Two particular aspects of our planet’s cosmic circumstances are important here: the stretching of Earth’s orbit around the Sun (eccentricity) and the gyration of the Earth’s axis (precession). Every time Earth’s orbit was pulled into a more elongated shape (maximum eccentricity) the climate in East Africa became very unstable. During each of these phases of climatic variability, whenever the precession cycle cast a little more solar warming onto the Northern Hemisphere, more rain fell onto the walls of the Rift Valley. The amplifier lakes appeared and enlarged, their shores lined with woodland. And conversely, during the opposite phase of the precession cycle the rift received less rainfall and the lakes diminished or disappeared altogether. The Rift Valley then returned to an extremely arid state with minimal foliage.29 So overall, the environment in East Africa over the last few million years has largely been very dry, but this general state was interspersed with highly variable periods when the climate swung rapidly back and forth between being much wetter and then very arid again.
These variable phases occurred every 800,000 years or so, and during those periods the amplifier lakes flickered in and out like a loose lightbulb – each swing causing a considerable shift in the availability of water, vegetation and food, which had a profound influence on our ancestors. The rapidly fluctuating conditions favoured the survival of hominins who were versatile and adaptive, and so drove the evolution of larger brains and greater intelligence.30
The three most recent periods of such extreme climatic variability occurred 2.7–2.5, 1.9–1.7, and 1.1–0.9 million years ago.31 Looking at the fossil record, scientists have made a fascinating discovery. The timing of when new hominin species emerged – often associated with an increase in brain size – or fell extinct again, tends to coincide with these periods of fluctuating wet–dry conditions. For instance, one of the most important episodes in human evolution occurred in the variable period between 1.9 and 1.7 million years ago, a phase when five of the seven major lake basins in the rift repeatedly filled and emptied. It was during this time that the number of different hominin species reached its peak, including the emergence of H. erectus with its dramatic increase in brain size. Overall, of the fifteen hominin species we know of, twelve first appeared during these three variable phases.32 What’s more, the development and spread of the different stages of tool technologies that we discussed earlier – Oldowan, Acheulean, Mousterian – also correspond with the eccentricity periods of extreme climate variability.33
And not only did the variable periods determine our evolution, they are also thought to have been the force driving several hominin species to migrate out of their birthplace and into Eurasia. We’ll explore in detail in the next chapter how our species Homo sapiens were able to disperse around the entire globe, but the conditions propelling hominins out of Africa in the first place again lie with the climate fluctuations in the Great Rift.
During each wet phase the filling of the large amplifier lakes and the extra availability of water and food would cause a population boom, while at the same time limiting the amount of space available for habitation along the tree-lined rift shoulders. This would have squeezed hominins along the tube of the Rift Valley and eventually pushed them out of East Africa with each wet pulse of the precessional cycle, like a climate pump. Moister conditions would also have allowed hominin migrants to move north along the Nile tributaries and across the greener corridors of the Sinai Peninsula and Levant region to spill into Eurasia.34 Homo erectus left Africa during the variable climate phase around 1.8 years ago, eventually spreading as far as China. In Europe, H. erectus evolved into the Neanderthals, while the H. erectus population that remained in East Africa eventually gave rise to anatomically modern humans 300,000–200,000 years ago.
Our own species dispersed out of Africa around 60,000 years ago, as we’ll see in the next chapter. We encountered the descendants of previous waves of hominin migrants – Neanderthals and Denisovans – as we moved through Europe and Asia. But both of these had died out by around 40,000 years ago, and only anatomically modern humans remained. From a peak in the diversity of different hominin species in Africa about 2 million years ago,35 through our interactions (and interbreeding) with closely related human species as we moved through Eurasia, Homo sapiens became a lonely species. We are today the sole survivor of our genus, and indeed of the entire hominin tree.
This in itself is a curiosity. We know from the extensive archaeological evidence that the Neanderthals were themselves an extremely adaptable and intelligent species. They crafted stone tools and hunted with spears, controlled fire, and may have decorated their bodies and even buried their dead. They were also physically stronger than us Homo sapiens. And yet almost as soon as we arrived in Europe the Neanderthals disappeared. They may have succumbed to the punishing climatic conditions in the depths of the Ice Age (although the uncanny coincidence with our arrival would seem to discount that explanation), or perhaps anatomically modern humans violently clashed with these pre-existing Europeans and we slaughtered them into oblivion. But the most likely explanation is that we simply outcompeted them for resources in the shared environment. Modern humans are thought to have had a much better capability with language and thus social coordination and innovation, as well as more advanced tool-making abilities. And despite having dispersed from tropical Africa more recently, we could craft sewing needles and so were able to make warmer, body-hugging clothing as the Ice Age dipped into particularly bitter spells.36
Humans prevailed over the Neanderthals with our brains not our brawn, and subsequently came to dominate the world. And the reason for this is probably the fact that our ancestors had a longer evolutionary history in the extreme fluctuating climate of East Africa, forcing them to developing greater versatility and intelligence than the Neanderthals. We had spent longer adapting to the wet–dry variability of the Rift Valley, and that also made us better able to cope in the different climates we encountered around the rest of the world, including the Ice Age climes of the Northern Hemisphere.37
All in all, the human animal was forged by a peculiar combination of planetary processes all coming together in East Africa over the past few million years. It wasn’t just that the region had dried out as Earth’s crust bulged with the magma plume rising beneath, shifting from the relatively flat, forested habitat of our primate ancestors into arid savannah. The entire landscape had become transformed into a rugged terrain cut through with steep fault scarps and ridges of solidified volcanic lava: this was a world fragmented into a complex mosaic of different habitats that continued to change with time. In particular, the extensional tectonics of East Africa ripped open the Rift Valley to create a particular geography of tall walls collecting rainfall and a hot valley floor. Cosmic cycles in Earth’s orbit and spin axis periodically filled basins on the rift floor with amplifier lakes that responded rapidly to even modest climate fluctuations to create a powerful evolutionary pressure on all life in this region.
These unique circumstances of our hominin homelands drove the development of adaptable and versatile species. Our ancestors came to rely more and more on their intelligence and on working together in social groups. This diverse landscape, varying greatly across both space and time, was the cradle of hominin evolution, and out of it emerged a naked and chatty ape smart enough to come to understand its own origins. The hallmarks of Homo sapiens – our intelligence, language, tool use, social learning and cooperative behaviour, which would allow us to develop agriculture, live in cities and build civilisations – are consequences of this extreme climatic variability, itself produced by the special circumstances of the Rift Valley. Like all species, we are a product of our environment. We are a species of apes born of the climate change and tectonics within East Africa.38
WE ARE THE CHILDREN OF PLATE TECTONICS
Plate tectonics did not just create the diverse and dynamic environment of East Africa in which we evolved as a species; they were also to be a factor that defined where humanity embarked on
building our early civilisations.
If you look at a map of the tectonic plate boundaries grinding against each other and superimpose the locations of the world’s major ancient civilisations, an astonishingly close relationship reveals itself: most are located very close to plate margins. Considering the amount of land available for habitation on Earth, this is a startling correlation, and is very unlikely to have come about by chance. Early civilisations seem to have chosen to snuggle up close to tectonic fractures, millennia before scientists identified their existence. There must be something about the plate boundaries that made them so favourable for the establishment of ancient cultures, despite the dangers of earthquakes, tsunamis and volcanoes posed by these fractures in the Earth’s crust.
Major early civilisations and their proximity to plate boundaries.
In the Indus Valley, the Harappan civilisation emerged around 3200 BC as one of the three earliest in the world (alongside those in Mesopotamia and Egypt),39 in a depressed trough running along the foot of the Himalayas. The collision between tectonic plates creases up ranges of high mountains – such as the Himalayas, created by the crashing of India into Eurasia – but the immense weight of the mountain range also flexes the crust alongside it downwards to create a lowlying subsiding basin. The Indus and Ganges rivers flowing off the Himalayas run through this foreland basin, where they deposit sediment eroded from the mountains to produce very fertile soils for early agriculture. You could say that the Harappan civilisation was born of the continental collision between India and the Eurasian plate.
In Mesopotamia, the Tigris and Euphrates rivers also flow along a subsiding foreland basin, pushed down by the Zagros mountains that formed as the Arabian plate was subducted beneath the Eurasian (shown here).40 Mesopotamian soils were therefore similarly enriched with sediment eroded out of this mountain range.41 The Assyrian and Persian civilisations both arose right on top of this junction between the Arabian and the Eurasian plate.
The Minoans, Greeks, Etruscans and Romans all also developed very close to plate boundaries within the complex tectonic environment of the Mediterranean basin. Within Mesoamerica, the Mayan civilisation emerged from around 2000 BC and spread across much of south-eastern Mexico, Guatemala and Belize, with major cities built among the mountains raised by the subduction of the Cocos plate beneath the North American and Caribbean plates. And the later Aztec culture flourished close to the same convergent plate boundary, with its earthquakes and volcanoes like Popocatepetl, the ‘Smoking Mountain’, sacred to the Aztecs.fn5
And it is not just depressed basins at the feet of mountain ranges raised by continental collisions, like Mesopotamia, that hold rich arable land. Volcanoes also produce fertile agricultural soil. They arise in a broad line 100 kilometres or so away from the subduction line, as the swallowed plate sinks deeper into the hot interior and melts to release rising bubbles of magma to feed eruptions on the surface above. Civilisations in the Mediterranean, such as the Greek, Etruscan and Roman, arose in areas of rich volcanic soil in the band where the African plate is being subducted under the smaller plates making up the Mediterranean region.42
Tectonic stresses also hold open fractures in rocks or push up blocks of crust in what is known as a thrust fault, which often create water springs. The long line of linked mountains along southern Eurasia, crumpled up by the collision of the African, Arabian and Indian plates, happens to coincide with the arid band across the Earth’s surface. This includes the Arabian and Great Indian deserts, and is created by the dry, descending portion of circulation in the atmosphere (which we’ll come back to in Chapter 8). Here these thrust faults frequently lie at the junction between lowlying barren deserts and high-rising inhospitable mountains or plateaus, and so trade routes often pass along these geological boundaries. Towns dotted along the way accommodate the travelling merchants, supported by the water springs at the foot of the mountains.43 But while tectonic movements can provide water sources in otherwise arid environments, these settlements are also vulnerable to destructive earthquakes with each new slip of the crust.44
In 1994 the small desert village of Sefidabeh in south-eastern Iran was utterly destroyed by an earthquake. The curious thing was that Sefidabeh is exceedingly remote: one of the few stops on a long trade route to the Indian Ocean, it’s the only settlement for 100 kilometres in any direction. And yet the earthquake seemed to target the village with uncanny precision. It turns out that Sefidabeh had been built right on top of a thrust fault lying far underground. The fault was so deep that it had created no obvious signs of its existence on the surface, such as a tell-tale scarp, and so hadn’t been previously identified by geologists. In hindsight, the only sign was an unremarkable, gently-folded ridge running alongside the town, that had slowly been built up over hundreds of thousand years of earthquake movements. The settlement had grown here because this continual tectonic up-thrusting maintained springs at the base of the ridge – the only water source for miles around. The tectonic fault had created the conditions allowing life in the desert, but it also had the potential to kill.45
The sources of water provided by these thrust faults have been used for thousands of years, and explain the location of many ancient settlements on tectonic boundaries. They are becoming an increasing cause for concern in the modern world, however. The capital of Iran, Tehran, began as a cluster of small towns on a major trade route at the base of the Alborz mountain range. The city grew rapidly from the 1950s and today is densely populated with a permanent population of over 8 million residents, rising to over 10 million during working hours.46 But the small trading towns originally occupying this site through the centuries were repeatedly damaged or levelled outright by the jerk of earthquakes as this thrust fault shifts to relieve mounting tectonic stress. The city of Tabriz, further along the mountain chain to the north-east of Tehran, was devastated by earthquakes in 1721 and 1780, each killing more than 40,000 people at a time when the population of any city was only a tiny fraction of what it is today. If, or indeed when, another large earthquake jolts on this thrust fault, the effects on Tehran could be devastating. People have settled at such thrust faults for millennia, drawn by the water supply they create and the trade routes running along the landscape boundary, and the large modern cities that have developed here are now particularly vulnerable from this geological heritage.47
We are the children of plate tectonics. Some of the largest cities in the world today rest on tectonic faults, and indeed many of the earliest civilisations in history emerged along the boundaries of the plates that make up Earth’s crust. And more fundamentally, tectonic processes in East Africa were critical for the evolution of hominins and the forging of our particularly intelligent and adaptable species. Let’s turn now to the peculiar period of our planet’s history that enabled humanity to migrate from our birthplace in the Great Rift Valley and come to dominate the entire globe.
Chapter 2
Continental Drifters
We are currently living in something of a peculiar geological age. It is a time that is distinguished by a single, dominant feature: ice. This may sound surprising, given our current concerns over global warming. That average temperatures have been rising since the Industrial Revolution, and particularly rapidly over the past sixty years, is undeniably true. But this recent jump caused by human activity is occurring within the general time frame of the long-term glaciation of the Quaternary Period. About 2.6 million years ago, at the start of the latest geological period, the Earth slid into a new climatic regime, characterised by the pulse of recurring ice ages. These conditions have had profound effects on the world we find today, and how we took our place in it.
At present we are living through an interglacial period, with relatively warm conditions, shrunken ice caps and consequently higher sea levels. But the average conditions over the past 2.6 million years have been much icier than today. We are perhaps familiar from museum exhibits and TV documentaries with how the world looked in the last ice age – a
time when great ice sheets expanded over much of the northern hemisphere, woolly mammoths strode across the tundra-like landscape preyed upon by sabre-toothed tigers, and fur-clad Palaeolithic humans hunted with stone-tipped spears.
Yet this was only the latest phase of glaciation in our recent planetary history. There have been between forty and fifty ice ages over the past 2.6 million years,1 and they’ve been getting progressively longer and colder over time. In fact, the Quaternary is an exceptionally unstable time for the planet’s climate,2 which has been see-sawing between bitter ice ages and warmer interglacial intervals, driving the periodic expansion and contraction of huge ice sheets. The freeze-ups last on average 80,000 years, the shorter respites between ice ages only around 15,000 years.3 Each interglacial period, such as the current Holocene Epoch we entered 11,700 years ago, is no more than a brief thermal intermission before the climate plunges back into another frosty episode. We’ll see later why our planet has entered this erratic climatic phase, but let’s look first at the conditions of the last ice age.
CHILLY TIMES
This began about 117,000 years ago, and lasted around 100,000 years until the beginning of the current Holocene interglacial.4 At its peak, between 25,000 and 22,000 years ago, immense ice sheets up to 4 kilometres thick extended from the north to smother northern Europe and America.5 Another smaller ice sheet expanded across Siberia, and great glaciers spread down from mountain ranges such as the Alps, Andes and Himalayas, as well as the rugged backbone of New Zealand.
Origins Page 3