As described in a letter published in the February 17, 2005 issue of Nature, scientists from Australia and the United States have estimated the age of anatomically modern human fossils in Ethiopia to be 195,000 years old, plus or minus 5000 years. McDougall, et al, reached this estimate after correlating rock layers at different sites and using radiometric dating to determine the ages of layers above and below the layer that contained the hominid fossils.

The image to the left is a composite stratigraphy of the Kibish Formation, reconstructed by comparing rock layers at various locations along a section of the Omo River in Ethiopia. The hominid fossils, labeled “Omo I” and “Omo II”, were discovered at two separate locations near the bottom of the sequence (around 16 meters using the meter scale on the far left). Samples from the rock layers labeled “Member I Tuff” and “Member III Tuff” were dated using radiometric dating to 196,000 years old plus or minus 2000 years and 104,000 years old plus or minus 7000 years.

Using this data and a geology principle called superposition, scientists were able to restrict the age of the fossils. The principle of superposition is based on common sense: a rock layer is younger than the layer below it and older than the layer above it. After all, a younger rock layer cannot somehow slip underneath an older rock layer. Since the layer including the fossils was above the Member I Tuff and below the Member III Tuff, the age of the fossils must be somewhere between these two age limits. The scientists then used other evidence to determine that the age of the fossils is likely closer to the lower limit of 196,000 years old. They conclude in their paper that “[o]ur preferred estimate of the age of the Kibish hominids is 195 +/- 5 kyr, making them the earliest well-dated anatomically modern humans yet described.”