The transitional forms from Homo habilis to Homo ergaster in Africa is the subject of great debate. The term Early humans, frequently associated to H. habilis, tends to include the transitional form to H. ergaster, with the boundary between the two species being placed at the 1.8 ma time split. Early humans share arcaic traits with the australopithecines, such as large teeth, molar size increase from M1 to M3, Y5 cusp pattern in all lower molars, large hypocone, retained subnasal prognathism, and relatively small brain (around 600 to 800 cc). Arcaic humans dating after 1.8 ma show more derived traits, such as reduced facial prognathism, human-like molar cusp pattern (Y5 most frequent in the first lower molar, though reduced in the M2 and M3 teeth), and more slender and taller bodies, larger brain size, and longer hind limbs compared to forearms.
The most ancient fossil specimens of H. ergaster, also referred to as the African H. erectus, to distinguish them from the Asian ones, date bach to 1.9 ma at the Lake Turkana sites in Kenya. They lived along the late H. hailis representatives as well as with Paranthropus boisei in East Africa. The extinction of the robust australopithecines has been attributed to predation and/or competition with arcaich humans, though climatic shifts bwtween 1,5 and 1,0 ma might have significantly reduced food availability and preferred habitats as open savannahs progresively spread throughout East and Soth Africa. However, H. ergaster thrived in such open, harsh environments, likely with the aid of the Acheulian Mode 2 industry. At aroun 1,8 ma it had already migrated out of Africa, ariving to the Caucasus (Georgia), found at the site of Dmanisi (1.77 ma), China and Soth East Asia (at around 1.8 to 1,6 ma). The dating of Dmanisi has been well stated, whereas the sites in ontinental China and those in Southeastern Asia have been controversial.
The oldesst Acheulian technological mode, also known as Mode 2, has been described at Olduvai in Tanzania, dating to 1.4 ma, in association to the OH-9 hominin cranium. The most characteristical trait of Mode 2 lithic technology is the bifacial tool, that remains stable, both in Africa and certain areas of Europe and Asia till 0,4 ma.
The earliest dispersal of the arcaic humans out of Africa (1.8 ma) predates the expansion of Mode 2 technology (1.4 ma). This would explain why the Acheulian technology is not found in most parto of continental China and in Sotheast Asia. The distribution of the Acreulian industry icludes the adjacent geographic areas of Africa, Europe, and the Near and Middle East, whereas in Eastern Europe, China and Southeast Asia the bifacial tool and the Levallois technique used to build it are not present. This fact has been used to support the contention that the African and Asian arcaic humans belong to two distinct species: H. ergaster and H. erectus. In this regard, the OH-9 skull is seen as an African H. erectus, given its strong supraorbital turus, long cranium (from the glabella to the opistocranium), inclined forehead, and angulated occipital. All these traits have been associated to the Asian forms rather than to the African ones. In this regard, OH-9 could represent a reverse migration, from Asia back to Europe. However, detailed comparisons between the eastern and western forms of arcaich humans have shown that differences in size and robusticity, more significantly developed in the Asian forms, are likely related to allometric, size differences. The Asian H. erectus in China have more recent datings than the African H. ergaster, and they show larger body sizes and greater robusticity in their skull and postcranial skeletons. Most authors now recognize that the two taxa likely represent a single, closely related group. However, the use of two distinct taxonomic names seems to prevail, thus putting enphasis in their geographical and chronological separation.
The uneven distribution of Mode 2 technology in Eastern Asia contrasts with the homogeneity of the bifacial tool, that remains unchanged during 1 million years, despite with minor local technological variations. The Acheulian sites abound close to water sorces, most represented by occupational base sites with butchering activitis, such as described in the site of Olorgesailie in Kenya, with evidence of gelada baboons butchering, where the Acheulian lithic industry persistet from 1.8 ma to the onset of the Neolithic period at around 0.01 ma. Thus, the Acheulian cannot be exclusively ascribed to H. ergaster, that was replace by H. sapiens between 300,000 and 100,000 years ago, when Mode 2 technology was not abandoned. The wide distribution of H. erectus in Asia suggests the existence of a network of somewhat isolated populations with a significant exchange of cultural practices and gene flow throughout the vast territory.
a. Fossil evidence of H. erectus «sensu lato»
The term sensu lato is frequently used to reffer to both H. ergaster (Africa) and H. erectus «sensu stricto» (Asia). The fossil evidence in both areas greatly overlap in their chrology, since at 1.8-1,4 ma they are found at either Turkana Lake (Kenya), Olduvai (Tanzania), Omo (Ethiopia), Dmanisi (Georgia), Modjokerto and Sangiran (Java), and Longgupo (China). More recent dates have been obtained for several Asian sites, such as Zoukoudien (China, 0.55 ma) and Ngandong (Solo river, Java, 0.053-0.027 ma). There is no evidence of the presence of Homo erectus in Europe. The earliest evidences of the human presence in Europe date to 1.3 ma at Barranco León and Fuente Nueva (Orce, Granada), Trinchera del Elefante (1.1 ma) and Dolina (0.8 ma) at Atapuerca (Burgos, Spain), followed by sites dated to less than 500,000 years, cuch as Sima de los Huesos (Spain), Arago (France) or Cepprano (Iyaly). None of these sites hold fossil remains that could be attributed to Homo erectus «sensu lato», despite some authors have claimed so. The ost significant sites with clear evidences of this taxon are, though, widely spread though Africa and Asia.
- Turkana Lake (Kenya), including the sites of East Rudolf (ER) and West Turkana (WT). Significan fossil specimens from these sites are: ER-3228 (a coxal bone with a stron «pilastra», an apomorphy of the taxon, in the illiac bone), ER-3733 (1,8 ma, skull with 848 cc brain capacity), ER-3883 (1.8 ma, skull with 804 cc), ER-1805 (cranial vault, 582 cc), and WT-15000 (Noriokotome boy, almost complete skeleton, 12 years old, 1.6 m tall).
- Olduvai Bed II (Tanzania), dating to 1.4 ma and including specimen OH-9 (calvaria, 1,067 cc).
- Bouri Formation (Middle Awash, Ethiopia), including specimen BOU-WP-2/66 (calvaria of Daka, 1 ma, 99 cc).
- Omo (Shungura Formation, Ethiopia), dating to 1.4 ma.
- Melka-Kunture (Gomboré I Formation, Ethiopia), with 1.5 ma.
- Swartkrans (Level II, Southafrica), 1.5 ma.
- Dmanisi (Georgia), 1.77 ma, includes fossil remains of at least 20 individuals, along with 5 almost complete skulls (cranial capacity ranging from 650 to 775 cc). These specimens have been attributed to a new taxon, Homo georgicus, since no H. ergaster not H. erectus had been found in this region before. In addition, their anatomical traits, both for the skull and the teeth, overlaps with the variability seen in both H. habilis and H. ergaster in Africa.
- Dmanisi Skull 1 (D2280) – Skull (775 cc) of an adult individual interpreted as male on account of the thick brow ridges.
- Dmanisi Skull 2 (skul D2282 650 cc, and mandible D211) -Gracile features, interpreted as the skull of an adolescent female.
- Dmanisi Skull 3 (D2700 600 cc, mandible D2735) – Skull of a young individual, with a gracile morphology, but the upper canine teeth have large crowns and massive roots; some traits suggests an interpretation as male.
- Dmanisi Skull 4 (D3444 625 cc, mandible D3900) – Skull of an elderly individual that had lost all but one tooth, interpreted as male.
- Dmanisi Skull 5 (D4500 546 cc, mandible D2600) – Skull of an adult individual, interpreted as male on account of its massive and prominent cranial features.