14 November 2013
IMAGINE for a moment that the last 125,000 years of Earth’s history exist somewhere on a tape – a thick, old-fashioned ribbon loaded between two metal drums. With every second that passes, more tape slowly unspools from one drum and is wound onto the other. Now suppose it’s possible to stop the tape, to intercede, and to reverse its direction. Rewind.
Gradually, with each turn of the drum, our existence is removed.
Every minute, an area of natural forest and woodland the size of 10 football fields is restored. At first, for each year that is regained an area slightly larger than Denmark is reforested. It takes only about 150 years of this to restore most of what has been lost. At the same time, urban sprawl retreats like a concrete tide. Megacities shrink to cities and then dwindle into towns and villages, green swathes of pristine undeveloped land reappearing in their wake. The world’s rivers are undammed. The seafloor is cleared of its wrecks and its tangled cables. The ozone layer is restored. The remains of most of the estimated 108 billion people who have ever lived are removed from the ground, and fossil fuels, precious stones and metals, and other mined materials are put back in. Tonnes of pollutants, including carbon and sulphur dioxide, are sucked out of the atmosphere.
Finally, we arrive at a point that seems incredibly distant to us: 125,000 years ago. In geological terms it might as well be yesterday, but the span of time between then and now represents the entirety of modern human existence. By running the tape backward to this point, we have removed almost all human impact on Earth. What is it like?
A hundred-and-twenty-five thousand years ago, Earth was part way through the Eemian interglacial period – a 15,000-year-long temperate phase bookended by two much longer, colder glacials. Suddenly, it had become a warm and green world. In the northern hemisphere, continental ice sheets had retreated from as far south as Germany in Europe and Illinois in North America.
“It got a little bit warmer than at present, and sea levels were maybe a little bit higher at their maximum,” says Ian Tattersall, curator of anthropology at The American Museum of Natural History in New York City.
One of the beneficiaries of this warm and stable climate was Homo sapiens. Our species had first appeared around 200,000 years ago in east Africa. By 125,000 years ago the population was probably somewhere between 10,000 and 100,000, surviving by foraging and hunting and making its first forays out of its ancestral home.
But we were not alone. “There were at least three lineages of hominids around,” says Tattersall, an expert in early human evolution. “There was Homo sapiens in Africa; there was the lineage of Homo erectus in eastern Asia, which later became extinct; and there were the Neanderthals in Europe.”
Other human species too, both unknown and partly known to us, were struggling to survive elsewhere. “Who knows what was going on in Africa?” says Tattersall. “There were hominids in Africa that didn’t look exactly like a modern Homo sapiens.”
The world also would have been teeming with large animals – whales in the ocean, giant herds of herbivores across on land. “I think if you could just teleport into this world, the thing you’d notice right away would be the megafauna,” says environmental historian Jed Kaplan at the University of Geneva’s Institute for Environmental Sciences in Switzerland. “You would find all of these massive herds of big animals roaming around all over the world,” he says. “There would be woolly mammoths roaming the Arctic. For sure you would see things like bison. You would have big cats living in Europe, maybe horses in the Americas, certainly many more bears, wolves, and all of these kinds of herd animals.”
Stepping outside nature
But then, without warning, everything changed. Or more precisely, humans changed first, and then so did the world. “The shit really didn’t hit the fan until humans started behaving in a modern fashion, about 100,000 years ago,” Tattersall says. “And it was after this that humans sort of stepped outside nature and found themselves in opposition to it, and started all the shenanigans that we’re familiar with today.”
It is sobering to read even an incomplete list of the shenanigans that Tattersall is talking about. As recently as about 2000 BC, world population was counted in the tens of millions. By AD 1700, it was at about 600 million; it is now slightly more than 7 billion and grows by an estimated 220,000 people every day. And that’s just the humans. According to the United Nations Food and Agriculture Organisation (FAO), the global cattle population is 1.4 billion, there are roughly a billion pigs and sheep, and 19 billion chickens worldwide at any one time, almost three for every person.
As befits our numbers, we consume energy like never before. In the 20th century alone, energy use grew 16-fold. According to an article published in 2009 in the International Journal of Oil, Gas and Coal Technology, since 1870, an estimated 944 billion barrels – or 135 billion tonnes – of oil have been extracted from beneath the Earth’s surface. In 2011 alone, the US mined more than a billion tonnes of coal, and China three times as much.
We have also altered the landscape in untold ways (see diagram). Together, agriculture and the use of fire have tamed and shaped the environment almost everywhere (see diagram). In many regions, farmed land has replaced the natural vegetation. Between 30 and 50 per cent of the planet’s land surface is used in one way or another by humans (see diagram), and we are tapping more than half of the world’s accessible fresh water.
Rice production, in particular, has flattened entire ecosystems. “People produce little dams,” says Erle Ellis, an environmental scientist at the University of Maryland. “And that changes the whole sediment movement in a watershed. The goal is to create wetlands everywhere to grow rice. And that has flattened a lot of places. It’s impressive.”
In the modern world, we are left with very few places that look the way they would if humans had not intervened. “There’s very few landscapes that are really left, especially in Europe,” says Kaplan. “There are hardly any forests where you find big dead trees just laying down on the floor. It’s incredibly rare.”
Ever since modern humans began to oppose the rest of nature, they moved, dispersing across the world like seeds in the wind, settling in the Near East 125,000 years ago, South Asia 50,000 years ago, Europe 43,000 years ago, Australia 40,000 years ago and the Americas between 30,000 and 15,000 years ago. The final significant, habitable land mass to be settled was New Zealand about 700 years ago.
Everywhere they went, humans took animals with them, some deliberately (dogs, cats, pigs) and others by accident (rats). The introduction of a non-native species to a delicately balanced ecosystem can have irreversible effects, says Ellis. Especially rats. “They have a huge effect. Anything that nests on the ground or in any place where a rat can get to it – those species are toast.”
We are also efficient killers in our own right, of course. Many species are known to have been hunted or persecuted to oblivion, most famously the dodo (last confirmed sighting in 1662). Also gone: Steller’s sea cow (1768); the bluebuck (~1800); the Mauritius blue pigeon (1826); the great auk (1852); the sea mink (~1860); the Falkland Islands wolf (1876); the passenger pigeon (1914) and the Caribbean monk seal (1952). Many more species have disappeared on our watch. The human march across the globe was followed by wave after wave of megafauna extinctions. The causes are still debated, but many point the finger at us. “I really think that humans had a role in tipping a lot of these megafauna populations toward extinction,” says Kaplan.
See what these animals looked like before we killed them all: “Hall of shame: Seven species that humans wiped out“
Fifteen thousand years ago, for example, humans were entering North America from Siberia. “There was an unprecedented pulse of extinction,” says Bill Ruddiman, a climate scientist at the University of Virginia. “That requires something brand new, and humans were brand new.”
“The American west, the plains, had a variety that was far richer than the Serengeti today,” says Ruddiman. “It was an amazing place. Aside from mammoths and mastodons, there were sabre-toothed tigers, horses, camels, gigantic ground sloths – all kinds of animals that went extinct in a pretty brief interval. The best data on that suggests it happened about 15,000 years ago.”
Today, wide open – and mostly empty – the American West looks vastly different from the way it did 125,000 years ago.
Removal of large animal species by humans has had effects on the landscape that are apparent almost everywhere. “A lot of land would be semi-open, kept partly open by these big herds of grazers and browsers and predators,” says Kaplan. “It’s important to keep in mind that landscape is also shaped by animals. These giant herds of bison would be trampling down little trees and keeping the landscape open, certainly not as much as people who are using fire, but definitely having an effect.”
We have also emptied the oceans. According to a 2010 report, the UK’s fishing fleet works 17 times harder than it did in the 1880s to net the same amount of fish. The FAO estimates that more than half of world’s coastal fisheries are over-exploited.
Whaling has also changed the oceans beyond recognition. During the 20th century, several species were hunted to the brink of extinction, and populations have still not recovered. A controversial study published in Science claimed that pre-whaling populations were dramatically higher than previously thought. By this estimate there were once 1.5 million humpback whales, rather than the 100,000 estimated by the International Whaling Commission. It is a similar story for minke, bowhead and sperm whales.
We have also shifted the climate. In May, atmospheric carbon dioxide levels topped 400 parts per million for the first time in millions of years; 125,000 years ago they were 275 parts per million. The increase comes partly from the burning of fossil fuels but also from the stripping of the world’s forests, which have acted as an almost bottomless carbon sink for millions of years.
The impact is etched dramatically on Earth’s ice. Across the world, glaciers are retreating and in some places have disappeared. The US National Snow and Ice Data Center at the University of Colorado in Boulder maintains an inventory of more than 130,000 glaciers around the world. Some are growing; many more are shrinking. Worldwide, for every glacier that is advancing, at least 10 are retreating. At its creation in 1910, Glacier National Park in Montana had an estimated 150 glaciers. Today there are about 30, all of which have shrunk. In 2009, the Chacaltaya glacier in Bolivia – once the location of the world’s highest ski-lift – disappeared. The polar ice sheets are breaking apart, calving city-size blocks of ice into the oceans. In July, a 30-kilometre crack in the Pine Island glacier in Antarctica created an iceberg the size of New York.
By running the tape of time backwards, almost all of these human impacts on Earth are gone. Now, just for fun, let’s do something else: let’s remove Homo sapiens. Imagine that 125,000 years ago, our small band of ancestors in east Africa was wiped out by a catastrophe: a lethal virus, perhaps, or a natural disaster.
Now, let the tape run forward again. What would the world look like today if modern humans had never been here?
In some respects the answer is obvious: it would look a lot like the world of 125,000 years ago. “We would have a continuous biosphere – one that we can scarcely now imagine. That is, forest, savannahs and suchlike, extending across the Earth,” says Jan Zalasiewicz, a geologist at the University of Leicester, UK. “No roads. No fields. No towns. Nothing like that.” The land would teem with large animals, the seas with whales and fish.
But it wouldn’t last, says Ruddiman. If humans had died out 125,000 years ago, we would now be entering another ice age. Glaciers would be growing and advancing. It’s a controversial idea and it has earned Ruddiman his critics. But now, more than a decade since he first proposed it, many climate scientists agree with him.
“If you erase the human effect there would be considerably more sea ice and much more extensive tundra around the Arctic circle,” he says. “Boreal forest would have retreated and, most dramatically of all, you would have growing ice sheets in a number of northern regions – the northern Rockies, the Canadian archipelago, parts of northern Siberia. It’s the very early stages of an ice age. That’s the single most dramatic change.”
Or maybe not. Perhaps, in our absence, one of the other human species that was present – Neanderthals, Homo erectus, or an as-yet unidentified species – rises to prominence and begins to shape the world instead of us.
Tattersall is doubtful. “Having established themselves, would they have followed in our footsteps?” he says. “Would they have become an ersatz Homo sapiens, implying that there was some sort of inevitability on our having become what we became? I would guess no.”
But there is a delicious counterpoint to this argument.
“There is this idea – convergent evolution – that if we didn’t come along and do all this, somebody else would,” says David Grinspoon, curator of astrobiology at the Denver Museum of Nature and Science in Colorado. “There still would have been selective pressure for some other species to go through the same kind of development that we did, where there’s this feedback between big brains, and language, and symbolic thought, and developing agriculture. If the scenario is literally that just Homo sapiens goes extinct but it’s still the same general landscape, maybe something similar would have happened. It wouldn’t have been identical because there’s so much randomness, and it might have taken longer.”
In short, perhaps it all would have happened anyway. Maybe this modern version of Earth, and our place in it, was unavoidable. Remove Homo sapiensfrom the equation, reforest the world and repopulate it with megafauna, and maybe in a 100,000 years or so our greatest works, our advancements and our errors – or at least something like them – would still be the outcome.
“I wish I had a crystal ball, or an alternate-universe viewer,” says Grinspoon. “It would be great to know.”
This article appeared in print under the headline “Rewind, erase, rerun”
Christopher Kemp is a writer based in Michigan