Thursday, July 07, 2011

Redating of Homo erectus Remains Validates Original Dates

The 20-meter terrace of the Solo River was excavated in the 1930s, yielding a nice cache of Homo erectus remains from the site of Ngandong and Sambungmachan. Using biostratigraphy, these remains were thought to be around 150-200 thousand years old.

Approximately fifteen years ago, Carl Swisher dated the Solo River terrace deposits containing the Ngandong hominin fossil remains by Uranium series and electron spin resonance to between 35 and 50 000 years ago. This put the temporal range of the Indonesian hominins at some 1.8 million years and surprised the palaeoanthropological community mightily. Given that modern humans could be found in the region from 50 000 years ago (Willandra Lakes Hominid 50), this gave considerable ammunition to the argument that there had been no continuity between Homo erectus and Homo sapiens in the region, supporting the replacement model of the origins of modern humans. While support for the replacement model was considered to have support from other quarters, nobody was prepared for 50 000 yea-old Homo erectus remains. Two initial criticisms were leveled at Swisher's methodology. First, it was thought that the fossils had been redeposited from older sediments and, second, Swisher had dated the excavation remains, not the original undisturbed sediments.

Now it seems that a redating of the 20-meter terrace have vindicated the older dates. In an article in PLoS, Indriatti, Swisher and others have arrived at much older dates. A news release from New York University has this to say:
The team applied two different dating techniques to the sites. Like earlier work, they used the techniques—U-series and Electron Spin Resonance, or ESR—that are applied to fossilized teeth. They also used a technique called argon-argon dating that is applied to volcanic minerals in the sediments. All three methods use radioactive decay in different ways to assess age and all yielded robust and methodologically valid results, but the ages were inconsistent with one another.

The argon-argon results yielded highly precise ages of about 550,000 years old on pumices—very light, porous volcanic products found at Ngandong and Jigar.

“Pumices are hard to rework without breaking them, and these ages are qu good, so this suggests that the hominins and fauna are this old as well,” said project geochronologist Carl Swisher of Rutgers University.

By contrast, the oldest of the U-series and ESR ages, which were conducted at Australian National University by Rainer GrĂ¼n, are just 143,000 years.
The PLoS article is here. The authors conclude:
At the very least, it now seems possible to bracket the age of the deposits at Ngandong and Jigar with a maxima of 546 ka based on the argon results and a minima of 143 ka based on the oldest of our fully modeled combined ESR/U-series ages.
If the middle Pleistocene 40Ar/39Ar ages better reflect the age of the Solo River 20 meter terrace deposits and hominins, the site of Ngandong remains a relatively late source of H. erectus; however, these H. erectus would not be the contemporaries of Neandertals and modern humans, and their chronology would widen the gap between the last surviving H. erectus and the population from Flores – whose source population has been argued to be Indonesian H. erectus; although this point is contested. Instead, the Ngandong hominins would be contemporaries of the H. heidelbergensis from Atapuerca, Spain and elsewhere in Europe, and, possibly the archaic H. sapiens specimen from Bodo (Ethiopia), which might favor arguments that they are more closely affiliated with these taxa and differ from H. erectus. Such ages for Ngandong would suggest that a series of geographically relatively isolated lineages of hominins lived during the middle Pleistocene.
There was evidently a considerable amount of variation present in these hominins and it seemed to us1 that there was quite a bit in the Chinese Homo erectus sample as well. We wouldn't take the steps of calling them separate species but the differences were definitely there.

More pieces of the puzzle.

1Kidder, J. H., & Durband, A. C. (2004). A re-evaluation of the metric diversity within Homo erectus. Journal of human evolution, 46(3), 297-313.

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