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Mar. 18, 2005 The human race as it exists today belongs to a family of species called Homo sapiens. The origin of Homo sapiens is not yet resolved; but two extreme theories have been proposed. One of them is a "Multiregional Hypothesis", which states that all modern humans evolved in parallel from three different species originating from Africa, Europe and Asia around 50,000 years ago, with some genetic intermixing among these regions. The second speculation is a "Out of Africa Hypothesis" whereby a small, relatively isolated population of early humans evolved into modern Homo sapiens, and succeeded in spreading across Africa, Europe, and Asia - displacing and eventually replacing all other early human populations as they spread. Studies of the diversity and mutation rate of nuclear DNA and mitochondrial DNA in living human cells provide partial evidence in support of this theory. From these studies an approximate time of divergence from the common ancestor of all modern human populations is estimated to be around 200,000 years ago, too young for the "Multiregional Hypothesis, implying that the ancestral population of all living people migrated from Africa to other parts of the world. Irrespective of whichever speculation (if either) is correct; the oldest fossil evidence unearthed (from Africa) so far was about 130,000 years old, which falls short of the dating arrived at by genetic analyses of modern populations. However, recent findings based on modern dating techniques have stretched back the dawn of humans 65,000 years, crediting two Ethiopian fossils - Omo I and Omo II - as the oldest member of our species. The two skulls and some bones were first uncovered by Richard Leakey and his team of paleontologists at the Kibish Formation along the Omo River in southernmost Ethiopia, near the town of Kibish in 1967. They found the skull (minus the face) and partial skeleton (parts of arms, legs, feet and the pelvis) of Omo I, and the top and back of the skull of Omo II. These hominid cranial remains were identified as early anatomically modern humans, assigned to Homo sapiens. Dating techniques 38 years ago were not as sophisticated as they are now. As a result, nobody attempted to date the fossils' burial site more accurately, until recently the National Science Foundation, the L. S. B. Leakey Foundation, the National Geographic Society and the Australian National University funded a study. In their latest expedition, the researchers, led by Ian McDougall of the Australian National University in Canberra, collected samples of the rock where the Omo fossils were found and dated the mineral crystals in volcanic ash layers above and below the layers of river sediments that contained the early human bones. Using an improved dating method based on the rate of decay of radioactive argon, the fossils were evaluated much older than a 104,000- year-old volcanic layer and very close in age to a 196,000-year-old layer. In order to apply modern dating techniques, the researchers had to obtain specimens from the precise location where the fossil remains had been excavated in 1967. This posed a big challenge in itself. Besides relying on GPS technology, the research team resorted to the video footage captured by the National Geographic Society during the first excavation. Hand-drawn maps from the late Paul Abell and photographs taken by the geologist Karl Butzer, both members of the 1967 team, were also used to locate the original excavation site of 1967. The researchers sampled the volcanic ash on both sides of the river that lay above where the fossils were found. The ash was the same on both sides. The research team found feldspar crystals from a volcanic eruption inside pumice fragments, which indicated that the crystals have not been contaminated. Such unadulterated crystals can be dated using a technique called potassium-argon dating. Ian McDougall and his team used a dating technique called 40Ar/39Ar, which is a variant of potassium-argon dating. By dating the crystals held in the pumice, scientists can be sure that everything in that group of sediment layers is nearly the same date. The remains of Omo I and Omo II were buried in the lowest sediment layer, called Member 1, of the 330-foot-thick (100-meter- thick) Kibish rock formation near the Omo River. In the same Member 1 sediment layers, the team found additional Omo I bones, animal fossils, and stone tools. The rock layers were formed in rapid bursts, corresponding to wet periods during which huge amounts of organic matter were dumped in the region by the overflowing River Nile, implying that the fossils are likely to be only slightly younger than the rocks on which they were lying. The study confirmed that the Omo I and Omo II hominid fossils are from similar stratigraphic levels in Member I of the Kibish Formation, despite the view that Omo I is more modern in appearance than Omo II. 40Ar/39Ar ages on feldspar crystals from pumice clasts within a tuff in Member I below the hominid levels place a limit 184,000 to 212,000 years on the hominids. Geological evidence indicates rapid deposition of each member of the Kibish Formation. Isotopic ages on the Kibish Formation correspond to ages of Mediterranean sapropels, which reflect increased flow of the Nile River, and necessarily increased flow of the Omo River. Thus the 40Ar/39Ar age measurements, together with the sapropel correlations, indicate that the hominid fossils have an age in the range 190,000 to 200,000 years, making them the earliest well- dated anatomically modern humans yet described. Results of laser fusion 40Ar/39Ar dating of alkali feldspar crystals from pumice clasts obtained from the Kibish Formation, west of Omo River at Nakaa’kire, 1 km north of Aliyo, Ethiopia, is published in the February 16, 2005 issue of the journal Nature. ------------ About the author: Soumya Maitra is a Software Engineer at HCL Technologies, India. He has published several popular science articles in various magazines, newspapers, and journals in India, such as Science Reporter, The Statesman, The Telegraph, Wisdom, Bioinformatics India Journal, and many more. Email: soumya.maitra@rediffmail.com Tell a friend about this site! ------------ All articles are EXCLUSIVE to Useless-Knowledge.com. 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