Olduvai Gorge has yielded some of the earliest hominin remains, including remains of Homo habilis, the earliest human. But what drew them to an area in what is now northern Tanzania? A new botanical study has shown the area was grassland, with the attraction of a nearby lake, according to Rodríguez-Cintas and colleagues writing in Palaeogeography, Palaeoclimatology, Palaeoecology. The findings shed light on environmental conditions at the dawn of humanity.
The discovery is the result of study of a core taken from Olduvai Gorge. The sediments data from around two million years ago, so the remains need to be robust. Many of the ecofacts were phytoliths, small silicon deposits formed by plants. Effectively, phytoliths are microscopic rocks formed by plants in distinctive shapes, and so very slow to decay. The team also found pollen, connected to Poaceae and fungal spores.
In addition, the scientists and colleagues found evidence of more than just grassland. “The identification, mainly in claystone facies, of freshwater markers such as diatoms, sponge spicules and chrysophyte cysts, derived from the catchment area of Palaeolake Olduvai, attests to the presence of freshwater settings (rivers, marshlands or wetlands, and/or springs). The identification of Nitzschia and not much else in some samples, suggests that these diatoms were either growing in the lake during less saline periods, or were transported from lake hinterland,” write Rodríguez-Cintas and colleagues in their article.
The phytoliths reveal that the landscape was not static, but underwent a few changes in a (geologically) short time, indicating a change from wetter to drier conditions. Puzzlingly, this appears to have happened five times in just thirty thousand years, which the authors note is a sub-Milankovitch cycle frequency.
“When these events are compared with the Earth’s orbital precessional insolation patterns modulated for June at 30° N latitude, the three upper most events are closely spaced during the second half of the insolation minimum, suggesting a lag between the forcing climatic pattern and the vegetative response,” write the authors in their article. “Contrariwise the two lowermost events occurring during the transition to a wetter phase, are short and widely spaced compared to their later counterparts and are interpreted as short-term drier episodes, and are related to a decrease in precipitation, or changes in palaeolake drainage patterns. We conclude that it is lake-parasequential changes that force such sub-Milankovitch arid interludes.”
The team therefore have evidence that the earliest humans would have had to cope with a drying climate on the edge of a wetland. There were savannahs, with grasses. By the waters of the lake, there were also sedges. Lakeside habitats would have provided two locations to forage for food in difficult times. However, an increase in lake salinity in drier periods would have shifted the vegetation, meaning that behaviour would have had to change to exploit the locality. It is possible that it was how the earliest humans could adapt to less suitable conditions that gave them the advantage over their australopithecus cousins.