A little over a decade ago I was suffering through an extremely dull talk at a conference in Tucson, Arizona, with Professor Martin Trauth of Potsdam University. Like naughty schoolboys we were passing notes. One of these notes from Martin said that he was putting a team together to work in Kenya, and would I like to be involved? I said that I would. Little did we know then that ten years later we would have changed the way we think early humans evolved in Africa.
Martin’s plan was simple: find out what the climate of East Africa was like for our ancestors living in the heart of the Rift Valley. That, he thought, might help unlock one of the great mysteries of evolution: why did we get larger brains? It was an extraordinary leap that equipped our lineage with the cranial capacity to conquer the planet. But was it a response to an epic drought in the cradle of mankind, as scientists had argued for a generation? Or was it a result of something even more dramatic?
A year or so later, after fieldwork in Kenya, we were sitting on the shore of Lake Naivasha, drinking fresh passion fruit juice and thinking about the evidence for the rise and fall of the lakes in the Rift Valley, and we realised that we had a completely new and original explanation.
To reach that explanation we had travelled 4,500 miles from London to what is now the Kenyan Rift Valley, and about 20 million years back in time, to the great age of mountain building in Africa and Asia. That was when the mighty forces of tectonics and climate change transformed East Africa from a relatively flat, forested region into a mountainous, fragmented landscape dominated by the rapid appearance and disappearance of huge deep-water lakes. From this dramatically altered landscape emerged an ape able to walk upright and ask such fundamental questions as, “Where did I come from?” And “Why?”
Before these mountains grew, East Africa had a warm, humid climate and extensive forest. Then, thousands of miles away, the clash of the Indian and Asian continental plates produced the massive uplift of the Tibetan plateau. The building of this three mile-high plateau had a profound effect on climate by deflecting weather patterns around it. In summer, it also acts as a huge heat engine, absorbing solar energy which it transfers to the atmosphere, causing massive upward convection. As this hot air rises, more air is sucked in from surrounding areas, including moist air from the Indian Ocean, resulting in the monsoon. But there was a knock-on effect for East Africa — since so much of the moisture keeping it humid was pulled away to India it started to dry out. In terms of human evolution, this climate split between Africa and Asia coincides with the split between Asian and African apes – the latter eventually evolving into us.
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As Tibet was thrusting upwards, a “rifting” process started in Ethiopia. Extra-hot magma in the mantle moved up under northeast Africa making the Earth’s crust expand and split down the middle like an over-cooked apple pie. This produced a deep, wide hanging valley half a mile above sea level with mountain ranges on either side rising up to three miles high. The effect of this formation on local climate was dramatic. First, the Rift Valley’s eastern shoulders prevented moist air from the Indian Ocean from passing over East Africa, also causing the region to dry out. The new mountain range acted like a banked wall of a racetrack, speeding the moist air away from Africa towards India. Second, the topography of East Africa changed completely, from a homogeneous flat region covered in moist forest to a landscape with mountains, plateaus and deep valleys, with vegetation varying from cloud forest to desert scrub.
It is this incredible change in the landscape of East Africa that many have suggested caused humans to stand up on two feet about six million years ago. This is sometimes called the “savannah hypothesis”, linking our need to walk upright to the replacement of forest by a much more fragmented landscape and the need to travel longer distances between food sources as efficiently as possible. And there was an added advantage of bipedalism, according to Professor Peter Wheeler of Liverpool John Moores University. Walking upright made it easier to maintain body temperature in the middle of the day, since a smaller area of the body was exposed to the Sun compared with quadrupeds.
The savannah hypothesis has been questioned recently because many palaeoanthropological sites show that our ancestors, in fact, lived in a mixed environment with both grassland and open forest close by, and, most importantly, a source of water — either a river or a lake. But this much is clear: the ability to walk upright must have provided a strong survival advantage over others species, because the benefit had to outweigh the cost of a huge increase in mother and infant mortality.
While tectonic change was producing climate change in East Africa, the global climate was also undergoing a major evolution. About two and half million years ago the great ice ages started. For decades it was assumed that the start of this period of gigantic, cyclical glaciation coincided with major changes in human evolution. Originally it was thought that each new ice age intensified the drying out of East Africa. It was Professor Peter deMenocal of Columbia University who, in 1995, first suggested this “aridity hypothesis”. From dust collected from ocean sediments he showed that, during these climatic transitions, northwest Africa, northeast Africa and Arabia all became dustier and drier. Professor Rick Potts at the Smithsonian Institution in Washington suggested that early hominins may have evolved bigger brains during these dry periods as a way of dealing with a highly variable environment, and, in fact, the fossil record supports this theory. But this turns out to be a myth encouraged by our desire to have a neat reason for the next stage in our development, namely brain expansion. This step is linked to the ice age, but only indirectly as it is too early. At about 1.8 million years ago, 700,000 years after the ice age started, four new species of hominin, our human ancestors, appear in East Africa, including the first of the Homo lineage. Each showed a massive 80 per cent expansion in brain capacity. This was the great leap forward for humanity. It coincided not with the onset of the ice ages, but with their knock-on effect on Africa.
As Professor Christina Ravelo of the University of California has found, about 1.8 million years ago the global cooling trend did finally affect the tropics by intensifying the east-west atmospheric pattern known as the Walker circulation. This is driven by west to east circulation of the oceans and atmosphere. The intensity of the Walker circulation is linked directly to the intensity of tropical rainfall, the monsoons and the occurrence of El Niño (cyclical warming of the tropical eastern Pacific Ocean). This increase in rainfall is critical, as we found out when we started our Kenyan fieldwork.
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Because East Africa has been so tectonically active, with so much uplift and movement, fragments of the past are captured, and in many cases exposed to us geologists. Some of the striking elements of the landscape are its relic lake deposits. These white layers can be tens of metres thick and resemble the White Cliffs of Dover, but in the middle of Africa. But instead of being made of tiny calcite shells, they are made of tiny diatom shells made of silicate. Diatomite is mined in Kenya because it makes an extremely good filter for soft drinks, but it also provides a wealth of information on how deep and fresh the lakes were.
When all the layers of diatoms from Ethiopia down to Tanzania are dated and compared, it seems that large, deep freshwater lakes occurred during each one of the supposed aridity steps during the ice ages. Did this mean that Peter deMenocal was wrong? No. It meant that the full story was even more interesting: these lakes were not permanent features of the landscape. Instead, they appeared and disappeared. So our ancestors may have had an idyllic environment which was then cruelly taken away as the lake dried up over a few generations. Thousands of years later the lake would return and the cycle would begin again.
The coming and going of these immense lakes is driven by the wobbles of the Earth as it moves around the Sun. Geologists already knew that variations in the Earth’s orbit forced the great ice-age cycles, but we have only recently started to grasp that this also has had a major effect on tropical climate. The most important orbital variable for the tropics is “precession”, which influences the amount of sunlight received during any one season and has a cycle of about 20,000 years. Modelling by Professor Amy Clement of Miami University has shown that precession has very little effect on tropical temperatures but a huge effect on rainfall.
The most dramatic example of the rapid appearance and disappearance of these freshwater lakes occurred 1.8 million years ago when lakes more than 300m deep covered much of the Rift Valley. We have calculated, for example, that Lake Turkana would have expanded southwards to cover 6,000 square miles instead of its current 2,500. These lakes would have appeared and disappeared at least four times during this period and, we believe, may have caused the environmental stress that led to the big jump in hominin brain size, as our ancestors, I would suggest, evolved an extremely complex social brain with an immense capacity for adaptation and group problem-solving. (For fresh evidence, just think how easily we have absorbed social media and mobile phone technology.)
So the story that is now emerging from Africa is that we humans evolved to deal with periods of rapid and unpredictable climate change. Adaptability and the use of social groups to problem-solve became hard-wired into us.
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Looking back at how we dealt with our first climate crisis makes me hopeful that we can deal with the man-made climate crisis of today, because evolution has given us all the tools we need, and bigger brains to think our way out of trouble.
Mark Maslin is a professor of geography and Director of UCL’s Environment Institute