Orrorin tugenensis

Taxobox | name = "Orrorin"
fossil_range = Miocene


image_width = 250px
image_caption = "Orrorin tugenensis" fossils
regnum = Animalia
phylum = Chordata
classis = Mammalia
ordo = Primates
familia = Hominidae
subfamilia = Homininae
tribus = Hominini
subtribus = Hominina
genus = "Orrorin"
genus_authority = Senut et al, 2001
species = "O. tugenensis"
binomial = †"Orrorin tugenensis"
binomial_authority = Senut et al, 2001

"Orrorin tugenensis" is considered to be the oldest known hominin ancestor related to modern humans and is the only species classified in genus "Orrorin". The name was given by the discoverers who found "Orrorin" fossils in the Tugen Hills of Kenya. By using radiometric dating techniques, the volcanic tuffs and lavas, faunal correlation and magneto-stratigraphy, the strata in which the fossils were found were estimated to date between 6.1 and 5.8 million years ago, during the Miocene. This find is significant because it represents the earliest hominid species with evidence of bipedal locomotion.

The fossils found so far come from at least five individuals. They include a femur, suggesting that "Orrorin" walked upright; a right humerus shaft, suggestive of tree-climbing skills but not brachiation; and teeth that suggest a diet much like that of modern humans. The obturator externus groove on the posterior aspect of the neck of the fossil femur suggests that "Orrorin tugenensis" moved bipedally. The bunodont, microdont molars and small canines suggest that "Orrorin" ate mostly fruit and vegetables, with occasional meat. "Orrorin" was about the size of a modern chimpanzee.

The team that found these fossils in 2000 was led by Brigitte Senut and Martin Pickford from the Muséum national d'histoire naturelle. The discoverers conclude that "Orrorin" is a hominin on the basis of its bipedal locomotion and dental anatomy; based on this, they date the split between hominins and African great apes to at least 7 million years ago, in the Messinian. This date is markedly different from those derived using the molecular clock approach, but has found general acceptance among paleoanthropologists.

If "Orrorin" proves to be a direct human ancestor, then australopithecines such as "Australopithecus afarensis" ("Lucy") may be considered a side branch of the hominid family tree: "Orrorin" is both earlier, by almost 3 million years, and more similar to modern humans than is "A. afarensis". The main similarity is that the Orrorin femur is morphologically closer to that of "H. sapiens" than is "Lucy's"; there is, however, some debate over this point.

Other fossils (leaves and many mammals) found in the Lukeino Formation show that "Orrorin" lived in dry evergreen forest environment, not the savanna assumed by many theories of human evolution. Thus, the origins of bipedalism occurred in an arboreal precursor living in forest and not a quadrupedal ancestor living in open country. A recently published idea suggests that ancestral apes may have shared the technique used by modern orangutans of moving bipedally over small springy branches with the vertebral column oriented vertically (orthograde), using their arms for balance and keeping their legs straight. This kind of upright locomotion could have been used as a way of getting around on the ground when gaps opened in the forest canopy. Our closest extant relatives the gorillas and chimpanzees developed a flexed stance (with clinograde, (sloping) vertebral column) and are more adapted to tree climbing and to quadrupedal locomotion while on the ground. According to a minority of researchers, like humans, they have fused and strengthened wrist bones suggesting a shared period of knuckle walking. [cite web | url = http://education.guardian.co.uk/higher/research/story/0,,2093002,00.html | title = New theory rejects popular view of man's evolution - Research - EducationGuardian.co.uk | accessdate = 2007-11-05 | author = Sample, I. | date = June 1, 2007] [cite web | url = http://news.bbc.co.uk/1/hi/sci/tech/6709627.stm | title = BBC NEWS - Science/Nature - Upright walking 'began in trees' | accessdate = 2007-11-05 | date = 31 May 2007] [cite web | url = http://www.liv.ac.uk/premog/premog-sup-info-SCIENCE.htm | title = PREMOG - Supplementry Info | accessdate = 2007-11-01 | author = Thorpe S.K.S. | coauthors = Holder R.L., and Crompton R.H. | date= 24 May 2007 | work = Origin of Human Bipedalism As an Adaptation for Locomotion on Flexible Branches | publisher = Primate Evolution & Morphology Group (PREMOG), the Department of Human Anatomy and Cell Biology, the School of Biomedical Sciences at the University of Liverpool]

References

*cite journal |last=Senut |first=B. |authorlink= |coauthors="et al." |year=2001 |month= |title=First hominid from the Miocene (Lukeino Formation, Kenya) |journal=Comptes Rendus de l'Académie de Sciences |volume=332 |issue=2 |pages=137–144 |doi=10.1016/S1251-8050(01)01529-4 |url= |accessdate= |quote=
* [http://cogweb.ucla.edu/ep/Orrorin.html Orrorin Tugenensis: Pushing back the hominin line]

External links

* [http://www.esi-topics.com/fbp/comments/december-01-Martin-Pickford.html Martin Pickford answers a few questions about this month's fast breaking paper in field of Geosciences]
* [http://news.bbc.co.uk/2/hi/science/nature/4201666.stm BBC News: First chimpanzee fossils found]