Ardipithecus ramidus


CLASSIFICATION

This early fossil 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, noting the apparently extreme dissimilarities between ramidus and all other known Australopithecines. They proposed Ardipithecus (from “ardi”, which means “ground” or “floor” in the Afar language) to be the genus [White, et al, 1995].

GEOLOGICAL SETTING

The initial and most extensive publication [White, et al, 1994] concerning Ardipithecus. ramidus specified that 17 hominid fossils had been located by the end of 1993. These specimens were retrieved from a cluster of localities West of the Awash River, within the Afar Depression, Aramis, Ethiopia.

Hominid and associated fossil faunas, including wood, seed and vertebrate specimens, were found entirely within a single interval overlying the basal Gaala Tuff complex, and beneath the Daam Aatu Basaltic Tuff (these volcanic strata have produced dates of 4.389 and 4.388 million years, respectively) [Renne, et al, 1999]. This definitively places all Ardipithecine specimens just shy of 4.4 million years ago.

Additionally, the associated strata were most likely produced within the context of a heavily forested, flood plain environment. Evidence for this conclusion was derived from representative non-human fossil remains, particularly from those species whose present-day analogues are environment-specific.

ANATOMY

A morphological description of the initial, mainly dental, fossil remains of Ardipithecus ramidus was published by White et al, 1994. The physical attributes of this hominid show a range of primitive traits, which are most likely character retentions from the last hominid/chimpanzee ancestor. At the same time, some hominid innovations are equally apparent. The currently known traits of Ardipithecus ramidus, in general, can be placed within two categories: ape-like traits and Australopithecine-like traits.

Much of the dentition is ape-like and this hominid most likely had a significantly different dietary niche than did later hominids. 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 enamel is thin. 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. 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.

Finally, something can be said of the skeletal anatomy and how it relates to the potentiality for bipedalism in A. ramidus. 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, suggests that if this creature was not bipedal in the modern sense, it at least had key adaptations toward a similar end.

Scanty postcranial remains (most significantly, a partial humerus) indicate that A. ramidus was smaller in size than the mean body size of Australopithecus afarensis. However, this particular estimate falls within the range of variation of A. afarensis.

EPILOGUE

A mandible and partial postcranial skeleton of a single individual was found in 1994. Analysis and publication on this find has yet to be made. Once completed, this should provide significant insight into the positional repertoire of Ardipithecus ramidus, dispelling all doubt as to whether or not this truly was a bipedal hominid.

 

Renne, Paul R, Giday WoldeGabriel, William K Hart, Grant Heiken, and Tim D White (1999) Geological Society of America Bulletin, pp. 869–885.

White, Tim D, Gen Suwa, and Berhane Asfaw (1994) Nature, 371:306–312.

White, Tim D, Ben Suwa, and Berhane Asfaw (1995) Nature, 375:88.