Homo erectus

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Homo erectus Temporal range: 1.9–0.1Ma PreЄ Є O S D C P T J K Pg N ↓ Pleistocene
Reconstruction of a specimen from Tautavel, France
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Primates
Family:	Hominidae
Genus:	Homo
Species:	H. erectus
Binomial name
†Homo erectus (Dubois, 1892)
Synonyms
† Pithecanthropus erectus † Sinanthropus pekinensis † Javanthropus soloensis † Meganthropus paleojavanicus † Telanthropus capensis † Homo ergaster?

Homo erectus (meaning "upright man," from the Latin ērigere, "to put up, set upright") is an extinct species of hominin that lived throughout most of the Pleistocene, with the earliest first fossil evidence dating to around 1.9 million years ago and the most recent to around 143,000 years ago. The species originated in Africa and spread as far as Georgia, India, Sri Lanka, China and Java.^[1][2]
There is still disagreement on the subject of the classification, ancestry, and progeny of H. erectus, with two major alternative classifications: erectus may be another name for Homo ergaster, and therefore the direct ancestor of later hominids such as Homo heidelbergensis, Homo neanderthalensis, and Homo sapiens; or it may be an Asian species distinct from African ergaster.^[1][3][4]

Some palaeoanthropologists consider H. ergaster to be simply the African variety of H. erectus. This leads to the use of the term "Homo erectus sensu stricto" for the Asian H. erectus, and "Homo erectus sensu lato" for the larger species comprising both the early African populations (H. ergaster) and the Asian populations.^[5][6]

Origin

Homo erectus, Natural History Museum, Ann Arbor, Michigan

The first hypothesis is that H. erectus migrated from Africa during the Early Pleistocene, possibly as a result of the operation of the Saharan pump, around 2.0 million years ago, and it dispersed throughout much of the Old World. Fossilized remains 1.8 to 1 million years old have been found in Africa (e.g., Lake Turkana^[7] and Olduvai Gorge), Georgia, Indonesia (e.g., Sangiran in Central Java and Trinil in East Java), Vietnam, China (e.g., Shaanxi) and India.^[8]

The second hypothesis is that H. erectus evolved in Eurasia and then migrated to Africa. The species occupied a Caucasus site called Dmanisi, in Georgia, from 1.85 million to 1.77 million years ago, at the same time or slightly before the earliest evidence in Africa. Excavations found 73 stone tools for cutting and chopping and 34 bone fragments from unidentified creatures.^[9][10]

Discovery and representative fossils

Skulls of
1. Gorilla 2. Australopithecus 3. Homo erectus 4. Neanderthal (La Chapelle aux Saints) 5. Steinheim Skull 6. Modern Homo sapiens

The Dutch anatomist Eugène Dubois, who was especially fascinated by Darwin's theory of evolution as applied to man, set out to Asia (the place accepted then, despite Darwin, as the cradle of human evolution), to find a human ancestor in 1886. In 1891, his team discovered a human fossil on the island of Java, Dutch East Indies (now Indonesia); he described the species as Pithecanthropus erectus (from the Greek πίθηκος,^[11] "ape", and ἄνθρωπος,^[12] "man"), based on a calotte (skullcap) and a femur like that of H. sapiens found from the bank of the Solo River at Trinil, in East Java. (This species is now regarded as H. erectus).

The find became known as Java Man. Thanks to Canadian anatomist Davidson Black's (1921) initial description of a lower molar, which was dubbed Sinanthropus pekinensis,^[13] however, most of the early and spectacular discoveries of this taxon took place at Zhoukoudian in China. German anatomist Franz Weidenreich provided much of the detailed description of this material in several monographs published in the journal Palaeontologica Sinica (Series D).

Nearly all of the original specimens were lost during World War II; however, authentic Weidenreichian casts do exist at the American Museum of Natural History in New York and at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, and are considered to be reliable evidence.

Throughout much of the 20th century, anthropologists debated the role of H. erectus in human evolution. Early in the century, however, due to discoveries on Java and at Zhoukoudian, it was believed that modern humans first evolved in Asia. A few naturalists (Charles Darwin most prominent among them) predicted that humans' earliest ancestors were African: he pointed out that chimpanzees and gorillas, who are human relatives, live only in Africa.^[14]

From the 1950s to 1970s, however, numerous fossil finds from East Africa yielded evidence that the oldest hominins originated there. It is now believed that H. erectus is a descendant of earlier genera such as Ardipithecus and Australopithecus, or early Homo-species such as H. habilis or H. ergaster. H. habilis and H. erectus coexisted for several thousand years, and may represent separate lineages of a common ancestor.^[15]

Archaeologist John T. Robinson and Robert Broom named Telanthropus capensis in the 1950s, now thought to belong to Homo erectus.^[16] Robinson discovered a jaw fragment, SK 45, in September 1949 in Swartkrans, South Africa. In 1957, Simonetta proposed to re-designate it Homo erectus, and Robinson (1961) agreed.^[17]

Fossils skull D2700 and D2735 jaw, two of several found in Dmanisi in the Georgian Caucasus

The skull of Tchadanthropus uxoris, discovered in 1961 by Yves Coppens in Chad, is the earliest fossil human discovered in the North of Africa.^[18] This fossil "had been so eroded by wind-blown sand that it mimicked the appearance of an australopith, a primitive type of hominid".^[19] Though some first considered it to be a specimen of H. habilis,^[20] it is no longer considered to be a valid taxon, and scholars rather consider it to represent H. erectus.^[18][21]

Homo erectus georgicus

Homo erectus georgicus (Georgian: ქართველი ადამიანი) is the subspecies name sometimes used to describe fossil skulls and jaws found in Dmanisi, Georgia. Although first proposed as a separate species, it is now classified within H. erectus.^[22][23][24] A partial skeleton was discovered in 2001. The fossils are about 1.8 million years old. The remains were first discovered in 1991 by Georgian scientist, David Lordkipanidze, accompanied by an international team that unearthed the remains.
There have been many proposed explanations of the dispersion of H. erectus georgicus.^[25] Implements and animal bones were found alongside the ancient human remains.

At first, scientists thought they had found mandibles and skulls belonging to Homo ergaster, but size differences led them to name a new species, Homo georgicus, which was posited as a descendant of Homo habilis and ancestor of Asian Homo erectus. This classification was not upheld, and the fossil is now considered a divergent subgroup of Homo erectus, sometimes called Homo erectus georgicus.^{[26][27][28][29]}

Location of Dmanisi discovery, Georgia

At around 600 cubic centimetres (37 cu in) brain volume, the skull D2700 is dated to 1.77 million years old and in good condition, offering insights in comparison to the modern human cranial morphology. At the time of discovery the cranium was the smallest and most primitive Hominina skull ever discovered outside of Africa. However, in 2003 a significantly smaller brained hominid was found on the isle of Flores, H. floresiensis. Homo erectus georgicus exhibits strong sexual dimorphism with males being significantly larger than females.

Subsequently, four fossil skeletons were found, showing a species primitive in its skull and upper body but with relatively advanced spines and lower limbs, providing greater mobility. They are now thought not to be a separate species, but to represent a stage soon after the transition between Homo habilis and H. erectus, and have been dated at 1.8 million years before the present, according to the leader of the project, David Lordkipanidze.^[23][30] The assemblage includes one of the largest Pleistocene Homo mandibles (D2600), one of the smallest Lower Pleistocene mandibles (D211), a nearly complete sub‐adult (D2735), and a completely toothless specimen (D3900).^[31]

A further skull, the only intact skull ever found of an early Pleistocene hominin, was described in 2013.^[32] At just under 550 cubic centimetres, the skull had the smallest braincase of all the individuals found at the site. The variations in these skulls prompted the researchers to examine variations in modern human and chimpanzees. The researchers found that while the Dmanisi skulls looked different from one another, the variations were no greater than those seen among modern people and among chimpanzees. These variations therefore suggest that previous fossil finds thought to be of different species on the basis of their variations, such as Homo rudolfensis, Homo gautengensis, H. ergaster and potentially H. habilis, may be alternatively interpreted as belonging to the same lineage as Homo erectus.^[33]

Classification and special distinction

A reconstruction of Homo erectus (reconstruction shown in Westfälisches Landesmuseum, Herne, Germany, in a 2006 exhibition)

Many paleoanthropologists still debate the definition of H. erectus and H. ergaster as separate species. Several scholars suggested dropping the taxon Homo erectus and instead equating H. erectus with the archaic H. sapiens.^{[34][35][36][37]} Some call H. ergaster the direct African ancestor of H. erectus, proposing that it emigrated out of Africa and immigrated to Asia, branching into a distinct species.^[38] Most dispense with the species name ergaster, making no distinction between such fossils as the Turkana Boy and Peking Man.^{[citation needed]} Although "Homo ergaster" has gained some acceptance as a valid taxon, these two are still usually defined as distinct African and Asian populations of the larger species H. erectus.

While some have argued (and insisted) that Ernst Mayr's biological species definition cannot be used here to test the above hypotheses, one can, however, examine the amount of morphological cranial variation within known H. erectus / H. ergaster specimens, and compare it to what one sees in disparate extant groups of primates with similar geographical distribution or close evolutionary relationship. Thus, if the amount of variation between H. erectus and H. ergaster is greater than what one sees within a species of, say, macaques, then H. erectus and H. ergaster may be considered two different species.

The extant model of comparison is very important, and selecting appropriate species can be difficult. (For example, the morphological variation among the global population of H. sapiens is small,^[39] and our own special diversity may not be a trustworthy comparison). As an example, fossils found in Dmanisi in the Republic of Georgia were originally described as belonging to another closely related species, Homo georgicus, but subsequent examples showed their variation to be within the range of Homo erectus, and they are now classified as Homo erectus georgicus.

H. erectus had a cranial capacity greater than that of Homo habilis (although the Dmanisi specimens have distinctively small crania): the earliest remains show a cranial capacity of 850 cm³, while the latest Javan specimens measure up to 1100 cm³,^[39] overlapping that of H. sapiens.; the frontal bone is less sloped and the dental arcade smaller than the australopithecines'; the face is more orthognatic (less protrusive) than either the australopithecines' or H. habilis's, with large brow-ridges and less prominent zygomata (cheekbones). These early hominins stood about 1.79 m (5 ft 10 in),^[40] (Only 17 percent of modern male humans are taller)^[41] and were extraordinarily slender, with long arms and legs.^[42]

The sexual dimorphism between males and females was slightly greater than seen in H. sapiens, with males being about 25% larger than females, but less than that of the earlier Australopithecus genus. The discovery of the skeleton KNM-WT 15000, "Turkana boy" (Homo ergaster), made near Lake Turkana, Kenya by Richard Leakey and Kamoya Kimeu in 1984, is one of the most complete hominid-skeletons discovered, and has contributed greatly to the interpretation of human physiological evolution.

For the remainder of this article, the name Homo erectus will be used to describe a distinct species for the convenience of continuity.

Use of tools

Homo ergaster used more diverse and sophisticated stone tools than its predecessors. H. erectus, however, used comparatively primitive tools. This is possibly because H. ergaster first used tools of Oldowan technology and later progressed to the Acheulean^[43] while the use of Acheulean tools began ca. 1.8 million years ago,^[44] the line of H. erectus diverged some 200,000 years before the general innovation of Acheulean technology. Thus the Asian migratory descendants of H. ergaster made no use of any Acheulean technology. In addition, it has been suggested that H. erectus may have been the first hominid to use rafts to travel over oceans.^[45]

Use of fire

East African sites, such as Chesowanja near Lake Baringo, Koobi Fora, and Olorgesailie in Kenya, show some possible evidence that fire was utilized by early humans. At Chesowanja, archaeologists found red clay sherds dated to be 1.42 Mya.^[46] Reheating on these sherds show that the clay must have been heated to 400 °C (752 °F) to harden. At Koobi Fora, sites FxJjzoE and FxJj50 show evidence of control of fire by Homo erectus at 1.5 Mya, with the reddening of sediment that can only come from heating at 200–400 °C (392–752 °F).^[46] A "hearth-like depression" exists at a site in Olorgesailie, Kenya. Some microscopic charcoal was found, but it could have resulted from a natural brush fire.^[46] In Gadeb, Ethiopia, fragments of welded tuff that appeared to have been burned were found in Locality 8E, but re-firing of the rocks may have occurred due to local volcanic activity.^[46] These have been found amongst H. erectus–created Acheulean artifacts. In the Middle Awash River Valley, cone-shaped depressions of reddish clay were found that could have been created by temperatures of 200 °C (392 °F). These features are thought to be burned tree stumps such that they would have fire away from their habitation site.^[46] Burnt stones are also found in the Awash Valley, but volcanic welded tuff is also found in the area.

A site at Bnot Ya'akov Bridge, Israel, has been claimed to show that H. erectus or H. ergaster made fires between 790,000 and 690,000 BP.^[47] To date this has been the most widely accepted claim, although recent reanalysis of burnt bone fragments and plant ashes from the Wonderwerk Cave have sparked claims of evidence supporting human control of fire by 1 Ma.^[48]

Cooking

There is no archaeological evidence that Homo erectus cooked their food. The idea has been suggested,^[49] but is not generally accepted.^[50][51] It is known, from the study of microwear on handaxes, that meat formed a major part of the erectus diet. Meat is digestible without cooking, and is sometimes eaten raw by modern humans. Nuts, berries, fruits are also edible without cooking. Thus cooking cannot be presumed: the issue rests on clear evidence from archaeological sites, which at present does not exist.

Sociality

Homo erectus was probably the first hominid to live in a hunter-gatherer society, and anthropologists such as Richard Leakey believe that it was socially more like modern humans than the more Australopithecus-like species before it. Likewise, increased cranial capacity generally coincides with the more sophisticated tools occasionally found with fossils.

The discovery of Turkana boy (H. ergaster) in 1984 gave evidence that, despite its Homo-sapiens-like anatomy, it may not have been capable of producing sounds comparable to modern human speech. Ergaster likely communicated in a proto-language lacking the fully developed structure of modern human language but more developed than the non-verbal communication used by chimpanzees.^[52] Such inference has been challenged by the discovery of H. ergaster/erectus vertebrae some 150,000 years older than the Turkana Boy in Dmanisi, Georgia, that reflect vocal capabilities within the range of H. sapiens.^[53] Both brain-size and the presence of the Broca's area also support the use of articulate language.^[54]

H. erectus was probably the first hominid to live in small, familiar band-societies similar to modern hunter-gatherer band-societies.^[55] H. erectus/ergaster is thought to be the first hominid to hunt in coordinated groups, use complex tools, and care for infirm or weak companions.

There has been some debate as to whether H. erectus, and possibly the later Neanderthals,^[56] may have interbred with anatomically modern humans in Europe and Asia. See Neanderthal admixture theory.^[57]

Descendants and subspecies

Homo erectus remains one of the most long-lived species of Homo, having existed over a million years, while Homo sapiens so far has existed for 200,000 years. As a distinct Asian species, however, no consensus has been reached as to whether it is ancestral to H. sapiens or any later hominids.

A model of the face of an adult female Homo erectus. Reconstruction by John Gurche, Smithsonian Museum of Natural History, based on KNM ER 3733 and 992.

• Homo erectus
  • Homo erectus erectus
  • Homo erectus yuanmouensis
  • Homo erectus lantianensis
  • Homo erectus nankinensis
  • Homo erectus pekinensis
  • Homo erectus palaeojavanicus
  • Homo erectus soloensis
  • Homo erectus tautavelensis
  • Homo erectus georgicus

Related species

• Homo ergaster
• Homo floresiensis
• Homo antecessor
• Homo heidelbergensis
• Homo sapiens
  • Homo sapiens idaltu
  • Homo sapiens sapiens
  • Homo sapiens neanderthalensis
• Homo rhodesiensis
• Homo cepranensis

Previously referred taxa

• Homo erectus wushanensis (actually a stem-orangutan)

The discovery of Homo floresiensis in 2003 and of the recentness of its extinction has raised the possibility that numerous descendant species of Homo erectus may have existed in the islands of Southeast Asia and await fossil discovery (see Orang Pendek). Homo erectus soloensis, who was long assumed to have lived on Java at least as late as about 50,000 years ago but was re-dated in 2011 to a much higher age,^[58] would be one of them. Some scientists are skeptical of the claim that Homo floresiensis is a descendant of Homo erectus. One explanation holds that the fossils are of a modern human with microcephaly, while another one holds that they are from a group of pygmies.

Individual fossils

Original fossils of Pithecanthropus erectus (now Homo erectus) found in Java in 1891.

Some of the major Homo erectus fossils:

• Indonesia (island of Java): Trinil 2 (holotype), Sangiran collection, Sambungmachan collection,^[59] Ngandong collection
• China ("Peking Man"): Lantian (Gongwangling and Chenjiawo), Yunxian, Zhoukoudian, Nanjing, Hexian
• Kenya: ER 3883, ER 3733
• Vértesszőlős, Hungary "Samu"
• Vietnam: Northern, Tham Khuyen,^[60] Hoa Binh^{[citation needed]}
• Republic of Georgia: Dmanisi collection ("Homo erectus georgicus")
• Ethiopia: Daka calvaria
• Eritrea: Buia cranium (possibly H. ergaster)^[61]
• Denizli Province, Turkey: Kocabas fossil^[62]

Gallery

Homo erectus tautavelensis skull.  Replica of lower jaws of Homo erectus from Tautavel, France.  Calvaria "Sangiran II" Original, Collection Koenigswald, Senckenberg Museum.  A reconstruction based on evidence from the Daka Member, Ethiopia.

Out of Africa I

 

From Wikipedia, the free encyclopedia
 

In paleoanthropology, Out of Africa I is the first hominin expansion into Eurasia, taking place between 1.8 and 0.8 million years ago. It is thought that Homo erectus developed a flexible adaptation to the open grounds, descending from the older Homo habilis lineage, which was strictly adapted to the dense woodlands. Such an adaptation would have allowed Homo erectus to leave Africa and expand its range into Eurasia. According to the recent African origin of modern humans hypothesis (Out of Africa II), the first hominin expansion out of Africa is followed by the dispersal into Eurasia and replacement of these previous hominins by anatomically modern humans, starting about 100,000 years ago. Without further specification, Out of Africa is usually held to mean Out of Africa II, the expansion of modern humans into Eurasia.

Movements out of Africa by early hominins seem to have occurred in at least three waves. Primitive chopper producers were first out by c. 1.8 Ma, followed by early Acheulean industries c. 1.4 Ma, and various cleaver-producing Acheulean groups around 0.8 Ma.^[1]

Until the early 1980s, hominins were assumed to have been restricted to the African continent for the whole of the Early Pleistocene (until about 0.8 Ma), and so much archaeological effort has disproportionately focused on Africa. Compounded with hominins probably being rare out of East Africa in the Early Pleistocene,^[2] we are left with a sequence of events broken in space and time.

Sites

Hominin sites are oldest in East Africa. The earliest evidence for retouched tools are from Kada Gona, Ethiopia, dating back to 2.6 – 2.5 Ma in the very early Pleistocene. They might be the product of Australopithecus garhi or Paranthropus aethiopicus, the two known hominins contemporary with the tools.^[3]

Homo habilis is the first member of the Homo line and could have descended from the Australopithecus as early as 2.3 Ma; it is first attested in Lake Turkana, Kenya. Homo erectus seems to appear a little later, its earliest remains dating to c. 1.9 – 1.6 Ma, from Koobi Fora, Kenya.^[4] The two species would have lived face to face in East Africa for nearly half a million years.^[5]

Dmanisi in Georgia, the earliest hominin site out of Africa, dates back to 1.81 Ma^[6]

Well before Homo habilis disappeared (c. 1.4 Ma), Homo erectus had made it into Eurasia. The earliest well-dated Eurasian site is that of Dmanisi in Georgia, and is securely dated to 1.81 Ma.^[6]
There, some evidence of caring for the old was found. The skull of an old Homo erectus had lost all but one teeth years before his death, and is perhaps unlikely to have survived on his own (but such caring is not yet certain – a partially paralysed chimpanzee at the Gombe reserve survived for years without help^[7]).

Early Pleistocene sites in North Africa, the geographical intermediate of East Africa and Georgia, are in poor stratigraphic context. The earliest of the dated is Ain Hanech in northern Algeria (c. 1.8^[8] – 1.2 Ma^[9]), an Oldowan grade layer. It attests that early hominins have crossed the northern African tracts, which are usually hot and dry. Hominins were part of the East African biome, and so a flux in climate could have momentarily expanded their environment, giving them the chance to move north.
There is very little time between Homo erectus’ apparent arrival in South Caucasus, and its probable arrival in East and Southeast Asia. There is evidence of hominins in Yuanmou, China, dating to 1.7 Ma and in Sangiran, on Java, Indonesia, dating to 1.66 Ma.^[10] It appears hominins took longer to move into Europe, with the earliest site in Barranco León in southeastern Spain dated to 1.4 Ma^[11] and a more controversial Pirro Nord in southern Italy, allegedly from 1.3 – 1.7 Ma.^[12]

In any case, by 1 Ma, hominins had settled in most of the Old World. In Western Europe, it is hard to say, however, if settlement was continuous or if successive waves repopulated the territory in glacial interludes. Early Acheulean tools were present at Ubeidiya by 1.4 Ma^[13] and it seems likely that successive waves out of Africa after then would have brought Acheulean technology to Western Europe, but handaxes, which are typical of the Acheulean industry, are absent in early Western European sites.

Routes out of Africa

Sinai Peninsula

The Sinai Peninsula should be the African exit route par excellence, being since the Pliocene the only land bridge between the two continents of the Old World. As detailed below, unless we argue for boats on behalf of Homo erectus, it is surely the only way out. However, it was hard to access until the Middle Pleistocene. The Nile followed a different and pitiful course.

There are two Eurasian entryways that take advantage of the Sinai. First, the Levantine corridor, which moves north along the Eastern Mediterranean. Second, down the eastern bank of the Red Sea. Archaeological efforts in Arabia is limited, and attention is usually given to the Levantine corridor.

Due to the presence of Eurasian sites securely dated to the Early Pleistocene, to reject the Sinai is effectively to affirm that early hominins crossed straits, and this hypothesis has problems of its own.

Bab-el-Mandeb

The Bab-el-Mandeb is a 30 km strait parting East Africa from 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 in direct proximity with Eurasia. It doesn’t require hopping from water body to the next across the North African desert.

The land connection with Arabia has disappeared in the Pliocene,^[14] and though it may have reformed momentarily,^[15] 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.^[16] 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,^[17] implying that the strait could have been crossed in the Early Pleistocene, though these finds have yet to be confirmed.^[18]

Strait of Gibraltar

The Strait of Gibraltar is the Atlantic entryway to the Mediterranean, where Spanish and Moroccan banks are only 14 km apart. A decrease in sea levels in the Pleistocene due to glaciation would not have brought this down to less than 10 km. Water treadmills at Gibraltar. Deep currents push westwards, and surface water flows strongly back into the Mediterranean. The current would likely lose a swimmer or an unsteered raft.

Entrance into Eurasia across the strait of Gibraltar could explain the hominin remains at Barranco León in southeastern Spain dated to 1.4 Ma^[11] and Sima del Elefante in northern Spain dated to 1.2 Ma.^[19][20] But the site of Pirro Nord in southern Italy, allegedly from 1.3 – 1.7 Ma,^[12] suggests a possible arrival from the East. Resolution is insufficient to settle the matter.^[21]

Strait of Sicily

The modern Strait of Sicily separates Tunisia and Sicily by 145 km, but is shallow and would have been much narrower in glacial maxima. We have a poor understanding of plate tectonics of this area for the greater part of the Pleistocene. But while plate tectonics could have made the strait narrower than predicted by the lowering of sea levels alone, contrast of Pleistocene fauna strongly argues against an actual land bridge. Since the strait is only 400 km away from the North African hominin site of Ain Hanech in Algeria, dating to 1.8 Ma^[8] or 1.2 Ma,^[9] it remains a plausible route for Early Pleistocene expansion into Eurasia. But there is close to no evidence for an actual passing. Alimen based most of his argument^[22] in favour of such migration on Bianchini’s discovery^[23] of Sicilian Oldowan grade tools. But radiometric dates have not been produced, and the artefacts might as well be from the Middle Pleistocene.^[24]

Crossing straits

Presence of hominin 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.^[25] He has reproduced a primitive dirigible 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 boats are deemed beyond the capacities of Homo erectus.

It might be tempting to consider a one-off event getting a few hominins across a strait – perhaps an Homo erectus family drifting on flood debris to land on a Eurasian bank. But successful population of Eurasia by such a beginning is unlikely. There are biological constraints to the minimum size a population must maintain to avoid extinction. That is to say, if less than 50 hominins at once made it into Eurasia and loss contact with African hominins, the population would likely undergo an extinction vortex, in part due to inbreeding.^[26]

Causes for hominin dispersals

Climate change and hominin flexibility

For a given species in a given environment, available resources will limit the amount 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 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^[27][28]). This flexibility could then have been positively selected and amplified, leading to Homo erectus’ adaptation to the peripheral open habitats.^[29] A new and more flexible hominin population could have come back to the old niche and replace the ancestral population.^[30] 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.^[31][32]

With Homo erectus’ new environmental flexibility, it is likely that it saw favourable climate fluxes open it the way to the Levantine corridor at least sporadically in the Early Pleistocene.^[2]

Chasing fauna

Lithic analysis implies that Oldowan hominins weren't predators.^[33] 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 cat Megantereon was an apex predator of the Early and Middle Pleistocene (before MIS 12). It went extinct in Africa c. 1.5 Ma,^[34] but had already moved out through the Sinai, and is among the faunal remains of the Levantine hominin site of Ubeidiya, c. 1.4 Ma.^[13] It couldn’t break bone marrow and its kills were likely an important food source for hominins,^[35] especially in glacial periods.^[36]

In colder Eurasian times, the hominin diet would have to be principally meat-based and Acheulean hunters must have competed with cats.

Coevolved zoonotic diseases

Bar-Yosef and Cohen^[1] 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 won't 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. Fifty-five percent of chimps at the Gombe reserve die of disease, most of them zoonotic.^[37] The majority of these diseases are still restricted to hot and damp African environments.
Once hominins had moved out into dryer and colder habitats of higher latitudes, one major limiting factor in population growth was out of the equation.

Hominin biology

Homo habilis reconstruction

While Homo habilis was certainly bipedal, its long arms are indicative of an arboreal adaptation.^[38] 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.^[39] Sheer body size, on the other hand, seems to have allowed for better walking energy efficiency and endurance.^[40] A larger Homo erectus would also dehydrate more slowly and could thus cover greater distances before facing thermoregulatory limitations.^[41] The ability for prolonged walking at a normal pace would have been a decisive factor for effective colonisation of Eurasia.^[42]

Brain thermoregulation

Homo erectus reconstruction

Thermoregulation and dehydration are major problems that need to be dealt with to move into the open grasslands. In particular, vascularisation of the brain is crucial in maintaining it in the narrow frame of tolerable temperatures.

Bones of the higher cranium grow in response to expansion of cerebral mass, in such a way that brain tissue and blood vessels mold the inner brain case. Endocranial casts of fossil skulls allow
approximating brain vascularisation.^[43] Dean Falk noticed that a single large vessel, the occipital marginal sinus, was responsible for irrigating most of the brain in early australopiths
(Australopithecus afarensis, Paranthropus robustus and boisei).^[44] The vessel grew smaller with time so as to be progressively replaced by a network of small veins in later hominins, starting with Homo habilis and continuing well into Eurasia. She interprets the change as an adaptation to cool the brain,^[45] which she uses to advance her “radiator theory” for accelerated encephalisation from Homo habilis onwards.^[46] To Falk, bipedalism, which predates large brains, favoured a rewiring of cerebral blood vessels into a gravity-assisted irrigation network, itself allowing the cool down needed for encephalisation.

Endocranial casts of Homo habilis and Homo erectus differ in the organisation of the frontal lobe, in particular in the prefrontal cortex where higher mental functions of consciousness and abstraction occur.^[47] By themselves, mental capacities have likely played a role in the success of Eurasian colonisation. They would have allowed for greater social complexity,^[48] predation and sharing prey,^[49] and an overall higher quality diet.^[50] If we are to believe Bednarik and his seafaring Indonesian Homo erectus, then the brain must have played a role in crossing channels.

According to Wheeler,^[51] loss of functional body hair would have helped prevent hyperthermia, since hair will obstruct air flow over the skin and restrict cooling by evaporation. He further suggests that body cooling due to hair loss has relieved a thermal constraint on brain size (but in a response to Falk’s radiator hypothesis, Ralph Holloway maintains that there is no evidence for a temperature constraint on brain size^[52]). However, differences in body hair between Homo habilis and Homo erectus are impossible to test, and it will remain unclear whether hair loss was part of the hominin adaptation or preadaptation to Eurasia.