This week’s discussion of the new fossils from Northern Kenya once again raises the issue of the critical role played by precise geological dating at this time period. This is hardly a new issue, but one that perhaps gets too often placed behind the fossils themselves in our discussions. When Lee Berger and I had a debate on the origins of the genus Homo at Boston University more than a year ago, this was one of the issues we were in lockstep agreement on–the Plio-Pleistocene is a dynamic period of evolutionary change within the genus Homo (and possible related taxa), meaning the correct temporal sequence of fossils is extremely important. At the time, Lee and I were discussing the potential of Australopithecus sediba–from Malapa, South Africa–to give rise to later Homo, with the specimens from Dmanisi representing some of the best examples. One of the challenges of such an interpretation is temporal distance between the two localities. The Malapa specimens have been very narrowly dated to just under two million years of age, while the Dmanisi fossils come from deposits between 1.78-1.76 million years of age, though with archaeological evidence of a presence older than 1.8 million years. In this case, these dates provide the temporal framework in which evolutionary hypotheses for the relationship between these fossils might be tested.
Going back to East Africa, the issue of dating is equally critical and even more challenging given the large number of individual localities fossils have been recovered from. The temporal sequencing of fossils is important regardless of whether your interpretive leanings go more towards a multiple lineage or a single lineage perspective. Under both scenarios, evolutionary change is a central component, whether it be change within a lineage or the diversification associated with a limited radiation of species across time and space. From a fossil perspective, evolutionary change means morphological variation. Morphological variation, however, is the usual criteria we use for the differentiation of species in the fossil record. Thus, the temporal position of fossils becomes intrinsically connected to attempts to understand and explain fossil variation. Consider the following schematic:
I have not altered the position of any of my red dots (i.e. “fossils”) with respect to where they sit in relation to the X-axis (morphological variation), but have clearly altered each dot’s temporal position in A, B and C, with obvious impacts on how we might interpret the variation. Of course the reality is much more challenging as we do not have discretely distributed dots, but instead have specific fossils with uncertainty, either historical or technical, in their exact temporal position. This is an issue that John Hawks has harped on repeatedly at this blog, including in this post from seven year’s ago (!) talking about the problem with temporal correlation:
To some extent, this begs the question about dates in paleoanthropology. Two things that are plausibly causally associated might still be as much as several hundred thousand years different in dates. So how are we to resist the hypothesis that two events with the same date are causally associated? We may never confirm the hypothesis that two events actually do have the same date — there is no statistical test for “significantly the same”, just “not significantly different”.
I bring up all of this, in part, to highlight several articles in this month’s Journal of Human Evolution looking at the stratigraphic sequence in Olduvai Gorge. Alan Deino has an article looking at 40Ar/39Ar dates for Olduvai Beds I and II with implications for climate change and human evolution. Godwin Mollel and Carl Swisher have an article on volcanic activity associated with the Ngorongoro volcanic system and its significance for Olduvai. Lindsay McHenry reinvestigates the bed I deposits based on tuff geochemistry. Ian Stanistreet tries to develop a fine-resolution scale for Beds I and II.
None of these articles are likely to generate headline stories in the New York Times without the direct association of new fossils, but this kind of research is essential for correctly interpreting such fossils.
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1. Deino, A. L. (2012). “40Ar/39Ar dating of Bed I, Olduvai Gorge, Tanzania, and the chronology of early Pleistocene climate change.” Journal of Human Evolution 63(2): 251-273. http://dx.doi.org/10.1016/j.jhevol.2012.05.004
2. McHenry, L. J. (2012). “A revised stratigraphic framework for Olduvai Gorge Bed I based on tuff geochemistry.” Journal of Human Evolution 63(2): 284-299. http://dx.doi.org/10.1016/j.jhevol.2011.04.010
3. Mollel, G. F. and C. C. Swisher Iii (2012). “The Ngorongoro Volcanic Highland and its relationships to volcanic deposits at Olduvai Gorge and East African Rift volcanism.” Journal of Human Evolution 63(2): 274-283. http://dx.doi.org/10.1016/j.jhevol.2011.09.001
4. Stanistreet, I. G. (2012). “Fine resolution of early hominin time, Beds I and II, Olduvai Gorge, Tanzania.” Journal of Human Evolution 63(2): 300-308. http://dx.doi.org/10.1016/j.jhevol.2012.03.001
