The Sahara pump theory is a hypothesis that explains how

flora Flora is all the plant life present in a particular region or time, generally the naturally occurring (indigenous (ecology), indigenous) native plant, native plants. Sometimes bacteria and fungi are also referred to as flora, as in the terms '' ...

and

fauna Fauna is all of the animal life present in a particular region or time. The corresponding term for plants is ''flora'', and for fungi, it is ''funga''. Flora, fauna, funga and other forms of life are collectively referred to as ''Biota (ecology ...

migrated between Eurasia and Africa via a

land bridge In biogeography, a land bridge is an isthmus or wider land connection between otherwise separate areas, over which animals and plants are able to cross and colonize new lands. A land bridge can be created by marine regression, in which sea leve ...

in the

Levant The Levant () is an approximate historical geographical term referring to a large area in the Eastern Mediterranean region of Western Asia. In its narrowest sense, which is in use today in archaeology and other cultural contexts, it is ...

region. It posits that extended periods of abundant rainfall lasting many thousands of years (

pluvial In geology and climatology, a pluvial is either a modern climate characterized by relatively high precipitation or an interval of time of variable length, decades to thousands of years, during which a climate is characterized by relatively high ...

periods) in Africa are associated with a "wet- green Sahara" phase, during which larger lakes and more rivers existed. This caused changes in the flora and fauna found in the area. Migration along the river corridor was halted when, during a desert phase 1.8–0.8 million years ago ( mya), the

Nile The Nile, , Bohairic , lg, Kiira , Nobiin: Áman Dawū is a major north-flowing river in northeastern Africa. It flows into the Mediterranean Sea. The Nile is the longest river in Africa and has historically been considered the longest ...

ceased to flow completely and possibly flowed only temporarily in other periods due to the

geologic uplift Orogeny is a mountain building process. An orogeny is an event that takes place at a convergent plate margin when plate motion compresses the margin. An ''orogenic belt'' or ''orogen'' develops as the compressed plate crumples and is uplifted ...

(

Nubian Swell The Nubian Swell is a geologic structural uplift in northern Africa that trends east to west and separates the lower Nile of Egypt from the Sudan basin. The Nubian Swell has been geologically active since early Mesozoic times, and portions of it a ...

) of the Nile River region.

Mechanism

During periods of a wet or '' Green Sahara'', the Sahara and

Arabia The Arabian Peninsula, (; ar, شِبْهُ الْجَزِيرَةِ الْعَرَبِيَّة, , "Arabian Peninsula" or , , "Island of the Arabs") or Arabia, is a peninsula of Western Asia, situated northeast of Africa on the Arabian Plat ...

become a

savanna A savanna or savannah is a mixed woodland- grassland (i.e. grassy woodland) ecosystem characterised by the trees being sufficiently widely spaced so that the canopy does not close. The open canopy allows sufficient light to reach the ground to ...

grassland and African flora and fauna become common. Following inter-pluvial arid periods, the Sahara area then reverts to desert conditions, usually as a result of the retreat of the

West African Monsoon A monsoon () is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with annual latitudinal oscill ...

southwards. Evaporation exceeds precipitation, the level of water in lakes like Lake Chad falls, and rivers become dry

wadi Wadi ( ar, وَادِي, wādī), alternatively ''wād'' ( ar, وَاد), North African Arabic Oued, is the Arabic term traditionally referring to a valley. In some instances, it may refer to a wet (ephemeral) riverbed that contains water ...

s. Flora and fauna previously widespread as a result retreat northwards to the Atlas Mountains, southwards into

West Africa West Africa or Western Africa is the westernmost region of Africa. The United Nations defines Western Africa as the 16 countries of Benin, Burkina Faso, Cape Verde, The Gambia, Ghana, Guinea, Guinea-Bissau, Ivory Coast, Liberia, Mali, M ...

, or eastwards into the Nile Valley and thence either southeast to the

Ethiopian Highlands The Ethiopian Highlands is a rugged mass of mountains in Ethiopia in Northeast Africa. It forms the largest continuous area of its elevation in the continent, with little of its surface falling below , while the summits reach heights of up to . ...

and

Kenya ) , national_anthem = " Ee Mungu Nguvu Yetu"() , image_map = , map_caption = , image_map2 = , capital = Nairobi , coordinates = , largest_city = Nairobi ...

or northeast across the Sinai into

Asia Asia (, ) is one of the world's most notable geographical regions, which is either considered a continent in its own right or a subcontinent of Eurasia, which shares the continental landmass of Afro-Eurasia with Africa. Asia covers an are ...

. This separates

population Population typically refers to the number of people in a single area, whether it be a city or town, region, country, continent, or the world. Governments typically quantify the size of the resident population within their jurisdiction using a ...

s of some of the species in areas with different

climate Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologi ...

s, forcing them to adapt, possibly giving rise to

allopatric speciation Allopatric speciation () – also referred to as geographic speciation, vicariant speciation, or its earlier name the dumbbell model – is a mode of speciation that occurs when biological populations become geographically isolated from ...

Plio-Pleistocene

The

Plio-Pleistocene The Plio-Pleistocene is an informally described geological pseudo-period, which begins about 5 million years ago (Mya) and, drawing forward, combines the time ranges of the formally defined Pliocene and Pleistocene epochs—marking from about 5&nb ...

migrations to Africa included the

Caprinae The subfamily Caprinae, also sometimes referred to as the tribe Caprini, is part of the ruminant family Bovidae, and consists of mostly medium-sized bovids. A member of this subfamily is called a caprine, or, more informally, a goat-antelope (a ...

in two waves at 3.2 Ma and 2.7–2.5 Ma; ''

Nyctereutes ''Nyctereutes'' (Greek: ''nyx, nykt-'' "night" + ''ereutēs'' "wanderer") is a genus of canid which includes only two extant species both known as raccoon dogs; the common raccoon dog (''Nyctereutes procyonoides'') and the Japanese raccoon dog (' ...

'' at 2.5 Ma, and '' Equus'' at 2.3 Ma. ''

Hippotragus ''Hippotragus'' is a genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and ...

'' migrated at 2.6 Ma from Africa to the

Siwaliks The Sivalik Hills, also known as the Shivalik Hills and Churia Hills, are a mountain range of the outer Himalayas that stretches over about from the Indus River eastwards close to the Brahmaputra River, spanning the northern parts of the Indi ...

of the Himalayas. Asian

bovid The Bovidae comprise the biological family of cloven-hoofed, ruminant mammals that includes cattle, bison, buffalo, antelopes, and caprines. A member of this family is called a bovid. With 143 extant species and 300 known extinct species, ...

s moved to Europe and to and from Africa. The primate '' Theropithecus'' experienced contraction and its fossils are found only in Europe and Asia, while ''

Homo ''Homo'' () is the genus that emerged in the (otherwise extinct) genus '' Australopithecus'' that encompasses the extant species ''Homo sapiens'' ( modern humans), plus several extinct species classified as either ancestral to or closely relat ...

'' and '' Macaca'' settled wide ranges.

185,000–20,000 years ago

Between about 133 and 122 thousand years ago (kya), the southern parts of the Saharan-Arabian Desert experienced the start of the Abbassia Pluvial, a wet period with increased monsoonal precipitation, around 100-200 mm/year. This allowed Eurasian biota to travel to Africa and vice versa. The growth of

speleothems A speleothem (; ) is a geological formation by mineral deposits that accumulate over time in natural caves. Speleothems most commonly form in calcareous caves due to carbonate dissolution reactions. They can take a variety of forms, depending on ...

(which requires rainwater) was detected in Hol-Zakh, Ashalim, Even-Sid, Ma'ale-ha-Meyshar, Ktora Cracks, Nagev Tzavoa Cave. In Qafzeh and Es Skuhl caves, where at that time precipitation was 600–1000 mm/year, the remains of Qafzeh-Skhul type

anatomically modern humans Early modern human (EMH) or anatomically modern human (AMH) are terms used to distinguish '' Homo sapiens'' (the only extant Hominina species) that are anatomically consistent with the range of phenotypes seen in contemporary humans from exti ...

are dated from this period, but human occupation seems to end in the later arid period. The

Red Sea The Red Sea ( ar, البحر الأحمر - بحر القلزم, translit=Modern: al-Baḥr al-ʾAḥmar, Medieval: Baḥr al-Qulzum; or ; Coptic: ⲫⲓⲟⲙ ⲛ̀ϩⲁϩ ''Phiom Enhah'' or ⲫⲓⲟⲙ ⲛ̀ϣⲁⲣⲓ ''Phiom ǹšari''; ...

coastal route was extremely arid before 140 and after 115 kya. Slightly wetter conditions appear at 90–87 kya, but it still was just one tenth the rainfall around 125 kya.

Speleothem A speleothem (; ) is a geological formation by mineral deposits that accumulate over time in natural caves. Speleothems most commonly form in calcareous caves due to carbonate dissolution reactions. They can take a variety of forms, depending ...

s are detected only in Even-Sid-2. In the southern Negev Desert speleothems did not grow between 185–140 kya (

MIS MIS or mis may refer to: Science and technology * Management information system * Marine isotope stage, stages of the Earth's climate * Maximal independent set, in graph theory * Metal-insulator-semiconductor, e.g., in MIS capacitor * Minimally ...

6), 110–90 (MIS 5.4–5.2), nor after 85 kya nor during most of the interglacial period (MIS 5.1), the glacial period and

Holocene The Holocene ( ) is the current geological epoch. It began approximately 11,650 cal years Before Present (), after the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene togeth ...

. This suggests that the southern Negev was arid to hyper-arid in these periods. The coastal route around the western

Mediterranean The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western Europe, Western and Southern Europe and Anatolia, on the south by North Africa ...

may have been open at times during the last glacial; speleothems grew in Hol-Zakh and in Nagev Tzavoa Caves. Comparison of speleothem formation with calcite horizons suggests that the wet periods were limited to only tens or hundreds of years. From 60–30 kya there were extremely dry conditions in many parts of Africa.

Last Glacial Maximum

An example of the Saharan pump has occurred after the Last Glacial Maximum (LGM). During the Last Glacial Maximum the Sahara desert was more extensive than it is now with the extent of the tropical forests being greatly reduced. During this period, the lower temperatures reduced the strength of the

Hadley Cell The Hadley cell, named after George Hadley, is a global-scale tropical atmospheric circulation that features air rising near the equator, flowing poleward at a height of 10 to 15 kilometers above the earth's surface, descending in the subtropics ...

whereby rising tropical air of the Inter-Tropical Convergence Zone (ITCZ) brings rain to the tropics, while dry descending air, at about 20 degrees north, flows back to the equator and brings desert conditions to this region. This phase is associated with high rates of wind-blown mineral dust, found in marine cores that come from the north tropical Atlantic. Around 12,500 BC, the amount of dust in the cores in the Bølling/ Allerød phase suddenly plummets and shows a period of much wetter conditions in the Sahara, indicating a Dansgaard-Oeschger (DO) event (a sudden warming followed by a slower cooling of the climate). The moister Saharan conditions had begun about 12,500 BC, with the extension of the ITCZ northward in the northern hemisphere summer, bringing moist wet conditions and a savanna climate to the Sahara, which (apart from a short dry spell associated with the

Younger Dryas The Younger Dryas (c. 12,900 to 11,700 years BP) was a return to glacial conditions which temporarily reversed the gradual climatic warming after the Last Glacial Maximum (LGM, c. 27,000 to 20,000 years BP). The Younger Dryas was the last stag ...

) peaked during the Holocene thermal maximum climatic phase at 4000 BC when mid-latitude temperatures seem to have been between 2 and 3 degrees warmer than in the recent past. Analysis of Nile River deposited sediments in the delta also shows this period had a higher proportion of sediments coming from the

Blue Nile The Blue Nile (; ) is a river originating at Lake Tana in Ethiopia. It travels for approximately through Ethiopia and Sudan. Along with the White Nile, it is one of the two major tributaries of the Nile and supplies about 85.6% of the water to ...

, suggesting higher rainfall also in the

. This was caused principally by a stronger

monsoon A monsoon () is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with annual latitudinal osci ...

al circulation throughout the sub-tropical regions, affecting India, Arabia and the Sahara. Lake Victoria only recently became the source of the White Nile and dried out almost completely around 15 kya. The sudden subsequent movement of the ITCZ southwards with a

Heinrich event A Heinrich event is a natural phenomenon in which large groups of icebergs break off from glaciers and traverse the North Atlantic. First described by marine geologist Hartmut Heinrich (Heinrich, H., 1988), they occurred during five of the last ...

(a sudden cooling followed by a slower warming), linked to changes with the

El Niño-Southern Oscillation EL, El or el may refer to: Religion * El (deity), a Semitic word for "God" People * EL (rapper) (born 1983), stage name of Elorm Adablah, a Ghanaian rapper and sound engineer * El DeBarge, music artist * El Franco Lee (1949–2016), American p ...

cycle, led to a rapid drying out of the Saharan and Arabian regions, which quickly became desert. This is linked to a marked decline in the scale of the Nile floods between 2700 and 2100 BC. One theory proposed that humans accelerated the drying out period from 6,000–2,500 BC by pastoralists overgrazing available grassland.

Human migration

The Saharan pump has been used to date a number of waves of human migration from Africa, namely: * Lower Paleolithic: '' Homo erectus'' (ssp. ''ergaster'') into Southeast and East Asia, possibly twice, once with an

Oldowan The Oldowan (or Mode I) was a widespread stone tool archaeological industry (style) in prehistory. These early tools were simple, usually made with one or a few flakes chipped off with another stone. Oldowan tools were used during the Lower ...

technology, which travelled as far as China and India to create the Chopper tradition, the second with Acheulian hand axes, only as far as the Indian Subcontinent. *Middle Paleolithic: ''

Homo heidelbergensis ''Homo heidelbergensis'' (also ''H. sapiens heidelbergensis''), sometimes called Heidelbergs, is an extinct species or subspecies of archaic human which existed during the Middle Pleistocene. It was subsumed as a subspecies of '' H. erectus'' i ...

'' into the Middle East and Western Europe. *Upper Paleolithic: ''

Homo sapiens Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...

'' (possible early "

Out of Africa ''Out of Africa'' is a memoir by the Danish author Karen Blixen. The book, first published in 1937, recounts events of the seventeen years when Blixen made her home in Kenya, then called British East Africa. The book is a lyrical meditation on ...

" wave, receded before 80,000 years ago and eventually replaced by the "

coastal migration In the context of the recent African origin of modern humans, the Southern Dispersal scenario (also the coastal migration or great coastal migration hypothesis) refers to the early migration along the southern coast of Asia, from the Arabian Pen ...

" wave after 70,000 years ago) *Epipaleolithic: Possible ''

Afroasiatic The Afroasiatic languages (or Afro-Asiatic), also known as Hamito-Semitic, or Semito-Hamitic, and sometimes also as Afrasian, Erythraean or Lisramic, are a language family of about 300 languages that are spoken predominantly in the geographic su ...

'' migration into the Levant associated with the aridity of the 8.2 kiloyear event *Neolithic: 5.9 kiloyear event: sometimes associated with certain population movements of the Neolithic period * ''

Libu The Libu ( egy, rbw; also transcribed Rebu, Lebu, Lbou, Libou) were an Ancient Libyan tribe of Berber origin, from which the name ''Libya'' derives. Early history Their occupation of Ancient Libya is first attested in Egyptian language texts ...

and Meshwesh migrations'' attacking Egypt at the end of the New Kingdom that ushered in the

Bronze Age Collapse The Late Bronze Age collapse was a time of widespread societal collapse during the 12th century BC, between c. 1200 and 1150. The collapse affected a large area of the Eastern Mediterranean (North Africa and Southeast Europe) and the Near ...

and saw chariots appear in the Sahara.Anderson, Helen (2016) "Chariots in Saharan Rock Art: an aesthetic and cognitive review" (Journal of Social Archaeology Vol 16 no. 3)

References