of the defining features of humanity is our need to understand our origins.
We know from the fossil record that our ancestors evolved the abilities to walk
and think in Africa, but our thirst for knowledge means that we not only want
to know where and when, but also how we evolved. To attempt to answer these
huge questions, we need to understand how the landscape in East Africa has changed,
and this is where geologists can make an important contribution to the field
of human evolution.
East Africas topography creates a seasonal climate there, with two relatively short rainy periods and two long dry periods, which as pictured here, dry out the regions lake. The switch between dry and wet periods likely also affected human evolution in the area thousands of years ago. Courtesy of Martin Trauth.
From the accumulated work of the last two decades, it is clear that East Africa has one of the most dramatically altered landscapes in recent geological history. Tectonic movements, climate shifts and changes in the suns energy all influenced the planet over the period of time that humans evolved in Africa. Each of these processes works at a different timescale, and together they have produced a unique and highly variable environment. Those environmental changes could well be responsible for humans walking upright and developing bigger brains.
Moving plates and climate
The scene-setter for the East Africa environment is mountain building both in Tibet and in East Africa itself. Before the mountains grew, East Africa was relatively flat with a warm, humid climate probably supporting extensive rainforest, very similar to Amazonia today. Then, about 50 to 40 million years ago came the clash of the Indian and Asian continental plates, which from 20 million years ago to today has caused the massive uplift of the Tibetan plateau.
The building of this 5-kilometer-high plateau deflected weather patterns around it and thus had a major effect on climate. In summer, the Tibetan plateau also acts as a huge heat engine, absorbing lots of solar energy, which it transfers to the atmosphere, causing massive amounts of upward convection. With all this hot air rising, moist air is sucked in from the Indian Ocean and produces the intense Southeast Asian monsoons. The process, which continues today, also deflects moist air away from Africa toward India, progressively drying out East Africa. In terms of human evolution, this distinct climatic split between Africa and Asia coincides with the split between Asian and African apes, the latter of which eventually evolved into us.
At the same time as India collided with Asia, intense volcanism started in East Africa, and as Tibet thrusted upward, rifting started in Ethiopia. This rifting initiated in three directions and resulted in the Red Sea, the Gulf of Aden and the East African Rift. The initial effects of the rifting were small, and in much of East Africa there was little environmental change for the next 10 million years, while in terms of human evolution, nothing much happened. Even as late as 6 million years ago, hominin bones of Sahelanthropus tchadensis with its implied close affinity with gorillas, suggests a relatively mild and humid climate in Chad, an area that is currently desert.
According to the detailed work of Manfred Strecker at Universität Potsdam in Germany, from 12 to 1.5 million years ago, the rifting of East Africa moved from northern Ethiopia southward to Mozambique. This rifting process not only produced a deep and wide rift valley but also lifted up rift shoulders on either side, de facto mountain ranges rising up to 4 kilometers high. Interestingly, since 1.5 million years ago, the rifting process in East Africa has all but stopped, becoming the dead branch of the triple junction, while rifting remains active in the Red Sea and Gulf of Aden.
The effect of the formation of the Rift Valley on local climate was dramatic. First, the eastern rift shoulder impeded the moist air from the Indian Ocean from passing over East Africa, causing the region to dry even further. In fact, the new mountain range led to the strengthening of the Findlater Jet, the high-altitude, fast-moving air mass that crosses the equator northward close to East Africa and drags moist air toward Southeast Asia. That strengthening, in turn, led to an increased export of moisture to Southeast Asia and an intensification of the monsoons.
East Africas climate is dominated by two rainy seasons, one long one in April-May and a shorter one in October-November, which can make fieldwork to study the regions past more exciting than usual. Courtesy of Mark Maslin.
Second, the topography of East Africa completely changed from a flat region covered by rainforest to a landscape with mountains, plateaus and deep rift valleys with vegetation varying from cloud forest to desert scrub. This incredible change in the landscape may have caused the evolution of bipedalism the ability to walk upright sometime after 6 million years ago.
Sometimes called the savannah hypothesis, the idea is that our need to walk upright is linked to the replacement of rainforest by grasslands and the need to travel larger distances between food sources as efficiently as possible. Anyone who has walked all day can testify to the amazing amount of ground we can cover. An added advantage of bipedalism, according to Peter Wheeler of Liverpool John Moores University in the United Kingdom, is that it is easier to maintain body temperature in the middle of the day, as a smaller area of the body is exposed when standing upright compared to when on all fours. Calculations suggest a bipedal hominin would absorb 60 percent less heat in the midday sun than a hominin in a quadrupedal stance.
Recently, however, some paleoanthropologists have been questioning the savannah hypothesis, as research sites show that our ancestors, very sensibly, lived in a mixed environment with both grassland and open forest close to a source of water. Computer modeling work by Robin Dunbar at University of Liverpool in the United Kingdom provides a solution to this conundrum, however. He found that as long as the early hominins spent more than 60 percent of their time on the ground, they gained a significant benefit from being bipedal, for both energy efficiency and the ability to run. Despite our views that early bipedal hominins such as Ardipithecus ramidus or Australopithecus afarensis were primitive, we should note in fact that they were highly successful, colonizing at least half a continent, which we know based on remains found as far apart as South Africa and Ethiopia.
The effects of tectonics on human evolution are not just restricted to Africa and Asia. Gerald Haug of the GeoForschungsZentrum Potsdam in Germany and colleagues have shown that thousands of miles away, between 5 and 2 million years ago, the Panama ocean gateway between North and South America progressively closed. This sealing stopped the exchange of water between the Pacific and North Atlantic oceans. Its ultimate effect was to strengthen the Gulf Stream by pumping more warm water northward, providing the moisture source for the growth of the great Quaternary ice sheets that kicked off about 2.5 million years ago.
Debate continues concerning the ultimate cause of the great ice ages, but it seems clear that they were a culmination of a long-term cooling trend that started more than 10 million years ago and progressed in three major steps. The first transition was the intensification of northern hemisphere glaciation, about 2.5 million years ago, when for the first time, large ice sheets grew periodically on North America and Europe. Some of these ice sheets may have been more than 2 kilometers high, and global sea level dropped during the ice ages by as much as 60 meters.
The second transition is a new discovery by Christina Ravelo and her team at the University of California at Santa Cruz. They found that about 2 million years ago, the global cooling trend directly affected the tropics by intensifying the Walker circulation, which is an east-west atmospheric circulation driven by warm air convection overland and its subsequent sinking. The intensity of the Walker circulation is linked directly to the intensity of the monsoons and the occurrence of the El Niño phenomenon.
The final transition was about 1 million years ago, at the Mid-Pleistocene revolution. Prior to 1 million years ago, ice ages occurred every 41,000 years, but after then, they became more intense and longer. The ice ages or glacial periods were the culmination of slow cooling over 80,000 to 90,000 years, resulting in an ice sheet more than 3 kilometers thick over North America and Europe; so much water became locked up in the ice sheets that the worlds oceans dropped in sea level by more than 120 meters. These glacial periods were followed by short 10,000- to 20,000-year-long interglacial periods, with a climate similar to the present-day.
So how did these global climate changes affect Africa? Originally, geologists thought that each one of these climate transitions intensified the drying out of East Africa. In fact, we know that ice ages are not only cold periods but also very dry periods of time, an idea known as the aridity hypothesis, which was first suggested by Peter deMenocal of the Lamont-Doherty Earth Observatory at Columbia University. He showed from dust collected from ocean sediments that during these climatic transitions, northwest Africa, northeast Africa and Arabia all became dustier and thus drier.
Making the connection to early humans, Rick Potts at the Smithsonian Institution in Washington, D.C., suggested that early hominins may have evolved bigger brains during these dry periods as a way of dealing with a highly variable environment. The fossil record supports this hypothesis, as during these environmental crises, hominins evolved in two ways: developing bigger brains to think their way out of trouble, as seen in the skulls of Homo rudolfensis and Homo erectus, and getting much bigger jaws so they could eat almost anything to survive the worst droughts, as seen in the jaws of Paranthropus boisei and Paranthropus robustus.
An exposed outcrop of the ancient Lake Kariandusi shows volcanic fragments that suggest the lake existed between 970,000 and 940,000 years ago. The lake sediments are 35 meters deep and contain diatoms (inset), which are microfossils of surface dwellers, suggesting that the lake was several hundred meters deep before drying up at the end of a 20,000-year cycle. Such cycles may have caused early humans to adapt to climate extremes with bigger brains and jaws. Images by Martin Trauth.
That connection, however, may be a bit more complicated. Studying lake sediments in the East African Rift Valley where our ancestors lived, co-authors Martin Trauth and Mark Maslin, along with colleagues, have found evidence of large deep lakes during each one of the supposed dry periods. Today, what scientists do not know is whether humans evolved bigger brains while lounging around beautiful large lakes or if something more sinister was going on.
There is some tantalizing evidence from lake sediments at Baringo Basin in central Kenya found by Alan Deino from the Berkeley Geochronology Center that suggests that these lakes may have come and gone rapidly on a 20,000-year cycle. So our ancestors may have had an idyllic environment that was cruelly taken away from them as the lake dried up over a few generations. Thousands of years later, the lake would return and the cycle would begin again, requiring people to adapt to survive.
All these dramatic environmental changes are supported by the fossil record and the development of either big brains or big jaws to deal with intense drought periods, thus supporting Potts variability hypothesis.
The last factor influencing the climate of East Africa is orbital forcing. The variation of the Earths orbit around the sun forces glacial-interglacial cycles, and recently, geologists have started to appreciate that it also has an additional separate, but significant, effect on tropical climate.
The most important orbital variable for the tropics is precession (the variation of Earths axis), which influences the amount of sunlight received during any particular season and has a cycle of about 20,000 years. Modeling work by Amy Clement of the University of Miami has shown that precession has very little effect on tropical temperatures, but a huge effect on rainfall. In her models, changing precession had the same scale of influence on rainfall as shifting from a glacial to an interglacial period. This link is supported by recent results from lake deposits and speleothems (mineral formations in caves) in Brazil that shows that rainfall there is also controlled by a 20,000-year precession cycle.
The same evidence has led Maslin to suggest that this may be the key long-term repetitive climatic forcing that caused repeated bursts of evolution and thus the huge biodiversity in Amazonia. Therefore, even if there were no other changes in the East African environment over the last 10 million years, the amount of solar radiation received in any particular season could have varied by up to 20 percent, which would have had a significant influence on the timing and duration of the two wet seasons.
This precessional forcing of the East African climate is supported by tentative data from the lake studies by Deino, who has evidence of the appearance and disappearance of the deposition of lake sediments every 20,000 years. Moreover, the variable topography and global climate changes may have enhanced the sensitivity of East Africa to the precessional effects of seasons, thus impacting the rainfall of the region. This process would have had an important effect on human evolution, as the survival of any hominin is dependent on access to water.
Geologists have started to put together a coherent picture of how the environment of East Africa has changed over the last 10 million years. The region has changed beyond all recognition over this period of time: from a relatively flat homogeneous region covered with rainforest, to a spectacularly heterogeneous region with mountains more than 4 kilometers high and vegetation ranging from desert scrub to dense forest. Add to the mix major global cooling, dramatic intensification of tropical atmospheric circulation, and regular 20,000-year variations in rainfall, and it could have been a dynamic system for human evolutionary development.
We still do not know, however, how quickly these environmental changes occurred over centuries or decades, and whether they were fast enough to cause evolution. Also, we need to understand whether these changes were synchronous across eastern and southern Africa. Maslin and Trauth hope to address that first key question by dating the retreat of the large Rift Valley lakes that we know existed at the beginning of our current interglacial some 10,000 years ago. Preliminary evidence suggests that more than 1,000 square miles of lakes vanished in a very short period of time. As to the synchronicity of pan-African climatic changes, co-author Beth Christensen and Maslin hope to compare marine records of southwestern Africas climate with those that already exist for East Africa. Both of these studies, along with work from others, hopefully will help us untangle the complex and highly variable climate of Africa.
In many ways, with all this dramatic environmental change occurring in East Africa, it is not surprising it was a hotbed of evolution, producing an ape that not only can walk upright but can even ponder its own evolution.
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