Increasingly dry conditions in East Africa half a million years ago led to the local extinction of large mammals in the region. For early humans, the options were to adapt or die, and our ancestors’ response helped make us what we are today, initiating a major leap in the development of tools.
Anthropologists pondering the causes of humanity’s technological development have speculated that climate was a likely cause. For example, if the forests in which our ancestors once dwelled were replaced by savannahs, our gradual movement out of the trees would make sense. These ideas have been hard to test however, because we have lacked much evidence of past climate in the most relevant places.
Attempts have been made to extrapolate from more distant climate records, but Professor Bernhart Owen of Hong Kong Baptist University has provided something much better: an indication of conditions in the basin of Lake Magadi, Kenya, close to sites inhabited at the time.
About 575,000 years ago, Magadi started receiving less water, and the plant species around it became indicative of a dryer climate. The change was enormously slow compared to what we are witnessing today, but was still sufficient to cause the disappearance of large grazing animals from the fossil records at sites nearby. The flaked tools and spearheads identified with the Middle Stone Age appeared not long after.
Magadi becomes a salt pan in the dry season, filling with water when the rains come. Once, however, it remained wet all year round, albeit shrinking to an uncomfortably salty remnant at times. In Proceedings of the National Academy of Sciences, Owen and co-authors report that sediments at Magadi’s base reveal the local rainfall over a period of more than a million years.
The local climatic conditions can be read in Magadi’s sediments in several ways. The mix of salt and organic materials provides an indication of how much water drained to the lake from the surrounding hills. Pollen trapped in the mud reveals the dominant nearby plant species. Finally, the mix of plankton living in the lake tracks the rising salinity over time, as species more suited to fresh waters died out once the region’s rainfall shrank and evaporation rose.
The results line up well with evidence of global temperature change and climatic shifts elsewhere in Africa. However, confirmation that these shifts were being experienced in a place our ancestors lived adds credibility to theories that significant innovations, such as advanced tools and the creation of trading networks, came in response to reduced food availability.
Ironically, the slow natural climate change of the era contributed to giving humans the capacity to inflict such rapid climate change on the world today.