Early expansions of hominins out of Africa
Several expansions of populations of archaic humans (genus Homo) out of Africa and throughout Eurasia took place in the course of the Lower Paleolithic, and into the beginning Middle Paleolithic, between about 2.1 million and 0.2 million years ago (Ma). These expansions are collectively known as Out of Africa I, in contrast to the expansion of Homo sapiens (anatomically modern humans) 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 of Homo (or indeed any hominin) outside of Africa dates to close to 2 million years ago. A 2018 study claims human presence at Shangchen, central China, as early as 2.12 Ma based on magnetostratigraphic dating of the lowest layer containing stone artefacts.[2] The oldest known human skeletal remains outside of Africa are from
Later waves of expansion are proposed around 1.4 Ma (early
Until the early 1980s, early humans were thought to have been restricted to the African continent in the Early 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]
Early dispersals
Pre-Homo hominin expansion out of Africa is suggested by the presence of Graecopithecus and Ouranopithecus, found in Greece and Anatolia and dated to c. 8 million years ago, but these are probably Homininae but not Hominini. Possibly related are the Trachilos 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]
The earliest known hominin presence outside of Africa dates to close to 2 million years ago. A 2018 study claims evidence for human presence at Shangchen, central China, as early as 2.12 Ma based on magnetostratigraphic dating of the lowest layer containing stone artefacts.[2]
It has been suggested that
Homo erectus
Homo erectus emerges just after 2 million years ago.[11]
Early H. erectus would have lived face to face with H. habilis in East Africa for nearly half a million years.[12]
The oldest Homo erectus fossils appear almost contemporaneously, shortly after two million years ago, both in Africa and in the Caucasus.
The earliest well-dated Eurasian H. erectus site is Dmanisi 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 this Homo 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 African H. erectus, dubbed Homo ergaster, is a
single occipital bone (KNM-ER 2598), described as "H. erectus-like", and dated to about 1.9 Ma (contemporary with
Ferring et al. (2011) suggest that it was still Homo habilis that reached West Asia, and that early H. 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 appears H. erectus took longer to move into Europe, the earliest site being Barranco León in southeastern Spain dated to 1.4 Ma, associated with Homo antecessor,[22] and a controversial Pirro Nord in Southern Italy, allegedly from 1.7 – 1.3 Ma.[23] The paleobiogeography of early human dispersals in western Eurasia characterizes H. ex gr. erectus as a temperature sensitive stenobiont, that failed to disperse north of the Alpide Belt.[24] The geographically restricted earliest human presence in the Iberian Peninsula should be regarded as evidence of a sustainable presence of human population in this isolated area. The Pannonian plain, situated south-west of the Carpathian Mountains, was apparently characterized by a comparatively warm climate similar to that of the Mediterranean Area, while the climate of the western European paleobiogeographic area was mitigated by Gulf Stream influence and could support the episodic hominin dispersals toward the Iberian 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 the Bosporus and the Manych Strait, the climate barrier from the north of the Greater Caucasus range, and the 41 kyr glacial Milankovitch cycles that repeatedly closed the Bosporus and thus triggered the two-way faunal exchange between southeastern Europe and the Near 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 the 50th 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. Early Acheulean tools at Ubeidiya 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 of Lower Paleolithic human remains in Indonesian islands is good evidence for seafaring by Homo erectus late in the Early 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 the Lombok 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 by Homo erectus in the Early Pleistocene offers some strength to the suggested water routes out of Africa, as the Gibraltar, Sicilian, and Bab-el-Mandeb exit routes are harder to consider if watercraft are deemed beyond the capacities of Homo erectus.
Homo heidelbergensis
Archaic humans in Europe beginning about 0.8 Ma (
Routes out of Africa
Most attention as to the route taken from Africa to West Asia is given to the Levantine land corridor and the Bab-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 the Strait of Gibraltar. A route across the Strait of Sicily was suggested in the 1970s but is now considered unlikely.
Levantine corridor
The use by hominins of the
Horn of Africa to Arabia (Bab el-Mandeb)
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 the Pliocene,[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 and evaporite 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]
Strait of Gibraltar
The
Strait of Sicily
Passage across the Strait of Sicily was suggested by Alimen (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
Causes for dispersal
Climate change and hominin flexibility
For a given species in a given environment, available resources will limit the number of individuals that can survive indefinitely. This is the carrying 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 to Homo 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 the open grounds.[49][50] With Homo erectus' new environmental flexibility, favourable climate fluxes likely opened it the way to the Levantine corridor, perhaps sporadically, in the Early Pleistocene.[4] There is evidence that the Mid-Pleistocene Revolution facilitated mammalian turnovers during the Late Early and Early Middle Pleistocene that may have included the hominin dispersals observed in the fossil record around this time.[51]
Chasing fauna
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]
Coevolved zoonotic diseases
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 of zoonotic 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 the Gombe reserve die of disease, most of them zoonotic.[59] The majority of these diseases are still restricted to hot and damp African environments. Once hominins had moved out into drier and colder habitats of higher latitudes, one major limiting factor in population growth was out of the equation.
Physiological traits
While Homo 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 walking energy efficiency and endurance.[62] A larger Homo erectus would also dehydrate more slowly and could thus cover greater distances before facing thermoregulatory limitations.[63] The ability for prolonged walking at a normal pace would have been a decisive factor for effective colonisation of Eurasia.[64]
See also
References
- ^ The term "Out of Africa I" is informal and somewhat rare.
The phrase Out of Africa used on its own generally refers to "Out of Africa II", the expansion of anatomically modern humans into Eurasia.
"Out of Africa I" is used in 2004, in Marco Langbroek, 'Out of Africa': an investigation into the earliest occupation of the Old World, p. 61,
and as the title of a collection of essays, J. G. Fleagle et al. (eds.), Out of Africa I: The First Hominin Colonization of Eurasia (2010).
see also: Herschkovitz, Israel; et al. (26 January 2018). "The earliest modern humans outside Africa". PMID 29371468.; Hurtley, Stella; Szuromi, Phil (2005). "Out of Africa Revisited". Science. 308 (5724): 922.S2CID 220100436.
- ^ S2CID 49670311. "Eight major magnetozones are recorded in the Shangchen section, four of which have normal polarity (N1 to N4) and four of which have reversed polarity (R1 to R4). By comparison with the geomagnetic polarity timescale [...] magnetozone N4 corresponds to the Réunion excursion (2.13–2.15 Ma) in L28."
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Further reading
- Antón, Susan C.; Swisher, Carl C. III (2004), "Early Dispersals of Homo from Africa", Annual Review of Anthropology, 33: 271–296, .
- 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, S2CID 85227705.
- Ciochon, Russell L. (2010), "Divorcing Hominins from the Stegodon–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, ISBN 978-90-481-9036-2(online).
- Dennell, Robin (2009), The Palaeolithic Settlement of Asia, Cambridge World Archaeology, Cambridge: Cambridge University Press, ISBN 978-0-521-61310-1(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, 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, ISBN 978-90-481-9036-2(online).