Orrorin tugenensis - Grade: A+ PDF

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Summary

Description of fossil orrorin tugenensis and current debates of its involvement in Evolution...

Description

ANTH-5500/BIOS-5970

FOSSIL TOPIC: ORRORIN TUGENENSIS: FEATURES AND CLASSIFICATION

ANELIS CHILUISA

FALL 2019

November 6, 2019

Features, time period, and local discoveries There are multiple ways to classify primitive hominins, primate ancestors, and human ancestors. But when it comes to Orrorin tugenensis, there’s a major debate on whether this and his ancestor Sahelanthropus tchadensis, and his further successors like Ardipithecus ramidus s. s. and Ar. kadabba were primate or human ancestors. It was later suggested that by looking at the different features of these species like the brow ridges, jaws, canines, face, femur length, foramen magnum presence, and others made them indeed primitive hominins (Ugar P., 2007).

Orrorin tugenensis was found around the 2000’s in the sediments of Tugen Hills of northern Kenya and was dated to exist around 6 mya based on radiometric decay [K-Ar dating] (Ugar P., 2007). At first, the fossils found were in poor conditions; there were only one cranial, one postcranial, and three partial femur fossils that researchers did their studies on. However, up to date other fossils have been found in four other sites in East Africa that has helped in the study of this species such as: posterior part of the mandible in two pieces, a symphys and several isolated teeth, three fragments of femurs, a partial humerus, a proximal phalanx, and a distal thumb phalanx (Dorey, 2018).

O. tugenensis is considered a primitive hominin because its enamel was thicker than in normal panins. In addition, another strong feature of O. tugenensis is his femoral neck. In primates that are normally climbers, the outer bone is equally thick all around the neck of the femur whereas in bipeds, this femur was thicker at the top and bottom of the neck. Additionally, by looking at the best conserved bone, Bar1002’00, it was shown that this specimen had a larger femur head

relative to other preserved parts (e.g. neck and shaft) yielding higher length estimates (Nakatsukasa M. et al., 2007). Regarding brain size, it was probably similar to a modern chimpanzee (320-400 cc), but because of poor fossil findings, it remains inconclusive. The same has been said for their overall body size and shape, since most researchers assume is probably similar in size to a modern chimpanzee. By looking at the molar fossils, they have primitive dental anatomy, molars found are relatively small although their canines are relatively large and pointed compared to humans. (Dorey, 2018). Their dentition also showed that their diet was probably based on fruit and vegetables but because of the canines they could have been also meat eaters. Because of their diet assumption, they probably lived in green areas rather than in the dessert. (Dorey, 2018).

New taxonomic designation When the features were analyzed for bipedalism, it was accepted that O. tugenensis could be a primitive hominin rather than an ape ancestor. Moreover, they claim that this species belong to the human family tree because of the strong evidence seen in the femoral neck of one of the fossils, and therefore, it should be its own genus since it doesn’t fully agree with Australopithecus and in fact the latter should be a side branch in the phylogenetic tree leaving O. tugenensis as the earliest direct ancestor of humans (Dorey, 2018). Because there is not enough evidence to support this argument due to the poor condition of the fossils found on this species, this taxonomy has not been fully accepted.

There are two different taxonomies that currently place O. tugenensis as a primitive hominin but not as a direct ancestor of humans. The splitter taxonomy is more detailed and classifies O.

tugenensis as a possible primitive hominin along with Sahelanthropus, Ardipithecus ramidus s. s. (sensu stricto), and Ar. kadabba, whereas the lumper taxonomy is more general and includes O. tugenensis with the same species but this time Ar. ramidus s. l. (sensu lato) is the representing species.

Controversy to current classification There are multiple debates as to where this species should be classified and why. The reason for O. tugenensis to be bipedal is very clear and well supported by multiples studies on the most conserved femur. In the femur, the head is spherical and rotated anteriorly; the neck is elongated and oval in section; and the lesser trochanter protrudes medially. These characteristics are major findings in bipedal hominins. However, the rest of the post cranium suggests that they were also able to climb trees. The phalanx found is also similar to a human phalanx, but it could just indicate that they had grasping abilities to climb trees (Senut et al., 2001).

Another suggestion was that this species had small molars with thick enamel, a feature that persists into the Homo line but is absent in the australopithecines. This suggests that a split between the two groups had already occurred and therefor Australopithecus afarensis, or “Lucy” could be just a side branch of the Homo line and O. tugenensis the direct ancestor of humans. DNA of ape and human was compared and suggested the split occurred around 6 mya, which happened around O. tugenensis time. In contrast, Lucy appeared between 2.9 and 3.9 mya, much earlier than O. tugenensis (Schuster A., 2001).

References

Ungar, P. S. (2007). Evolution of the Human Diet: The Known, the Unknown, and the Unknowable. Oxford: Oxford University Press. Dorey, F. (2018, April 11). Orrorin tugenensis. Retrieved from https://australianmuseum.net.au/learn/science/human-evolution/orrorin-tugenensis/. Nakatsukasa, M., Pickford, M., Egi, N., & Senut, B. (2007). Femur length, body mass, and stature estimates of Orrorin tugenensis, a 6 Ma hominid from Kenya. Primates, 48(3), 171–178. doi: 10.1007/s10329-007-0040-7 Senut, B., Pickford, M., Gommery, D., Mein, P., Cheboie, K., & Coppens, Y. (2001). First hominid from the Miocene (Lukeino Formation, Kenya). Earth and Planetary Sciences, 137–144. Schuster, A. M. H. (2001). Ancient Ancestors? Archaeology, 54(4), 24–25.

Appendix I: Homo Habilis- 3D structure The 3D structure printed was the skull known as KNM ER 1813 which belongs to Homo habilis. This species in general was discovered by Louis Leakey at Olduvai Gorge but the specimen on the 3D print was found outside East Africa and dates around 1.8 to 2.5 mya. It was considered the first hominin with a large brain in the range of 500 and 700 cc. There is the hypothesis that due to severe climate change they needed to hunt and developed a sense of communication (Broca’s area in the brain) and because of this they evolved from A. afarensis type to A robustus and Homo lineages. They were also found with Oldowan tools.

By looking at the fossil, it can be seen that it presents also slight prognathism, but it is less than A. africanus or A afarensis. H. habilis also has thicker brow ridges. Another important difference is also the molars, their dentition is narrower than australopithecines and have a thinner enamel. Hence why it is suggested that they used tools to process animal and plant materials or had softer diet. There is a phylogeny confusion as to what hominin evolved after A. afarensis and it is concluded that H. habilis can be one of the different lines for Homo evolution. It can also be suggested that A. africanus evolved after A afarensis or A. garhi evolved after A. afarensis. There’s no straight line or a concrete consensus on this.

The quality of the print is fairly good in resolution, however, is not as sophisticated as the one seen in class. A few general traits can be identified better than others. For example, the zygomatic flare and brow ridges compared to the molars. In contrast, the fossils found in lab are multiple and can demonstrate the different fossils found for this species and we can also compare with the other fossils from different hominin lineages....