Ardipithecus kadaba Essay

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April 23, 2010

Ardipithecus kadaba

Ardipithecus kadabba is an early hominin species recovered from sediments in the Middle Awash Valley of Ethiopia dated to between 5.2 and 5.8 million years ago.  These fossils are of particular importance because fragments from both the skull and body have been found and are argued to demonstrate some of the earliest signs of bipedalism and hominin dental morphology.  As one of the oldest species of human ancestors, Ar. kadabba helps to push back the origin of hominins into the late Miocene Epoch (roughly 11.6 to 5.3 million years ago).

There are relatively few Ar. kadabba fossils and the majority are teeth and fragments of the mandible (lower jaw).  Ar. kadabba teeth are noticeably more primitive (meaning they retain traits found in their ancestors, in this case represented by living apes) than Australopithecus afarensis and Australopithecus anamensis.  The incisors  (front teeth) are smaller than extant apes but are wider than those of australopiths and later hominins. Ar. kadabba canines are primitive in size—i.e.,  slightly smaller than female chimpanzee canines.  The upper canines of  Au. afarensis, Au. anamensis, and Ardipithecus ramidus, in contrast, are smaller than those of female chimpanzees, exhibiting a shift to smaller canines found in all later hominins.  The shape of the upper canine in Ar. kadabba, however, is relatively derived (meaning it differs from that found in their ancestors and more closely resemble later hominins) with a more symmetrical, or circular, crown (enamel-covered part of the tooth, that protrudes above the gum); this shape differs from other early hominin species—e.g., Orrorin tugenensis—which exhibits the primitive, ape-like shape.  The other Ar. kadabba teeth are similar in many general features with its suggested descendant, Ar. ramidus (see below), including overall size, proportions and wear patterns.  The Ar. kadabba dentition, however, retains some primitive features—e.g., thinner dental enamel and a more asymmetrical lower third premolar (cheek teeth that lie between the canines and the molars) crown shape.  The Ar. kadabba mandible is small yet broad, and the overall shape is similar to other early hominin species such as Sahelanthropus tchadensis and Ar. ramidus.  When viewed from the side, the front of the mandible is relatively primitive in shape, retreating backward from top to bottom.  The mandibular ramus (the vertical plates of bone at the rear of the mandible, behind the teeth) has a weaker outward flare than later hominin species. 

Ar. kadabba postcrania ( the parts of the skeleton other than the skull) are limited to fragmentary pieces of the forearm, two finger bones, a clavicle (collar bone) fragment, and a bone from the fourth toe.  At least five individuals are represented among the eleven postcranial elements recovered.  The forelimb (arm and hand) bones are fairly primitive, resembling living great apes.  For example, the finger bones are relatively large with strongly-built joint surfaces (where two bones meet to form a joint) than later hominins and the ulna (bone of the forearm closest to the body) is more curved and ape-like.  Another primitive feature that distinguishes this species from more derived hominin species is found in the morphology of the elbow joint, permitting increased mobility, characteristic of living apes and unlike the less mobile elbow of later hominins.

Ar. kadabba has been argued to have walked bipedally based on the characteristics of a single bone from the fourth toe, specifically the upward orientation of the joint surface closer to the rest of the foot.  This feature is similar to the condition found, not only in Au. afarensis but also in other later hominins, including Homo sapiens; living great apes, on the other hand, have more downwardly-tilted joint surfaces.  The morphology of fourth toe has led some researchers to suggest Ar. kadabba walked bipedally—that this feature allowed this species to  “toe off” (push off of the ground during bipedal walking, mostly with the big toe).  However, as noted by other researchers, similar joint shape occurs in non-hominin Miocene apes (e.g., Sivapithecus) which may or may not have walked bipedally along limbs when in trees.  In addition, the Ar. kadabba toe bone is dated to several hundred thousand years before the rest of the fossils, and was found in a locality 16 km from the main site.  This fact has caused some researchers to question the validity of grouping the toe bone with the rest of the Ar. kadabba fossils.

The evolutionary relationships between Ar. kadabba and other early hominin species are of great interest to paleoanthropologists.  A more recent hominin taxon, Ar. ramidus, which is also found in the Middle Awash of Ethiopia, is argued by some researchers to be a direct descendant of Ar. kadabba because these two species share many features, such as relatively thin tooth enamel and larger canines.  Some researchers further suggest that Ar. kadabba represents the earliest species in a single lineage of descent of East African hominin taxa, starting with Ar. kadabba to Ar. ramidus to Au. anamensis and ending with Au. afarensis.  These species are all found in eastern Africa and some morphological trends, such as the reduction in canine size and premolar morphology, support this hypothesis, but the dates of Ar. ramidus and Au. anamensis (see essay on Ar. ramidus) cast some doubt on this explanation.

Ar. kadabba appears to have occupied a closed, densely-wooded habitat close to permanent sources of water (e.g., lakes and/or rivers) with swampy conditions and floodplain grasslands.  This reconstruction is based on the fossil remains of non-hominin animals found in the same layers as the Ar. kadabba material and suggests strongly against the once proposal, called the "savannah hypothesis", that bipedalism initially evolved in open grassland environments.