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''Ardipithecus'' is a genus of an extinct hominine that lived during Late Miocene and Early Pliocene in Afar Depression, Ethiopia. Originally described as one of the earliest ancestors of humans after they diverged from the main ape lineage, the relation of this genus to human ancestors and whether it is a hominin is now a matter of debate.〔 Two fossil species are described in the literature: ''A. ramidus'', which lived about 4.4 million years ago during the early Pliocene, and ''A. kadabba'', dated to approximately 5.6 million years ago (late Miocene).〔 Behavioral analysis showed that ''Ardipithecus'' could be very similar to those of chimpanzees, indicating that the early human ancestors were very chimpanzee-like in behaviour. == ''Ardipithecus ramidus'' == ''A. ramidus'' was named in September 1994. The first fossil found was dated to 4.4 million years ago on the basis of its stratigraphic position between two volcanic strata: the basal Gaala Tuff Complex (GATC) and the Daam Aatu Basaltic Tuff (DABT). The name ''Ardipithecus ramidus'' stems mostly from the Afar language, in which ''Ardi'' means "ground/floor" (borrowed from the Semitic root in either Amharic or Arabic) and ''ramid'' means "root". The ''pithecus'' portion of the name is from the Greek word for "ape". Like most hominids, but unlike all previously recognized hominins, it had a grasping hallux or big toe adapted for locomotion in the trees. It is not confirmed how much other features of its skeleton reflect adaptation to bipedalism on the ground as well. Like later hominins, ''Ardipithecus'' had reduced canine teeth. In 1992–1993 a research team headed by Tim White discovered the first ''A. ramidus'' fossils—seventeen fragments including skull, mandible, teeth and arm bones—from the Afar Depression in the Middle Awash river valley of Ethiopia. More fragments were recovered in 1994, amounting to 45% of the total skeleton. This fossil was originally described as a species of ''Australopithecus'', but White and his colleagues later published a note in the same journal renaming the fossil under a new genus, ''Ardipithecus''. Between 1999 and 2003, a multidisciplinary team led by Sileshi Semaw discovered bones and teeth of nine ''A. ramidus'' individuals at As Duma in the Gona Western Margin of Ethiopia's Afar Region. The fossils were dated to between 4.35 and 4.45 million years old. ''Ardipithecus ramidus'' had a small brain, measuring between 300 and 350 cm3. This is slightly smaller than a modern bonobo or female common chimpanzee brain, but much smaller than the brain of australopithecines like Lucy (~400 to 550 cm3) and roughly 20% the size of the modern ''Homo sapiens'' brain. Like common chimpanzees, ''A. ramidus'' was much more prognathic than modern humans. The teeth of ''A. ramidus'' lacked the specialization of other apes, and suggest that it was a generalized omnivore and frugivore (fruit eater) with a diet that did not depend heavily on foliage, fibrous plant material (roots, tubers, etc.), or hard and or abrasive food. The size of the upper canine tooth in ''A. ramidus'' males was not distinctly different from that of females. Their upper canines were less sharp than those of modern common chimpanzees in part because of this decreased upper canine size, as larger upper canines can be honed through wear against teeth in the lower mouth. The features of the upper canine in ''A. ramidus'' contrast with the sexual dimorphism observed in common chimpanzees, where males have significantly larger and sharper upper canine teeth than females. The less pronounced nature of the upper canine teeth in ''A. ramidus'' has been used to infer aspects of the social behavior of the species and more ancestral hominids. In particular, it has been used to suggest that the last common ancestor of hominids and African apes was characterized by relatively little aggression between males and between groups. This is markedly different from social patterns in common chimpanzees, among which intermale and intergroup aggression are typically high. Researchers in a 2009 study said that this condition "compromises the living chimpanzee as a behavioral model for the ancestral hominid condition."〔 ''A. ramidus'' existed more recently than the most recent common ancestor of humans and chimpanzees (CLCA or ''Pan''-''Homo'' LCA) and thus is not fully representative of that common ancestor. Nevertheless, it is in some ways unlike chimpanzees, suggesting that the common ancestor differs from the modern chimpanzee. After the chimpanzee and human lineages diverged, both underwent substantial evolutionary change. Chimp feet are specialized for grasping trees; ''A. ramidus'' feet are better suited for walking. The canine teeth of ''A. ramidus'' are smaller, and equal in size between males and females, which suggests reduced male-to-male conflict, increased pair-bonding, and increased parental investment. "Thus, fundamental reproductive and social behavioral changes probably occurred in hominids long before they had enlarged brains and began to use stone tools," the research team concluded.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Ardipithecus」の詳細全文を読む スポンサード リンク
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