Orrorin Tugenensis:
Pushing back the hominin line

The Kenya Palaeontology Expedition (KPE) report in December 2000 the discovery of what is almost certainly a new species of hominid at Kapsomin in Kenya's Baringo district; see the BBC news story; for background, see family trees. The excavating team includes Martin Pickford from the KPE and Brigitte Senut from the Museum of Natural History in Paris. The remains have not yet been dated, but they were found in found in six-million-year-old rocks. They include a left femur, pieces of jaw with teeth, isolated upper and lower teeth, arm bones, and a finger bone; the excavations are ongoing. Preliminary analyses suggest the hominid, the size of a chimpanzee, was an agile climber and that it walked on two legs when on the ground. The tentative date of six million years indicate a date very close to the common ancestor of humans and chimpanzees, although this date may now need to be pushed back.

The find was published by Senut et al. in January 2001 (see bibliography below). The authors report the recovery of remains of an early hominid from four localities in the Lukeino Formation in the Tugen Hills, Kenya. Radioisotopic age determinations from lavas underlying and overlying the Lukeino Formation and from crystals from the sediments themselves indicate an age of ca 6 million years for these hominids. The Lukeino hominids are thus the oldest known members of the family (Pickford & Senut, 2001).

The find comprises thirteen fossils belonging to at least five individuals. "The femora indicate that the Lukeino hominid was a biped when on the ground, whilst its humerus and manual phalanx show that it possessed some arboreal adaptations. The upper central incisor is large and robust, the upper canine is large for a hominid and retains a narrow and shallow anterior groove, the lower fourth premolar is ape-like, with offset roots and oblique crown, and the molars are relatively small, with thick enamel." (Senut et al., 2001). A new genus and species is proposed: Orrorin tugenensis.

The authors note that the molars of Orrorin are smaller than those of Australopithecines and are closer in size to those of Ardipithecus. "The anterior teeth, upper incisor and canine, as well as the lower P4 are less hominid-like and more ape-like, being closer in morphology to teeth of female chimpanzees. The molar enamel is thick. Another important feature is the relatively great depth of the corpus mandibularis, which is an archaic feature among hominids. Compared to later hominids, it seems that small jugal teeth relative to body size would be a primitive feature, inherited from the common ancestor of African apes and hominids, and retained in the Homo lineage. If this is so, then Australopithecines would have progressively developed megadonty -- large jugal teeth and relatively small bodies. The postcranial evidence suggests that Orrorin tugenensis was already adapted to habitual or perhaps even obligate bipedalism when on the ground, but that it was also a good climber. Many scholars have considered that the earliest hominids were small animals; the femur and humerus of Orrorin are 1.5 times larger than those of AL 288.1, probably equivalent in size to a female common chimpanzee, indicating that the ancestor may have been larger than previously envisaged." (Senut et al., 2001).

The authors conclude that on the basis of dental and postcranial morphology, it appears that Orrorin belongs to the hominid lineage, which was already present 6 million years ago. They suggest this confirms the hypothesis that the divergence between apes and humans took place prior to 6 million years ago, and probably between 9 and 7 million years ago, as they show in the diagram to the right. In this diagram, Ardipithecus ramidus is the ancestor of Pan and the Australopithecines an extinct line -- a view at odds with the view of most paleoanthropologists (cf. family trees and Ardipithecus ramidus). For an impression of the early discussion, see Michael Balter, Scientists Spar Over Claims of Earliest Human Ancestor, Science 291 5508 (23 Feb 2001): 1460-1461 (full text, external).

L.C. Aiello and M. Collard (2001), Our newest oldest ancestor? Nature 410 (29 Mar): 526, applaud the find but question the interpretations, as reported in ScienceWeek:

Aiello and Collard "point out that the announcement of the Orrorin find has caused considerable stir, partly because the deposits of the Tugen Hills are being prospected by two competing groups of researchers, and partly because Orrorin is claimed to be approximately 6 million years old. This makes Orrorin 1.5 million years older than Ardipithecus ramidus, the oldest previously recognized candidate for the earliest hominin. Orrorin's apparent age falls within the molecularly determined range of the last common ancestor between humans and the African apes."

"Aiello and Collard point out that the great age of Orrorin does not seem to be in serious question. The geology of the Lukeino Formation is well known; the volcanic tuffs in this formation have been securely dated at 6.2 to 5.6 million years old by radiometric techniques; and there is little doubt that the specimens come from the Lukeino Formation sediments. But Aiello and Collard suggest that it is difficult to have the same confidence in the conclusions of Senut et al about human evolutionary history, since they adopt a scheme that contrasts sharply with prevailing ideas and that glosses over many areas of controversy and uncertainty."

"Aiello and Collard point out that at least 13 known hominin species from Africa existed before Homo erectus, and that currently in consensus among most paleoanthropologists are only the following ideas concerning the main evolutionary paths: a) There are 5 hominin genera (Ardipithecus, Australopithecus, Paranthropus, Kenyanthropus, Homo). b) The large-toothed and massive-jawed genus Paranthropus represents a dead-end branch" (cf. hominoid taxonomy).

Aiello and Collard conclude,

It... appears that cranial and dental anatomy does not necessarily mirror molecularly determined phylogenies in modern primates, which casts considerable uncertainty on anatomically based evolutionary trees. For now, at least, it is probably best to avoid naming ancestors, and maintain a simple division: that between hominins of archaic aspect (Orrorin, Ardipithecus, Australopithecus -- including Paranthropus -- and Kenyanthropus) and hominins of modern aspect (Homo sapiens and the remaining species of Homo).

Source: ScienceWeek 13 July 2001.

Contacts

Leslie C. Aiello, University College London, UK

Martin Pickford, Chaire de Paléoanthropologie, Prehistoire du College de France, GDR 983 et UMR 8569 du CNRS, 8 rue Buffon, 75005 Paris, France

Brigitte Senut, Laboratoire de paléontologie du Muséum national d'histoire naturelle, UMR 8569 et GDR 983 du CNRS, 8, rue Buffon, 75005 Paris, France.

Bibliography of the orrorin tugenensis find

Pickford, M. and Senut, B. (2001). 'Millennium ancestor', a 6-million-year-old bipedal hominid from Kenya. South African Journal of Science 97. 1-2: 22.

Senut, Brigitte; Martin Pickford; Dominique Gommery; Pierre Mein; Kiptalam Cheboi; Yves Coppens (2001). First hominid from the Miocene (Lukeino Formation, Kenya). Comptes Rendus de l'Academie des Sciences, Series IIA - Earth and Planetary Science 332. 2 (30 January 2001): 137-144. Full text (subscription required).

Senut, Brigitte and Martin Pickford (2001). The geological and faunal context of Late Miocene hominid remains from Lukeino, Kenya. Comptes Rendus de l'Academie des Sciences, Series IIA - Earth and Planetary Science 332. 2 (30 January 2001): 145-152. Full text (subscription required).