HUMANITY today is actively reshaping the planet. Our appetite for natural resources and large-scale industrial activity is eradicating species, warming the oceans and disrupting the global climate on an unprecedented scale. So profound is our impact that some have called for the times we live in to be declared a new geological period: the Anthropocene, the age of humanity.
But this ability to shape our environment on such a scale is a recent phenomenon. For most of our history, it is our environment that has shaped us. The physical features of the planet we live on enabled our species to arise, nurtured our remarkable brains, facilitated our spread across the planet and even encouraged the birth of the first cities. This is the remarkable story of the way Earth has moulded humanity, and how we have turned the tables to shape the world.
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How we emerged
Some 55.5 million years ago, Earth’s thermostat did something unexpected. In the space of 100,000 years, barely the blink of an eye on geological timescales, the temperature of the planet jerked up by between 5°C and 8°C, hovered for a bit, and then came back down. This brief planetary fever was hugely disruptive to life on Earth, driving the rapid evolution and divergence of whole new orders of animals, including our own.
The principal culprit is thought to have been methane. As a powerful greenhouse gas, its presence in air makes our atmosphere trap more of the sun’s heat than it usually would, raising the temperature on the surface. Then, as now, huge deposits of methane lay on the sea floor, a by-product of decaying organic matter. Under the extreme pressures and low temperatures found at such depths, the gas became trapped within crystals of ice, safely locked away so long as the ice didn’t melt. In planetary terms, it was a barrel of gunpowder waiting for a match.
The fateful spark is thought to have been a cluster of volcanic eruptions that peppered the atmosphere with enough carbon dioxide to cause an initial temperature rise. This, in turn, melted ice on the sea floor, causing methane to bubble up through the water and into the atmosphere, leading to a further temperature rise that melted yet more methane.
The sweltering climate resulted in a burst of evolutionary diversification. The fossil record shows that ungulates, which include modern species like the cow, goat, pig, sheep, llama, camel and horse, first emerged during this period. These families of large herbivores are utterly critical to human societies around the world, providing not just a reliable source of meat, milk, hide, wool and leather, but also means of transport. We ride them, load them with packs and put them to work hauling carts or ploughs, all in the service of human development.
But the most significant group of mammals that sprang up during this heatwave were the primates, the group that our own species belongs to. These early ancestors of ours, physically similar to lemurs, emerged and then rapidly dispersed across Asia, Europe and North America. But it was in the unique geology of East Africa that they took their first unsteady steps towards humanity.
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How we got smart
All animals evolve in response to their natural environment, and our own species, Homo sapiens, is no different. The emergence of our large brains millions of years ago is a case in point. These supercharged organs require a lot of resources and energy, and so would have needed a very good reason to emerge. Their development would have been driven by necessity, probably as a response to complex and rapidly changing surroundings that required high intelligence and adaptability to survive. But what has long puzzled palaeontologists is what made the corridor of the East African Rift Valley such an ideal incubator for these intelligent apes (see “Narrow escape”).
The major factor that led our evolutionary family, the hominins, to split from our tree-swinging, primate ancestors was the general drying out of East Africa and the transformation of the densely forested habitat into grassy savannah. This drying trend was driven largely by forces beneath the ground. As the African continental plate bulged upwards and ripped apart to form the East African Rift Valley, wall-like mountainous ridges rose to line the valley. These peaks blocked the movement of moisture-laden clouds and kept East Africa arid.
The amount of rainfall available to our ancestors in this dry, equatorial region would have varied with cyclical shifts in Earth’s tilt and orbit around the sun, known as the Milankovitch cycles. But these cosmic variations happen over thousands of years – too slow to have a significant effect over an individual lifetime. So why would big, adaptable brains be needed to navigate this relatively stable new status quo?
The answer that has been emerging in recent years relies on a powerful, combined effect of tectonic geography and cosmic cycles. While the mountainous walls of the rift collect rainfall on their flanks, the valley bottom is hot and dry. That means the many lakes strung along the rift floor are highly sensitive to the delicate balance between precipitation and evaporation, and their water levels fluctuate significantly with the Milankovitch cycles. During particularly unstable climatic periods, these bodies of water rapidly flicker in and out of existence, like the light of a dying bulb.
This changing availability of water, and therefore vegetation, animal life and food, is thought to be what favoured the evolution of humanity’s versatile behaviour.
The fossil evidence backs up this hypothesis. The three most recent periods of extreme climate variability in this region occurred around 2.6, 1.8 and 1 million years ago, and are each associated with the emergence of new hominin species in East Africa and increases in brain size. And once the unstable, fluctuating environment had started encouraging the growth of intelligence, it rapidly led to other big gains, including increasingly complex social interaction, language and tool use. It was in these specialised surroundings that our own species eventually emerged from other hominins around 300,000 years ago.
How we took over the world
Our ancestors didn’t remain in their cradle. The narrow, tube-like rift valley squeezed migrating populations out of its northern end. During wetter, greener spells, they would have been able to walk across the usually barren plains of North Africa to Sinai, in what is now north-eastern Egypt, or the Arabian peninsula beyond. But to truly take over the world, including by crossing the oceans that separated continents, an entirely different set of planetary conditions would be needed.
The first of our ancestors’ big migrations began nearly 2 million years ago, when Homo erectus spread across Asia, reaching as far as China and Indonesia. There they gave rise to at least two other hominin species – the Neanderthals in Europe and the Denisovans in central Asia. Anatomically modern humans left Africa around 65,000 years ago, spreading up into Europe and along the southern margin of Eurasia to today’s India and South-East Asia.
During this great dispersal, Earth was in the depths of its most recent freeze, one of at least 40 such glaciations that have taken place over the past 2.6 million years. Not only were global temperatures much lower during these periods, but the climate was also much drier. Even beyond the reach of the advancing ice sheets, much of the land was desolate tundra. Such conditions would have been undeniably brutal for palaeolithic humans living across Eurasia, but the glaciation offered one crucial advantage to our early ancestors. The great ice sheets locked up vast amounts of water, lowering global sea levels by up to 120 metres.
Large tracts of the shallow continental shelves became dry land, and this offered flat plains for hunting and highways for migration (see “Frosty reception”). Early humans in Asia were able to simply walk across the Sunda land bridge to populate current-day Malaysia, as well as Sumatra, Java and other parts of Indonesia, while the Sahul land bridge offered easy access between what is now New Guinea and Australia. But perhaps most significantly for the human story, a wide corridor of land linking eastern Siberia and Alaska – the Bering land bridge – also emerged. This gave our ancestors a route of entry from Eurasia into the otherwise unreachable continent of North America, where they soon worked their way down to cross the isthmus of Panama and reach South America as well.
As they moved into Europe and central Asia, our ancestors encountered – and interbred with – their Neanderthal and Denisovan cousins. But after crossing into the Americas, humanity was walking where no hominin species had ever trodden before. When the last glaciation ended and the Bering land bridge disappeared back beneath the waves, east and west became severed. Two isolated human populations, essentially identical in terms of genetics and abilities, but with access to different sets of plants and animals, began independent experiments in putting down roots and building cities.
How we built cities
From around 11,500 years ago, people all over the world began abandoning their hunter-gatherer ways and started to settle down. There were good reasons for this lifestyle change. The most recent glacial period was coming to an end, marking the first interglacial period that early humans had experienced since migrating out of Africa. The period of relative climatic stability that has lasted since then proved ideal for the emergence of agriculture and city life, spawning the civilisations that have irrevocably shaped present-day humanity.
In short order, we learned to tame the natural world to provide domesticated animal and plant species for our farms. The staple of most of our diets became cereal crops such as wheat, rice and maize – grass species that had proliferated around the world as Earth cooled and dried over the past few tens of millions of years. And the animals we domesticated were mostly ungulates – herbivores like the sheep, pig, cow and horse, whose ancestors had come to dominate these new grassy ecologies.
Settled farming allowed populations to expand quickly, and soon people began congregating in dense clusters that became the first cities. But it was planetary forces that came to dictate where these first cities formed and the earliest civilisations began.
“Plate tectonics provided the spawning points for our first civilisations”
Take Mesopotamia. This was where the organisation, governance and cities of Sumerian civilisation began emerging around 6000 years ago, in an area roughly corresponding with the northern part of the modern day Middle East. The Tigris and Euphrates rivers reliably delivered the lifeblood for irrigating agriculture, and deposited thick alluvial soil carried from the highlands to the north-west.
The reason so much fertile soil built up in Mesopotamia is because it lies along what is known geologically as a foreland basin. This feature is the result of continental drift that caused the Arabian peninsula to swing away from north-east Africa. It slammed into the southern margin of the Eurasian tectonic plate, forming the Zagros mountains, and the immense weight of this mountain range flexed Earth’s crust to create a low-lying subsiding basin that filled with sediment: Mesopotamia.
The Harappan civilisation emerged along the Indus valley at around the same time as the Sumerians in Mesopotamia, and did so in an essentially identical tectonic scene. The Indus flows along the foreland basin that sags down alongside the mighty Himalayas, which were created by the crashing of India into Eurasia. Active plate tectonics therefore provided not only the cradle for the very evolution of humanity in Africa, but also the spawning points for our first civilisations.
How we are risking it all
Today, the power dynamic between Earth and humanity has flipped. Human-driven climate change is the most severe jolt in global temperatures in 55 million years. That earlier climatic spasm was crucial to the development of our early ancestors, but a similar event today would have catastrophic consequences for our present way of life.
The effects of global warming will be far from uniformly distributed, and different regions will experience opposite effects. Many will see increasing droughts or the loss of fertile land as deserts form, while others will experience more intense bursts of rainfall and flooding. Both these extremes threaten to severely disrupt reliable agriculture. Melting polar ice caps will raise sea levels and risk inundating coastal areas and cities, while the disappearance of mountain glaciers will significantly affect water availability. All the while, tropical diseases will reach further from the equator, and intensifying heatwaves will kill more vulnerable members of society.
Taken together, these factors will drive widespread displacement of people and mass migration. Left unchecked, human-driven climate change will shape our future as much as the planet itself has shaped our past. To navigate it safely, we must understand the deep connections that tie us to our planet, and come to terms with how Earth made us.
A geological legacy
The Inner Sea
Throughout history, the Mediterranean has buzzed with a multitude of notable cultures and civilisations. Many flourished on its northern shores – the Greek islands or Italy – more so than on the southern lip along the African coastline. This may be because the northern Med has an intricate geometry full of natural harbours and islands, perfect for early sea-faring societies, whereas the coastline to the south is mostly smooth and less accommodating. This fundamental difference is due to continental drift: Africa is being subducted underneath the Eurasian plate as it rides north, crumpling up the European coastline and leaving that of Africa smoother.
Island Nation
The UK’s island status is a defining part of its identity. The natural moat that surrounds it has protected it from most invasions and so maintained the balance of power on the continent by making it hard for any one state to consolidate a European empire. All the while, its relative proximity has allowed it to prosper from trade. But Britain hasn’t always been an island – it used to be joined to France by a land bridge. This physical connection was eroded away by a megaflood that began almost half a million years ago, as a glacial lake rapidly drained away during a previous cold spell. The iconic white cliffs of Dover are this land bridge’s northern stumps.
Bedrock of democracy
The impact of the planet’s geology is even visible in how people vote. Running through the staunchly Republican south-eastern US states, for example, is a distinct crescent of Democrat-voting counties (see graphic, below). This curve closely follows an exposed band of 75-million-year-old rocks laid down during the Cretaceous period. These have produced a very rich, fertile soil, which was found to be perfect for growing cotton. In the mid-1800s, cultivation of this crop led to a boom in slave labour. A high proportion of African-Americans still remain in these areas, where they favour the more liberal policies of the Democratic party, and continue to vote for its presidential candidates.
Article amended on 26 April 2019
We clarified that the sculpture of a human figure is by Antony Gormley
