Current Research

Human evolution shows a close link to the ecology of those ecosystems where it took place during more than six million years. North Tanzania preserves some of the best continuous fossiliferous records of human and mammal community evolution in the same region of East Africa. From the base of the Laetoli beds (almost 4,5 Ma) to the top of Olduvai Gorge (Upper Pleistocene), the fossiliferous record spans more than 4 million years of hominin evolution linked to the environmental evolution of different biomes.

Paleoenvironmental reconstructions, based on macromammal taxonomy and ecomorphology (Shipmand & Harris, 1988; Plummer & Bishop, 1994; Spencer, 1997), micrommamal taxonomy and taphonomy (Fernández-Jalvo et al., 1998), soil isotopic analyses (Cerling & Hay, 1986; Cerling, 1992; Sikes, 1994; Sikes & Ashley, 2008), fossil pollen (Bonnefille, 1984; Bonnefille et al., 1982) and phytoliths (Bamford et al., 2006), have provided partial interpretive windows on how the landscapes evolved both at Laetoli and Olduvai Gorge for slightly more than the earliest half of this long period (Table 1). When lumping these studies together, a picture emerges of increasing aridity and openness of habitats across time (Table 1). However, this change in climate has never been proven to be neither regular nor progressive. An alternative scenario of highly fluctuating climatic changes (Figure 1) can also be inferred if attention to fluctuations in ecological conditions in modern savannas is lent.

This has recently received support from the analysis of wet/dry cycles detected in lowermost Bed II (Olduvai Gorge) in response to astronomical forcing effects of eccentricity and precession of the equinox (Ashley et al., 2009). Rhythmic variability of the monsoon system produced wet periods followed by a transition into dry periods followed by a transition back into wet climate (Figure 2). These cycles have been detected through high-resolution analysis to stratigraphy. Hominin activities in the landscape vary according to dry/wet cycle, as documented in the characteristics of their discard of stone tools. This variation would remain unnoticed if a broad climatic interpretation for all (50 Ka) of lower Bed II would have been used.

If these wide climatic fluctuations are detected in very small stratigraphic intervals, it is logical to assume that they may have been fairly common across extensive stratigraphic sections. This could have had important evolutionary consequences both for hominins and the mammal community. Potts (1996) argued that Neogene mammals faced selective criteria according to their capability of adapting to highly variable and fluctuating ecological conditions. This “variability selection” would have determined the adaptation of organisms and, especially, of hominins and savannah mammal communities. The hypothesis of variability selection has undergone limited testing. In the present project we intend to provide the empirical basis for testing it by reconstructing is as much details as possible the succession of climatic conditions during most of the 4 million years of preserved fossiliferous record.

Current paleoecological evidence is insufficient for testing this hypothesis as it also is for understanding the evolution of climate and ecology in the region. There are several reasons that account for this deficiency:

1. The reconstruction in an important part of the stratigraphic sequence is not existent (as for Olduvai Gorge Beds II (upper and middle II), III, IV, Masek, Ndutu and Naisiusiu and Laetoli Nadolanya and Ngaloba beds).
2. In several portions of the sequence the paleoecological interpretation is based on the application of one or few of the available techniques. This is insufficient given that some of the techniques produce different results when applied to the same problem.
3. Even in the same stratigraphic units where most of the available techniques to reconstruct paleoecology and paleoclimate have been used (e.g., Olduvai Gorge Bed I), they have never been jointly applied to the same stratigraphic interval. This means that every analysis is basically reconstructing stochastic climatological or paleoecological conditions that belong to different moments and are, therefore, not necessarily comparable.

In the current research program, we introduce a comprehensive research effort which goal is to reconstruct the landscape and climatic evolution in the Laetoli-Olduvai Gorge region (with emphasis in the latter area for the current proposal) for the past 4 million years. This comprehensive approach involves:

1. The simultaneous use of all the available techniques to reconstruct landscape and paleoecology in the same stratigraphic intervals.
2. Targeting specific stratigraphic units within each bed to undertake the study, based on the presence of hominin fossils and/or archaeological record.
3. Approachig these specific units in a lateral way following outcrops of each stratigraphic interval.
4. Combining the strategy described in point 3 with a thorough traditional diachronic approach, selecting stratigraphic type sections to which analyses of soil isotopes, pollen and phytoliths as well as of detailed high-resolution lithostratigraphic analyses (including research into soil micromorphology) are carried out in every single layer to detect climatic fluctuation to the highest degree of accuracy.

If a more comprehensive picture of the evolution of landscapes and climate can be made over 4 million years, both diachronically on every single geological layer preserved in the whole stratigraphic sequence as well as synchronically on targeted layer with sufficient lateral exposure and containing hominin presence, hominin habitat selection could be linked to human speciation. One of the goals of the research presented here is also to link speciation to climatic and ecological change.