Several expansions of populations ofarchaic humans (genusHomo) out of Africa and throughout Eurasia took place in the course of theLower Paleolithic, and into the beginningMiddle Paleolithic, between about 2.1 million and 0.2 million years ago (Ma).These expansions are collectively known asOut of Africa I, in contrast to the expansion ofanatomically modern humans (Homo sapiens) into Eurasia, which may have begun shortly after 0.2 million years ago (known in this context as "Out of Africa II").^[1]

The earliest presence ofHomo (or indeed anyhominin) outside of Africa dates to close to 2 million years ago.A 2018 study identified possible hominin presence atShangchen, central China, as early as 2.12 Ma based onmagnetostratigraphic dating of the lowest layer containing what may possibly be stone artefacts.^[2]The oldest known human skeletal remains outside of Africa are fromDmanisi,Georgia (Dmanisi skull 4), and are dated to 1.8 Ma. These remains are classified asHomo erectus georgicus.

Later waves of expansion are proposed around 1.4 Ma (earlyAcheulean industries), associated withHomo antecessor and 0.8 Ma (cleaver-producing Acheulean groups), associated withHomo heidelbergensis.^[3]

Until the early 1980s, early humans were thought to have been restricted to the African continent in theEarly Pleistocene, or until about 0.8 Ma; Hominin migrations outside East Africa were apparently rare in the Early Pleistocene, leaving a fragmentary record of events.^[4][5]

Pre-Homo hominin expansion out of Africa is suggested by the presence ofGraecopithecus andOuranopithecus, found in Greece and Anatolia and dated to c. 8 million years ago, but these are probablyHomininae but not Hominini. Possibly related are theTrachilos footprints found in Crete, dated to close to 6 million years ago;^[6] the age of the footprints was later reestimated to be 6.05 million years ago, 0.35 million years older than previous estimations.^[7] Another reestimation by Willem Jan Zachariasse and Lucas Lourens interpret the purported footprints to have originated 3 million years ago and doubt if they were footprints or the hominins had made the footprints because of the shallow marine setting and the separation of Crete from mainland Greece and Turkey in the Late Pliocene by the South Aegean Basin.^[8]

Australopithecina emerged about 5.6 million years ago, in East Africa (Afar Depression).Gracile australopithecines (Australopithecus afarensis) emerged in the same region, around 4 million years ago.The earliest known retouched tools were found inLomekwi, Kenya, and date back to 3.3 Ma, in the latePliocene. They might be the product ofAustralopithecus garhi orParanthropus aethiopicus, the two known hominins contemporary with the tools.^[9] GenusHomo is assumed to have emerged by around 2.8 million years ago, withHomo habilis being found atLake Turkana,Kenya. The delineation of the "human" genus,Homo, from Australopithecus is somewhat contentious, for which reason the superordinate term "hominin" is often used to include both. "Hominin" technically includeschimpanzees as well aspre-human species as old as 10 million years old (the separation ofHomininae into Hominini andGorillini).

The earliest known hominin presence outside of Africa dates to close to 2 million years ago. A 2018 study claims evidence for human presence atShangchen, central China, as early as 2.12 Ma based onmagnetostratigraphic dating of the lowest layer containing stone artefacts.^[2]

It has been suggested thatHomo floresiensis was descended from such an early expansion. It is not clear whether these earliest hominins leaving Africa should be consideredHomo habilis, or a form of earlyHomo or lateAustralopithecus closely related toHomo habilis, or a very early form ofHomo erectus. In any case, the morphology ofH. floresiensis has been found to show greatest similarity withAustralopithecus sediba,Homo habilis andDmanisi Man, raising the possibility that the ancestors ofH. floresiensis left Africa before the appearance ofH. erectus.^[10] Aphylogenetic analysis published in 2017 suggests thatH. floresiensis was descended from a species (presumably Australopithecine) ancestral toHomo habilis, making it a "sister species" either toH. habilis or to a minimallyhabilis-erectus-ergaster-sapiensclade, and its line is older thanH. erectus itself. On the basis of this classification,H. floresiensis is hypothesized to represent a hitherto unknown and very early migration out of Africa, dating to before 2.1 million years ago. A similar conclusion is suggested by the date of 2.1 Ma for the oldestShangchen artefacts.^[2]

Homo erectus emerges just after 2 million years ago.^[11]EarlyH. erectus would have lived face to face withH. habilis in East Africa for nearly half a million years.^[12]The oldestHomo erectus fossils appear almost contemporaneously, shortly after two million years ago, both in Africa and in the Caucasus.The earliest well-dated EurasianH. erectus site (if the fossils are indeedH. erectus — seeDmanisi hominins) isDmanisi in Georgia, securely dated to 1.8 Ma.^[13][14] A skull found at Dmanisi is evidence for caring for the old. The skull shows that thisHomo erectus was advanced in age and had lost all but one tooth years before death, and it is perhaps unlikely that this hominid would have survived alone. It is not certain, however, that this is sufficient proof for caring – a partially paralysed chimpanzee at the Gombe reserve survived for years without help.^[15]The earliest known evidence for AfricanH. erectus, dubbedHomo ergaster, is asingle occipital bone (KNM-ER 2598), described as "H. erectus-like", and dated to about 1.9 Ma (contemporary withHomo rudolfensis). This is followed by a fossil gap, the next available fossil beingKNM-ER 3733, a skull dated to 1.6 Ma.^[16]Early Pleistocene sites in North Africa, the geographical intermediate of East Africa and Georgia, are in poor stratigraphic context. The earliest of the dated isAin Hanech in northernAlgeria (c. 1.8^[17] – 1.2 Ma^[18]), anOldowan grade layer. These sites attest that earlyHomo erectus have crossed the North African tracts, which are usually hot and dry.^[4]: 2There is little time betweenHomo erectus' apparent arrival inSouth Caucasus around 1.8 Ma, and its probable arrival in East and Southeast Asia. There is evidence ofH. erectus inYuanmou, China, dating to 1.7 Ma and inSangiran, onJava, Indonesia, from 1.66 Ma.^[19]

Ferring et al. (2011) suggest that it was stillHomo habilis that reached West Asia, and that earlyH. erectus developed there.H. erectus would then have dispersed from West Asia, to East Asia (Peking Man) Southeast Asia (Java Man), back to Africa (Homo ergaster), and to Europe (Tautavel Man).^[20][21]

It appearsH. erectus took longer to move into Europe, the earliest site beingBarranco León in southeasternSpain dated to 1.4 Ma, associated withHomo antecessor,^[22] and a controversialPirro Nord in Southern Italy, allegedly from 1.7 – 1.3 Ma.^[23] The paleobiogeography of early human dispersals in western Eurasia characterizesH. ex gr.erectus as a temperature sensitivestenobiont, that failed to disperse north of theAlpide Belt.^[24] The geographically restricted earliest human presence in theIberian Peninsula should be regarded as evidence of a sustainable presence of human population in this isolated area. ThePannonian plain, situated south-west of theCarpathian Mountains, was apparently characterized by a comparatively warm climate similar to that of theMediterranean Area, while the climate of the western Europeanpaleobiogeographic area was mitigated byGulf Stream influence and could support the episodic hominin dispersals toward theIberian Peninsula.^[24] Apparently, the faunal exchanges between southeastern Europe and the Near East and southern Asia were controlled by the complex interaction of such geographic obstacles as theBosporus and theManych Strait, the climate barrier from the north of theGreater Caucasus range, and the 41 kyr glacialMilankovitch cycles that repeatedly closed the Bosporus and thus triggered the two-way faunal exchange betweensoutheastern Europe and theNear East, and, apparently, the further westward dispersal of the archaic hominins in Eurasia.^[25]

By 1 Ma,Homo erectus had spread across Eurasia (mostly restricted to latitudes south of the50th parallel north^[26]).It is hard to say, however, whether settlement was continuous in Western Europe, or if successive waves repopulated the territory in glacial interludes. EarlyAcheulean tools atUbeidiya from 1.4 Ma^[27] is some evidence for a continuous settlement in the West, as successive waves out of Africa after then would likely have brought Acheulean technology to Western Europe.^{[citation needed]}

The presence ofLower Paleolithic human remains inIndonesian islands is good evidence for seafaring byHomo erectus late in theEarly Pleistocene. Bednarik suggests that navigation had appeared by 1 Ma, possibly to exploit offshore fishing grounds.^[28] He has reproduced a primitive dirigible (steerable) raft to demonstrate the feasibility of faring across theLombok Strait on such a device, which he believes to have been done before 850 ka. The strait has maintained a width of at least 20 km for the whole of the Pleistocene. Such an achievement byHomo erectus in the Early Pleistocene offers some strength to the suggested water routes out of Africa, as theGibraltar,Sicilian, andBab-el-Mandeb exit routes are harder to consider if watercraft are deemed beyond the capacities ofHomo erectus.

Archaic humans in Europe beginning about 0.8 Ma (cleaver-producing Acheulean groups)are classified as a separate, erectus-derived species, known asHomo heidelbergensis.^[3]H. heidelbergensis from about 0.4 Ma develops its own characteristic industry, known asClactonian.H. heidelbergensis is closely related toHomo rhodesiensis (also identified asHomo heidelbergensis sensu lato or AfricanH. heidelbergensis), known to be present in southern Africa by 0.3 Ma.

Homo sapiens emerges in Africa before about 0.3 Ma from a lineage closely related to earlyH. heidelbergensis.^[29] The first wave of "Out of Africa II and "earliest presence ofH. sapiens in West Asia, may date to between .3 and 0.2 Ma,^[29] and ascertained for 0.13 Ma.^[30] Genetic research also indicates that a later migration wave ofH. sapiens (from .07-.05 Ma) from Africa is responsible for all to most of the ancestry of current non-African populations.^[31][32][33]

Most attention as to the route taken from Africa to West Asia is given to theLevantine land corridor and theBab-el-Mandeb straits. The latter separates the Horn of Africa and Arabia, and may have allowed dry passage during some periods of the Pleistocene. Another candidate is theStrait of Gibraltar. A route across theStrait of Sicily was suggested in the 1970s but is now considered unlikely.

The use by hominins of theLevantine corridor, connectingEgypt via theSinai peninsula with theEastern Mediterranean, has been associated to the phenomenon of rising and declining humidity of the desert belt of northern Africa, known as theSahara pump. The numerous hominin sites in the Levant, such asUbeidiya andMisliya cave, are used as indicators of this migration route.^{[citation needed]} As of 2012, the genetic analysis of human populations in Africa and Eurasia supports the concept that during thePaleolithic andMesolithic periods, this route was more important for bi-directional human migrations between Africa and Eurasia than was the Horn of Africa.^[34]

Bab-el-Mandeb is a 30 km strait between East Africa and the Arabian Peninsula, with a small island,Perim, 3 km off the Arabian bank. The strait has a major appeal in the study of Eurasian expansion in that it brings East Africa close to Eurasia. It does not require hopping from one water body to the next across the North African desert.^{[citation needed]}

The land connection with Arabia disappeared in thePliocene,^[35] and though it may have briefly reformed,^[when?][36] the evaporation of the Red Sea and associated increase in salinity would have left traces in the fossil record after just 200 years andevaporite deposits after 600 years. Neither have been detected.^[37] A strong current flows from the Red Sea into the Indian Ocean and crossing would have been difficult without a land connection.

Oldowan grade tools are reported from Perim Island,^[38] implying that the strait could have been crossed in the Early Pleistocene, but these finds have yet to be confirmed.^[39]

TheStrait of Gibraltar is the Atlantic entryway to the Mediterranean, where Spanish and Moroccan banks are only 14 km apart. A decrease insea levels in the Pleistocene due toglaciation would not have brought this down to less than 10 km. Deep currents push westwards, and surface water flows strongly back into the Mediterranean.Entrance into Eurasia across the strait of Gibraltar could explain the hominin remains atBarranco León in southeastern Spain (1.4 Ma)^[22] andSima del Elefante in northern Spain (1.2 Ma).^[40][41] But the site of Pirro Nord in southern Italy, allegedly from 1.3 – 1.7 Ma,^[23] suggests a possible arrival from the East. Resolution is insufficient to settle the matter.^[5]

Passage across theStrait of Sicily was suggested byAlimen (1975)^[42] based on the 1973 discovery of Oldowan grade tools in Sicily.^[43]Radiometric dates, however, have not been produced, and the artefacts might as well be from the Middle Pleistocene,^[44]and it is unlikely that there was a land bridge during the Pleistocene.^[4]: 3

For a given species in a given environment, available resources will limit the number of individuals that can survive indefinitely.This is thecarrying capacity. Upon reaching this threshold, individuals may find it easier to gather resources in the poorer yet less exploited peripheral environment than in the preferred habitat.Homo habilis could have developed some baseline behavioural flexibility prior to its expansion into the peripheries (such as encroaching into the predatory guild^[45][46]). This flexibility could then have been positively selected and amplified, leading toHomo erectus' adaptation to the peripheral open habitats.^[47] A new and environmentally flexible hominin population could have come back to the old niche and replaced the ancestral population.^[48] Moreover, some step-wise shrinking of the woodland and the associated reduction of hominin carrying capacity in the woods around 1.8 Ma, 1.2 Ma, and 0.6 Ma would have stressed the carrying capacity's pressure for adapting to theopen grounds.^[49][50] WithHomo erectus' new environmental flexibility, favourable climate fluxes likely opened it the way to the Levantine corridor, perhaps sporadically, in the Early Pleistocene.^[4] There isevidence that theMid-Pleistocene Revolution facilitated mammalian turnovers during the Late Early and Early Middle Pleistocene that may have included the hominin dispersals observed in thefossil record around this time.^[51]

Lithic analysis implies that Oldowan hominins were not predators.^[52] However,Homo erectus appears to have followed animal migrations to the north during wetter periods, likely as a source of scavenged food. The sabre-tooth catMegantereon was anapex predator of the Early and Middle Pleistocene (beforeMIS 12). It became extinct in Africa c. 1.5 Ma,^[53] but had already moved out through the Sinai, and is among the faunal remains of theLevantine hominin site ofUbeidiya, c. 1.4 Ma.^[27] It could not breakbone marrow and its kills were likely an important food source for hominins,^[54] especially in glacial periods.^[55] According to this hypothesis, hominins would also have successfully competed with thekleptoparasitic giant short-faced hyena,Pachycrocuta, for these carcasses.^[56] In colder Eurasian times, the hominin diet would have to be principally meat-based and Acheulean hunters must have competed with cats.^{[citation needed]}

Some papers have argued against this hypothesis, showing that the dispersals of hominins from Africa into Eurasia were asynchronous with those of other land mammals and that the latter was thus unlikely to be the cause of the former.^[57][58]

Bar-Yosef and Belfer-Cohen^[3] suggest that the success of hominins within Eurasia once out of Africa is in part due to the absence ofzoonotic diseases outside their original habitat. Zoonotic diseases are those that are transmitted from animals to humans. While a disease specific to hominins must keep its human host alive long enough to transmit itself, zoonotic diseases will not necessarily do so as they can complete their life cycle without humans. Still, these infections are well accustomed to human presence, having evolved alongside them. The higher an African ape's population density, the better a disease fares. 55% of chimps at theGombe reserve die of disease, most of them zoonotic.^[59] The majority of these diseases are still restricted to hot and damp African environments. When hominins moved out into drier and colder habitats of higher latitudes, one major limiting factor in population growth was removed.

WhileHomo habilis was certainly bipedal, its long arms are indicative of an arboreal adaptation.^[60]Homo erectus had longer legs and shorter arms, revealing a transition to obligate terrestriality, though it remains unclear how this change in relative leg length might have been an advantage.^[61] Sheer body size, on the other hand, seems to have allowed for better walkingenergy efficiency andendurance.^[62] A largerHomo erectus would alsodehydrate more slowly and could thus cover greater distances before facingthermoregulatory limitations.^[63] The ability for prolonged walking at a normal pace would have been a decisive factor for effective colonisation of Eurasia.^[64]

The appearance of early hominins in Eurasia coincided with a reduction in the diversity of the continent's carnivore guild. It has been postulated that this was related to the Oldowan-Acheulean transition, as the development of Acheulean technology signifies a change in human ecology from a passive, scavenging role to that of more active predation.^[65]

• Archaic humans in Southeast Asia
• Early human migrations
• Interbreeding between archaic and modern humans

• Antón, Susan C.; Swisher, Carl C. III (2004), "Early Dispersals ofHomo from Africa",Annual Review of Anthropology,33:271–296,doi:10.1146/annurev.anthro.33.070203.144024.
• Eudald Carbonell; Marina Mosquera; Xosé Pedro Rodríguez; José María Bermúdez de Castro; Francesc Burjachs; Jordi Rosell; Robert Sala; Josep Vallverdú (2008), "Eurasian Gates: The Earliest Human Dispersals",Journal of Anthropological Research,64 (2),(subscription required):195–228,doi:10.3998/jar.0521004.0064.202,JSTOR 20371223,S2CID 85227705.
• Ciochon, Russell L. (2010), "Divorcing Hominins from theStegodon–Ailuropoda Fauna: New Views on the Antiquity of Hominins in Asia", in John G. Fleagle; et al. (eds.),Out of Africa I: The First Hominin Colonization of Eurasia, Vertebrate Paleobiology and Paleoanthropology Series, Dordrecht: Springer, pp. 111–126,doi:10.1007/978-90-481-9036-2_8,ISBN 978-90-481-9035-5.ISBN 978-90-481-9036-2 (online).
• Dennell, Robin (2009),The Palaeolithic Settlement of Asia, Cambridge World Archaeology, Cambridge: Cambridge University Press,ISBN 978-0-521-84866-4.ISBN 978-0-521-61310-1 (paperback).
• Dennell, Robin (2010), "'Out of Africa I': Current Problems and Future Prospects", in John G. Fleagle; et al. (eds.),Out of Africa I: The First Hominin Colonization of Eurasia, Vertebrate Paleobiology and Paleoanthropology Series, Dordrecht: Springer, pp. 247–274,doi:10.1007/978-90-481-9036-2_15,ISBN 978-90-481-9035-5,OCLC 762628720.
• Rabett, Ryan J. (2012),Human Adaptation in the Asian Palaeolithic: Hominin Dispersal and Behaviour during the Late Quaternary, Cambridge: Cambridge University Press,ISBN 978-1-107-01829-7.
• Zaim, Yahdi (2010), "Geological Evidence for the Earliest Appearance of Hominins in Indonesia", in John G. Fleagle; et al. (eds.),Out of Africa I: The First Hominin Colonization of Eurasia, Vertebrate Paleobiology and Paleoanthropology Series, Dordrecht: Springer, pp. 97–110,doi:10.1007/978-90-481-9036-2_7,ISBN 978-90-481-9035-5.ISBN 978-90-481-9036-2 (online).

• Archaeological theory
• Homo erectus
• Peopling of the world
• Prehistoric migrations
• History of Africa

• Webarchive template wayback links
• Articles with short description
• Short description matches Wikidata
• All articles with unsourced statements
• Articles with unsourced statements from February 2016
• Articles with unsourced statements from January 2020
• Articles with unsourced statements from March 2016
• All articles with vague or ambiguous time
• Vague or ambiguous time from September 2018
• Pages containing links to subscription-only content