I'm finally back from various unexpectedly lengthy pursuits south of the border, and now enjoying the light snow lightly caking everything in MTL, like so much powdered sugar on a delicious sfogliatella...
Two weeks ago I posted about a new study on the potential impact of volcanism on the accumulation of hominin remains at the Lower Pleistocene site of Dmanisi, Georgia. One of the appealing aspects of the conclusions of that study by de Lumley et al. (in press) is that it accounted for the presence of fully five distinct individuals at the site, since large accumulations of hominin remains at single localities tend to be rare.
The situation at Dmanisi may, however, be slightly more complex than this, suggest M. Calvo-Rathert and his colleagues, in a paper in Quaternary Research. They claim that, in fact, the individuals recovered at Dmanisi come from two distinct geological units. How distinct, you ask? Distinct enough that one (Unit A, the volcanogenic one) has normal magnetic polarity, while the other (Unit B, ) evidences reversed polarity. This, they infer, means that Unit B could be as young as 1.07 mya, while Unit A dates to 2.0-1.8 mya, which they interpret as meaning that the various hominins found at the site oculd have been deposited over a very span of time.
If the results from the present study are compared with the
geomagnetic polarity scale, and available radiometric data (1.8
to 2.0 Ma for the underlying lavas and the volcanic ash level)
are considered, the lower part of the section shows a clear
correlation with the Olduvai subchron (Fig. 1). If considered
reversed, the upper levels cannot be correlated with any specific
point and could be as young as 1.07 Ma (age of Jaramillo)
because the whole section is not continuous and so the smaller
events might not be recorded, even though the data set from this
study is composed of continuous subsections. If directions of
the upper-lying Unit B samples are considered intermediate,
those data might correspond to a polarity change between
Olduvai and any of the normal polarity subchrons shown,
although because of the reasons outlined before we do not favor
that interpretation.
Despite the morphologic differences observed between mandible
D2600 found in layer A1 and the remaining hominin
findings (Rightmire et al., 2006), different data sources (i.e.,
stratigraphic and sedimentological) suggest that the time frame
spanning the Dmanisi lithostratigraphic section is not long (e.g.,
Gabunia et al., 2001). On the other hand, human remains and
artifacts have been found in volcanic ashes (Unit A, normal
polarity), pipe features (reversed polarity) and Unit B (reversed
polarity), and the Olduvai/Matuyama reversal is not recorded in
the discontinuous sequence presented in this study. A conservative
analysis of these observations suggests that the age of the
Dmanisi site could at least span several ten thousands of years,
although a much wider period of hundreds of thousands of years
cannot be excluded. All this might point to more than one
human population occupying the studied area. (Calvo-Rathert et al. 2008: 96)
It therefore seems that the Dmanisi hominin sample might have accumulated under much less 'catastrophic' conditions that implied by de Lumley et al. (in press), and that some of the morphological variability in said sample might be due to the fact that it contains individuals belonging to distinct populations potentially separated in time by hundreds of thousand years.
References:
CALVORATHERT, M., GOGUITCHAICHVILI, A., SOLOGASHVILI, D., VILLALAIN, J., BOGALO, M., CARRANCHO, A., MAISSURADZE, G. (2008). New paleomagnetic data from the hominin bearing Dmanisi paleo-anthropologic site (southern Georgia, Caucasus). Quaternary Research, 69(1), 91-96. DOI: 10.1016/j.yqres.2007.09.001
DELUMLEY, M. (2008). Impact probable du volcanisme sur le décès des Hominidés de Dmanissi. Comptes Rendus Palevol DOI: 10.1016/j.crpv.2007.09.002
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