Archaeological and genetic evidence suggests that modern humans (the modern form of Homo sapiens, our species) originated in Africa during the Stone Age, between 30,000 and 280,000 years ago. The latest archaeological excavations in southern Africa have shown that technological innovation, linked to the emergence of culture and modern behaviour, took place abruptly: the beginnings of symbolic expression, the making of tools from stone and bone, jewellery or the first agricultural settlements.
An international team of researchers has linked these pulses of innovation to the climate that prevailed in sub-Saharan Africa in that period.
Over the last million years the global climate has varied between glacial periods (with great masses of ice covering the continents in the northern hemisphere) and interglacial periods, with changes approximately every 100,000 years. But within these long periods there have been abrupt climate changes, sometimes happening in the space of just a few decades, with variations of up to 10ºC in the average temperature in the polar regions caused by changes in the Atlantic ocean circulation. These changes affected rainfall in southern Africa.
The researchers have pieced together how rainfall patterns varied in southern Africa over the last 100,000 years, by analysing river delta deposits at the edge of the continent, where every millimetre of sediment core corresponds to 25 years of sedimentation. The ratio of iron (dissolved from the rocks by the water during the rains) to potassium (present in arid soils) in each of the millimetre layers is a record of the sediment carried by rivers and therefore of the rainfall throughout the whole period.
The reconstruction of the rainfall over 100,000 years shows a series of spikes that occurred between 40,000 and 80,000 years ago. These spikes show rainfall levels rising sharply over just a few decades, and falling off again soon afterwards, in a matter of centuries. This research has shown that the climate changes coincided with increases in population, activity and production of technology on the part of our ancestors, as seen in the archaeological records. In turn, the end of certain stone tool industries of the period coincides with the onset of a new, drier climate.
The findings confirm one of the principal models of Palaeolithic cultural evolution, which correlates technological innovation with the adoption of new refuges and with a resulting increase in population and social networks.
For these researchers, the bursts of demographic expansion caused by climate change in southern Africa were probably key factors in the origin of modern humans' behaviour in Africa, and in the dispersal of Homo sapiens from his ancestral home.
The study forms part of the GATEWAYS project of the European Commission's 7th Framework Programme, coordinated by Rainer Zahn, a researcher with the Institute for Environmental Science and Technology (ICTA-UAB) and the UAB's Department of Physics, and taking part in it was Martin Ziegler, a post-doctoral researcher at the School of Earth and Ocean Sciences of the University of Cardiff (UK) and scientists from the Natural History Museum, London (UK).