Our ancient ancestors seem to have survived some pretty harsh arid spells in East Africa’s Rift Valley over five million years. Quite how they kept going has long been a mystery, given the lack of water to drink. Now, new research shows that they may have been able to survive on a small networks of springs.
The study from our inter-disciplinary research team, published in Nature Communications, illustrates that groundwater springs may have been far more important as a driver of human evolution in Africa than previously thought.
The study focuses on water in the Rift Valley. This area – a continuous geographic trench that runs from Ethiopia to Mozambique – is also known as the “cradle of humanity”.
Here, our ancestors evolved over a period of about five million years. Throughout this time, rainfall was affected by the African monsoon, which strengthened and weakened on a 23,000-year cycle. During intense periods of aridity, monsoon rains would have been light and drinking water in short supply. So how did our ancestors survive such extremes?
Previously, scientists had assumed that the evolution and dispersal of our ancestors in the region was solely dependent on climate shifts changing patterns of vegetation (food) and water (rivers and lakes). However, the details are blurry – especially when it comes to the role of groundwater (springs).
We decided to find out just how important springs were. Our starting point was to identify springs in the region to map how groundwater distribution varies with climate. We are not talking about small, babbling springs here, but large outflows of groundwater. These are buffered against climate change as their distribution is controlled by geology – the underlying rocks can store rainwater and transfer it slowly to the springs.
We figured that our ancestors could have stayed close to such groundwater in dry times – playing a greater part in their survival than previously thought. When the climate got increasingly wet, groundwater levels would have risen and made springs more plentiful – feeding smaller rivers and leading to lakes becoming less saline. At this point, our ancestors would have roamed across the landscape free of concerns about water.
Life and death decisions
To test this idea, we embarked on a computer experiment. If the springs and water bodies are thought of as the rest stops, or service stations, then the linkages between can be modelled by computers. Our model was based on what decisions individuals would have taken to survive – and what collective behaviours could have emerged from thousands of such decisions.
Individuals were give a simple task: to find a new source of water within three days of travel. Three days is the time that a modern human and, by inference, our ancestors could go without drinking water. The harder and rougher the terrain, the shorter the distance one can travel in those vital three days.
We used the present landscape and existing water springs to map potential routes. The detailed location of springs may have changed over time but the principles hold. If our agent failed to find water within three days, he or she would die. In this way we could map out the migration pathways between different water sources as they varied through 23,000-year climate cycles. The map shows that there were indeed small networks of springs available even during the driest of intervals. These would have been vital for the survival or our ancestors.
The model also reveals movement patterns that are somewhat counter-intuitive. One would assume that the easiest route would be along the north to south axis of the rift valley. In this way, hominins could stay at the bottom of the valley rather than crossing the high rift walls. But the model suggests that in intermediate states between wet and dry, groups of people may have preferred to go from east to west across the rift valley. This is because springs on the rift floor and sides link to large rivers on the rift flanks. This is important as it helps explain how our ancestors spread away from the rift valley. Indeed, what we are beginning to see is a network of walking highways that develop as our ancestors moved across Africa.
Human movement allows the flow of gossip, know-how and genes. Even in modern times, the water-cooler is often the fount of all knowledge and the start of many budding friendships. The same may have been true in ancient Africa and the patterns of mobility and their variability through a climate cycle will have had a profound impact on breeding and technology.
This suggests that population growth, genetics, implications for survival and dispersal of human life across Africa can all potentially be predicted and modelled using water as the key – helping us to uncover human history. The next step will be to compare our model of human movement with real archaeological evidence of how humans actually moved when the climate changed.
So next time you complain about not finding your favourite brand of bottled spring water in the shop, spare a thought for our ancestors who may died in their quest to find a rare, secluded spring in the arid African landscape.
This research was carried out in partnership with our colleagues Tom Gleeson, Sally Reynolds, Adrian Newton, Cormac McCormack and Gail Ashley.