Two articles in the Journal of Human Evolution reanalyze the geochronology of the Kibish Formation, in the Omo Valley, Ethiopia. One is titled, “Microstratigraphy of the Kibish hominin sites KHS and PHS, Lower Omo Valley, Ethiopia,” and the other is titled, “Sapropels and the age of hominins Omo I and II, Kibish, Ethiopia.”
Excavations of Kibish Formation between 1967–1974, yielded two very important Homo sapiens 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. These two were first indirectly dated to be 130,000 years old by a U-series disequilibrium analysis (a measurement of the decay of uranium into thorium over time) from layer near the Omo I site. Why Omo II wasn’t dated? I don’t know… later on I quote how Omo II’s exact location is kinda uncertain. Regardless, this date wasn’t well accepted. Richard Klein raised the possibility, in his book “The Human Career,” that Omo I and Omo II effectively sank into a older sediment layer (Member I) because the site was once a prehistoric delta.
About 40 years later, Frank Brown and Ian McDougall along with John Feagle returned to the Omo Valley and re-dated mineral crystals from volcanic tuffs where Omo I and II came from. Why? One reason is that the 160,000 year old Homo sapiens idaltu crania (BOU-VP-16/1, BOU-VP-16/2 and BOU-VP-16/5) from Herto, Ethiopia dethroned their claim to the oldest known Homo sapiens specimens. The team dated the 40Ar/39Ar feldspar crystals from pumice clasts above (Member III) and below layers (Member I) of river sediments that contained the early human bones. The argon-argon dates yielded a 195,000 year old age. They published their results in Nature, “Stratigraphic placement and age of modern humans from Kibish, Ethiopia.”
The big problem with this re-dating has been the uncertainty of whether or not they were getting crystals exactly from where the fossils came from. A National Geographic article summarized how the team estimated where the fossils came from,
“They were able to do this using National Geographic Society video footage taken during the first excavation. They also used photographs taken by Karl Butzer, a geologist currently at the University of Texas, who did the original geological studies of the site. Also helpful were hand-drawn maps from the late Paul Abell, another member of the 1967 team.”
As you can imagine, a substantial amount of erosion and deformation can happen to geological formations during 40 years, that would alter the stratigraphic context. And looking at videos, and photos, and hand-drawn maps is an hardly exact science. Believe me, exactly 1 year ago I was trying to find a locality based off of hand-drawn maps and missed. Frank Brown was even quoted in this same National Geographic article saying that the original locations have been lost. But they were able to find more of Omo I, including part of the femur that fit a piece found in 1967.
In the two new papers, both sets of authors reanalyze the best available evidence to pinpoint where Omo I and II came from: Member 1 of the Kibish Formation. Craig Feibel reconstructs the microstratigraphy of the formation and writes that he cannot falsify Klein’s claim but reaffirms that,
“The documented abundance of vertebrate fossils deriving from restricted levels in upper Member I provides an accumulational/preservational context within which the hominins are not out of place. Intrusion during the short period of terminal delta-plain development in latest Member I times would be temporally insigniﬁcant in any case.”
McDougall, Brown and Fleagle return to the argon-argon dating. They turn to checking out alkali feldspar crystals in pumice clasts and figure out that Nakaa’kire Tuff in Member I, and directly below the hominin levels, is 198,000 years old — extending the age of Omo I and II by 3,000 years. I do not know why PHS was not dated, but in the new paper the authors do write that the site of Omo II was mislocated by Butzer in his maps. In their conclusion, they make a snarky/competitive comment to the Herto discoverers, saying,
“Thus, the hominin fossils from Kibish are about 40 ka older than those from Herto, making Omo I and Omo II the oldest well-dated anatomically modern human fossils yet recovered.”
FEIBEL, C. (2008). Microstratigraphy of the Kibish hominin sites KHS and PHS, Lower Omo Valley, Ethiopia. Journal of Human Evolution DOI: 10.1016/j.jhevol.2008.05.011
MCDOUGALL, I., BROWN, F., FLEAGLE, J. (2008). Sapropels and the age of hominins Omo I and II, Kibish, Ethiopia. Journal of Human Evolution DOI: 10.1016/j.jhevol.2008.05.012
McDougall, I., Brown, F.H., Fleagle, J.G. (2005). Stratigraphic placement and age of modern humans from Kibish, Ethiopia. Nature, 433(7027), 733-736. DOI: 10.1038/nature03258