The history of the interpretation of the genus Homo from its introduction in 1758 by Carolus Linnaeus in the tenth edition of his Systema Naturae to the inclusion in 1964 of Homo habilis, has been one of episodic relaxation of the criteria used for including taxa in Homo. Each of these episodes has resulted in one, or more, species being added to Homo.
As originally conceived by Linnaeus, the genus Homo incorporated two species, Homo sapiens and Homo sylves-tris (also called Homo troglodytes or Homo nocturnus). The latter apparently referred to a mythical nocturnal cave-dwelling form from Java, but the discovery of Homo flore-siensis (Brown et al., 2004; Morwood et al., 2005) may mean that Homo sylvestris might not have been mythical after all (Collard and Wood, 2006).
William King was the first to suggest that an extinct hominin species, namely Homo neanderthalensis, should be included within Homo (King, 1864). Researchers still argue about the taxonomic significance of the differences between H. sapiens and H. neanderthalensis, but there is no gainsaying that the inclusion of the latter within Homo meant expanding the definition of Homo so that it included archaic and derived morphology (e.g., discrete and rounded supraorbital margins, midline facial projection, a distinctive parietal and occipital morphology, and robust limb bones with relatively large joint surfaces) that is not seen at all, or is not seen in this combination, in H. sapiens.
The next modification of the interpretation of the genus Homo took place nearly half a century later, when in 1908, the sole item of fossil evidence, the Mauer mandible, for the taxon H. heidelbergensis (Schoetensack, 1908), was added to the genus. This meant that Homo now embraced at least one individual with a mandible that combined the absence of a true chin with evidence a robust mandibular corpus. Then came the addition of Homo rhodesiensis from the site now known as Kabwe (Woodward, 1921), and subsequently the addition of Homo soloensis from Ngandong (initially included in the subgenus Javanthropus [Oppenoorth, 1932], but later included in the genus Homo [Oppenoorth, 1937]). The addition of the Ngandong fossils, meant that crania substantially more robust than those of modern humans and H. neanderthalensis were now included in Homo. Nonetheless, the endocranial volumes of all the crania associated with H. neanderthalensis, H. rhodesiensis and H. soloensis are still within, or close to, the modern human range (de Sousa and Wood, 2006).
When Franz Weidenreich formally proposed in 1940 that two existing extinct hominin hypodigms, Pithecanthropus erectus and Sinanthropuspekinensis, should be merged into a single species and transferred to Homo as Homo erectus (Weidenreich, 1940) the addition of these taxa resulted in Homo subsuming an even wider range of morphology. Subsequently the hypodigms of Meganthropus (Mayr, 1944), Telanthropus (Robinson, 1961), and Atlanthropus (Le Gros Clark, 1964) were also transferred to H. erectus. The addition of the H. erectus hypodigm at this time, even though it was well before the discovery of the small H. erec-tus crania from East Africa (Anton, 2004: Leakey et al.,
2003; Potts et al., 2004; Spoor et al., 2007) and Dmanisi (Rightmire et al., 2006), substantially increased the range of endocranial volume within the genus Homo. Compared with most of the pre-1940 hypodigm of Homo, fossils attributed to H. erectus have a smaller neurocranium, a lower vault, a broader base relative to the vault, and more complex premolar roots. They also have a substantial, shelf-like torus above the orbits, and there are often both sagittal and angular tori, although the expression of some, or all, of this morphology may be size-related (Antón et al., 2007). The occipital sagittal profile is sharply angulated in H. erectus, with a well-marked supratoral sulcus, and the inner and outer tables of the vault are thickened. The cortical bone of the postcranial skeleton is generally thick. The long bones are robust, and the shafts of the femur and the tibia are flattened from front to back relative to those of other Homo species. However, all the postcranial elements of H. erectus are consistent with a habitually upright posture and obligate long-range bipedalism.
In 1964, Louis Leakey, Phillip Tobias and John Napier announced the discovery at Olduvai Gorge of specimens they assigned to a new species of Homo, called Homo habilis (Leakey et al., 1964). The type specimen of H. habilis, the sub-adult OH 7, recovered in 1960, preserves most of both parietal bones, a partial mandible and several hand bones of a juvenile skeleton. Between 1960 and 1963 further evidence of a "non-robust" fossil hominin was unearthed in Bed I (OH 8) and Bed II (OH 13, OH 14, OH 16) of Olduvai Gorge. The inclusion of this group of specimens in Homo substantially widened the range of morphology within the genus, and meant that the Le Gros Clark (1955) diagnosis of Homo needed to be amended. In particular, to accommodate H. habilis in the genus Homo Leakey et al. (1964) were forced to reduce the lower end of the range of brain size of fossils attributed to Homo to 600 cm3. Leakey and his colleagues claimed that other criteria, such as dexterity, an erect posture and a bipedal gait, did not need to be changed because their interpretation of the functional capabilities of the type specimen and the paratypes of H. habilis was consistent with these functional criteria (Leakey et al., 1964). Ultimately, however, fresh evidence, and fresh interpretations of existing evidence, has led others to offer rather different functional assessments of the same material (see below).
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