Saharan corridors and their role in the evolutionary geography of ‘Out of Africa I’

Full text

27
J.G. Fleagle et al. (eds.), Out of Africa I: The First Hominin Colonization of Eurasia, Vertebrate Paleobiology and Paleoanthropology,
DOI 10.1007/978-90-481-9036-2_3, © Springer Science+Business Media B.V. 2010
Abstract In any discussion of hominin dispersal it is possible,
and important, to examine the event at many different scales.
This paper examines the initial dispersal out of Africa at the
scale of populations rather than species, looks at dispersal
between ecological zones rather than continents, and consid-
ers dispersal within Africa prior to any dispersal out of Africa.
Before hominins could disperse out of Africa they needed to
disperse out of their likely area of endemism in sub-Saharan
Africa to North Africa, the most likely departure point for
Eurasia. Prior to the Middle Pleistocene, successful long term
colonizations of North Africa by hominins were very rare, and
apparently less successful than their colonizations of Eurasia.
The Early Pleistocene hominin dispersal into Eurasia was most
probably along the western coast of the Red Sea. The ability
of hominins to successfully disperse into Eurasia and success-
fully colonize northern continents was made possible by the
ecological and climatic diversity within Africa.
Keywords Climate • Dispersal • Ecology • North Africa
• Population
Introduction
Most discussions of the first hominin dispersals out of Africa
are, understandably, driven by the discovery of fossil and
archeological remains in Eurasia. However, dispersal events
are a problem in evolutionary geography (Lahr and Foley
1998), and they are shaped by the interaction through time of
the species under study and the community ecology of the
source and recipient areas of the dispersal event. Particularly
when dealing with inter-continental movements, understand-
ing the interaction of a target group and its environment
requires that the dispersal process is analyzed in terms of
fluctuating corridors and barriers, so as to obtain insights
into the phylogeographic structuring of any species and the
evolution of its adaptive pattern(s).
In the particular case of hominin dispersals into Eurasia,
one of the problems lies, in my view, in the scale at which we
analyze the dispersal event. In this paper, I want to experi-
ment with an attempt at changing the scale at which the prob-
lem can be examined, from species to populations, from
continents to ecological zones, and explore what this new
look at what has become known as ‘out of Africa I’ may
offer. The starting point for the scalar adjustment is that
before hominins disperse out of Africa, they must first dis-
perse out of their region of endemism – sub-Saharan Africa,
and more specifically, eastern Africa. This implies that the
starting point of what we usually consider ‘out of Africa
dispersal’ is more accurately described as ‘within Africa dis-
persal’. To put more strongly, Eurasian colonisation could be
seen as a by-product of dispersals within Africa.
Hominins, then, did not disperse out of Africa, but out of
sub-Saharan Africa, and in particular, out of East Africa.1
This is not just a statement of fact. Only by reducing the
question to the ecological scale at which the process actu-
ally operated (the scale at which populations share a partic-
ular set of competitors and resources), can we hope to find
the causes and conditions for demographic fluctuations
through time, as well as the evidence for interpreting the
dispersal response in terms of niche expansion or exclusion.
Furthermore, the identification of geographical corridors,
barriers and refugia should set the parameters of the dispers-
ing process in terms of speed of movement, degree of loca-
lized selective pressures (and thus progressive adaptive
differentiation), spatial constraints on home range size (and
thus extent of demographic bottlenecking and genetic drift
effects), as well as asynchronicities in evolutionary trends.
M.M. Lahr (*)
Leverhulme Centre for Human Evolutionary Studies,
University of Cambridge, Cambridge, UK
e-mail: mbml1@cam.ac.uk
Chapter 3
Saharan Corridors and Their Role in the Evolutionary
Geography of ‘Out of Africa I’
Marta Mirazón Lahr
1This does not negate the existence of other directions of hominin
movement (particularly between South and East Africa, or between
East and West Eurasia), but focuses on a set of dispersal events (the ‘out
of Africa’ set) that significantly shaped the evolutionary history of the
genus Homo.

28 M.M. Lahr
Fig. 3.1 A simplified model of the temporal sequence of early homi-
nin dispersals out of East Africa given changing environmental condi-
tions driven by climatic change after the formation of the Saharo-Arabian
belt in the Late Pliocene. (a) Phase I – Late Pliocene formation of
Saharo-Arabian desert belt (red dotted line) resulting in the separation
of Africa into two zones – sub-Saharan and North Africa, of very
unequal size and variability. Drawn is the hypothetical ecological
range of a given hominin population in East Africa. Note that the
ancestral range is characterized by moderate altitude, and the latitudi-
nal extent of desert (and thus ecological zonation to be traversed) is
much greater on the western (Atlantic) than the eastern (Strait of
Hormuz) end of the Saharo-Arabian desert belt. (b) Phase II – a hypo-
thetical ‘wet episode’ in which the Saharan barrier is latitudinally com-
pressed (thick red lines) in relation to its maximum aridity extent (thin
red dotted lines), and paleodrainage systems are formed (McCauley
et al. 1997). The E-W expansion of forests in equatorial sub-Saharan
Africa (Foley 1999; indicated by the thin bilateral horizontal arrows),
together with the N-S expansion of savannahs on the northern and
southern forest edges alters the range of the East African population
and promotes a northward expansion that leads to colonization of new
ecological zones by the use of temporary available corridors to the
East, West and North. These ‘jump dispersals’, as defined by Tchernov
(1992a) and Lahr and Foley (1994), may be followed by subsequent
jump dispersals through corridors leading towards high carrying
capacity environments, and thus to the establishment of ‘daughter’
populations in new ecological zones, normally discontiguous from the
parental source. Note that during the last 2 million years, ‘wet epi-
sodes’ have varied markedly in frequency and extent. During the
intense and stable arid period between 2 and 1 Ma, two periods of
significantly increased precipitation have been identified. This differs
markedly from the climatic pattern of Middle Pleistocene glacial
cycles, during which the amplitude and periodicity of change increased,
leading to shorter, more frequent and more pronounced high and
low atmospheric water periods (major arid and wet episodes)

293 The Role of Corridors in ‘Out of Africa’
All these issues are prominent in the evolution of the genus
Homo, with its recurrent pattern of ‘out-of-Africa’ move-
ments, apparent simultaneous large-scale expansions (Plio-
Pleistocene and Late Pleistocene dispersals), geographically
differentiated contemporaneous lineages (Middle Pleistocene
Homo heidelbergensis and erectus, Late Pleistocene Homo
sapiens and neanderthalensis), pronounced localized differ-
entiation (Homo floresiensis), unequal population levels of
genetic variance (Homo sapiens), and so on. Dispersing
behaviour can be seen as one of the major characteristics of
the genus, one which arguably shaped its evolution to a greater
extent than any other.
In this paper, I want to explore these ideas using the record
of the colonization of North Africa by hominins in the
Pleistocene. At its heart lies the simple observation that,
compared to eastern Africa, the evidence for early hominin
presence in northern Africa is remarkably scant. Two views
can be taken on that observation. One, that the sparse early
evidence is a research taphonomy, and that greater fieldwork
efforts should change the picture dramatically. The other,
that the scarcity of early sites is at least partly real, and thus
part of the solution. Undoubtedly, more field research will
improve the record. Nevertheless, given that paleoanthropo-
logical and paleontological work in North Africa has not
been unsubstantial, I would argue that the available informa-
tion can form the basis of a working model for analyzing the
parameters shaping early dispersals of hominins from East
Africa.
A Biogeographical Model for Dispersals
Out of East Africa
Whole Cenozoic records of dust transport show that the most
significant change in the last 65 million years – an order of
magnitude increase in dust generation – occurred as a conse-
quence of the onset of northern hemisphere glaciation in the
Late Pliocene, reflecting major continental drying (Rea
1994). There are three main sources of dust today, eastern
and central Asia, northwest Africa and Arabia. The Late
Pliocene major dust episode reflects the establishment of two
of these, the formation of the Sahara and Arabian desert belt.
Therefore, since the Late Pliocene, Africa has been sharply
divided by a major biogeographical barrier.
The degree of separation between sub-Saharan Africa and
both the Mediterranean Basin and the South Asian landmass
varied throughout the Pleistocene according to the interac-
tion of precipitation levels and altitudinal profiles. That inter-
action, in turn, shaped corridors and refugia which promoted
population isolation and movement. Furthermore, that same
interaction between altitude and varying precipitation levels
also acted on the source populations, by either expanding or
contracting their ecological niche. Figure 3.1 shows a model
of a simplified possible temporal sequence in the biogeogra-
phy of populations dispersing from East to North Africa and
Eurasia following climatic change.
The critical aspect of the model2 is the identification of
potential corridors and refugia. Although these are based
on Late Pleistocene data, their definition as such derives
from the interaction of fluctuating aridity and the Saharan
landscape, and depending on the development of tempo-
rally specific barriers, should have had a similar ecological
role during earlier periods. At this point, the presence of
barriers beyond the Saharan-Arabian desert belt itself is not
drawn, as these would be specific to geomorphological pro-
cesses at different times. It is expected that, if these existed
in such a way as to affect the extent or indeed existence of
a given corridor, they would be identified through the
examination of the Early Pleistocene record itself or from
specific paleoenvironmental studies.
With the aim of testing whether this model fits the record
of Early Pleistocene dispersals of hominins, and thus throws
light onto their phylogeography and adaptive trajectories, the
next part of this paper will consider the paleoanthropological
record of North Africa, as well as the evidence for dispersals
from North Africa into Eurasia. Finally, I will use the ‘fitted’
model to explore how an evolutionary geography theoretical
framework might contribute to our understanding of hominin
evolution at the time.
Fig. 3.1 (continued) Because of the sharpness and rapid succession
of arid-wet episodes in the Middle Pleistocene, the expansion of East
African populations early during wet episodes would have taken place
after periods of major environmental fragmentation, with the potential
for increased inter-population variance through drift. The greater
potential for demographic fluctuations and drift during the Middle
Pleistocene probably accelerated the rate of evolutionary and cultural
change in relation to earlier periods. (c) Phase III – the re-establish-
ment of arid conditions, resulting in the contraction of equatorial for-
ests, development of barriers and disappearance of the ecological
corridors that had allowed for jump dispersals to take place in the pre-
vious phase. The East African population would expand southwards at
this point, recolonizing niches vacated by the retracting forest belt.
Populations in the high carrying capacity areas colonized during the
dispersal phase may survive for different lengths of time depending on
the size, competitive community and resource structure of the refugia.
Depending on the length of time until another wet episode opens the
dispersal corridors, surviving populations in the refugia will differenti-
ate from the parental population through both selection and drift. Only
in this case, the dispersal event would have become a vicariant process
of evolutionary consequence, leading to the establishment of an evolu-
tionary lineage in the Simpsonian sense (Simpson 1961; Foley and
Lahr 2007). Map, public domain (http://commons.wikimedia.org/wiki/
File:Africa_topography_map.png)
2 This model was built on the basis of paleoenvironmental reconstruc-
tions of the Sahara during the second half of the last glacial cycle.

30 M.M. Lahr
Plio-Pleistocene North Africa
Any discussions of dispersals from eastern Africa to North
Africa and Eurasia have to assume that such movements
took place when paleoenvironmental conditions differed
dramatically from those today, so as to eliminate, completely
or partially, the Saharan desert barrier.
As briefly alluded to above, global climatic events charac-
terize the period around 2.5 Ma. These led to major global
cooling that resulted in significantly increased aridity in
Africa and eustatic sea-level changes (Brunner and Maniscalco
1998; Shackleton et al. 1990; DeMenocal 1995). The effects
on northern African environments were very pronounced and
widespread (Sarnthein et al. 1982; Stein 1985; Tiedemann
et al. 1989; LeHouerou 1992; Morel 1992). The establish-
ment of the Saharo-Arabian desert belt (Rea 1994) was
followed by a period of very dry conditions between 2 and
1 Ma, interrupted by two wet episodes. These pronounced
climatic and consequently environmental changes led to faunal
turnovers, the two most significant of which in Africa occurred
~2.5 Ma and 1.8 Ma (Vrba 1995).
During the Pliocene, hominins expanded at least once into
Central Africa from the East, as shown by the mid-Pliocene
remains of Australopithecus bahrelghazali (Brunet et al. 1995).
The fauna suggests an open environment, drier than that of
East African sites at the time, and a degree of endemism.
However, no evidence exists that they expanded further north
at the time. Stone-tools are conspicuously absent from the
rich Maghrebian paleontological localities of Aïn Boucherit
(~2.3 Ma) and Ahl al Oughlam (~2.4 Ma) (Raynal et al.
2001). Indeed, signs of hominin presence in the Early
Pleistocene of northern Africa are extremely rare altogether.
For the period between 1.8 and 1.0 Ma, several archeological
sites are known across Eurasia, distributed from Spain to Java.
In contrast, the North African record of this time is almost
non-existent (Fig. 3.2). Sites with Mode 1 assemblages
Fig. 3.2 Distribution of main Early Paleolithic sites in northern Africa
(from various sources as discussed in the text). Sites include: (1) Aïn
Hanech & El-Kherba, (2) Abassieh, (3) Shatti, BJJ6, (4) Saoura valley,
(5) Richat Massif, Ouadane, Aftassa-Amzeili, (6) Sherda, Enneri group,
(7) Beli, (8) Yayo, (9) Jos Plateau, (10) Mekrou group, (11) Faleme
group (Djita, Sandande, Sare, Kare, Kidira), (12) Dhar Tichitt, (13)
Richat group (Khatt Takfoil, Adrar, Tazazmout, Aderg Motleh,
Hammami), (14) ACH1 Erqueiz Lahmar, (15) Sidi Abderrahman, Rabat,
Khebibat, Sale, (16) Aïn Fritissa, (17) Tighenif, Ouzidane, (18) Lake
Karar, (19) Sidi Zin, Sidi Mansour group, (20) Wadi Merdum, (21)
Soura group, (22) Tachengit, (23) Tihodane group, (24) Fazzan group
(Wadi al-Ajjal, Tadrart Acacus, Messak Settafet, NUS, BJJ, Murzuq),
(25) Ounianga group, (26) Fayum group, (27) Saffagah, (28) Kharga
Oasis, (29) Bir Tarfawi, Bir Kiseiba group, (30) Arkin eight group
13 5
6
8
9
7
11
12
14
15 16 17
18 19
20
1
3
21
22
23
25
2
27
30
28
29
Kharga
Oasis
Bir Tarfawi &
Bir Kiseiba
26
24
10
Mode 1 only
Mode 2
Mode 2 (only late Acheulean - all late Mid
Pleistocene?)
4

31
3 The Role of Corridors in ‘Out of Africa’
are few, and none are uncontroversial in relation to their
chronology.3 The best contextualized of these is the site of
Aïn Hanech, near Sétif in northern Algeria, and the recently
described site of El-Kherba in its vicinity (Sahnouni et al.
2002). Aïn Hanech was discovered in 1947 by Camille
Arambourg, who described an archeological assemblage
from the site as Oldowan, with affinities towards those found
at Olduvai Bed I/lower Bed II.
The site has been re-excavated in recent years by Sahnouni
and colleagues, who have identified another Oldowan local-
ity in the same wadi (El-Kherba). Sahnouni and colleagues
have clarified a number of outstanding issues about the
archeology of this site (notably that argued by Clark [1975]
as not in primary context), especially the confirmation of the
stratigraphy and the source of the Acheulean tools found
(Sahnouni et al. 1996; 2002). The site has yielded 2,156 arte-
facts (Fig. 3.3), including some retouched pieces in which a
microwear analysis identified evidence of meat processing.
The controversial aspect of the recent studies has been the
dating proposed by Sahnouni and colleagues. On the basis of
normal paleomagnetism and the associated fauna (which
includes the species Mammuthus meridionalis, Equus tabeti
[but see comments below], Sivatherium maurusium,
Kolpochoerus phacochoeroides, and the absence of Anancus
[present at Aïn Boucherit]), Sahnouni and colleagues have
proposed a date for the site within the Olduvai subchron
(1.95–1.78 Ma) (Sahnouni et al. 1996, 2002). This dating is
strongly disputed by Raynal, Geraads and colleagues, who
believe the site to be closer to 1.2 Ma (Raynal et al. 2001;
Geraads et al. 2004). Whether the earlier or later Early
Pleistocene date of Aïn Hanech is confirmed, the site still
represents the oldest stratigraphically contextualized homi-
nin occupation of North Africa.
Besides the numerous surface finds of pebble tools and
flaked stones, very few other Oldowan sites have been
described in some detail for this vast region – of particular
interest those of Abassieh, Cairo, and in the Richat Massif of
Mauritania. The former was described in 1925 by Bovier-
Lapierre (1925), who identified a number of archeological
occurrences, some in stratigraphic sequence, on the plains to
the East of the city of Cairo (‘Plaine de l’Abassieh’) and the
surrounding highlands. Exciting as the finding of an Oldowan
site at the ‘door’ of the Levantine corridor is, unfortunately
no work followed these early observations, and the area has
now been covered by the suburbs of Cairo. In the Massif des
Richât, East of Ouadane, Mauritania, a number of Oldowan
sites have been recorded, and some described (Vernet and
Naffé 2003). These sites are located on the edge of paleo-
lakes, such as those of El Beyedh and Guelb Er Richât
(Baouba M., personal communication, 2004). The main sites
among these are Wadi Akerdeil, the artefacts of which are des-
cribed by Monod as showing affinities towards those of Olduvai
3 It should be noted that a very conservative list of Mode 1-Oldowan
sites is presented here. Another thirty or so localities where pebble tools
and Oldowan cores and flakes have been observed could be added to
this list (see Aumassip 2004). However, the majority of these are early
observations without study or quantification, and as is the case in so
many North African areas, part of mixed surface assemblages on the
deflated landscapes.
Fig. 3.3 Comparison of the Early Pleistocene lithic technology (Mode 1 and Mode 2) from North Africa. Aïn Hanech redrawn from Sahnouni
et al. 2002; Thomas Quarry 1 Unit L and the Oulad Hamid 1 Quarry redrawn from Raynal et al. 2001 by R.A. Foley

32 M.M. Lahr
Bed I, and Aftassa-Amzeili, to the SE of Zouerate (Monod
1976). Also important from a biogeographical perspective
are the surface finds of Oldowan artifacts in Chad (Sherda,
Enneri group) and, in the last two years, in Fazzan, Libya
(Shatti, Barjuj 6). None of these findings have chronological
control.
Clearly, the Early Pleistocene occupation of northern
Africa by Mode 1 stone-tool making hominins was ephem-
eral at best, particularly in the eastern Sahara. As succinctly
put by Veermersch “… if, during the Early Pleistocene,
hominids followed the Nile Valley on their journey out of
Africa, no clear traces of their presence have yet been found”
(Veermersch 2001).
This rarity of early sites in North Africa can also be
extended to the early Acheulean (early Mode 2). At present,
the best Quaternary stratigraphic sequence is that of the
complex of sites at Casablanca, Morocco, which comprises
a series of deposits and terraces, ranging from 180 m a.s.l. to
the coast, and variously cover the last 5.5 million years
(Raynal et al. 2001). The oldest archeological occurrence at
Casablanca is that of Thomas Quarry 1, Unit L, dated to ~1
Ma. The archeology is Mode 2, consisting of chopping-
tools, polyhedrons, and some cleavers, trihedrons and
bifaces. The bifaces are not symmetrical or carefully fin-
ished (Fig. 3.3).
The remaining sites are all likely to be Middle Pleistocene
in age, ranging from early to very late. The earliest Middle
Pleistocene sites in the Casablanca sequence are those of
Oulad Hamida 1 Quarry, the Grotte de Rhinoceros at Thomas
Quarry 1 (OH1-GDR), and the Hominid Cave. These deposits
are currently dated as early Middle Pleistocene, between
0.7–0.6 Ma (Rhodes et al. 1994), of similar age and faunal
associations as Tighenif. At OH1-GDR, a rich Acheulean
assemblage, in which cleavers are rare and bifaces are large,
was found (Fig. 3.3). The Thomas Quarry mandible probably
derives from a pink breccia from the earliest levels of
OH1-Th1, in which three new Homo teeth have been discov-
ered (Raynal et al. 2003).
At both Tighenif and the neighbouring site of Lake Karar
(a small spring) in Oran Province, northern Algeria, Acheulean
tools vary from simple quartzite forms to carefully finished
lanceolate bifaces. The assemblages contain cleavers, thought
to show similarities to those of Olduvai Bed III by McBurney
(1960), and evidence of the use of the Kombewa technique,
thought to have developed in East Africa ~1 Ma (as evidenced
at Olorgesailie; Potts 1989). The site of Sidi Zin, at the mar-
gin of a paleo-spring or water hole at the extreme eastern
foothills of the Atlas Mountains in northern Tunisia, contains
four stratigraphic units (Layers I to IV) (Clark 1975;
McBurney 1960; Gobert 1950). In Layer I, beautiful narrow
lanceolate handaxes were found, like the best at Lake Karar,
together with flake tools and flattened river pebbles coarsely
trimmed into massive scraping or chopping edges (which
according to McBurney are similar to other such in sub-Saharan
African industries); Layer II probably represents a short occu-
pation, and contains elliptical, carefully finished cleavers and
small flake tools, lacking pebble scrapers and choppers as
well as lanceolate handaxes; Layer III contains an industry
similar to that of Layer I (Layer IV is MSA, with Levallois
prepared cores) (Freeman 1975). The site of Arkin 8, on the
West margin of the Nile at the Sudanese-Egyptian border
(Chmielewski 1968), is undated. However, the character of
the stone-tool industry (dominated by heavy duty tools, cor-
diform, ovate and lanceolate handaxes, as well as trihedrons;
see Sahnouni et al. 1996) tentatively aligns it with this early
Middle Pleistocene group of sites. Similarly undated, early
bifaces and trihedral pieces have recently been recorded for
the Fazzan, Libya.
Of younger Middle Pleistocene age (~400 ka?) a number
of sites have been found, including the localities at Sidi
Abderrahman – Cap Chatelier, Grotte d’Ours, and Grotte des
Littorines (locality of the Sidi Abderrahman Homo mandi-
ble) (Raynal et al. 2001). McBurney (1960) described two
technological phases within the Sidi Abderrahman complex;
an early one containing beautifully finished pear-shaped
handaxes, and cleavers made on very large flakes; and a later
one, which would correspond to the pink breccia horizons at
the Grotte d’Ours and Grotte des Littorines. The site of
Tihodaïne at the edge of the Tassili Massif in southern
Algeria, contains a late Acheulean industry, with carefully
made handaxes and cleavers, in association with a water
dependent fauna, and some indications of more open envi-
ronments (Elephas recki, Rhinoceros simus, Equus zebra,
Hippopotamus amphibious, Bubalus antiquus?, Bos primi-
genius, Gazella dorca, and several antelopes including
Kobus). The fauna and archeology at Tihodaïne has been
argued to show correlations to those of Olduvai Bed IV
(McBurney 1960). At Bir Kiseiba, Egypt, the assemblage
has also been likened to that of Olduvai Bed IV. It contains
cleavers and handaxes made on large flakes (Kombewa tech-
nique) (Haynes et al. 1997), similar to those at Bed IV at
Olduvai (Leakey 1975) and other East African sites (Clark
1975), as well as to those of Gesher Benot Ya’aqov in the
Dead Sea Rift (Goren-Inbar and Saragusti 1996), dated to
780 Ka (Verosub et al. 1998). The undated site of Ouzidane,
near Tighenif, has been referred to as middle or late Acheulean
in character (Vaufrey 1955).
Among the possibly later Acheulean sites, we find those
at the Fayum depression (associated with the 40 m lake), the
Wadi Midauwara in the Kharga depression, and Bir Tarfawi
(Hill 2001). At Bir Tarfawi and Bir Sahara East, geochrono-
metric dating of the Acheulean deposits suggest a minimum
age of 350 Ka (Wendorf et al. 1994), while recent work on
the geochronology of the fossil-spring tufas of the Kharga
Oasis have provided U-series minimum ages of 300 ka
(Smith et al. 2004).

33
3 The Role of Corridors in ‘Out of Africa’
The numerous Acheulean sites recorded (but minimally
studied) in Mauritania, as well as those along southern tribu-
taries of the Senegal River, have no chronological control,
and the fauna of only one of these has been reported (Coppens
et al. 1972). Similarly, the recently reported Acheulean
deposits in the Tadrart Acacus and Messak Settafet in the
westernmost Fazzan, Libya, are so far undated (Garcea
1997), as are the various Acheulean localities within the
Edeyen Awbari and Murzuq (Lahr et al. 2007, 2008, 2009).
The character of the assemblages found at Tachengit,
another central Saharan site, and that at the site of ACH1
Erqueiz Lahmar, on the edge of a paleoriver in the Sahrawi
Arab Democratic Republic (Polisario-controlled western
Sahara) is unclear, and it may represent a very early Middle
Stone Age site in which bifaces are found together with
flakes produced through Levallois methods (as in the second
occupation levels of the Kharga Oasis, or the 9 m terraces of
the Nile, or the uppermost levels at Cap Chatelier). Similar
assemblages of small, thin bifaces of possible early MSA
context have recently been recorded in Fazzan (Lahr et al.
2007, 2008, 2009).
The main aspects of the early North African record can be
summarised as follows:
1. North Africa was not occupied by hominins in the Late
Pliocene – at the richest Northwest African paleontologi-
cal site, that of Ahl al Oughla (~2.4 Ma), as well as at the
somewhat later site of Aïn Boucherit, no evidence of
hominin occupation exists.
2. The earliest evidence for hominin dispersals into North
Africa comes from the site of Aïn Hanech (northern
Algeria), with Oldowan stone tools and a controversial
date of 1.8 Ma, although they are certainly older than
1 Ma (the conservative estimate is 1.2 Ma). Therefore,
current evidence suggests that the colonization of North
Africa is synchronous with that of Eurasia.
3. The pre-Acheulean occupation of the region was minimal,
and most probably extremely ephemeral.
4. The early Acheulean (of Early Pleistocene age) occupa-
tion was also sparse, and currently only documented with
chronological certainty in the Casablanca sequence
(Thomas Quarry 1 Unit L).
5. Of the few described Early Pleistocene sites (or localized
groups of sites) – three Oldowan and one Acheulean,
three are located along the Atlantic border of North Africa,
while the fourth, on the edge of the Nile delta, must be
considered as only tentative given that it is not available
for new studies that could confirm its stratigraphic integ-
rity or dating.
6. A later Acheulean tradition, of early to middle Middle
Pleistocene age (700–400 kya?), is more widespread, rep-
resented at sites in the eastern, as well as the western
Sahara, extending also into the Central Sahara (Bir Kiseiba,
Sidi Zin, Tighenif, Lake Karar, the Oulad-Thomas Quarry
deposits, the Sidi Abderrahman deposits, Tihodaïne,
Wadi el-Ajal, Fazzan). In some of these sites, technologi-
cal affinities with eastern African industries (through the
Kombewa technique) are apparent, also pointing towards
possible links with the Levant (Gesher Benot Ya’aqov).
A somewhat later group of sites (400–200 kya?), or
perhaps part of a northeastern African continuous Middle
Pleistocene Acheulean tradition, is found between the Nile
and the Libyan desert and plateau (Fayum, Kharga, Bir
Tarfawi, Bir Sahara East). None of these sites show
evidence of use of the Kombewa technique.
7. Later sites, of very late Acheulean or early MSA charac-
ter are equally or more widespread as the early Middle
Pleistocene Acheulean.
Across Deserts and Mountains, Seas
and Deltas
The brief review of the Early Stone Age/Early Paleolithic
record of North Africa clearly indicates that the Saharan des-
ert remained a very major barrier to hominins until the Middle
Pleistocene, when the establishment of glacial-interglacial
cycles changed again, albeit intermittently and temporarily,
the environmental conditions of the region. Clearly, getting
out of sub-Saharan Africa was as much of a challenge as
getting out of Africa.
However ephemeral the occupation, hominins did cross
the Sahara during the Early Pleistocene and their presence
along the southern Mediterranean coast, at least along its
western portion, before 0.8 Ma is not disputed. These North
African hominin populations have been generally assumed
to have been part of the dispersal into Eurasia.
Nevertheless, once on the Mediterranean coast, getting
out of Africa is also not uncontroversial. There is presently
only one land connection between Africa and Eurasia,
namely the Sinai Peninsula, and although this land bridge
has been in existence since the Miocene (Tchernov 1992b) it
involved the crossing of the Nile Delta, which paleontologi-
cal evidence suggests acted as a partial barrier to faunal
movements, reflected in the paucity of Eurasian elements in
North African faunas (Tchernov 1992b). Besides the Sinai
route, the crossings of both the Sicily-Tunisian Strait and the
Strait of Gibraltar have also been proposed (Alimen 1975;
Martínez-Navarro and Palmqvist 1995, 1996; Strauss 2001).
Figures 3.4 and 3.5 show the bathymetric outline of the
Gibraltar and Tunisian/Sicilian straits respectively. As dis-
cussed by several authors before, both areas currently reach
depths of 200 m or more, and would have, therefore, not
been fully exposed during the currently estimated low sea-
level stands of the later Pleistocene. Nevertheless, because of

Fig. 3.4 Bathymetric outline of the Gibraltar Strait (From Lahr 1996)
Fig. 3.5 Bathymetric outline of the Sicily-Tunisia Strait (From Lahr 1996)

35
3 The Role of Corridors in ‘Out of Africa’
uncertainties associated with the degree of tectonic move-
ments, and the fact that the Sicilian Channel has been subsid-
ing since the Middle Pleistocene (Alexander 1988; Bonfiglio
and Berdar 1970), the question has not been settled.
From the Late Pliocene to ca. 1 Ma, three major drops in
sea-level associated with important changes in the European
paleoenvironment are known to have taken place – the
‘Aquatraversan’ or ‘Elephant-Equus Event’; the ‘Aullan’
(the Italian ‘Wolf Event’); and the ‘Cassian’ or ‘end-
Villafranchian’ Event (Azzaroli 1995; Arribas and Palmqvist
1999). These have been dated to 2.6–2.4 Ma, 1.8–1.6 Ma,
and 1.2–0.9 Ma respectively. The first of these had very
pronounced consequences at both marine and terrestrial
levels, and clearly correlates with the major global cooling at
this time. In Europe it is associated with the arrival of the
Asian elephants and monodactyl horses (Arribas and
Palmqvist 1999), thus marking a period of E-W trans-
Eurasian dispersals. However, the arrival in Eurasia of
African faunas during the ‘Aullan’ Event (at the Neogene-
Quaternary boundary) and their particular distributions
across Europe have been argued to suggest possible trans-
Mediterranean exchanges (Martínez-Navarro and Palmqvist
1995; 1996). These are considered in more detail below.
Alimen’s (1975) argument for a crossing between Tunisia
and Sicily during the Early Pleistocene was strongly influ-
enced by the description of Acheulean findings in Sicily
(Bianchini 1973). Several Mode 1 tools and assemblages
have been reported from Sicily. These include both surface
finds and in association with marine terraces. However, in
the absence of radiometric dates, stratigraphic context and
faunal associations, later Middle Pleistocene ages cannot be
ruled out (Villa 2001). Contrastingly, the Quaternary pale-
ontological record of Sicily strongly suggests that a land
bridge between Tunisia and Sicily did not exist. The island
has a rich Pleistocene paleontological record (Agnesi et al.
2004), usually divided into four stratigraphic units. The ear-
liest of these is found in the northwestern half of the island
(which was separated from the southeastern half until the
Middle Pleistocene; see Bonfiglio and Piperno 1996). The
small mammal component shows signs of endemism, while
the large one is European in origin (Villa 2001). The excep-
tion is the large-sized ctenodactylid rodent Pellegrinia
panormensis, a form of African affinities that has never been
observed elsewhere in Europe (Thaler 1972). The second
unit has been dated to the earliest Middle Pleistocene
(Bonfiglio et al. 1997). It shows even more endemic fea-
tures, including the dwarf elephant E. falconeri (Roth 1990).
The subsequent assemblages are of certain Middle
Pleistocene age, and are evidence of the uplift of the Hyblean
Plateau and fusion of the two Early Pleistocene Sicilian
landmasses (Bonfiglio and Piperno 1996). Therefore, the
hypothesis of a connection between North Africa and Europe
across the Sicilian Strait during the Early Pleistocene is not
supported by existing data.
The Strait of Gibraltar is currently ~14.5 km wide
(Fig. 3.4). It would require a sea-level drop of approximately
300 m to virtually close it, while a 200 m drop would narrow
it to ~6.5 km (see Arribas and Palmqvist 1999; although sev-
eral islands would become exposed whenever the sea-level
dropped more than 100 m, making for several short crossings,
see Martinet and Searight 1994). At present, positive evidence
of such low sea-level stands during the Plio-Pleistocene does
not exist. However, the argument for multiple Mediterranean
crossings is based on the Early Pleistocene circum-Mediterra-
nean paleontological record (Martínez-Navarro and Palmqvist
1996; Arribas and Palmqvist 1999). Palmqvist and colleagues
have argued that the colonization of the Northern
Mediterranean zones and western Asia through multiple
routes from northern Africa would explain the chronologi-
cally close appearance of the same African mammals in Spain
(Orce), Italy (Pirro Nord), Greece (Apollonia) and Georgia
(Dmanisi), as well as their absence from Central Europe; such
multiple dispersals would also be consistent with the apparent
latitudinal constraint of Megantereon whitei to 40°N.
While the Sicilian faunal evidence strongly suggests that
the area was not connected to North Africa in the Early
Pleistocene, the argument proposed by Martínez-Navarro,
Palmsqvist and colleagues for trans-Gibraltar crossings can-
not be ruled out. However, an outstanding issue remains. The
critical African species that dispersed into Eurasia between
1.8 and 1.6 Ma are the carnivores Megantereon whitei (a sabre-
toothed cat) and Pachycrocuta brevirostris, the horse Equus
numidicus (= E. altidens), the primate Theropithecus oswaldi,
and the hippo Hippopotamus antiquus. At present, a particu-
lar link between these forms in northwestern Africa and
southwestern Europe is not strong.
Equus numidicus is found in East Africa (Omo Shungura,
Olduvai, East Turkana) and in the Late Pliocene Maghrebian
site of Aïn Boucherit (? ~2 Ma; Sahnouni et al. 2002), but its
presence at Aïn Hanech (possibly dated to the critical Plio-
Pleistocene boundary; Sahnouni et al. 2002), instead of the
derived form Equus tabeti (as reported by Sahnouni et al. 1996,
2002) or even E. cf mauritanicus, is controversial (Geraads et al.
2004). Although they are both species of the “simplicidens type”
(Guerrero-Alba and Palmqvist 1997), the one dispersing into
Eurasia in the Early Pleistocene (found at Orce, Cúllar de Baza,
Cueva Victoria, Huéscar 1 in Spain; Pirro Nord and Selvella in
Italy; Sainzelles in France, and Süssenborn in Germany) has
been clearly identified as E. numidicus, a form that may have
not been present in northwest Africa at ~1.8 Ma.4
Theropithecus oswaldi is not recorded in the Late Pliocene
faunas of Ahl al Oughlam (Morocco, ~2.4 Ma), or at Aïn
4However, if the short chronology of Geraads, Raynal and colleagues
(in which Aïn Boucherit would not be older than 2 Ma, and thus closer
to the Plio-Pleistocene boundary, and Aïn Hanech not older than
1.2 Ma) is considered, then E. numidicus would be present in northwest
Africa at the critical time.

36 M.M. Lahr
Boucherit, where the species Theropithecus atlanticus was
found (Alemseged and Geraads 1998; Raynal et al. 2001).
According to Raynal and colleagues, T. oswaldi is first
recorded in the Maghrebian stratigraphy at Tighenif, cur-
rently estimated as very late Early Pleistocene or earliest
Middle Pleistocene in age. Although for some time the only
European site in which this African primate had been found
was Cueva Victoria, Spain (Arribas and Palmqvist 1999),
thus suggesting a Maghrebian-Iberian exchange, remains of
T. oswaldi have now been identified at Pirro Nord (Rook
et al. 2004) and 'Ubeidiya (Belmaker 2002), as well as its
previously known presence at Mirzapur, India (Delson 1993).
Altogether, the distribution points to a western Asian point
of entry followed by trans-Eurasian dispersals.
Similarly, the record of the two main large carnivores that
dispersed into Eurasia in the Early Pleistocene (Pachycrocuta
and Megantereon), as well as Hippopotamus antiquus, are
clear evidence of an African dispersal at this time, but not
particularly across the Mediterranean straits. The oldest
record of Pachycrocuta, although still debated, is now con-
sidered to be in East Africa (Werdelin 1999). Its appearance
in Europe pre-dates the beginning of the Pleistocene, having
been found at Olivola, Italy, dated to ~2 Ma (Napoleone et al.
2003) and a generally earlier Late Pliocene date (from a time
when hominins are absent from North Africa) cannot be
ruled out. The identification of the Megantereon Eurasian
form which replaced M. cultridens in the Early Pleistocene
as the African Megantereon whitei has been strongly argued
by Martínez- Navarro and colleagues (Martínez-Navarro and
Palmqvist 1995, 1996); this species is thought to have entered
Eurasia through the Levantine corridor around 1.8 Ma
(Palmqvist and Arribas, 2001).
A possibly stronger case for a dispersal across the Strait
of Gibraltar can be made for the Late Pliocene, when an
African-Eurasian faunal exchange took place. As part of
this exchange, a small number of African species (Giraffa,
Struthio and Kuabebihyrax) are recorded at the sites of
Bethlehem, Kuabebi in the Caucasus (2.6–2.5 Ma), and pos-
sible derivative forms in Europe (see Martínez-Navarro
2010), while Equus disperses to Africa, possibly through the
Bab el Mandab (Tchernov, 1992b). Most interesting is the
presence of Eurasian forms at Ahl al Oughlam (Geraads,
1997; Raynal et al. 2001), namely Ursus cf. etruscus and
Nyctereutes abdeslami. Although they may have dispersed
from either the Iberian Peninsula (both are present, see
Kurtén and Crusafont 1977) or western Asia (Nyctereutes is
found at Bethlehem, see Hooijer 1958), the absence of Ursus
from other African sites is noticeable. Martínez-Navarro fur-
ther points to the presence of the Eurasian Capra primaeva
in the Late Pliocene Tunisian site of Aïn Brimba. Together
with the presence (survival?) of the African rodent Pellegrinia
in Sicily amidst an endemic Early Pleistocene fauna, the
presence of these Eurasian forms in Late Pliocene Maghrebian
sites may indicate a time when the Mediterranean straits
were exposed. However, if those trans-Mediterranean
exchanges occurred, they preceded the earliest hominin
colonization of the Mediterranean basin.
If the Mediterranean straits were not exposed since the
Late Pliocene, all potential Pleistocene hominin (and faunal)
dispersals out of and into North Africa had to take place via
the Sinai Peninsula. In this scenario, northeastern (the east-
ern Sahara) and northwestern Africa (the Maghreb) acquire
extremely different biogeographical roles – one as a corridor,
the other as a cul-de-sac.
Out of Africa Across the Sahara
Integrating the archeological information above with the
lack of evidence for trans-Mediterranean crossings during
the Pleistocene, it is a strong hypothesis that northwestern
Africa acted as a cul-de-sac throughout the period, receiving
intermittent faunal (and hominin) dispersals from Central
and West Africa when climatic conditions allowed the for-
mation of bodies of water (in the form of wadis, paleolakes,
springs and water holes – all of which form the geomorpho-
logical context of all Quaternary sites in the region), and
more rarely from Eurasia along the southern Mediterranean
coast. This would make the Maghreb, or in particular the
Atlantic and Mediterranean plains of the Atlas Mountains, a
faunal refugium throughout the period. This may also have
promoted the recurrent extinction, as well as extended survi-
vorship, of species (including hominins and their cultural
traditions) at times when these had already become extinct
or become technologically different elsewhere. In a recent
simulation study of extinction rates of large European carni-
vores, O’Regan and colleagues show the strict correlation
between the spatial extent of a refugium and the likelihood
of extinction, largely driven by inbreeding (O’Regan et al.
2002). Similar models could be built for the Maghreb, and
tested again the paleontological record to assess the extent to
which the area acted as a refugium during the Pleistocene.
In this context, it is interesting to note that the Maghrebian
hominin fossil remains, traditionally attributed to Homo
erectus, are all of Middle Pleistocene age (Oulad-Thomas
Quarry, Tighenif, Sidi Abderrahman, Sale), when Homo
heidelbergensis is found in sub-Saharan Africa (Rightmire
1996). If these affinities are correctly identified, that would be
evidence for the relatively late survivorship and eventual
extinction of Homo erectus in this area. On the other hand,
these remains were studied a long time ago, within a different
theoretical, comparative and chronological framework. A
new assessment of their affinities would be extremely useful.
The outstanding question is whether northeast Africa was
a corridor for dispersals between East Africa and the Levant

37
3 The Role of Corridors in ‘Out of Africa’
during the Pleistocene. Geomorphological and paleontologi-
cal evidence indicates that the Nile linked East African water
sources with the Mediterranean in the Middle Pliocene, also
supported by the molecular phylogenies of the single haplo-
chromine cichlid fish found out of Africa, the Levantine spe-
cies Astatotilapia flaviijosephi (Werner and Mokady 2004).
During the Late Pliocene, the Paleo-Nile cut through Egypt
(along a different course), but probably had its sources within
Egypt itself (Said 1981), i.e., there was not a water corridor
connecting the East African highlands to the Mediterranean
at the time. With the onset of major aridity at the Plio-
Pleistocene boundary, the traversal of the Nile along the
Egyptian desert stopped, and it is not thought to have been
formed again until the early Middle Pleistocene ~0.7 Ma
(Said 1981; Rzóska 1976). Indeed, paleo-ichtyological stud-
ies show that several hydrological systems in Africa lost their
pan-African faunas in the Early Pleistocene, reflecting the
disappearance of trans-regional waterways due to increased
aridity and geomorphological change (Stewart 2001). The
formation of the Proto-Nile, and subsequent Pre-Nile, in the
Middle Pleistocene is characterized by the renewed connec-
tion to sub-Saharan African watersheds.
The lack of Early Pleistocene sites (with the exception of
the already discussed poorly contextualized and undated site
of Abassieh), together with the evidence for (a) the establish-
ment of the Saharan desert barrier during the Plio-Pleistocene
(Tchernov 1988), (b) the lack of a water corridor connecting
East Africa to the Mediterranean shores until the Middle
Pleistocene, and (c) the strikingly different faunistic composition
of the sites of 'Ubeidiya and Maghrebian sites such as Irhoud
Ocre and Sis Abdalla (Tchernov 1992b), suggests that north-
eastern Africa was not a dispersal corridor between East
Africa and the Levant during the Early Pleistocene.
Significantly, the African component of Early Pleistocene
Eurasian faunas has been argued to be of East, rather than
North African affinity (“the East African stamp upon the
fauna of 'Ubeidiya”, see Tchernov 1992b).
If this assessment was correct, the few Early Pleistocene
northwest African sites may have been the result of short-
term northward expansions of hominins and fauna from
Central Africa, using either the Central Saharan (through
Chad, Tibesti, Hoggar-Tassili, southern Atlas wadis) or more
probably the Western Saharan (Senegal, Mauritania) water-
ways (a complex series of paleolakes, paleodrainage systems
and highland springs), and be thus unrelated to the events
taking place in western Asia and eventually to the East, West
and North of the Levant.
The available data suggest very different conditions at and
after the beginning of the Middle Pleistocene. Not only the
restoration of the Nile re-established a waterway between East
Africa and the Levantine corridor, but the increased amplitude
and frequency of climatic change initiated at the time (when
not only the known ~100 and ~41 ka orbital precession-scale
mechanisms were operating, but a marked increased in sub-
Milankovtich variability on a millennial scale, probably related
to monsoon dynamics and the size of northern hemisphere ice
sheets; Larrasoana et al. 2003) led to markedly greater levels
of precipitation for short periods of time. Two examples illus-
trate the unique temporary paleoenvironmental conditions in
the Sahara during the Middle and Late Pleistocene. In the east-
ern Sahara, sedimentary evidence shows that the Middle
Pleistocene sites in the Darb el Arba’in desert were associated
with a period of deposition at least two orders of magnitude
wetter than the present (Wendorf et al. 1993), while the com-
bined extents of Lake Megachad and Lake Megafezzan, joined
by extensive wadis, not only created a continuous waterway
between the Mediterranean and Central Africa, but submerged
6% of the Sahara (Drake et al., in press). The presence of per-
manent (at the scale of hominin generations) bodies of water
in the Sahara during certain periods would have allowed the
colonization of North Africa by sub-Saharan hominins. These
Middle Pleistocene expansion and dispersal movements across
the Sahara were not trans-Saharan faunal exchanges, as the few
Eurasian elements that appear in the southern Mediterranean
faunas (such as the presence of Ursus cf. arctos and possibly
Mammuthus meridionalis at Tighenif, see Jaeger 1975; Geraads
1982) do not seem to have dispersed southwards (as also
shown by the recent work on the molecular phylogenies of
North African elephant shrews by Douady et al. 2003). Robert
Foley and myself have argued that this ‘Out of Africa’ direction-
ality is consistent with several dispersals of African Middle
Pleistocene Acheulean (and later MSA) stone-tool making
hominin populations into North Africa during wet episodes
(Foley and Lahr 1997; Lahr and Foley 1998); although not all of
these dispersals would have been of great extent, some would
reach the Levant and lead to the recurrent immigration of Middle
and Late Pleistocene hominins (and humans) into Eurasia.
‘Out of Africa I’: Different Routes
and Directions
Given that there is undisputed evidence of hominins in
Eurasia at and after 1.8 Ma, if Plio-Pleistocene hominins did
not disperse out of Africa across northern Africa, they would
have to have done so through the Bab el Mandab strait, across
from the Horn of Africa to the Arabian Peninsula. The Bab
el Mandab strait and the Red Sea were flooded in the Early
Pliocene, and appear to have remained so until the present
(Tchernov 1992b). However, the local geology is extremely
complex due to the extent of tectonic activity, and it has been
suggested that a land bridge was exposed at certain times
(Haq et al. 1987) and that faunal exchanges took place
(Tchernov 1992b; Turner 1999).

38 M.M. Lahr
Several Early Paleolithic sites have been identified in the
Arabian Peninsula along the margins of ancient river drain-
age systems and lakes (see Petraglia 2003 for a comprehen-
sive review). Mode 1 sites are few, but significantly more
numerous than those of the vastly greater area of North
Africa, including a complex of six Oldowan sites in the prox-
imity of the Bab el Mandab (Whalen and Schatte 1997).
Acheulean artifacts and/or assemblages have been found in
sites along both East and West southern margins of the Red
Sea (Faure and Roubet 1968; Zarin et al. 1981; Whalen et al.
1988; Whalen and Pease 1992; Walter et al. 2000), but have
so far been only identified along the eastern (Arabian) mar-
gin further North (for example, at the site of Saffaqah; see
Whalen et al. 1984). Together with the East African charac-
ter of the African fauna dispersing into Eurasia at ~1.8 Ma
(Tchernov 1992b), the occupation of Arabia by hominins
manufacturing Early Paleolithic artefacts makes a dispersal
across the Bab el Mandab during the Plio-Pleistocene likely.
In this scenario, the eastern coast of the Red Sea could have
acted as a low carrying capacity corridor (i.e., promoting a
rapid linear movement) leading to the Levant, while the coast
of Yemen and Oman, or alternatively across the middle of
Arabia (North of the Rub’ al-Khali desert) could have taken
other hominin populations to the Strait of Hormuz and thus
the coastal corridor (also of low carrying capacity) towards
South Asia.
Integrating Model and Data
The above discussion can be used to assess and refine the
corridor/refugium model presented in Fig. 3.1.
1. In this model, five immediate potential corridors from
East Africa were identified (ignoring a southward dimen-
sion) – (a) towards Central Africa along a Sahelian high-
land edge; (b) towards the Mediterranean along the Nile;
(c) towards the Sinai Peninsula along the western coast of
the Red Sea (or through the Red Sea Hills); and (d) towards
the Sinai Peninsula along the eastern coast of the Red Sea
across the Bab el Mandab; and (e) towards the Strait of
Hormuz, along the southern coast of Arabia. The latter
case will not be considered further here, as the relevant
data were not discussed. The first four corridors would
have taken hominins into two high carrying capacity envi-
ronments – the Lake Chad basin, and the Delta-Sinai-
Levant area (Fig. 3.6).
The available record provides only circumstantial
evidence that the first corridor (towards the Lake Chad
Basin) was used. This evidence consists of the earlier,
mid-Pliocene use of this dispersal route by an australo-
pithecine; and the presence (but undated) of both Oldowan
and Acheulean sites in the area to the North and South of
the Lake Chad Basin (Beli, Jos Plateau, Yayo). Of the
three northward corridors, the Nile did not exist as such
until the Middle Pleistocene, after which time the evi-
dence suggests that it was indeed used; the western coast
of the Red Sea is insufficiently known to be assessed
(although the possible Mode 1/Oldowan site at Thébes, if
confirmed, could change this picture); finally, the use of
the eastern coast of the Red Sea and immediate highlands
is supported by the substantial number of Early Paleolithic
sites and the possibly greater affinities of Levantine fau-
nas towards East rather than North African communities,
as well as the presence of Levantine forms in East but not
North Africa. However, the crossing of the Bab el Mandab
remains speculative at this point, and if found to be unre-
alistic it would strongly point towards the western coast
of the Red Sea as the main Early Pleistocene dispersal
route from East Africa to Eurasia.
2. If hominin populations reached and survived in the area of
the Lake Chad Basin, they would again disperse when pre-
cipitation increased and or if conditions at the refugium
deteriorated. Dispersals from the Lake Chad Basin could
take any of three corridors – (a) the Sahelian highland edge
corridor towards East Africa; (b) a corridor towards the
Central Sahara following the megalake paleodrainage sys-
tem into the Central Sahara highlands and from there along
wadis towards the southern foothills of the Atlas Mountains
and the Mediterranean; and (c) a corridor along the northern
edge of the Niger River. Only the second and third of these
corridors are discussed (Fig. 3.7).
Fig. 3.6 Out of East Africa model, stage I. See Fig. 3.1a–c for climatic
patterns

39
3 The Role of Corridors in ‘Out of Africa’
The record of the Central Sahara supports only a very
ephemeral or rare use of this route during the Early
Pleistocene. Although a few Mode 1 and early Mode 2
sites have been identified in Fazzan in 2007–2008, these
are rare in comparison with more recent localities, and
remain so far undated. Middle Pleistocene sites are more
frequent, and the few that have paleontological remains,
such as Tihodaïne, these are associated with a riverine/
lacustrine fauna of African affinities. It is likely that only
during the Middle Pleistocene the magnitude of climatic
change from glacial to interglacial conditions allowed for
the formation of the Saharan megalakes and associated
paleorivers, and thus a more substantial hominin occupa-
tion of the area. The westward corridor is supported by
circumstantial evidence in terms of the number of Early
Paleolithic sites in the Senegal River drainage system and
Mauritanian highlands. This corridor would have taken
hominins to a high carrying capacity environment, and
was a likely dispersal route of early hominins.
3. If hominin populations survived in the western Saharan
refugium, the model predicts that under conditions simi-
lar as those described above, hominins would disperse
further. Dispersals from the Western Saharan refugium
could take place along two corridors – (a) towards the
Lake Chad Basin from where they had originated; and
(b) towards the Maghrebian Atlantic plain along water-
ways paralleling the Atlantic coast. The second of these is
discussed below (Fig. 3.8).
The presence of two Early Pleistocene sites in the
Maghreb (Aïn Hanech, Thomas Quarry 1, Unit L) dated
to ~1.8 and ~1.0 Ma suggests that this route was used,
albeit very rarely. It is likely that this route, like all trans-
latitudinal corridors, would have been open for very short
periods of time, since renewed aridification would have
affected the mid-Saharan latitudes first, thus cutting-off
the parental and daughter populations. This corridor
would have taken hominins towards a high carrying
capacity refugium, and yet the Early Pleistocene occupa-
tion of which does not appear to have become permanent.
It is possible that the distance of this area from other,
more populous hominin localities would have restricted
the gene pool of the dispersing groups to the point of
compromising their survival. If this hypothesis is correct,
it has implications for the interpretation of the dispersals
into Eurasia. The Early Pleistocene colonization of
Eurasia, although probably not permanent in the very
long-term, shows that hominin populations did survive
for a significant period of time in certain areas. This
implies that they would have had to either sustain demo-
graphically a period of continuous contact with their
parental population until the daughter group reached a
sufficient size, or have dispersed as one of many groups
following the same route and destination. In the case of
the first occupations of northwestern Africa, neither con-
dition seems to have been fulfilled.
4. Finally, hominin populations in the Maghreb could have
dispersed yet further, perhaps in relation to low population
density resource stress. In this case, it is unlikely that homi-
nins could have used again the western Saharan corridor.
Indeed, the endemism of North African faunas, the absence
Fig. 3.7 Out of East Africa model, stage II. See Fig. 3.1a–c for climatic
patterns
Fig. 3.8 Out of East Africa model, Stage III. See Fig. 3.1a–c for
climatic patterns

40 M.M. Lahr
in sub-Saharan Africa of the few Eurasian forms that reach
the southern Mediterranean coast, the deep phylogenetic
history of some North African species studied, and the fact
that the onset of the effects of climatic amelioration (and
thus faunal and hominin demographic growth) occurs ear-
lier in the tropics, support the view that most dispersals
between sub-Saharan and North Africa had a northward
direction. Therefore, if Maghrebian hominins were to dis-
perse further, they could have used three corridors – (a)
across the Strait of Gibraltar into Iberia; (b) across the Sicily-
Tunisian Strait; and (c) along the southern Mediterranean
coast towards northeastern Africa (Fig. 3.9).
As discussed before, dispersals across the Mediterranean
straits are very unlikely, except perhaps for a Late Pliocene dis-
persal between Northwest Africa and Iberia, although if this took
place it pre-dated the first evidence for hominins in the area. A
corridor towards the East, along the Mediterranean coast, could
have taken hominins into other smaller refugia, and potentially
as far as the Nile Delta and Sinai Peninsula. This has to remain
an open possibility at present, since this corridor was clearly
used in an opposite direction by the small number of Holarctic
species that are found in Maghrebian sites. Nevertheless, it
should be stressed that the Libyan desert (between Cyrenaica
and the Nile oases) is today the most arid part of the Sahara, and
it would have constrained hominins to using an extremely nar-
row band of coastal plains as a dispersal corridor.
The summary of the discussion presented above is in
Fig. 3.10. The main aspect of this particular model is the
geographic direction and consequence of the Early versus
Middle Pleistocene hominin dispersals out of East Africa. The
former, would have, on the one hand, taken more than one
population of hominins to North Africa, but without resulting
in the permanent colonization of the region and leading to the
eventual extinction of these lineages. The short duration of
these early North African occupations probably implies that
the dispersing groups did not adapt successfully to their new
environments, and would thus appear morphologically (and
culturally?) relatively undifferentiated. On the other hand,
Early Pleistocene dispersals would have also taken hominin
groups into Eurasia for the first time, resulting in the establish-
ment of a number of populations distributed across a vast area,
from Southeast Asia to eventually the Iberian Peninsula. These
various Eurasian early hominins would have also eventually
become extinct, but at least in some areas, after the long-term
adaptation to new conditions. Therefore, this Eurasian disper-
sal would probably be best described as an Early Pleistocene
adaptive radiation, leading to the evolution of a number of
differentiated hominin lineages. Especially in Europe and
Southeast Asia, where significant subsequent dispersals prob-
ably did not take place until the Middle and Late Pleistocene
respectively, local populations would show the greatest degree of
differentiation among these early groups (such as H. antecessor,
Javanese H. erectus, and H. floresiensis).
By contrast, Middle Pleistocene hominin movements
were more frequent and of lesser longitudinal and greater
latitudinal geographical extent. Their frequency was probably
Fig. 3.9 Out of East Africa model, Stage IV. See Fig. 1a–c for
climatic patterns
Fig. 3.10 Model for Early and Middle Pleistocene dispersals of homi-
nins out of East Africa. Red line indicates Early Pleistocene hominin
dispersals out of East Africa associated with Mode 1 and Mode 2
Industries. Blue line indicates Middle Pleistocene hominin dispersals
out of East Africa associated with derived Mode 2 industries

41
3 The Role of Corridors in ‘Out of Africa’
driven by the cyclicity of climatic change after 0.8 Ma. The
limits of these early Middle Pleistocene dispersals are prob-
ably responsible for the archeological distribution of the
Acheulean in Eurasia or the Movius Line.
Causes, Conditions, Constraints
and Consequences
The aim of this paper was to develop a model that addressed
the problem of the early dispersals of hominins out of Africa
at a scale closer to that at which the process took place, i.e.,
the scale of populations and ecological units, rather than
species and continental areas. In order to do so, the areas
surrounding East Africa to the North and East were described
in terms of corridors, barriers and refugia, which when inte-
grated with the record for hominin occupation of North
Africa and African-Eurasian faunal exchanges, led to the
proposal of a new hypothesis regarding the spatial and tem-
poral pattern of these early hominin movements. In order to
finish this discussion, I would like to use Rob Foley’s “4 Cs”
(Foley 1990, 1995) to explore the evolutionary context of the
population movements under discussion (Fig. 3.11).
Conditions
The first dispersals into North Africa and Eurasia took place
against a background of major climatic change. The model
places much emphasis on the role of climatic change in alter-
ing the competitive environments of hominin (and other)
populations which would have led to demographic processes
of microevolutionary consequence. Climatic change clearly
shaped the conditions in which these dispersals could take
place, but was not the driving force. The most significant
climatic shift of the Plio-Pleistocene period occurred 2.5 Ma,
but it was the period between 2.5 and 1.8 Ma that set the
ecological competitive conditions behind hominin trans-
continental dispersals. These dispersals involved hominins
that could successfully exploit grasslands, and indeed is in
such open environments that many of the first hominin sites
in Eurasia are found (Dennell 2004). The reconstruction of
East African Pliocene hominin sites suggests woodland envi-
ronments (Cerling 1992; Kappelman et al. 1997; Reed 1997),
and that the evolution of extensive grasslands was one of the
consequences of the onset of global glaciation 2.5 Ma.
However, the paleoenvironmental reconstructions of East
African hominin sites would suggest that hominins (or a
population among them) did not evolve a successful grass-
land adaptation until 1.8 Ma (as argued by Dennell), and thus
lagging behind the actual climatic driven environmental
change by a period of time (Foley 1993, 1994). The evolu-
tion of the Late Pliocene East African grasslands would have
changed the competitive circumstances of hominins;
increased interspecific competition leads to ecological dif-
ferentiation as a means of conflict avoidance (MacArthur
and Levins 1967; MacArthur 1968, 1970; Pianka 1978). This
process eventually led to the evolution of a dietary strategy
among one group of hominins more strongly based on ani-
mal food (Foley 1987, 2001). Therefore, one of the ancestral
adaptations of the hominins that dispersed out of Africa was
a carnivory-dependent strategy (Stiner 2002). Stone tools
were probably part of this adaptation, allowing certain homi-
nins to break into the carnivoran guild (Brantingham 1998).
As argued by Rob Foley, Mary Stiner and others, it was prob-
ably this carnivoran, stone-tool making adaptation that estab-
lished the right conditions for dispersals out of East Africa.
The evolution of carnivory had major biological conse-
quences with feedback effects and shaped a striking novel
adaptive package (Fig. 3.12). This involved changes in life-
history, body size and proportions, brain and gut size, and
most likely cognition. Although these other traits, particu-
larly when found in combination, came to define some of the
most successful species in the planet, I would argue that they
were not a necessary condition for the early dispersal of
hominins out of Africa. Indeed, the fossil remains from
Dmanasi strongly suggest that hominins could disperse out
of Africa without having the entire adaptive complex.
Causes
What caused certain populations of hominins to disperse has
been asked many times. The causes of evolutionary change
are established though proximate mechanisms. As argued
before, dispersals into North Africa and Eurasia could only
take place during a period of increased precipitation, so that
the Saharan barrier could be crossed. Therefore, the immedi-
ate mechanism leading hominins to disperse towards the
North could be the reduction and fragmentation of their
range as equatorial forests expanded (see Adams 2005 for a
discussion on this semi-controversial topic), leading to
increased inter- and intraspecific competition in East Africa
among non-woodland adapted groups, and the consequent
ecological differentiation through dispersal into neighbouring
Fig. 3.11 Rob Foley’s ‘4 Cs’ evolutionary ecology model
CONSTRAINTS
EVOLUTIONARY
CHANGE
CONDITIONS
CONSEQUENCESCAUSES

42 M.M. Lahr
zones. Similar processes did occur before, even the use of
one of the immediate corridors out of East Africa (A. bahrel-
ghazali). The difference in the case of Plio-Pleistocene Homo
was probably on the success with which the ecological shift
met, leading to demographic growth that sustained an exten-
sive dispersal event.
Constraints
The pattern of dispersals suggests a strong dependence on
water on the part of the hominins (maybe as one of the con-
sequences of carnivory dependence?). This water depen-
dence means that, in areas without significant lake basins,
the primary areas of colonization were often away from
plains and in highlands. This preference for higher grounds
in which springs and streams could be found would also fit
other important selective pressures, pressure for availability
of raw material for stone tools, shelter and possibly defense.
Consequences
Hominin dispersals had many consequences, from evolu-
tionary to ecological. However, the main immediate conse-
quence of the Pleistocene dispersals out of Africa was the
structuring of the species into a number of populations, many
of which developed independent evolutionary trajectories.
Population structure increases significantly the genetic vari-
ance of a species, and thus the rate of evolutionary change. In
the case of Pleistocene Homo, that change involved both bio-
logical and cultural dimensions.
This interpretation of the evolutionary ecology of the
early dispersals of hominins out of East Africa is summa-
rized in Fig. 3.13.
Concluding Thoughts
This paper tries to make two main points. The first of these is
that the evolutionary geography of populations is the correct
scale at which to analyze evolutionary problems of dispersals
and contractions. As a theoretical framework, it brings
together ecology, geography, phylogeny and biology, mak-
ing it a powerful theoretical and analytical tool. The second
key point is that the answers to the problem of how hominins
were capable of colonizing Eurasia need to be sought in
African ecology, not Eurasian. The evolutionary geography
model developed in this paper attempts to trace the pattern of
early hominin dispersals into North Africa; its comparison to
the first occupation of Eurasia shows that the pattern of hom-
inin colonization of areas beyond East Africa differed mark-
edly from one area to another. In particular, it shows that,
contrary to what happened in Eurasia, hominins were possi-
bly not able to colonize North Africa successfully until the
Fig. 3.12 Homo adaptive package. A consequence of carnivory?

43
3 The Role of Corridors in ‘Out of Africa’
Middle Pleistocene. It is probably the case that until the
Middle Pleistocene there were major constraints on the abil-
ity of hominins to establish populations in Africa beyond the
East African Rift Valley and the South African Transvaal,
possibly as a result of the joint effects of forests and deserts.
Ironically, Eurasian migrant hominin populations were prob-
ably demographically more successful and stable that their
African ancestors, but then again, it was that African insta-
bility that set the conditions for the evolution of novelties
which dispersed out of Africa again and again.
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Out of Africa I
The First Hominin Colonization
of Eurasia
Contributions from the Second Stony Brook
Human Evolution Symposium and Workshop, September 27–30, 2005
Edited by
John G. Fleagle
Department of Anatomical Sciences and Turkana Basin Institute,
Stony Brook University, Stony Brook, NY, USA
John J. Shea
Department of Anthropology and Turkana Basin Institute,
Stony Brook University, Stony Brook, NY, USA
Frederick E. Grine
Departments of Anthropology and Anatomical Sciences,
Stony Brook University, Stony Brook, NY, USA
Andrea L. Baden
Interdepartmental Doctoral Program in Anthropological Sciences,
Stony Brook University, Stony Brook, NY, USA
Richard E. Leakey
Department of Anthropology and Turkana Basin Institute,
Stony Brook University, Stony Brook, NY, USA

Editors
John G. Fleagle
Department of Anatomical Sciences
and Turkana Basin Institute
Stony Brook University
Stony Brook, NY 11794-8081
USA
john.fleagle@stonybrook.edu
Frederick E. Grine
Departments of Anthropology
and Anatomical Sciences
Stony Brook University
Stony Brook, NY 11794-4364
USA
frederick.grine@stonybrook.edu
Richard E. Leakey
Department of Anthropology
and Turkana Basin Institute
Stony Brook University
Stony Brook, NY 11794-4364
USA
richard.leakey@stonybrook.edu
John J. Shea
Department of Anthropology
and Turkana Basin Institute
Stony Brook University
Stony Brook, NY 11794-4364
USA
john.shea@sunysb.edu
Andrea L. Baden
Interdepartmental Doctoral Program
in Anthropological Sciences
Stony Brook University
Stony Brook, NY 11794-4364
USA
abaden@ic.sunysb.edu
ISBN 978-90-481-9035-5 e-ISBN 978-90-481-9036-2
DOI 10.1007/978-90-481-9036-2
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