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Ardipithecus ramidus

Ordo: Primates
Subordo:Anthropoidae
Superfamilia:Hominoidae
Familia:Hominidae
Ardipithecus ramidus


Ardipithecus ramidus is the most primitive hominid, discovered in Aramis in the Middle Awash region of Ethiopia at the end of 1994 by paleonthropologist Tim White and his two colleagues Berhane Asfar and Gen Suwa. Approximately 17 different specimens were found with many bones present. These specimens included part of a child's mandible, some isolated teeth, a fragment of basi-cranium, and three bones of a left arm of a single individual (on the right an image is shown of the bones which they found on the site). In 1998 in Gona (the site is located about 500 km to the northeast of Addis Ababa, the capital of Ethiopi) ,more than 30 specimens belonging to at least 9 individuals were recovered with dental anatomy (diamond-shaped maxillary canines) including enamel thickness, jaws (one possibly a female), teeth, and toe and finger bones (on the left an image is shown of the sites where found fossils).




Ardipithecine specimens are 4.4 milion years old (fossil was found entirely within a single interval overlying the basal Gaala Tuff comlex, and beneath the Daam Aatu Basaltic Tuff, these volcanic strata have producted dates of 4.389 and 4.388 milion years) [Renne, et al, 1999].

This hominid was initially placed within the Australopithecus genus, with a new specific epithet - ramidus (from the Afar word "ramid", meaning "root") [White, et al, 1994]. Tim White and associates have subsequently reassigned the hominid to a new genus. More primitive, chimplike morphology hence the change of genus name. They proposed Ardipithecus (from "ardi", which means "ground" or "floor" in the Afar language) to be the genus [White, et al, 1995].

Ardipithecus ramidus probably lived in a wooded environment, because it was found in strata with preserved fossils of wood, seeds and more than 60 mammal species including primitive elephants, rhinos, horses, rats, woodland forest antelope and monkeys (White et al., 1994). This discover is important because it place very early hominids in a woodland setting rather than a savanna one.

The physical attributes of Ardipithecus ramidus show a range of primitive features, which are most likely character retentions from the last hominid /chimpanzee ancestor. The features of this species, in general, can be placed within two categories: ape-like features and Australopithecine-like features. Research done by biochemists Allan Wilson and Vincent Sarich have revealed that the hominid-ape line diverged between 5 to 7 million years ago, making Ardipithecus ramidus the possible "missing link" between humans and apes.

The dentition of Ardipithecus ramidus is more ape-like than those of Australopithecines. A small canine-incisor to postcanine dental ratio, typical of all other known hominids, is strikingly absent in Ardipithecus ramidus. In addition to the presence of a relatively large anterior dentition, tooth have thin, ape-like enamel. Though slightly greater than in teeth of modern chimpanzees, enamel thickness of A. ramidus is extremely thin by hominid standards. Premolar and molar morphology also point to niche affinities with the great ape ancestors. Strong crown asymmetries, in particular enlarged buccal cusps, characterize the upper and lower premolars. Additionally, an ape-like molar shape prevails. The length (in the mesiodistal plane) to breadth (in the buccolingual plane) ratio, which is roughly equal to 1 in later hominids, is much greater in A. ramidus.

Some important derived features, link Ardipithecus ramidus with the Australopithecines. Hominid-like canines are present. These are low, blunt, and less projecting than the canines of all other known apes, but the canines are lined up with posterior teeth, which is a trait found in apes. Upper and lower incisors are larger than those of the Australopithecines, but are smaller than those of chimpanzees. This character state can thus be considered transitional between apes and Australopithecines. Additionally, the lower molars are broader than those of a comparably-sized ape. This trait, too, approaches the common hominid condition. [White, et al, 1995 ].

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Pieces of the cranial bones that have been recovered, including parts of the temporal and the occipital, strongly indicate an anterior positioned foramen magnum. The fact that the skull of A. ramidus rested atop the vertebral column, rather than in front of it. That feature indicate that A. ramidus was an obligate biped. Also, the neurocranium pieces show features that are similar to modern chimpanzees. For example, the spongy bone on the temporal that is pneumatized extends into the temporal squama. The external auditory meatus is small, and the mandibular fossa is flat. The morphology seems to indicate heavy anterior tooth loading, which indicates regular use of the front teeth in clamping and pulling. This could be indicative of aggressive biting, or more likely, leaf stripping - which would be an important adaptive behavior in their woodland environment. A. ramidus probably fed by fruit, leaves, insects and small animals.

The humeri, radii, and ulni show that the species had very powerful arms and forearms, which were not used to support body weight (they weren't quadrapeds), and which closely resemble modern human and large-bodied apes. The forearm features indicate that it was used in climbing (the increased strength and robusticity due to the loss of grasping feet and the need to make up for weight load in climbing). Also, morphology of the toe bone indicates that Ardipithecus walked erect on two feet.

The humerus indicate that A. ramidus was smaller in size than the mean body size of their relatives species Australopithecus afarensis. It weighed roughly 40 kg and its height probably was 122 cm. Its exact brain size is unknown, but it was approximately the size of a chimpanzee's. However, someone has suggested that the brain size is estimated to be around 450 cc[cubic centimeter].

It is fairly sure that Ardipithecus ramidus is a hominid, as the very earliest hominines were expected to be apelike (or even possibly chimplike) in many ways such as dentition anyway. It has thus been decided that Ardipithecus ramidus is not the direct ancestors of the genus Homo, but that they share a common ancestor.