HOMINIDS AND PALEOENVIRONMENTS: THE VIEW FROM THE TUGEN HILLS, KENYA

The postulated association of astronomically forced global climatic shifts with faunal evolutionary change has a long history, back at least to Croll and Wallace, being used to explain apparent problems with Darwinian theory. Since then several researchers have suggested a correlation of world climate change with human evolutionary events. Such ideas are attractive, but give rise to some critical questions. Is astronomical forcing a dominant influence in the interior of continents at the equator? Some Milankovitch effects are felt less at low than at higher latitudes. Also, in the east African Rift Valley, which produces most data for human evolution, we know that local topographic and tectonic factors considerably influence local climate and environment. Even if such forcing is influential, is the resolution of the African Pliocene terrestrial record, geological and faunal, good enough to be able to detect it? And then, if it does have an effect, and is detectable, does it influence evolution of mammals in a simple direct way? The Tugen Hills, in the Kenya Rift Valley, preserve a continuous succession of vertebrate faunas ranging from 16 Ma to recent times. One of several interests of the Baringo Paleontological Research Project is the period between 3-2 Ma, significant for the origin of such hominid lineages as Homo and Paranthropus. Exposures in the Barsemoi River, west of Lake Baringo, reveal a sequence of five major diatomites. Single-crystal ⁴⁰Ar/³⁹Ar determinations show they occur between 2.66 Ma and 2.56 Ma, and reflect depositional cycles that match the 23 ka Milankovitch precessional periodicity. Other data strongly suggest these cycles are not due to local tectonics. Over 35 fossil sites correlate into this section, three of which incorporate hominids, one with the earliest known member of genus Homo. This situation provides an opportunity to look at the effect, if any, of such climatic fluctuations on the late Pliocene fauna.