The East African Rift runs for thousands of kilometres from Ethiopia to Mozambique. As the swelling from the magma plume bulging beneath it continues, the Rift is still being pulled apart. This ‘extensional tectonic’ process is causing whole slabs of rock to fracture along faults and break off, with the flanks being pushed up as steep escarpments and the blocks in between subsiding to form the valley floor. Between about 5.5 and 3.7 million years ago this process created the current landscape of the Rift: a wide, deep valley half a mile above sea level and lined on both sides with mountainous ridges.6
One major effect of the swelling of this crustal bulge and the high ridges of the Rift was to block rainfall over much of East Africa. Moist air blowing over from the Indian Ocean is forced upwards to higher altitudes where it cools and condenses, falling as rain near the coast. This creates drier conditions further inland – a phenomenon known as a rain-shadow.7 At the same time, the moist air from the central African rainforests is also blocked from moving eastwards by the highlands of the Rift.8
The upshot of all these tectonic processes – the creation of the Himalayas, the closing of the Indonesian Seaway, and in particular the uplift of the high ridges of the African Rift – was to dry out East Africa. And the formation of the Rift changed not only the climate but also the landscape, in the process transforming the ecosystems of the area. East Africa was remoulded from a uniform, flat area smothered in tropical forest, to a rugged, mountainous region with plateaus and deep valleys, its vegetation ranging from cloud forest to savannah to desert scrub.9
Although the great rift started to form around 30 million years ago, much of the uplift and aridification happened over the past 3–4 million years.10 Over this time, the same period that saw our evolution, the scenery of East Africa shifted from the set of Tarzan to that of The Lion King.11 It was this long-term drying out of East Africa, reducing and fragmenting the forest habitat and replacing it with savannah, that was one of the major factors that drove the divergence of hominins from tree-dwelling apes. The spread of dry grasslands also supported a proliferation of large herbivorous mammals, ungulate species like antelope and zebra that humans would come to hunt.
But it wasn’t the only factor. Through its tectonic formation the Rift Valley became a very complex environment, with a variety of different locales in close proximity: woods and grasslands, ridges, steep escarpments, hills, plateaus and plains, valleys, and deep freshwater lakes on the floor of the Rift.12 This has been described as a mosaic environment, offering hominins a diversity of food sources, resources and opportunities.13
The widening of the Rift and the upwelling of magma was accompanied by strings of violent volcanoes spewing pumice and ash across the whole region. The East African Rift is dotted with volcanoes along its length, many of which formed in just the last few million years. Most of these lie within the Rift Valley itself, but some of the largest and oldest are growing on the edges, including Mt Kenya, Mt Elgon, and Mt Kilimanjaro, the tallest mountain in Africa.
The frequent volcanic eruptions spilled lava flows that solidified into rocky ridges cutting across the landscape. These could be traversed by nimble-footed hominins, and along with the steep scarp walls within the Rift may have provided effective natural obstacles and barriers for the animals they hunted. Early hunters were better able to predict and control the movements of their prey, constraining escape routes and directing them into a trap for the kill. These same features may also have offered vulnerable early humans a degree of protection and security from their own predators that prowled the landscape.14 It seems that this rough and varied terrain provided hominins with the ideal environment in which to thrive. Early humans, who, like us, were relatively feeble and did not have the speed of a cheetah or the strength of a lion, learned to work together and take advantage of the lie of the land, with all its tectonic and volcanic complexity, to help them hunt.
It is active tectonics and volcanism that have created and then sustained these features of a varied and dynamic landscape over the course of our evolution. In fact, because the African Rift is such a tectonically active region, the landscape has changed greatly since the times of earliest human habitation. As the Rift has continued to widen, the areas once populated by hominins on the valley floor have now become uplifted onto the flanks of the Rift; today it is here that we find hominin fossils and archaeological evidence, completely removed from their original settings. And it is this great rift, the most substantial and long-lived region with extensional tectonics in the world today, that is believed to have been crucial to our evolution.
FROM TREES TO TOOLS
The first indisputable hominin for which we have discovered good fossil remains is Ardipithecus ramidus, which lived around 4.4 million years ago in forest lining the Awash river valley in Ethiopia. This species was roughly the same size as modern chimpanzees, with an equivalent-sized brain, and teeth that suggest they had an omnivorous diet. The fossilised skeletons indicate they still lived in trees and had only developed a primitive bipedality – the ability to walk upright on two feet. About 4 million years ago, the first members of the genus Australopithecus – the ‘southern ape’ – shared several traits with modern humans, such as a slender and gracile body-form (but still with more primitive skull shapes), and they were competent at walking bipedally. Australopithecus afarensis, for example, is well known from surviving fossils. One of these is the remarkably complete skeleton of a female who lived 3.2 million years ago in the Awash river valley, which came to be known as Lucy.fn2
Lucy would have stood at only about 1.1 metres, but had a spine, pelvis and leg bones very similar to those of modern humans. So while Lucy, and other members of A. afarensis,fn3 still had a small, chimpanzee-sized brain, their skeleton clearly indicates a lifestyle of long-distance bipedal walking. Indeed, a bed of volcanic ash in Laetoli, Tanzania, has preserved three sets of footprints from 3.7 million years ago. These were probably created by members of A. afarensis and look remarkably like those you might leave in the sand during a stroll along the beach.
In human evolution, the development of bipedalism clearly came a long way before significant increases in brain size – we walked the walk before we could talk the talk. These Australopithecus fossils, together with those of the earlier Ardipithecus species, also show that bipedality didn’t evolve as an adaptation to walking in open, grassy savannah environments as had been thought previously, but first emerged with hominins still living closely among trees in wooded areas.15 But bipedalism certainly became an increasingly useful adaptation as the forests shrank and became more fragmented. Our early hominin ancestors were able to move between islands of woods, and then venture out into the grasslands. Bipedalism allowed them to see over the tall grass, and minimised the area of their bodies exposed to the hot sun, helping them to keep cool in the savannah heat. And the opposable thumbs that became so useful for holding and manipulating tools are also an evolutionary inheritance from our forest-dwelling primate ancestors. The hand crafted by evolution to grasp a tree branch pre-adapted us for holding the shaft of a club, an axe, a pen, and ultimately the control stick of a jet plane.
By around 2 million years ago the hominin species of the Australopithecus genus had all fallen extinct and our own genus, Homo, had emerged from them. Homo habilis (‘handy man’) was the first, with a gracile body-form similar to the earlier australopithecines and a brain only slightly larger.16 A dramatic increase in the size of the body and brain, as well as a major shift in lifestyle, however, came with Homo erectus, which appeared around 2 million years ago in East Africa. Below the skull, the skeleton of H. erectus is very similar to that of anatomically modern humans, including adaptations for long-distance running and a shoulder design that would have allowed the throwing of projectiles. They are also thought to have exhibited other traits shared with us, like long childhoods of slower development and advanced social behaviour.
H. erectus was probably also the first hominin to live as a hunter-gatherer an
d to control fire – not just for warmth but possibly also for cooking their food.17 They may even have used rafts to travel over large bodies of water.18 By 1.8 million years ago H. erectus had spread across Africa and then became the first hominin to leave the continent and disperse through Eurasia, probably in several independent waves of migration.19 This species persisted for almost 2 million years. By contrast, anatomically modern humans have only been around for a tenth of that time – and at the moment we’d be lucky to survive the next 10,000 years, let alone 2 million.
H. erectus gave rise to Homo heidelbergensis around 800,000 years ago, which by 250,000 years ago had developed into Homo neanderthalensis (the Neanderthals) in Europe and the Denisovan hominin in Asia. The first anatomically modern human, Homo sapiens, emerged in East Africa between 300,000 and 200,000 years ago.
Over the course of human evolution, hominins became increasingly bipedal, and then more efficient long-distance runners,20 with changes to the skeleton including an S-shaped spine, a bowl-like pelvis and longer legs to support this upright posture and mode of locomotion. Body hair became reduced, except on the scalp. The shape of the head also transformed, producing a smaller snout with a more pronounced chin, and a more bowl-shaped brain case.21 Indeed, the major difference between the earlier Australopithecus genus and our Homo lineage was this increase in brain size. Throughout their 2 million years of evolution the australopithecines’ brain size was strikingly constant at around 450 cm3, roughly equivalent to that of a modern chimpanzee. But H. habilis had a brain a third larger, at about 600 cm3, and brain size doubled again from H. habilis through H. erectus to H. heidelbergensis. By 600,000 years ago, H. heidelbergensis had a brain roughly the same size as that of modern humans, and three times larger than that of australopithecines.22
As well as increasing brain size, another defining feature of the hominins was how we applied our intelligence to tool-making. The earliest widespread stone tools – known as Oldowan technology – date back to about 2.6 million years ago, and were used by the later Australopithecus species as well as H. habilis and H. erectus. Rounded cobbles from a river were used for cracking open bones or nuts against another, flat, anvil stone. Sharpened edges were created by chipping off flakes, and this shaped stone was then used for cutting and scraping meat from a kill, or for wood-working.fn4
A revolution in Stone Age technology came when H. erectus inherited Oldowan tools and refined them into the Acheulean industry 1.7 million years ago. Acheulean tools are more carefully worked by knocking off smaller and smaller flakes to create more symmetrical and thinner, pear-shaped hand-axes. They have represented the dominant technology for the vast majority of human history. A later transformation produced the Mousterian technology, used by Neanderthals and anatomically modern humans through the Ice Age. Here, the core stone was carefully prepared and trimmed by knapping around the edge, before a final, large flake was skilfully knocked off. It was the removed flake rather than the shaped core stone that was the goal: a thin, pointed shard was perfect as a knife or could be used as a spear point or arrowhead.23
These stone tools, as well as wooden spear shafts, enabled hominins to become fearsomely effective hunters without needing to develop large teeth or claws on their own bodies like other predators. We exploited sticks and stones as artificial teeth and claws to hunt for food or to defend ourselves, all whilst being able to keep a safe distance from prey and predators to minimise the risk of injury.
These developments in body-form and lifestyle enhanced each other. More efficient running and sophisticated cognitive abilities, coupled with tool use and control of fire, enabled more effective hunting and a diet with an ever greater proportion of meat for powering a larger brain. This in turn enabled us to develop more complex social interaction and cooperation, cultural learning and problem solving, and perhaps most significantly, language.24
THE CLIMATE PENDULUM
Many of these landmark transitions in our evolution are preserved within the Afar region – the triangular depression that as we saw sits right at the intersection of the tectonic triple junction – at the northern, and oldest, end of the Rift. The first hominin fossils, those of Ardipithecus ramidus, were discovered in the Awash river valley that runs north-east from the Ethiopian plateau towards Djibouti, flowing right through the middle of the Afar triangle. This same river valley preserved the 3.2 million-year-old remains of Lucy – indeed, her entire species, Australopithecus afarensis, was named for this region. And the oldest known Oldowan tools were found in Gona, Ethiopia, which also lies within the Afar triangle. But the whole length of the East African Rift Valley has been a hotbed of hominin evolution.
The drying climate and the rift system with its mosaic of varied features, including volcanic ridges and fault scarps, were clearly instrumental in providing the environmental conditions to drive our evolution. But while this complex, tectonic landscape may have provided opportunities for roaming hominins, it doesn’t explain sufficiently how such incredible versatility and intelligence emerged in the first place. The answer is thought to come down to a particular quirk of the extensional tectonics of the great Rift Valley, and how it interacts with fluctuations in the climate.
As we have seen, the world has been getting generally cooler and drier for the past 50 million years or so, and the tectonic uplift and formation of the Rift Valley has meant that East Africa in particular dried out and lost its former forests. But within this global cooling and drying trend, the climate became very unstable and swung back and forth dramatically. As we will discover in more detail in the next chapter, around 2.6 million years ago the Earth slid into the current epoch of the ice ages, with its alternating glacial and interglacial phases driven by rhythmical shifts in Earth’s orbit and tilt known as the Milankovitch cycles. East Africa was too far from the poles to encounter the advancing ice sheets themselves, but this doesn’t mean it wasn’t greatly affected by these cosmic cycles. In particular, the periodic stretching of Earth’s orbit around the Sun into a more elongated egg shape – known as the eccentricity cycle – has produced periods of highly variable climate in East Africa. During each of these phases of extreme variability, the climate oscillates back and forth between very arid and wetter conditions, with the faster beat of the precession cycle of Earth’s tilted axis, which we’ll come back to.25
Still, these cosmic periodicities and the swings in climate they drive operate over thousands and thousands of years. If we want to understand human evolution, the mystery is that processes which have had the biggest influence on East Africa – such as the overall drying effect from tectonic uplift and rifting within the region, or climate rhythms like the precession of Earth’s axis – operate on an exceedingly slow timescale compared to the lifespan of an animal. Yet intelligence, and the extremely versatile behaviour it allows, is an adaptation similar to the use of a multi-tool Swiss army knife, helping an individual cope with diverse challenges as the environment varies significantly within its lifetime. Environmental changes over a much longer timescale can be met by evolution adapting the body or physiology of a species over the generations (such as the camel adapting to constantly arid conditions). Intelligence on the other hand is the evolutionary solution to the problem of an environment that shifts faster than natural selection can adapt the body. So for there to have been a strong evolutionary pressure driving hominins to ever more flexible and intelligent behaviour, something must have been affecting our ancestors over very short timescales.
What might have been special about the circumstances in East Africa that drove evolution towards highly intelligent hominins such as ourselves? The answer that has been emerging in recent years comes down once again to the peculiar tectonic environment of the region. As we have seen, East Africa was bulging upwards with the magma plume rising beneath and this stretched the crust until it fractured and faulted. The geography of the Great African Rift is therefore characterised by a flat valley floor where great chunks of crust have sunk down, and which is l
ined on both sides by mountainous ridges. In particular, from about 3 million years ago numerous large, isolated basins formed on the valley floor that could fill with lakes if the climatic conditions were wet enough.26 These deep lakes are important because they provided hominins with a more reliable source of water through the dry seasons each year than that supplied by streams.27 But many of them were also ephemeral: they appeared and disappeared over time with the shifting climate.
The East African rift valley system, with major lakes and amplifier lake basins shown.
The landscape of the tectonic rift creates a sharp contrast in the conditions between the high ground and the bottom of the valley. Rain falls over the tall rift walls and volcanic peaks, where it then flows into the lakes dotting the valley floor, a much hotter environment with high rates of evaporation. This means that many of the lakes in the Rift Valley are exceedingly sensitive to the balance between precipitation and evaporation, and even a slight shift in climate causes their water levels to respond very considerably and rapidly – far more so than other lakes around the world and even elsewhere in Africa.28 As small changes in the regional climate cause very large changes in the levels of these vital bodies of water, they are known as ‘amplifier lakes’ – they act like a hi-fi amplifier intensifying a weak signal. And it is these peculiar amplifier lakes that are thought to provide the key link between the long-term trends of tectonics creating the rift valley and the Earth’s climate swings and the rapid fluctuations of habitats that directly, and dramatically, affected our evolution.
Origins Page 2