Responding to a crisis often brings out the best in people. Certainly it has in the past, when sudden changes in climate during the Middle Stone Age sparked off surges of cultural evolution and innovation in early modern humans, some 100,000 to 40,000 years ago.
The earliest stone tool-making had developed by at least 2.6 million years ago. However, the pace of innovation really started to accelerate in short, rapid bursts around 100,000 years ago. The use of symbolism and personal adornments became more complex as human culture developed - but what lay behind these periods of rapid development has been harder to establish.
Our study was based on evidence drawn from marine sediment cores - a cross-section through many layers of earth extracted off the South African coast.
We know that during the last ice age the northern hemisphere experienced a series of dramatic cooling events linked to a substantial slowdown in the Atlantic Ocean circulation, which reduced the amount of warm water reaching the northern high latitudes. These abrupt cooling events caused large parts of sub-Saharan Africa to experience very hot, dry conditions, and this cycle of cooling and warming was repeated many times throughout the last ice age.
By comparing our climate reconstructions produced from the cores with models of climate variation, we re-created the regional climate patterns of the last 100,000 years. We found that our new data shows that in what is now South Africa the climate conditions moved in the opposite direction: cool weather in the north drove the tropical monsoon weather systems southwards bringing a wetter, not drier, climate. And in comparison, when the northern hemisphere thawed, the hotter, drier conditions returned.
Culture and climate
When we compared the timing of these climate changes with the archaeological data from the cores, we found remarkable coincidences. Several major Middle Stone Age industries fell tightly together at the same time as the abrupt change to a wetter climate. Similarly, the industries' disappearance appears to coincide with the later transition to drier conditions.
For example, the Still Bay culture was one of the most sophisticated Middle Stone Age groups in Africa when it emerged during an interval of wetter conditions around 78,000 years ago. The archaeological record revealed the groups' use of distinctive tools, including carefully worked stone points and rock fragments engraved with symbolic patterns. But evidence of this culture abruptly disappears some 71,000 years ago as drier condition prevailed.
It would take another 7,000 years, and a further northern hemisphere cold event that brought back wetter conditions, before a new culture emerged. This new culture brought with them a notably different, more advanced toolkit that included blades that were probably used as arrowheads. The correspondence between climatic change and cultural innovation supports the view that population growth led to greater human interaction and subsequent rapid cultural advancement.
Lessons for today
The South African archaeological record is important because it shows some of the oldest evidence for modern behaviour in early humans. Particularly the use of symbols, which has been linked to the development of complex language, and personal adornments made of sea shells. The quality of the data allowed us to make these correlations between climate and behavioural change.
But comparable data from elsewhere is needed before it can be established whether this region was uniquely important. Nonetheless, these results present the most convincing evidence so far that abrupt climate change was instrumental in our ancestors' cultural development.
Perhaps there is a lesson to be learnt from this, given the situation in which we find ourselves now. Once more humans face rapid, potentially disastrous climate change. Our ancestors were probably reduced to a fairly small number, but dealt with the situation with communication, collaboration and invention.
It is these three qualities that would eventually make us the most successful species on the planet, and it is these three qualities that we must rely on to help us tackle our modern climate crisis.