Ian McDougall of the Australian National University in Canberra and his colleagues worked with two well-known fossil finds known as Omo I and Omo II, which were recovered from Ethiopia's Kibish Formation by Richard Leakey. The remains include two partial skulls as well as arm, leg, foot and pelvis bones for Omo I. "Anthropologists said they looked very different in their evolutionary status," remarks study co-author Frank Brown of the University of Utah. "Omo I appeared to be essentially modern Homo sapiens and Omo II appeared to be more primitive." At the time, the bones were dated to 130,000 years ago, based on radioactive decay of uranium and thorium from oyster shells found nearby. This time the scientists returned to the southern Ethiopian site and identified the resting places of both individuals. They also unearthed another part of a femur bone for Omo I that fits with the original remains.
The researchers then analyzed the volcanic ash layers above and below the river sediment that contained the fossils using argon dating. They determined that the rock just below the fossils dated to 196,000 years ago. Because the layers of the Kibish Formation formed quickly during wet seasons that inundated the area with organic matter, the team posits that the bones are only slightly younger than this underlying layer. In addition, a layer of ash more than 150 feet above the burial sites dates to 104,000 years old, putting a lower limit on their age. Using other evidence, which drained from the Nile and the Omo rivers onto the Mediterranean seafloor, the researchers attest that the Omo fossils are most likely no younger than 190,000 years old.
Previously the oldest known traces of our species were fossils from Herto, Ethiopia, that date to about 160,000 years ago. The older age of the Omo remains is concordant with dates suggested by genetic studies for the origin of our species, says study co-author John Fleagle of Stony Brook University. He adds that "as modern human anatomy is documented at earlier and earlier sites, it becomes evident that there was a great time gap between the appearance of the modern skeleton and 'modern' behavior."