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Orrorin tugenensis

Introduction

The year 2001 was filled with several big announcements on the early hominid front. The first big announcement was the "Millenium Man" find by Senut et al. from the ca. 6 myr Lukeino formation in the Tugen Hills of Kenya. The find included 12 fossil fragments uncovered in October and November of 2000 (one additional fossil attributed to the same species from the same formation had been discovered in 1974 and been the focus of contention as to its phylogenetic status) from at least 5 individuals, which included several limb bone fragments, several mandibular fragments, a single manual phalanx, and some assorted isolated teeth. The material was discovered by an expedition (the Kenya Paleontology Expedition) led by French paleoanthropologists Brigette Senut and Martin Pickford. While the find is extremely interesting due to several tantalizing characteristics and its date, the politics of the find perhaps received the most press.

The Kenya Paleontology Expedition is a joint venture between the Collêge de France, Paris and the Community Museums of Kenya (founded in part by Pickford). However, most research in Kenya has been conducted under the auspices of the National Museum of Kenya. Pickford worked at the National Museum until 1984 when he was fired by then-director Richard Leakey. Pickford later co-authored a "tell all" type book about R. Leakey (Master of Deceit). To say that there is bad blood between the two would be a definite understatement. The Community Museums of Kenya has been reported as a "name only" organization that does not actually constitute a museum, however, the Community Museums is a nonprofit nongovernmental organization licensed by the Kenyan government with equal legal status to the National Museum of Kenya. Until a recent change put the jurisdiction of permits in the hands of the Ministry of Education, permits were issued by the Office of the President. President Daniel arap Moi of Kenya granted permits in 1998 that are valid through December 31, 2001, to Pickford making the Senut/Pickford team perfectly legitimate legally. The problem comes in the form of the National Museums of Kenya (with Meave Leakey as primary paleoanthropologist) through which Andrew Hill of Yale was issued a permit in 1978 and has conducted research there over the past two decades. Hill claims that his permit covers the land on which the Orrorin material was discovered. The situation has involved very unprofessional actions and statements over the past year, including the way in which permits were obtained by Pickford et al. to do the fieldwork, the control (or influence) by the Leakeys within the National Museum that likely prevented the provision of permits by "cordial" means through the Museum, editorial comments in December 2000 and February 2001 issues of Science, reports by Richard Leakey to the government of Kenya that Pickford was illegally excavating in the area that led to Pickford's arrest and detention for several days in prison (Pickford is now reportedly - or was - sueing Leakey over the incident), and many vicious rumors and comments spread by both sides to colleagues and the media.

Regardless of the petty jealousies, political maneuvering, etc. by both sides of these issues, the fact remains that a very early hominid has been found by the Senut et al. team, the material is or was in their possession, initial publication has taken place, and it would be constructive if the material were to speak for itself rather than the egos of the participants in the dispute, all paleoanthropologists should be interested in anything that furthers knowledge of our evolutionary history. Since what little is known about the situation merely indicates that the Pickford/Senut team's only real infraction was stepping on toes in order to get access to fieldwork, it would be a good idea if those with hurt feelings put that aside and accepted the fact that they didn't find the specimens and they don't get the credit for them.

The material that was found is interesting in several respects, and several claims have been made by Senut et al. regarding the interpretation of the material. It has been claimed that the femora indicated obligate bipedalism, the size of the femora indicate a body size larger than expected for a stem hominid, and that the teeth show features that indicate thick enamel and small teeth and a primitive condition in the hominid line. The initial reports in Comptes Rendus positions the material as ancestral to Homo through "Praeanthropus" (a taxa used to refer to A. afarensis by those that do not consider it ancestral to later australopithecine lineages), with Ardipithecus a decendent of Orrorin that is ancestral to Pan. In this scenario the australopithecines are left out in the cold as dead-end side branches. "Orrorin" means means "original man" in Tugen, and the specific name "tugenensis" refers to the Tugen Hills locality where the material was discovered.

Diagnostic Features

As with any newly discovered "species", details are sketchy, access to material is severely restricted, and interpretations abound. The dating for this material is relatively secure, and as such, the spatial distance between this material and other known hominid material makes it likely that this species designation will stick over the long haul (though as a "true" species or a chronospecies, or with a change in genus designation always possible). The interpretations of the material ands it phylogenetic significance are much more uncertain. As with all the hominid pages on this web site, all perspectives contained within are opinion, either mine, "accepted" opinion, or the opinion of the person(s) they are attributed to. Since so very little is known I will present what Senut et al. have published make my guarded comments on those interpretations.

The O. tugenensis material originates from four localities within the Lukeino Formation at Tugen Hills, Kenya. An isolated lower molar (KNM LU 335) was uncovered at Cheboit in 1974, a manual phalanx (BAR 349'00) was uncovered at Kapcheberek, an small fragment of a proximal right femur (BAR 1215'00) was uncovered at Aragai, and the remainder of the material, which includes 2 mandibular fragments (BAR 1000a'00 and BAR 1000b'00) that make up the holotype of O. tugenensis, a proximal left femur (BAR 1002'00), a right distal humerus (BAR 1004'00), a second proximal left femur (BAR 1003'00), an I1 (BAR 1001'00), a P4 (BAR 1390'00), an upper right canine (BAR 1425'00), and a left M3 (BAR 1426'00) and a right M3 (BAR 1900'00), was uncovered at the Kapsomin locality. Of basic interest in terms of dating are the Rormuch Dolerite Sill (5.62 ± 0.5 myr) that runs through the Lukeino Formation, the lower levels of the Kararaina Basalts (5.65 ± 0.13 myr) that caps the Lukeino Formation, and the upper levels of the Kabarnet Trachytes (6.2 ± 0.13 myr) that runs just underneath the Lukeino Formation. These dates indicate that the material dates from approximately 6.2-5.65 myr. Additional supporting evidence comes in the form of volcanogenic crystals from the Lukeino Formation that yielded an age of 6.06 ± 0.13 myr. This makes the tugenensis material the oldest known hominids if the classification of the material as hominid is accepted (and the material definitely looks hominid, at least as much as any other early australopithecine material does that lacks extensive crania).

The upper incisor (BAR 1001'00) is described as robust, massive, relatively large mesio-distally, smaller in size than Australopithecus and similar in size to Ardipithecus. Some of the reported characteristics include:

The upper canine (BAR 1425'00) is described as triangular in labial view with a cervical outline that is not strongly mesio-distally compressed and with a swelling, but not a true cingulum, above the cervix. Some of the reported characteristics include:

The two upper third molars (BAR 1426'00 and BAR 1900'00) are described as trapezoidal - nearly triangular - in occlusal outline with small metacone and large protocone. The molars are moderately to heavily worn, making details of the crown uncertain. Some of the reported characteristics include:

The premolar (BAR 1390'00) is described as ovoid in occulsal outline and compressed mesiodistally with two offset roots. The enamel is missing from the posterior, anterior, and lingual surfaces. Some of the reported characteristics include:

The three lower molars from the recent excavations are lightly worn and preserved in the mandibular fragment (BAR 1000'00). The teeth are broken with enamel missing from the anterior and lingual surfaces of the left molars and from the mesial surface of the right molar. The right molar is also missing a small piece of enamel distally. The M2 is rectangular in occlusal outline with slight elongation mesiodistally. Some of the reported characteristics include:

The lower molar (KNM LU 335) is either an M1 or an M2 and was excavated in 1974. Its phylogenetic position and significance has been debated and similarities to both modern humans and chimpanzees have been pointed out.

The two left femora (BAR 1002'00 and BAR 1003'00) are the best preserved specimens from the Lukeino material. BAR 1002'00 is the most complete and preserves the femoral head and two thirds of the shaft. Both specimens lack the greater trochanter. The visible line of fusion on the BAR 1002'00 specimen indicates that it is likely from a young adult. Some of the reported characteristics of the femora include:

Senut et al. claim that the femora show evidence of bipedal locomotion, if not obligate bipedalism. Comparisons are made to A.L. 288-1 and modern humans to show that Orrorin has features that indicate bipedalism. However, much of this evidence is equivocal and does not provide sufficient support for obligate bipedalism. The presence of an intertrochanteric grooves is provided as support, but the presence - or lack there of - in proving bipedalism is not particularly clear. Features that can be used as unequivocal evidence of bipedalism are not present due to the lack of a distal femur, proximal tibia, and the missing greater trochanter. The only clear interpretation of this material with regards to bipedalism at the moment is that its morphology does not rule out bipedalism. Bipedalism is not an unlikely interpretation based on the material evidence, but it is a premature and unclear claim.

The upper limbs are represented by a distal shaft (BAR 1004'00). This specimen shows a straight lateral crest, onto which inserts the m. brachioradialis muscle. This feature is present in modern chimpanzees as well as in afarensis, and has been linked with climbing adaptations. The manual phalanx (BAR 349'00) is curved, similar to afarensis, further evidence of climbing adaptations. Senut et al. conclude that tugenensis was a bipedal hominid that was adapted also to arboreal activities.

Most of the fossil material from the Kapsomin locality have a thin carbonate coating of algal or bacterial origin that indicates they were deposited in water before being covered in sediments. Some of the specimens (including the hominid mandibular fragments) are severely cracked, likely due to desiccation through exposure prior to burial. The faunal assemblage is dominated by small to medium size bovids and small colobines. The presence of impalas suggest an open woodland site with the colobine species indicating there were denser strands of trees in the vicinity. Marginal woodland environment is supported as the likley environment of this O. tugenensis material.

Conclusions

As with all new and scrappy finds, controversy is everywhere. Senut et al. take great pains to show disimilarity between Ororrin and the australopithecines, and similarity between Orrorin and Homo. The teeth are relatively small, similar to Ardipithecus, however, retain thick enamel like all australopithecine and Homo, perhaps making it more likely an ancestor of the hominids and Ardipithecus an ancestor of modern chimpanzees (a possibility that has been pointed out before in reference to the Ardipithecus material). However, the claim that it indicates australopithecines as a dead-end side branch due to the larger body size of Orrorin compared to A.L. 288-1 and the larger teeth of the australopithecines is very weak. A.L. 288-1 ("Lucy") is a female of a highly dimorphic species, and since sexual dimorphism decreased gradually through the australopithecine line it would be expected that an earlier ancestor would be even more dimorphic. If the Orrorin femora belong to males it is unreasonable to make claims of a decline in body size in the australopithecine line. The size of the teeth is also extremely weak support, as tooth size increased in all African ape lineages over time, and shifts in diet is well within reason to describe a small increase and then a decrease in the line leading to modern humans. It is also important to keep in mind that with such a small sample size any such argument about the population these individuals belonged to is weak at the very least.

The date of this material makes it extremely important in the study of human evolution prior to and after the split with the line leading to Pan. Hopefully, more material will be discovered, including better evidence for or against bipedalism, and more complete crania material.


Bibliography

This bibliography contains the sources of the information cited above, as well as any sources that could provide any other information on the subject. If you know of any other sources that are pertinent to O. tugenensis, please e-mail me the citation in the format used below, and I will add it to the list. Any problems with information I presented above can be sent to me here. I don't want to provide disinformation, and any corrections are gladly accepted (with proper documentation of what is wrong and why, with sources). Thanks!

Pickford, M. 1975. "Late Miocene sediments and fossils from the northern Kenya Rift, Valley." In Nature, vol. 256, pp. 279-284.

Pickford, M., and B. Senut. 2001. "The geological and faunal context of Late Miocene hominid remains from Lukeino, Kenya." In Comptes Rendus de l'Académie de Sciences, vol. 332, pp. 145-152.

Senut, B., M. Pickford, D. Gommery, P. Mein, K. Cheboi, and Y. Coppens. 2001. "First hominid from the Miocene (Lukeino Formation, Kenya)." In Comptes Rendus de l'Académie de Sciences, vol. 332, pp. 137-144.


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