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Sahara Desert Research Paper


Evidence increasingly suggests that sub-Saharan Africa is at the center of human evolution and understanding routes of dispersal “out of Africa” is thus becoming increasingly important. The Sahara Desert is considered by many to be an obstacle to these dispersals and a Nile corridor route has been proposed to cross it. Here we provide evidence that the Sahara was not an effective barrier and indicate how both animals and humans populated it during past humid phases. Analysis of the zoogeography of the Sahara shows that more animals crossed via this route than used the Nile corridor. Furthermore, many of these species are aquatic. This dispersal was possible because during the Holocene humid period the region contained a series of linked lakes, rivers, and inland deltas comprising a large interlinked waterway, channeling water and animals into and across the Sahara, thus facilitating these dispersals. This system was last active in the early Holocene when many species appear to have occupied the entire Sahara. However, species that require deep water did not reach northern regions because of weak hydrological connections. Human dispersals were influenced by this distribution; Nilo-Saharan speakers hunting aquatic fauna with barbed bone points occupied the southern Sahara, while people hunting Savannah fauna with the bow and arrow spread southward. The dating of lacustrine sediments show that the “green Sahara” also existed during the last interglacial (∼125 ka) and provided green corridors that could have formed dispersal routes at a likely time for the migration of modern humans out of Africa.

Traditionally, it has been assumed that the Nile was the primary corridor for the dispersal of hominins across the Sahara desert and then out of Africa (1). However, this route has proven hard to substantiate for three reasons. First, considerable research efforts have failed to uncover evidence for its consistent use and the movement of its inhabitants into the Levant (2). Second, the chronology of the evolution of the modern Nile drainage, which is essential to create a functional corridor, is controversial (3, 4). Finally, although many ancient fluvial deposits have been recognized in the Nile Valley, some with lithics that confirm the presence of hominins, their stratigraphic relationships and ages are poorly understood (5).

The need for a Nile migration corridor arises from the view that a belt of aridity has always separated the more humid regions north and south of the Sahara, yet the evidence for this arid barrier is contradictory. For example, genetics indicates that the Sahara has been a barrier to the dispersal of Macroscelidea (elephant shrews) since the late Miocene when the desert first formed (6), whereas pollen analysis of terrestrial sediments from the central Sahara demonstrates that during the Holocene humid period an arid belt existed north of 22° (7). Furthermore, arid plant taxa are found continuously through Atlantic Ocean cores between 23° and 27° north for the last 250 ka suggesting a long-lasting Saharan arid belt at this latitude (8). In contrast, a green Sahara route has been proposed whereby this currently hyperarid region could have been habitable during “green” periods of greater humidity (9,10). This humid period was first suggested by Duveyrier (11) to explain the existence of the Nile crocodiles in isolated Saharan oases and has periodically been promoted since, notably by Dumont (12) who noted the trans-Saharan distribution of numerous aquatic animal species. Recent evidence for this view is provided by isotopic data from a Mediterranean marine core and aquatic snails that suggest a river corridor across the Sahara during marine isotope stage (MIS) 5e (13) and variations in dust concentrations in a North Atlantic marine core that indicates humid conditions during the last two interglacials (14).

The idea of the Sahara as a barrier to faunal dispersal is becoming increasingly important because of its role in the out-of-Africa debate. Here we use numerous lines of evidence to evaluate the barrier hypothesis, and use this evidence to propose models for the peopling of the Sahara during past humid phases. In the next section, we map the palaeohydrology of the Sahara and show that during humid periods it contains an extensive interconnected hydrological system that animals appear to have used to migrate across the green Sahara. The following section compares the spatial distribution of archaeology and languages to that of the fauna and palaeohydrology to develop theories of the peopling of the green Sahara during its last humid phase. Finally, we evaluate dates of palaeolake sediments from the previous humid phase and show that it corresponds with the age of the first permanent modern human occupation of the North African coast and the Levant, suggesting a crossing of the Sahara at this time.

African Humid Period Biogeography and Palaeohydrology

Reanalysis of the Saharan zoogeography (SI Appendix, Section 1 and Table S1) suggests that many animals, including water-dependant creatures such as fish and amphibians, dispersed across the Sahara recently. For example, 25 North African animal species have a spatial distribution with population centers both north and south of the Sahara and small relict populations in central regions. This distribution suggests a trans-Saharan dispersal in the past, with subsequent local isolation of central Saharan populations during the more recent arid phase. If a diverse range of species (including fish) can cross the Sahara, it is impossible to envisage the Sahara functioning as barrier to hominin dispersal. The zoogeography of the Nile suggests that it was a much less effective corridor (SI Appendix, Table S1). Only nine animal species that occupy the Nile corridor today are also found both north and south of the Sahara. This paucity of Nileotic fauna highlights the importance of a green Sahara corridor and indicates that the Nile route is, potentially, less significant. A further 13 species of animal are not found in the Nile corridor but are found both north and south of the Sahara yet not in the center (SI Appendix, Table S1). Although they must have dispersed across the Sahara, whether they used the Nile or Green Sahara route cannot be determined without additional fossil evidence.

Fish species and molluscs are found both in oases across the Sahara and along the Nile, suggesting they crossed the Sahara using both the Green Sahara and Nile routes. The majority of these species are thought to only disperse via water (15) (SI Appendix, Section 2), thus current drainage only explains the Nile populations and another mechanism needs to be established to explain the present trans-Saharan distribution.

Mapping the palaeohydrology of the Sahara, using satellite imagery and digital topographic data, yields a tenable model (Fig. 1 and SI Appendix, Section 3). The Sahara was formerly covered by a dense palaeoriver network with many channels containing very large alluvial fans where rivers divide into multiple branches in an inland area. Where these fans are located on the boundary between two river catchments, their distributary channels can temporarily link adjacent river systems, thus allowing water-dependant life to transfer from one basin to the next (SI Appendix, Section 4). Five palaeofans identified in Fig. 1 link river systems in this manner. The green Sahara also contained numerous closed basins that supported some very large lakes (10, 16–19) (Fig. 1). When these lakes were full, they would have overflowed, thereby linking adjacent catchments (20). Fig. 1 shows that many fans and lake spillways are located in strategic positions that link waterways across the Sahara (SI Appendix, Section 4). Furthermore, tectonic activity can cause river diversion that transfers aquatic biota between basins, which appears to have been an important process in many northern basins (SI Appendix, Section 4). These mechanisms not only link many Saharan basins together but also connect them to large rivers with their headwaters outside the desert that can feed aquatic biota into the Sahara during humid periods. This interlinked waterway explains the observation that the fish species of the Sahara, Niger, Chad, and Nile Basins form a single biogeographic province (20) (Fig. 2A). The low degree of endemism indicates that these palaeohydrological interconnections happened relatively recently (15), perhaps during the early Holocene “climatic optimum” when the Sahara was last humid (21).

Fig. 1.

Late Pleistocene and early Holocene palaeohydrology of the Sahara (∼11 to 8 ka). The catchments of the megalakes that form corridors across the Sahara are indicated with letters A–D, while the river catchments that link with many of these megalake basins to form the vast Saharan inland waterway are indicated with letters E–I: (A) Lake Megachad, (B) Lake Megafezzan, (C) Ahnet-Mouydir Megalake, (D) the Basin of the Chotts, (E) Senegal River, (F) Nile River, (G) Sahabi River, (H) Kufra River, and (I) Niger River. Numbers indicate the location of fan, tectonic and lake outflow links between basins: (1) Nile Basin/Chad Basin, (2) Chad Basin/Niger Basin, (3) Niger Basin/Senegal Basin, (4) Niger/Basin West Ahaggar Mountains, (5) West Ahaggar Mountains/Ahnet-Mouydir Basin, (6) Ahnet-Mouydir Basin/Chotts Basin, (7) Chotts Basin/Fezzan Basin, (8) Fezzan Basin/Serir Tibesti, and (9) Serir Tibesti/Kufra Basin. The 200-mm isohyet indicates the current limit of the Sahara Desert.

Fig. 2.

Late Pleistocene and early Holocene palaeohydrology and biogeography of the Sahara (∼11 to 8 ka). (A) Biogeographic provinces for the African fish species are indicated (1 Sudanian, 2 Upper Guinean, 3 Eburneo-Guinean, 4 Lower Guinean, 5 Congo, 6 Maghrebian) along with the distribution of Tilapia zillii, both recently, as indicated by the hatched area and from marking the location of Saharan refuges, and during the Holocene, as indicated by fossils and rock art. A trans-Sahara distribution is evident, both across the Sahara and down the Nile. (B) Distribution of Hippopotamus amphibious both historically (∼1 ka) as shown by the hatched area and during the Holocene by marking the location of older historical sightings, fossils, and rock art. A distribution restricted to the River Nile and the southern-central Sahara is evident.

Freshwater mollusc biogeographic provinces (22) have a similar spatial distribution to that of the fish (SI Appendix, Section 5 and Fig. S5) suggesting that they dispersed across the Sahara using the same routes (SI Appendix, Section 2). That they have no endemic genera, and only 11 endemic species (22), again indicates a recent dispersal into the Sahara. In contrast, the divide between amphibian biogeographic provinces (SI Appendix, Figs. S7 and S8) is located roughly equidistant from the headwaters of the Nile, Chari, and Niger rivers and the Atlas Mountains (23). The latter presumably acted as refuges for these animals during arid periods. The contrasting spatial distribution of fish to amphibians is explained by the restriction of fish to rivers and lakes but the ability of amphibians to disperse both along watercourses and overland. Consequently, during early Holocene climatic amelioration, Palearctic amphibian species moved south, tropical species moved north, each crossing catchment divides that formed insurmountable barriers to fish, allowing direct colonization routes and thus their meeting in the center.

Although there is evidence that animals crossed the Sahara relatively recently, the timing and routes taken are uncertain. To constrain these two issues, maps of the spatial distribution of the recent (∼1980) or historical ranges (∼1900) of selected North African aquatic (20, 22, 24) and savanna (25) animal species, found within and north and south of the Sahara were combined with maps of reported historical sightings (24, 26), Holocene fossils (15, 27–33), and their depictions in rock art (26, 28, 34) (SI Appendix, Section 5). The evidence suggests that many species inhabited the Sahara during the early Holocene. This appears to be the case for hardy fish such as Tilapia zillii (Fig. 2A) and Clarias gariepinus (SI Appendix, Fig. S8), for the Nile crocodile (Crocodylus niloticus) (SI Appendix, Fig. S9), and for savanna species such as giraffe (Giraffa camelopardalis) (SI Appendix, Fig. S10) and African elephant (Loxadona africana) (SI Appendix, Fig. S11). These distributions suggest that, during the early Holocene, the Sahara was a savannah with riparian corridors linking north and south. Patches of more arid terrain in mountain rainshadows may better explain the arid pollen found in North Atlantic cores (8).

Some sub-Saharan animals do not exhibit trans-Saharan spatial distributions (Fig. 2B and SI Appendix, Figs. S12 and S13). They are found only in the southern Sahara, with a northern limit near the Ennedi, Tibesti, Tasilli, and Ahoggar Mountains, representing a catchment divide in the central Sahara (SI Appendix, Fig. S12). These species all have specialized aquatic requirements, such as deep water in the case of Nile perch (Lates niloticus) and hippopotamus (Hippopotamus amphibius) or long-term water connections for mollusc species such as Bellamya unicolor, Cleopatra bulimonides Pila and Lanistes (22) (SI Appendix, Section 2). The fan to the west of the Ahaggar Mountains (Fig. 1, fan 5) provides the only hydrological connection across this divide to the northern Sahara, but appears to have been a barrier to specialized aquatic species, accounting for an impoverished aquatic fauna in the northern Sahara.

The Peopling of the Sahara During the Holocene

We hypothesize that the differences in animal resources between the northern and southern Sahara during the early Holocene influenced the way it was peopled by humans. The north–south contrast in Saharan species ranges are remarkably similar to some key lithic, bone tool, and linguistic spatial distributions, suggesting that the peopling of the region during the early Holocene humid phase was driven by cultural adaptations that allowed exploitation of specific fauna.

The early Holocene archaeology of the Sahara is characterized by a regional distribution of specific archaeological cultures, such as those defined by barbed bone points, fishhooks, Ounanian arrow-points, and, more controversially, pottery (32, 35–38). The Sahara today is largely populated by speakers of Afroasiatic languages, Berber and Arabic, with some Nilo-Saharan languages (Teda-Daza and Zaghawa) in the region of Northern Chad, and Songhay cluster languages scattered across Mali and Niger (Fig. 3). However, it is clear that this situation is recent; Berber-speaking Tuareg moved into the Central Sahara ∼1500 y ago and the spread of the Hassaniya Moors into Mauritania probably dates from the 15th Century (39). Before this time, the central and southern Sahara are thought to have been populated by Nilo-Saharan speakers. The Nilo-Saharan language phylum is both widespread and strongly internally divided, suggesting considerable antiquity (40) (Fig. 3). Its greatest diversity is in the east, where a large number of small branches are found (Fig. 3), suggesting the original locus of expansion. Although fragmented into enclave populations today, the presence and pattern of relic populations in the northern desert points strongly to a much wider distribution in the past, covering the region from the Ethio-Sudan borderland to Mauritania and southwest Morocco.

Fig. 3.

Sahara palaeohydrology overlaid with the spatial distribution of Ounanian and barbed bone points and Nilo-Saharan languages. The languages marked as “other” are too small to be depicted as separate colors; they are Nyimang, Temein, Hill Nubian, Daju, Berta, and Gumuz. Note the similarity between the distribution of barbed bone points and the distribution of species that require deep water (Fig. 2B and SI Appendix, Figs. S12 and S13).

It has long been suggested that Nilo-Saharan languages might correlate with barbed bone points, the so-called “Aqualithic” (35). Fig. 3 superimposes the sites of known barbed bone points on a map of current Nilo-Saharan languages, showing a remarkable similarity in spatial distribution, and also a notable correspondence with Holocene distribution of large aquatic species (e.g., Fig. 2A and SI Appendix, Fig. S12). It appears that the expansion of aquatic resources in the Holocene made the Sahara attractive to populations with existing fishing and riverine hunting skills (SI Appendix, Section 6). Their ability to hunt hippopotamus and crocodiles and to catch a wide variety of deepwater fish species would have propelled a rapid dispersal from east to west and into the central Sahara, to judge by the numerous branches of Nilo-Saharan in the east (Fig. 3 and SI Appendix, Section 6). Their movement further north would have been restricted by the absence of many of these species. However, the presence of an isolated Nilo-Saharan population (the Koranje, a branch of Songhay) and a barbed bone point in Northwest Africa near the headwaters of the catchment of the Soura River, the river that links the Atlas mountains to the lakes in the Ahnet-Mouydir basin (Fig. 1), forming a corridor from the central Sahara to Northwest Africa, indicates that a few groups may have traversed the green Sahara using the most promising routes. There is direct linguistic evidence that Nilo-Saharan populations exploited these aquatic resources in the form of a widespread cognate for “hippo” from Gumuz in Ethiopia, to Songhay in Mali (SI Appendix, Table S2). SI Appendix, Table S2 shows similar forms for “crocodile,” although in this case the cognates are split between eastern and western languages.

We hypothesize that the other economic revolution that occurred in the Sahara at approximately the same time was the southward spread of the bow and arrow. North African hunters would have observed the new abundance of large and unfamiliar land mammals to the south, notably elephant and giraffe. In a dispersal inverse to that of the Nilo-Saharans, they would have been attracted southward to hunt these animals with the bow and arrow. The “Ounanian” of Northern Mali, Southern Algeria, Niger, and central Egypt at ca. 10 ka is partly defined by a distinctive type of arrow point (37). These arrowheads are found in much of the northern Sahara (Fig. 3) and are generally considered to have spread from Northwest Africa. This view is supported by the affinity of this industry with the Epipalaeolithic that also appears to have colonized the Sahara from the north (41). No Ounanian points occur in West Africa before 10 ka, suggesting the movement of a technology across the desert from north to south around this time.

Our model envisages the initial Holocene repopulation of the Sahara being carried out by two separate populations practicing two quite different resource exploitation strategies: (i) aquatic foraging using bone point and fish hook technology, and (ii) savanna hunting using the bow and arrow. By linking the distribution of the Nilo-Saharan language phyla to the archaeological distribution of aquatic- and terrestrial-adapted technologies, we explain the pattern of human repopulation of the desert in terms of the changing faunal distribution, which is in turn dictated by the nature of trans-Saharan hydrological linkages.

Older Saharan Occupation and Crossings

The movement of people across the Sahara during the Holocene may well have been the last of several important hominin dispersals. To demonstrate the possibility of earlier trans-Saharan migrations, it is necessary to determine the timing of pre-Holocene humid phases in the Sahara. This chronological evidence can be obtained by dating the deposition of pre-Holocene lake sediments and demonstrating synchronous palaeohydrologic changes across the Sahara. MIS5 contains the most abundantly dated pre-Holocene humid sediment sequences, yet only two speleothems, two regions containing spring tufas, and nine separate palaeolakes have been dated to this period (SI Appendix, Sections 4 and 7), thus there are many gaps and it is not possible to say if the entire Sahara was humid. For example, before this study, there was a complete lack of last MIS5 ages in the southernmost part of the Sahara. One way to overcome this problem is to date lacustrine episodes in the Saharan megalakes because they are located within abutting catchments and these catchments span the desert (Fig. 1). Because the high stands indicate significantly increased moisture availability, fluvial systems within the megalake catchments must be active at these times. Consequently, synchronous lake high stands would yield a humid corridor across the Sahara, allowing northward dispersal of hominin populations located in sub-Saharan Africa.

To evaluate the timing of pre-Holocene humid phases, it is necessary to locate and date extensive sequences of lacustrine sediments in megalake basins. Fortunately, Saharan megalakes appear to preserve the required long-term lacustrine sequences (16–19, 42) despite the general assumption that such records are rare in the Sahara because of deflation of lacustrine sediments during arid periods (43). We have applied optically stimulated luminescence (OSL) dating to selected sediments (SI Appendix, Section  7) and have found evidence for synchronous humidity in the Fezzan–Chad–Chotts and Chad–Chotts–Ahnet-Moyer megalake corridors during MIS5 (Fig. 1). Humidity in the Chad Basin was investigated by dating Lake Megachad beach ridges (SI Appendix, Fig. S16). The Bama Ridge in northeastern Nigeria forms the southwestern shoreline of the Holocene palaeolake Megachad with an area of 361,000 km2 (10). To the southwest of the Bama Ridge, we identified a succession of older beach ridges that run roughly parallel to it (SI Appendix, Fig. S16). OSL ages for two of these ridges at Alhajari and Kawiya give ages of 114 ± 14 ka and 125 ± 12 ka, respectively, suggesting megalake conditions in MIS5. In the Fezzan Basin, OSL dating (SI Appendix, Section 7; ref. 16) of palaeolake sediments shows that two large lakes of 1,350 and 1,730 km2 existed in this basin at a similar time (SI Appendix, Fig. S16). The Fezzan Basin ages are consistent with a U/Th isochron age for lake sediments from the basin of the Chotts (98 ± 5 ka; ref. 18) and the Ahnet-Mouydir basin [92(+20 - 18) ka; ref. 19). At this time, the Chotts basin contained a lake with an area of 30,000 km2 while the Ahnet-Mouydir basin lake attained an area of at least 32,000 km2, thus suggesting two green corridors across the Sahara during MIS5 that could have been used by hominins to cross the Sahara (Figs. 1 and 4). Once hominins had achieved a trans-Saharan distribution, it would have been possible to pass into the Levant via the Mediterranean coast. Because the Levant was humid at this time (44), onward dispersal would have been possible.

Fig. 4.

Palaeohydrology of North Africa during MIS5 with the location of dated sites marked. The hatched area covers catchments containing large palaeolakes and thus represents the minimum area which experienced considerably enhanced humidity during MIS5.

It is probable that increased humidity within the megalake catchments is indicative of regional scale humidity and hence dispersal routes may have existed throughout the desert, as appears to have been the case during the early Holocene, rather than being restricted to corridors. Other studies of Saharan fluvial and lacustrine sediments provide some evidence for humidity in other areas. A humid corridor has recently been proposed in the Sahabi palaeoriver system (13) (Fig. 4) and there are 41 other ages for 12 areas that contain humid sediments scattered throughout the Sahara that are indistinguishable from these “corridor” ages. Their integration with the palaeohydrological map provides evidence for a wider green Sahara during MIS5 as well as the present interglacial (Fig. 4 and SI Appendix, Section 3).

Using the Holocene biogeography and palaeohydrology of the Sahara as an analogue for the MIS5 humid period, it is likely that an interconnected waterway would have been available for faunal and human dispersal. This humid period corresponds very closely with the age of the first modern human occupation of the North African coast (45) and the Levant (46) by sub-Saharan populations, who may have been crossing the Sahara at this time (9). The occupation of the Mediterranean coast of Africa by these early modern human migrants appears to have lasted from ∼110 to ∼30 ka (45), though the Levantine occupation appears to have finished by ∼70 ka (47). Some view the out-of-Africa dispersal into the Levant as the start of the spread of modern humans onward into Arabia and India in MIS5 (48), whereas others believe it to be a “dead end” that was followed by a later more successful dispersal of modern humans out of Africa at a later date: 60 ka in MIS4 (49). There is little evidence for a green Sahara in MIS4, so the desert is unlikely to have directly played a role in this postulated second dispersal, however, the modern humans north of the Sahara could have provided the population required to drive it. Indeed, “spatially” this hypothesis is attractive because the Sinai corridor out of North Africa is close to the location of the earliest archaeological sites associated with this dispersal, which are found in the Levant at ∼47 ka (50). However, an alternative dispersal route for modern humans out of Africa via the Bab el Mandab has been proposed at about this time, largely on genetic grounds (51), thus if this dispersal occurred, a route out of Africa via North Africa is not necessary. To further complicate matters, the possibility of multiple dispersals of modern humans both into and out of Africa has recently been suggested (52). If these proposed migrations did indeed happen, it is feasible that these different dispersals employed different routes.

Notwithstanding the as yet unresolved issue of the timing and route taken by modern humans out of Africa, both the use of the Green Sahara route to the North African coast by hominins and its long-term viability are supported by archaeological evidence (53–56). Stone tools from Oldowan to Neolithic have been found in all the megalake basins that form corridors across the Sahara, suggesting that the region was not only periodically habitable for hominins, but that it was also regularly occupied by them.


We thank the Society for Libyan Studies for funding the Fezzan Project that inspired this work and the Desert Migrations Projects that allowed research in the Fezzan to continue. Particular thanks go to David Mattingly who led these research projects and encouraged this research. We are also grateful to Marta Lahr, Robert Foley, and Kevin MacDonald for helpful discussions on the peopling of the Sahara and to David Blackburn for his help in understanding its biogeography. Grants from the Royal Geographical Society—Institute of British Geographers Peter Fleming Award, the National Geographic Society, Repsol/YPF Murzuk S A, and the Great Manmade River Authority also funded aspects of this research, for which they are thanked.


  • 1To whom correspondence should be addressed. E-mail: nick.drake{at}kcl.ac.uk.
  • Author contributions: N.A.D. and R.M.B. designed research; N.A.D., R.M.B., S.J.A., C.S.B., and K.H.W. performed research; N.A.D., R.M.B., and S.J.A. analyzed data; and N.A.D. and R.M.B. wrote the paper.

  • The authors declare no conflict of interest.

  • This article is a PNAS Direct Submission.

  • This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1012231108/-/DCSupplemental.


When most people imagine an archetypal desert landscape—with its relentless sun, rippling sand and hidden oases—they often picture the Sahara. But 11,000 years ago, what we know today as the world’s largest hot desert would’ve been unrecognizable. The now-dessicated northern strip of Africa was once green and alive, pocked with lakes, rivers, grasslands and even forests. So where did all that water go?

Archaeologist David Wright has an idea: Maybe humans and their goats tipped the balance, kick-starting this dramatic ecological transformation. In a new study in the journal Frontiers in Earth Science, Wright set out to argue that humans could be the answer to a question that has plagued archaeologists and paleoecologists for years.

The Sahara has long been subject to periodic bouts of humidity and aridity. These fluctuations are caused by slight wobbles in the tilt of the Earth’s orbital axis, which in turn changes the angle at which solar radiation penetrates the atmosphere. At repeated intervals throughout Earth’s history, there’s been more energy pouring in from the sun during the West African monsoon season, and during those times—known as African Humid Periods—much more rain comes down over north Africa.

With more rain, the region gets more greenery and rivers and lakes. All this has been known for decades. But between 8,000 and 4,500 years ago, something strange happened: The transition from humid to dry happened far more rapidly in some areas than could be explained by the orbital precession alone, resulting in the Sahara Desert as we know it today. “Scientists usually call it ‘poor parameterization’ of the data,” Wright said by email. “Which is to say that we have no idea what we’re missing here—but something’s wrong.”

As Wright pored the archaeological and environmental data (mostly sediment cores and pollen records, all dated to the same time period), he noticed what seemed like a pattern. Wherever the archaeological record showed the presence of “pastoralists”—humans with their domesticated animals—there was a corresponding change in the types and variety of plants. It was as if, every time humans and their goats and cattle hopscotched across the grasslands, they had turned everything to scrub and desert in their wake.

Wright thinks this is exactly what happened. “By overgrazing the grasses, they were reducing the amount of atmospheric moisture—plants give off moisture, which produces clouds—and enhancing albedo,” Wright said. He suggests this may have triggered the end of the humid period more abruptly than can be explained by the orbital changes. These nomadic humans also may have used fire as a land management tool, which would have exacerbated the speed at which the desert took hold.

It’s important to note that the green Sahara always would’ve turned back into a desert even without humans doing anything—that’s just how Earth’s orbit works, says geologist Jessica Tierney, an associate professor of geoscience at the University of Arizona. Moreover, according to Tierney, we don’t necessarily need humans to explain the abruptness of the transition from green to desert.

Instead, the culprits might be regular old vegetation feedbacks and changes in the amount of dust. “At first you have this slow change in the Earth’s orbit,” Tierney explains. “As that’s happening, the West African monsoon is going to get a little bit weaker. Slowly you’ll degrade the landscape, switching from desert to vegetation. And then at some point you pass the tipping point where change accelerates.”

Tierney adds that it’s hard to know what triggered the cascade in the system, because everything is so closely intertwined. During the last humid period, the Sahara was filled with hunter-gatherers. As the orbit slowly changed and less rain fell, humans would have needed to domesticate animals, like cattle and goats, for sustenance. “It could be the climate was pushing people to herd cattle, or the overgrazing practices accelerated denudation [of foliage],” Tierney says. 

Which came first? It’s hard to say with evidence we have now. “The question is: How do we test this hypothesis?” she says. “How do we isolate the climatically driven changes from the role of humans? It’s a bit of a chicken and an egg problem.” Wright, too, cautions that right now we have evidence only for correlation, not causation.

But Tierney is also intrigued by Wright’s research, and agrees with him that much more research needs to be done to answer these questions.

“We need to drill down into the dried-up lake beds that are scattered around the Sahara and look at the pollen and seed data and then match that to the archaeological datasets,” Wright said. “With enough correlations, we may be able to more definitively develop a theory of why the pace of climate change at the end of the AHP doesn’t match orbital timescales and is irregular across northern Africa.”

Tierney suggests researchers could use mathematical models that compare the impact hunter-gatherers would have on the environment versus that of pastoralists herding animals. For such models it would be necessary to have some idea of how many people lived in the Sahara at the time, but Tierney is sure there were more people in the region than there are today, excepting coastal urban areas.

While the shifts between a green Sahara and a desert do constitute a type of climate change, it’s important to understand that the mechanism differs from what we think of as anthropogenic (human-made) climate change today, which is largely driven by rising levels of CO2 and other greenhouse gases. Still, that doesn’t mean these studies can’t help us understand the impact humans are having on the environment now.

“It’s definitely important,” Tierney says. “Understanding the way those feedback (loops) work could improve our ability to predict changes for vulnerable arid and semi-arid regions.”

Wright sees an even broader message in this type of study. “Humans don’t exist in ecological vacuums,” he said. “We are a keystone species and, as such, we make massive impacts on the entire ecological complexion of the Earth. Some of these can be good for us, but some have really threatened the long-term sustainability of the Earth.” 

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