Classification as Homo
There has been scholarly debate regarding its placement in the genus Homo rather than the genus Australopithecus. The small size and rather primitive attributes have led some experts (Richard Leakey among them) to propose excluding H. habilis from the genus Homo, and renaming as "Australopithecus habilis".
In its appearance and morphology, H. habilis is the least similar to modern humans of all species in the genus Homo (except the equally controversial H. rudolfensis), and its classification as Homo has been the subject of controversial debate since its first proposal in the 1960s. H. habilis was short and had disproportionately long arms compared to modern humans; however, it had a less protruding face than the australopithecines from which it is thought to have descended. H. habilis had a cranial capacity slightly less than half of the size of modern humans. Despite the ape-like morphology of the bodies, H. habilis remains are often accompanied by primitive stone tools (e.g. Olduvai Gorge, Tanzania and Lake Turkana, Kenya).
Homo habilis has often been thought to be the ancestor of the more gracile and sophisticated Homo ergaster, which in turn gave rise to the more human-appearing species, Homo erectus. Debates continue over whether all of the known fossils are properly attributed to the species, and some paleoanthropologists regard the taxon as invalid, made up of fossil specimens of Australopithecus and Homo. New findings in 2007 seemed to confirm the view that H. habilis and H. erectus coexisted, representing separate lineages from a common ancestor instead of H. erectus being descended from H. habilis. An alternative explanation would be that any ancestral relationship from H. habilis to H. erectus would have to have been cladogenetic rather than anagenetic (meaning that if an isolated subgroup population of H. habilis became the ancestor of H. erectus, other subgroups remained as unchanged H. habilis until their much later extinction).
Newly published (as of 2015) virtual reconstruction estimated the endocranial volume at between 729 and 824 ml it is larger than any previously published value.
Compared to australopithecines, H. habilis' brain capacity of around 640 cm³ was on average 50% larger than australopithecines, but considerably smaller than the 1350 to 1450 cm³ range of modern Homo sapiens. These hominins were smaller than modern humans, on average standing no more than 1.3 m (4 ft 3 in) tall.
A fragment of fossilized jawbone, dated to around 2.8 million years ago, was discovered in the Ledi-Geraru research area in Afar Regional State in 2013. The fossil is considered the earliest evidence of the Homo genus known to date, and seems to be intermediate between Australopithecus and H. habilis. The individual in question lived just after a major climate shift in the region, when forests and waterways were rapidly replaced by arid savannah.
One set of fossil remains (OH 62), discovered by Donald Johanson and Tim White in Olduvai Gorge in 1986, included the important upper and lower limbs. Their finding stimulated some debate at the time.
KNM ER 1813
KNM ER 1813 is a relatively complete cranium which dates to 1.9 million years old, discovered at Koobi Fora, Kenya by Kamoya Kimeu in 1973. The brain capacity is 510 cm³, not as impressive as other early specimen and forms of H. habilis discovered.
OH 24 (Twiggy) is a roughly deformed cranium about 1.8 million years old discovered in October 1968 at Olduvai Gorge, Tanzania. The brain volume is just under 600 cm³; also a reduction in a protruding face is present compared to members of more primitive australopithecines.
OH 7 dates to 1.75 million years old, and was discovered by Mary and Louis Leakey on November 4, 1960 at Olduvai Gorge, Tanzania. It is a lower jaw complete with teeth; due to the size of the small teeth, researchers estimate this juvenile individual had a brain volume of 363 cm³. Also found were more than 20 fragments of the left hand. Tobias and Napier assisted in classifying OH 7 as the type fossil.
KNM ER 1805
KNM ER 1805 is a specimen of an adult H. habilis made of three pieces of cranium dating to 1.74 million years old from Koobi Fora, Kenya. Previous assumptions were that this specimen belongs to H. erectus based on the degree of prognathism and overall cranial shape.
Homo habilis is thought to have mastered the Lower Paleolithic Olduwan tool set which used stone flakes. These stone flakes were more advanced than any tools previously used, and gave H. habilis the edge it needed to prosper in hostile environments previously too formidable for primates. Whether H. habilis was the first hominid to master stone tool technology remains controversial, as Australopithecus garhi, dated to 2.6 million years ago, has been found along with stone tool implements.
Most experts assume the intelligence and social organization of H. habilis were more sophisticated than typical australopithecines or chimpanzees. H. habilis used tools primarily for scavenging, such as cleaving meat off carrion, rather than defense or hunting. Yet despite tool usage, H. habilis was not the master hunter its sister species (or descendants) proved to be, as ample fossil evidence indicates H. habilis was a staple in the diet of large predatory animals, such as Dinofelis, a large scimitar-toothed predatory cat the size of a jaguar.
Homo habilis coexisted with other Homo-like bipedal primates, such as Paranthropus boisei, some of which prospered for many millennia. However, H. habilis, possibly because of its early tool innovation and a less specialized diet, became the precursor of an entire line of new species, whereas Paranthropus boisei and its robust relatives disappeared from the fossil record. H. habilis may also have coexisted with H. erectus in Africa for a period of 500,000 years.
Evolutionary biologist Jeremy Griffith has drawn parallels between H. habilis and the psychological developmental evolution of modern humans as a manifestation of Ernst Haeckel's theory of ontogeny being a summarised recapitulation of phylogeny, suggesting elements of the phenotype of H. habilis relate to early adolescence (12-13 years of age) in modern humans.
- Friedemann Schrenk, Ottmar Kullmer, Timothy Bromage, "The Earliest Putative Homo Fossils", chapter 9 in: Winfried Henke, Ian Tattersall (eds.), Handbook of Paleoanthropology, 2007, pp 1611–1631, doi:10.1007/978-3-540-33761-4_52 This date range overlaps with the emergence of Homo erectus. New York Times article Fossils in Kenya Challenge Linear Evolution published August 9, 2007.
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