In 1967, a team of palaeontologists led by Richard Leakey travelled to the Kibish Formation along the Omo River in southern-most Ethiopia, near the town of Kibish. The expedition consisted of three contingents: French, under Camille Arambourg, American, under Clark Howell, and Kenyan, led by Richard Leakey. Louis Leakey, his father and world renowned paleoanthropologist and archaeologist could not go because of his arthritis. Crossing the Omo in 1967, Richard Leakey’s contingent was attacked by crocodiles, which destroyed their wooden boat. Expedition members barely escaped with their lives. Richard Leakey radioed Louis for a new, aluminium boat, which the National Geographic Society was happy to supply.
On site, Kamoya Kimeu, Richard Leakey’s right-hand man and a field-trained Kenyan was the first one to spot the bits of fossils leading to the discovery of the earliest Homo sapiens skull (Omo I), now dated to 195,000 years ago. Kimeu is now regarded as the most successful discoverer of hominid bones in the world. They found the skull (minus the face 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 excavations of Kibish Formation between 1967–1974, yielded two very important Homo sapiens (early human bones) calvariae, each from different localities — Omo I which resembles modern humans and Omo II which has more primitive features. Omo I’s site was called Kamoya’s Hominid Site (KHS) after the discoverer, and about 2.6 km northwest of KHS, Omo II’s site, Paul’s Hominid Site (PHS) is located.
Geologist Frank Brown was not part of the 1967 expedition, but was working nearby and got to look at the site and the fossils.
In 1967, these hominid fossils were dated as being 130,000 years old, although the scientists doubted the accuracy of their dating technique, which was based on the decay of uranium-238 to thorium-238 in oyster shells from a rock layer near the skulls.
Geologist and geochronologist Ian McDougall of Australian National University in Canberra, anthropologist John Fleagle of New York State’s Stony Brook University and geologist Frank Brown, dean of the University of Utah’s College of Mines and Earth Sciences co-authored a study titled: The Oldest Homo Sapiens: Fossils Push Human Emergence Back To 195,000 Years Ago, which was published by the journal Nature in its Feb. 17, 2005, issue. These three researchers and researchers from other universities returned to Kibish in 1999, 2001, 2002 and 2003. They identified sites where Omo I and Omo II were found in 1967, and obtained more of Omo I, including part of the femur (upper leg bone) that fit a piece found in 1967. They also found animal fossils and stone tools, and studied local geology. The Nature study includes initial results from those expeditions.
The fossil record of human ancestors may go back 6 million years or more, and the genus Homo arose at least 1.8 million years ago when australopithecines evolved into human ancestors known as Homo habilis. Brown says the fossil record of humans is poor from 100,000 to 500,000 years ago, so Omo I is significant because it now is well dated.
Both Omo I and Omo II were buried in the lowermost portion or “member” of the Kibish Formation, a series of annual flood sediments laid down rapidly by the ancient Omo River on the delta where it once entered Lake Turkana. Lake levels now are much lower, and the river enters the lake about kilometres south of Kibish.
The 100-meter-thick formation is divided into at least four members, with each of the four sets of layers separated from the other by an “unconformity,” which represents a period of time when rock eroded away instead of being deposited. For example, the lowermost Kibish I member was deposited in layers as the Omo River flooded each year. After thousands of years, rainfall diminished, lake levels dropped, and the upper part of Kibish I eroded away. Later, the lake rose and deposition resumed to create layers of Kibish member II.
Interspersed among the river sediments are occasional layers of volcanic ash from ancient eruptions of nearby volcanoes. Some ash layers contain chunks of pumice, which in turn contain feldspar mineral crystals. Feldspar has small amounts of radioactive potassium-40, which decays into argon-40 gas at a known rate. The gas, trapped inside feldspar crystals, allows scientists to date the feldspar and the pumice and ash encasing it.
Geologist, Frank Brown says potassium-argon dating shows that a layer of ash no more than 10 3 meters below Omo I’s and Omo II’s burial place is 196,000 years old, give or take 2,000 years. Another layer is 104,000 years old. It is almost 50 meters above the layer that yielded the Omo humans. The unconformities represent periods of time when rock was eroded, so the fossils must be much older than the 104,000-year-old layer and close in age to the 196,000-year-old layer, Brown says.
The clinching evidence, he says, comes from sapropels, which are dark rock layers on the Mediterranean seafloor that were deposited when floods of fresh water poured out of the Nile River during rainy times. The Blue Nile and White Nile tributaries share a drainage divide with the Omo River. During ancient wet periods, monsoons on the Ethiopian highlands sent annual floods surging down the Nile system, causing sapropels to form on the seafloor, and sent floods down the Omo, making Lake Turkana rise and depositing Kibish Formation sediments on the river’s ancient delta. (During dry periods, Lake Turkana was smaller, flood sediments were deposited farther south and rocks at Kibish were eroded.)
No other sediments on land have been found to record wet and dry periods that correlate so well with the same climate pattern in ocean sediments, Brown says. The new study found that the “members” – or groups of rock layers – of the Kibish formation were laid down at the same time as the Mediterranean sapropels. In particular, the volcanic layer right beneath Omo I and II dates to 196,000 years ago by potassium-argon dating, and it corresponds almost perfectly to a sapropel layer previously dated as 195,000 years old, Brown says.