martes, 11 de febrero de 2014

New dating pushes Atapuerca Homo antecessor to 900,000 BP (Burgos)

The caves of the Sierra de Atapuerca contain a rich fossil record of the earliest hominins in Europe starting nearly one million years ago. They represent an exceptional reserve of data, the scientific study of which provides priceless information about the appearance and the way of life of these remote human ancestors.
One of the problematic issues of the Atapuerca sites is the dating of the strata where the fossils are found. The most conspicuous hominin findings of the 1994 and 1995 field seasons include a partial face of a juvenile individual, ATD6-69, and a mandibular fragment of a juvenile individual, ATD6-5, together with a set of isolated teeth of the same individual, that constitutes the holotype of the species Homo antecessor.
A study published by the Journal of Archaeological Science has now clarified that the sediment of Gran Dolina, where the first remains of Homo antecessor were discovered, is around 900,000 years old. It was previously dated to around 780,000 years ago, and made public in the journal Nature in 1995.

Expanding the excavated area

A new excavation of TD6 made between 2003 and 2007 near the test pit of the 1990s allowed the researchers to expand the excavated area to about 13 m2.  As a result of this new excavation, the H. antecessor human hypodigm now includes nearly 150 fossil remains, belonging to a minimum of eleven individuals (eight immature and three adults).

A Challenge

The dating of this and other archaeological sites is the subject of scientific debate, and in 2012 Professor Chris Stringer, of the Natural History Museum (UK), wrote in the journal Evolutionary Anthropology that in his opinion, the researchers not only had the ages of the fossils wrong but had also incorrectly identified the species of ancient humans found there. He suggested that Juan Luis Arsuaga, the co-director was “distorting our picture of human evolution“. This prompted the researchers to work out how to date the fossils more precisely.
Josep M. Parés, from the Spanish National Research Centre for Human Evolution, who is leading this study on the new dating of level TD6 of the Gran Dolina, explained that: “We are applying new methods and techniques, and we also have better field and laboratory knowledge. We have published a study that represents a small step towards a large project which will take us longer: reviewing all the dates in order to refine them. We want to include it all within a more solid geochronological framework.”

A re-assessment

What this new study contributes is the combination of the technique of palaeomagnetism, which involved revising the polarity of the materials constituting stratigraphic layers and  re-assessing existing dating figures.
Consideration of the existing Thermo Luminescence (TL) and Optically Stimulated Luminescence  (OSL) dates for TD8 (∼820 ka) and TD7 (∼960 ka) gave maximum or finite age estimates to support the hypothesis that the intervening magnetozone represented the period∼890–940 ka. Taken together, the current evidence points towards the new magnetozone as the Kamikatsura/Santa Rosa subchrons, which have been dated as 0.899 and 0.936 Ma (million years ago) respectively.

Around 900,000 years

Previously the stratum layer was given a minimum age of 780,000 years and now it is known that they are referring more accurately to around 900,000 years. “The change might sound very small or very large,”  Parés continued, “but the TD6 stratum is known precisely as having been the place of discovery of the Homo antecessor and this further defines its age.”
The site has produced thousands of fossils and artefacts and has become a Pleistocene landmark in studies on early human settlement outside the African continent. Now the researchers are going to attempt to use individual fossils, especially teeth, to obtain direct dates for the remains, as well as those already known by their strata dating. The entire process has also advanced the methodologies and evaluation of long term dating strategies.

Source: Past Horizons:

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