO models, popularly known as Out of
Africa models, founded on a hoax [3]?
Models for modern human origins
First, it is necessary to revisit what the RAO and multiregional models stipulate, although this is not straightforward, because scientists presenting these models have
modified their views in the face of new data, and have
shifted positions. Accepting new data and modifying views
are commendable in scientists, of course, but if shifts of
position are not made explicit, confusion will inevitably
Corresponding author: Stringer, C. ([email protected]).
Keywords: Homo sapiens; human evolution; human origins; modern humans; Africa;
multiregionalism; hybridisation; ancient DNA.
0169-5347/$ – see front matter
ß 2014 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.tree.2014.03.001
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follow. In addition, if the shifts are significant, the models
might converge on each other, causing further confusion
[4]. Therefore, I next present models as they were during
the 1990s, as summarised by Aiello [5] and Stringer [4]
(Figure 1):
(i) the RAO model argues that modern humans first
arose in Africa approximately 100 000 years ago and
spread from there throughout the world. Indigenous
premodern populations in other areas of the world
were replaced by the migrating populations, with
little, if any, hybridisation between the groups
(Figure 1A);
(ii) the RAO and hybridisation model is similar to the
above, but allows for a greater extent of hybridisation
between the migrating population and the indigenous
premodern populations (Figure 1B);
(iii) the assimilation model also accepts an African origin
for modern humans. However, it differs from the
previous models in denying replacement, or population migration, as a major process in the appearance
of modern humans. Instead, this model emphasises
the importance of gene flow, admixture, changing
selection pressures, and resulting directional morphological change (Figure 1C); and
(iv) multiregionalism differs from the previous three in
denying a recent African origin for modern humans. It
emphasises the role of both genetic continuity over
time and gene flow between contemporaneous populations in arguing that modern humans arose not only
in Africa, but also in Europe and Asia from their
Middle Pleistocene forebears (Figure 1D).
To give a further perspective on these models, I refer the
reader to Figure 2, which compares the models in terms of
the extent of African versus nonAfrican genetic contributions to present-day humans worldwide. On the left of
Figure 2 is absolute RAO (sometimes dubbed by its opponents the ‘Eve Theory’, given that one version derived from
mitochondrial DNA (mtDNA) reconstructions of an ancestral ‘African Eve’), with the total replacement of nonAfrican genes. At the other extreme of Figure 2 are models that
give Africa no role in the evolution of modern humans (for
example, ones that have suggested a West Asian or southeast Asian centre of origin; see [6]). In between the polar
opposites is a ‘mostly out of Africa’ model in the form of
Bra¨uer’s [7] RAO + hybridisation, and Smith [8] and Trinkaus’s [9] assimilation model, whereas towards the right is
classic multiregionalism (i.e., the version generally espoused from 1984 to 2003, which denied Africa the major
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Africa
Asia
Europe
Africa
Asia
100% RAO
RAO+hybridizaon
Assimilaon
Mulregionalism
0% RAO
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Figure 2. A spectrum showing the percentage of recent African genetic
contribution to living humans under various evolutionary scenarios from 0%
recent African origin (RAO) under nonAfrican origin models to 100% under RAO
with complete replacement (see [6]).
(A)
Europe
(C)
(B)
Africa
Asia
Europe
Africa
Asia
(D)
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Figure 1. Evolutionary models of modern human origins. (A) Recent African origin;
(B) recent African origin and hybridisation; (C) assimilation; and (D) multiregional
evolution [4,5].
role in modern human origins [10,11]). Depending on the
extent of nonAfrican archaic gene flow envisaged (less
than, or greater than, 50%), the centrally positioned assimilation model could approach either RAO + hybridisation or classic multiregionalism. This is an important
consideration in evaluating the veracity of the assimilation
model, which has tended to argue that modern human
features were assimilated into indigenous populations
outside of Africa, rather than that early modern populations assimilated elements of the indigenous populations,
as argued by RAO + hybridisation.
An historical perspective on the models
As recently as 1970, no palaeoanthropologist of whom I am
aware held the view that Africa was the evolutionary home
of modern humans (as distinct from more ancient species,
such as Homo habilis or Homo erectus). The region was
considered marginal or unimportant, with the pendulum of
scientific opinion strongly swung toward nonAfrican, multiregional, or Neanderthal phase models (the latter postulated that human evolution everywhere had passed
through a Neanderthal-like stage) [6]. However, during
the course of the next two decades, workers such as
Howells [12], Clark [13], Beaumont et al. [14], Hublin
and Tillier [15], Stringer and Day [16], and Bra¨uer [17]
began to focus on Africa as the possible homeland of
modern humans. A convoluted African origin model was
also advanced by Protsch [18], but given that this relied on
dubious self-generated data to argue that modern humans
had originated first, and had then given rise to archaic
humans, it had little influence on the debate (contra Bednarik [3]). By the early 1990s, the pendulum was moving in
favour of RAO because fossil evidence began to be increasingly reinforced by the clear signals of mtDNA and Y-DNA
in recent human samples. By the late 1990s, the pendulum
had swung even further toward a pure RAO, with growing
fossil, archaeological, and genetic data supporting this
model, including the distinctiveness of the first Neanderthal mtDNA sequences recovered from 1997 onwards [6].
However, large amounts of autosomal DNA have now
been recovered from Eurasian Neanderthals and from
fragmentary fossils found in Denisova Cave, Siberia, revealing another ancient human population. Moreover,
traces of this nonAfrican DNA, attributed to ancient interbreeding events, have been found in recent human genomes, indicating that they are not purely of recent African
origin. These revelations have halted and reversed the
pendulum swing, away from an absolute RAO. I would
say that we are now looking at a version of RAO that most
closely resembles Bra¨uer’s early formulation (out of Africa + hybridisation), or perhaps a version of the assimilation model of Smith and Trinkaus, with a strong African
predominance. If the evidence for archaic assimilation in
living humans remains modest and is restricted to Africa
and to the dispersal phase of modern humans from Africa,
constituting less than 10% of our genome, I think ‘mostly
out of Africa’ is the appropriate designation and, for me,
that is still RAO.
Human species in the fossil record
However, given that interbreeding did occur between modern and archaic humans, out of Africa and perhaps within
Africa too [19,20], does this mean that we should abandon
the different species names and lump the fossils of the past
million years or more as H. sapiens, as multiregionalists
have suggested? If hybridisation events between the various lineages prove to have been widespread and significant
in both time and space, we might have to do that, but I do
not think that point has been reached yet. Personally, I
have never equated the use of a separate species designation for Neanderthals (morphological species concept) with
complete reproductive isolation from H. sapiens (biological
species concept) and, in my view, there are still good
scientific reasons to give populations that had long and
(largely) separate evolutionary histories different names,
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species or otherwise. We can measure the amount of
morphological variation in closely related primate species
today, and compare this with the differences between, say,
Neanderthals and recent humans (see, for example,
[21,22]). Such comparisons demonstrate that the latter
two are distinct enough to be classed as different species,
regardless of whether they meet the biological species
criterion of no interbreeding (a standard that numerous
recognised primate species today do not achieve).
If we nevertheless proceed to merge even Neanderthals
and modern humans as a species, we end up with a H.
sapiens that is simultaneously characterised by a high and
rounded skull, and a longer and lower skull; by no continuous brow ridge, and a strong continuous brow ridge; by a
well-developed chin even in infants, and minimal chin
development; by no suprainiac fossa in adults, and a
suprainiac fossa throughout ontogeny; by semicircular
canals of modern shape, and semicircular canals of Neanderthal shape; by a narrow pelvis with a short, thick
superior pubic ramus, and a wide pelvis with a long, thin
superior pubic ramus, and so on. The disparate nature of H.
sapiens would become even more extreme if we start to add
in the features of species such as Homo heidelbergensis,
antecessor, and erectus. Similar arguments have recently
been made in connection with the proposal to widen greatly
the diagnosis of H. erectus, based on new fossil discoveries
and analyses from the site of Dmanisi (compare [22] and
[23]).
What the multiregional model really meant
However, if our modern genes do come from more than one
region, why has multiregionalism not been proved correct,
as its supporters are now asserting? At this point, it is
worth reminding ourselves what classic multiregionalism
actually proposed. Here is a quotation from a paper written
in 1994 by Milford Wolpoff and four other prominent
advocates of the model at that time [10] (note that one
of these authors was Smith, who had by then also proposed
an early version of the assimilation model [8]):
The evolutionary patterns of three different regions
show that the earliest ‘modern’ humans are not Africans and do not have the complex of features that
characterize the Africans of that time or any other...
There is no evidence of specific admixture with Africans at any time, let alone replacement by them...
There is indisputable evidence for the continuity of
distinct unique combinations of skeletal features in
different regions, connecting the earliest human populations with recent and living peoples.
To reinforce that last point, we can add this caption from
Thorne and Wolpoff [11] for archaic and modern human
skulls from China spanning approximately 500 000 years:
Series of Chinese skulls shows continuity in form
without evidence of a replacement by African characteristics. From left to right, the male skulls are from
the Zhoukoudian Lower Cave (Middle Pleistocene
period), Dali site (early Upper Pleistocene period)
and Zhoukoudian Upper Cave (late Upper Pleistocene).
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Thus, multiregionalism gave Africa no special place in
our evolution and claimed specific anatomical connections
down to details of the skull and teeth between H. erectus
fossils that were more than 500 000-years old and humans
living in the same areas today. Despite recent revelations
that support its inference of interregional gene flow (although so far at a relatively low level; see below), the
multiregional model has otherwise been rather comprehensively falsified, as has the absolute recent African
origin (‘Eve theory’) model (given that we know that modern humans are not derived purely from African ancestors).
Trees and braids
Finlayson [2] has argued that the extent of reticulation
now apparent from genomic data is so great that we should
abandon specific distinctions, and envisage evolutionary
processes as braided rivulets rather than as branches.
However, the view that we should switch to braids and
abandon phylogenetic trees is also mistaken, because we
need both concepts to understand recent human evolution.
As Hawks [24] recognised:
I admit that the braided stream is not a perfect
analogy. Diverging rivulets within a valley almost
always come together again, forming a complicated
network as they form sandbars and islets. None of
them flow into a cul-de-sac. Some human populations
of the past did become extinct, they did not inexorably
flow back into the mainstream of our evolutionary
history. Some of them may have flowed back into the
mainstream only through very small channels of genetic exchange. When we go far enough back, some
populations really did branch off into their own direction.
Comparing 1000 recent human genomes reveals approximately 100 changes in amino acid coding shared by
living H. sapiens but absent from the Neanderthal and
Denisovan genomes [25], including ones that are known to
influence the development and maintenance of brain cells.
Over 3000 further genetic mutations that might have
affected the behavioural or morphological evolution of
modern humans are also uniquely fixed in our species.
Such distinctions are likely to have accumulated during
the several hundred millennia that the lineages of H.
sapiens and those of Neanderthals and Denisovans were
biogeographically separated (tree concept), and evidently
did not diffuse into the genomes of Neanderthals and
Denisovans known so far, despite any braids of gene flow
that might have existed. This brings us on to a fundamental issue: does the relatively low prevalence of Neanderthal
and Denisovan genes in H. sapiens reflect the rarity of
ancient hybridisation opportunities, or their lack of viability [26]? Whereas multiregional and assimilation models
often imply unconstrained interbreeding between ancient
human groups whenever they had the opportunity, the
reality is that many factors (demographic, cultural, and
biological) could have militated against reproductive success, and we are now learning more about these constraints, including the fact that exchanges between
genetically distant populations can have costs as well as
Opinion
benefits [27,28]. New analyses suggest that, when modern
humans and Neanderthals met and mixed, they were at
the edge of biological incompatibility, such that there was
reduced male fertility and rapid natural selection to remove the Neanderthal-derived variants that caused this
sterility [27]. Further evidence of the swift elimination of
much of the introgressed DNA comes from the partial
genome of the Tianyuan skeleton from China, dated to
approximately 40 000 years ago, because this shows a no
greater component of Neanderthal-derived DNA than recent Asian samples [29].
RAO is still the most appropriate model
The big picture is that we are predominantly of recent
African origin, and RAO is not just about the sources of our
shared modern morphology and most of our genes; it is also
about the genesis of our shared patterns of behaviour.
Inferred behavioural gaps between Neanderthals and modern humans have certainly narrowed from recent research,
but in my view they have not disappeared. I think that the
pre-eminence of Africa in the story of modern human
origins was primarily a question of its larger geographic
and human population size, which gave greater opportunities for morphological and behavioural variations, and for
innovations to develop and be conserved, rather than the
result of a special evolutionary pathway. By contrast,
genomic data suggest that the lineages of the Neanderthals
and Denisovans had much greater demographic attrition
[25], perhaps related to the challenges posed by the unstable climates of Eurasia, and this might well have inhibited
their cultural as well as physical evolution [6].
‘Modernity’ was not a package that had a single African
origin in one time, place, and population, but was a composite whose elements appeared, and sometimes disappeared,
at different times and places and then coalesced to assume
the form we see in extant humans [6]. However, during the
past 400 000 years, most of that assembly took place in
Africa, which is why a recent African origin still represents
the predominant (but not exclusive) mode of evolution for H.
sapiens. Rather than saying ‘we are all multiregionalists
trying to explain the out-of-Africa pattern’ [1], it would be
more appropriate to say ‘we are all out-of-Africanists who
accept some multiregional contributions’.
Acknowledgements
I would like to thank David Reich, Laura Buck, and two reviewers for
their helpful comments on this manuscript, and the Human Origins
Research Fund and the Calleva Foundation for support of my research.
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