Significations et usages de Ardipithecus

Ardipithecus Temporal range: Pliocene
Ardipithecus ramidus specimen, nicknamed Ardi
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Primates
Family:	Hominidae
Subfamily:	Homininae
Tribe:	Hominini
Genus:	Ardipithecus White et al., 1995
Species
†Ardipithecus kadabba †Ardipithecus ramidus

Ardipithecus is a fossil hominoid, described by its discoverers as a very early hominin genus. Two species are described in the literature: A. ramidus, which lived about 4.4 million years ago^[1] during the early Pliocene, and A. kadabba, dated to approximately 5.6 million years ago (late Miocene).^[2]

  Ardipithecus ramidus

  Map showing discovery location

A. ramidus was named in September 1994. The first fossil find 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).^[3] 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; cf English 'earth' or German 'erde') and ramid means "root". The pithecus portion of the name is from the Greek word for "monkey".^[4] Like most primitive, but unlike all previously recognized hominins, it had a grasping [hallux] or big toe adapted for locomotion in the trees. However, its describers claim that other features of its skeleton reflect adaptation to bipedalism on the ground as well. Like later hominins, Ardipithecus had reduced canine teeth. Its brain was small and comparable in size to that of the modern chimpanzee.

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.^[5] The fossils were dated to between 4.32 and 4.51 million years old.^[6]

Ardipithecus ramidus had a small brain, measuring between 300 and 350 cm³. This is about the same size as 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.^[7]

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 fibrous plants, ripe fruit or hard 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.^[8]

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."^[8]

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.^[2]

  Ardi

  Casts of Ardi's finger bones

On October 1, 2009, paleontologists formally announced the discovery of the relatively complete A. ramidus fossil skeleton first unearthed in 1994. The fossil is the remains of a small-brained 50-kilogram (110 lb) female, nicknamed "Ardi", and includes most of the skull and teeth, as well as the pelvis, hands, and feet.^[9] It was discovered in Ethiopia's harsh Afar desert at a site called Aramis in the Middle Awash region. Radiometric dating of the layers of volcanic ash encasing the deposits suggest that Ardi lived about 4.4 million years ago. This date, however, has been questioned by others. Fleagle and Kappelman suggest that the region in which Ardi was found is difficult to date radiometrically, and they argue that Ardi should be dated at 3.9 million years.^[10]

The fossil is regarded by its describers as shedding light on a stage of human evolution about which little was known, more than a million years before Lucy (Australopithecus afarensis), the iconic early human ancestor candidate who lived 3.2 million years ago, and was discovered in 1974 just 74 km (46 mi) away from Ardi's discovery site. However, because the "Ardi" skeleton is no more than 200,000 years older than the earliest fossils of Australopithecus, and may in fact be younger than they are,^[10] some researchers doubt that it can represent a direct ancestor of Australopithecus.

Some researchers infer from the form of her pelvis and limbs and the presence of her abductable hallux, that "Ardi" was a facultative biped: bipedal when moving on the ground, but quadrupedal when moving about in tree branches.^[2][11][12] A. ramidus had a more primitive walking ability than later hominids, and could not walk or run for long distances.^[13] The teeth suggest omnivory, and are more generalised than those of modern apes.^[2]

  Ardipithecus kadabba

  Ardipithecus kadabba fossils

Ardipithecus kadabba is "known only from teeth and bits and pieces of skeletal bones",^[9] and is dated to approximately 5.6 million years ago.^[2] It has been described as a "probable chronospecies" (i.e. ancestor) of A. ramidus.^[2] Although originally considered a subspecies of A. ramidus, in 2004 anthropologists Yohannes Haile-Selassie, Gen Suwa, and Tim D. White published an article elevating A. kadabba to species level on the basis of newly-discovered teeth from Ethiopia. These teeth show "primitive morphology and wear pattern" which demonstrate that A. kadabba is a distinct species from A. ramidus.^[14]

The specific name comes from the Afar word for "basal family ancestor".^[15]

  Lifestyle

The toe and pelvic structure of A. ramidus suggest to some researchers that the creature walked upright.^[5]

According to Scott Simpson, the Gona Project's physical anthropologist, the fossil evidence from the Middle Awash indicates that both A. kadabba and A. ramidus lived in "a mosaic of woodland and grasslands with lakes, swamps and springs nearby," but further research is needed to determine which habitat Ardipithecus at Gona preferred.^[5]

  Contrary Views

The fossils of Ardipithecus have not yet been studied by researchers beyond the original (2009) group of describers, and the paleobiology and relationships of these creatures are the subject of controversy.^[16][17] Skeptics claim that many of the allegedly hominin-like features seen in the Ardipithecus material are found elsewhere among living and fossil primates, and that claims about its hominin status and locomotor habits are not adequately supported by the available evidence.

  See also

• Lucy (Australopithecus), 3.2 million years extinct hominid
• Pan prior
• Ardi

  References

  External links

Wikispecies has information related to: Ardipithecus

Wikimedia Commons has media related to: Ardipithecus

• Science Magazine: Ardipithecus special (requires free registration)
• The Smithsonian Institution's Human Origins Program:
  • Ardipithecus kadabba
  • Ardipithecus ramidus
• Ardipithecus ramidus at Archaeology info
• Explore Ardipithecus at NationalGeographic.com