Orrorin tugenensis essay


February 10, 2010

Orrorin tugenensis

Orrorin tugenensis is represented by a collection of fossils from the Tugen Hills region of Kenya.  Specifically, O. tugenensis is known from four sites in this region: Cheboit, Kapsomin, Kapcheberek, and Aragai.  “Orrorin” means “original man” in the Tugen dialect, and “tugenensis” pays tribute to the Tugen Hills region.  The sediments in which this specimens have been found are dated to between 6 and 5.8 million years ago using radioisotopic methods, paleomagnetism (dating accomplished using the timing of reversals in Earth’s magnetic poles), and biochronology (dating that utilizes the relative time frames of extinct non-hominin animals).  Orrorin tugenensis is important to hominin evolution because it (along with Sahelanthropus tchadensis, from central Africa) may represent some of the earliest evidence for bipedalism in the human fossil record.

O. tugenensis is represented by 20 fossil specimens, coming from a minimum of five individuals.  Specimens include teeth and lower jaw fragments, pieces of the humerus (the bone of the upper arm), pieces of the femur (thigh bone), and a phalanx (finger bone).  The molar teeth of O. tugenensis are smaller than in species in the genus Australopithecus and, in this respect, are similar to Ardipithecus ramidus and Kenyanthropus platyops.  The thickness of the dental enamel in O. tugenensis, however, is similar to that of Australopithecus afarensis and K. platyops, which have thicker enamel than Ar. ramidus and living apes.  The canine teeth of O. tugenensis are very primitive, resembling those of female chimpanzees in shape.  The morphology (size and shape) of the O. tugenensis femur has led some researchers to suggest that it practiced bipedal locomotion.  In particular, the proximal femur (where the femur articulates with the pelvis) bears a femoral neck (the part of the bone that connects the shaft of the femur to the head, which articulates with the pelvis) that is longer than in living apes.  Some of the muscle markings on this portion of the femur have also been argued to indicate a shift toward bipedality, but arguments concerning the distinctiveness of these traits to bipeds abound.  In addition, computed tomography (CT, an imaging technique, similar to x-ray, that permits researchers to observe the internal structures of bones) scans of the O. tugenensis proximal femur show that the cortical bone (outer layer of bone) has differential thickness along the top and bottom surfaces; it is thickened inferiorly and thinner superiorly.  This morphology has been argued to indicate that O. tugenensis was a biped.  However, the published CT scans are somewhat ambiguous and the pattern found in O. tugenensis is also found in non-bipedal primates.  Furthermore, some of the fossils attributed to O. tugenensis possess features that are considered specializations for arboreal locomotion (moving around in the trees).  The O. tugenensis phalanx, for instance, is curved, similar to that of living apes.  It should be noted, however, that indisputably bipedal hominins (e.g., Au. afarensis) also exhibit curved finger bones.

Currently, the debate over whether or not O. tugenensis was bipedal is unresolved.  Thus, its evolutionary relationships with later hominin species are not well established.  While some researchers are convinced by the features in this species that seem to indicate that it practiced bipedalism, others remain skeptical.  If O. tugenensis is, in fact, a biped it would mark some of the earliest evidence for this form of locomotion in the human fossil record and would shed light on the evolutionary causes of the shift to bipedality.  In addition, due to the fact that O. tugenensis dates to the time period during which the lineages leading to humans and living chimps are thought to diverge (based on molecular studies), clear evidence of bipedalism in this species would imply that some of the earliest species to evolve after this split were bipedal.  Drawing connections between O. tugenensis and later hominin species, however, is difficult because it possesses a mixture of primitive features (more generalized, e.g., canine morphology and smaller molar teeth, shared with Ar. ramidus and living chimps) and derived (more specialized) features, e.g., thicker dental enamel, shared with Au. afarensis) that confound efforts to connect it directly to later hominin species.

The habitats in which O. tugenensis has been found are reconstructed as areas nearby lakes and streams.  Evidence from non-hominin animals found at O. tugenensis sites (e.g., impalas and leaf monkeys) corroborates this finding, suggesting that O. tugenensis inhabited woodland environments.  If O. tugenensis is bipedal, these environmental reconstructions would suggest that the earliest species practicing this form of locomotion evolved in more forested habitats, contrary to the once widely held belief that bipedality was an adaptation to life on the savanna.