3.5 to 3.0 million years ago: Early roots of adaptability
In a recent publication, Raymonde Bonnefille, Rick Potts, and their colleagues show that the best known early human species of the Pliocene (a period from 5 to 2 million years ago) survived wide environmental change for nearly half a million years, from 3.4 to 2.95 million years ago. The species Australopithecus afarensis was able to do so despite its small brain and the lack of stone tools.
The authors studied fossil pollen from the Hadar site, Ethiopia, where over the past three decades the Institute of Human Origins (Arizona State University) has discovered many fossilized bones of Australopithecus afarensis, the species of the famed fossil skeleton known as 'Lucy'. The authors' analysis of preserved tree, grass, and swamp-plant pollen revealed how vegetation ('biome scores' for different vegetation types) had changed and how rainfall ('P' for precipitation), temperature (T), and humidity (a) fluctuated over time. You can see measurements of these variables in the figure below, and how they relate to fluctuation in global ice sheets and African dust (right side of the figure).
Fossils of Australopithecus afarensis occur straight through these fluctuations. During 450,000 years at Hadar, the A. afarensis population survived several sizeable shifts in climate and in the distribution of forest, dry grass, wet grass, and bushland habitats. Lacking large brains and stone technology, A. afarensis could move capably on two legs and probably in the trees to find and eat diverse foods as environments changed.
The last known evidence of Lucy's lineage, however, marked the beginning of an era of more dramatic climate oscillation, during which new species of early humans arose, eventually including the genus Homo to which living humans belong.
High-resolution vegetation and climate change associated with Pliocene Australopithecus afarensis. Published in Proceedings of the National Academy of Sciences U.S.A. (2004), vol. 101, pp. 12125-12129. By: Raymonde Bonnefille, Richard Potts, F. Chalié, D. Jolly, and O. Peyron.