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The end of life on Earth

WHEN it comes to what could wipe out all life on Earth, asteroids and comets are the likeliest culprits. If that doesn’t do it, a swollen sun will (see “The end of the solar system“).

In the short term the greatest threat comes from comets, says . Comets hit at three times the speed of asteroids, he says, so they pack more of a punch. “Hale-Bopp is 50 kilometres in diameter. Had it hit, there would be no life on Earth.”

The energy released by a Hale-Bopp-sized impact would boil the oceans and vaporise rock. Earth’s surface would be sterilised. The only possible refuge would lie deep inside our planet, currently home to hardy bacteria and archaea. If life survived down there, maybe in time it could reseed the surface.

But it may never get the chance. “Some say the deep microbial biosphere couldn’t survive because if you wipe out the surface ecosystem, sooner or later the nutrients they need will disappear,” says Ward.

So if life here were extinguished, could it start again? “You could argue that it happened once, so it would likely happen again,” says Institute in Houston, Texas. “But it’s difficult to say because strictly speaking, we don’t know what the steps were from an abiotic world to a biological one.”

One thing’s for sure: conditions on Earth post-annihilation would be more favourable than first time around. Amino acids and other molecular building blocks of life would remain. And the sun is 30 per cent brighter than it was when life began, making liquid water a certainty.

If life did rise again, it may not be as we know it. Evolutionary biologist Stephen Jay Gould famously argued that if life started over, it wouldn’t evolve the same way. “It’s assured that the nature of life, even if it were to re-evolve on the same planet, would be different,” says Kring.

“If life rises again on Earth, it may not be as we know it: life wouldn’t evolve the same way again“

Ultimately, though, life on Earth is doomed. Our sun will eventually get so hot that it will start killing off plant life. Higher temperatures also mean more rainfall, accelerating the weathering of silicate rocks, which in turn draws more carbon dioxide from the atmosphere. Eventually photosynthesis becomes impossible, setting off a cascade of extinctions that starts with large mammals and ends with the hardiest microbes.

Life might restart once in a while when conditions improve for brief spells, but there will come a point when the supercharged sun creates conditions that couldn’t possibly support biology. There will be no coming back from that. Andy Ridgway

The end of you

The end of ice

THE last ice age ended 12,000 years ago, when the ice sheets retreated from Eurasia and North America. We are now basking in the warmth of a so-called interglacial period, but there is still a staggering amount of ice. The Antarctic ice sheet alone covers an area larger than the US and India combined, and it is nearly 5 kilometres thick in places.

Perhaps not for much longer. If we are foolish enough to burn all the fossil fuels we’ve found so far, , a study out last year concluded. It will take thousands of years, but when the last great chunk of Antarctic ice melts or slides into the sea, global sea levels will be about 70 metres higher than today.

On average, that is. Around Antarctica and Greenland, sea level will actually fall. That’s because the shrunken mass of ice will exert a weaker gravitational pull on the water than it did before. And as the weight of all that ice is removed, the land will slowly rise.

The retreat of the ice will be bad news for polar bears, but there will be winners too. Plants, animals and people are already moving in. Greenland, currently a barren white expanse with a few green strips on the coast, will finally be green all over.

It will also attract something more surprising: the pole. As Greenland’s ice is lost, the axis of rotation of the planet will shift towards the island. Earth’s rotation will also slow slightly, because rising sea levels around the equator act like a pirouetting dancer extending their arms to slow the spin. Dramatic as that sounds, neither of these changes will have serious consequences, says Richard Alley of Penn State University in University Park.

The same cannot be said for the rise in sea levels. For someone looking down from space, the changes wouldn’t seem that drastic. True, some large chunks of land will vanish beneath the waves, including Florida, Bangladesh, the Netherlands, Denmark, eastern England and a big part of China. In most places, though, only a relatively thin strip of coastal land will disappear.

Yet the land that will be lost is where , and where we have built our greatest cities. Venice, London, New York, Shanghai, Sydney and many more will be partly or wholly lost in a slow-motion catastrophe of barely imaginable magnitude.

This will not be the first time humanity has been forced to retreat. At the end of the last ice age, the sea swallowed vast coastal plains around the world – like Doggerland, which once connected Britain to mainland Europe, but now lies under the North Sea.

Just like the people of Doggerland, the residents of modern coastal cities will no doubt build anew. Perhaps one day Antarctica will be home to magnificent metropolises that make our cities seem nearly as primitive as the settlements drowned 10,000 years ago. Michael Le Page

The end of Homo sapiens

WE HUMANS are a successful bunch: no other species has guided its own fate or shaped its environment as completely as we have. In doing so, we have sidestepped many of the selection pressures that drove our evolution. But and will continue to do so. So what will be the fate of our species?

Predicting our future evolution is a tricky business: it’s hard to know what genetic novelties might arise, and which of them might take hold. Even so, scientists have started to weigh the possibilities by studying trends in health and reproduction.

A team led by Stephen Stearns at Yale University, for example, has found that over the past 60 years, relatively short and heavy women in the town of Framingham, Massachusetts, tended to have more children than women with the opposite traits. They also found that these physical characteristics are being passed on to their daughters, suggesting natural selection is alive and well in humans. It is difficult to say what is selecting for these traits, but it looks as if we can expect women in Western countries, on average, to become slightly shorter and stouter.

More radically, we might begin to direct our own evolution. In one sense we already do: by shaping our environment and culture, we inadvertently drive heritable changes in our genes. But if sophisticated gene editing technologies permit whole genome engineering in sperm and eggs, we would have more control than ever – we would get to choose which traits we pass on to the next generation.

It’s possible we will cut the whole enterprise short by bringing about our own extinction through nuclear annihilation, runaway climate change or some other cataclysm. In most apocalyptic scenarios, however, at least a few H. sapiens would survive, perhaps forced to retreat into remote refuges. From a deeply interconnected species of 7 billion individuals, we would become fragmented across various ecological settings, each population beset by local environmental pressures.

These are the conditions that favour the formation of new and distinct species, says , a palaeoanthropologist at the American Museum of Natural History in New York City. If those populations are small enough, over deep time the random mutations that prove advantageous might be incorporated into the genomes of surviving H. sapiens – and as the genetic novelties accumulate, populations might begin to diverge.

Eventually, for instance, humans in the northern reaches of Arctic Canada might adapt to the challenges of their environment to become something new. Meanwhile, in Australia, people might adapt in profoundly different ways. Eventually, members of one group could no longer be able to mate with those of the other – one of the key signifiers of a distinct species emerging.

In the event that those new hominin species ever came into contact again, it would be war. “We would have a situation pretty much like what we already had at the end of the last ice age,” says Tattersall. “Modern humans spread all over the world, encountered other hominid populations and eliminated them.” So history could repeat: like those we outcompeted in the past, our all-conquering species might itself be driven to extinction.

Making a new hominin would be a slow process. It would take hundreds of thousands if not millions of years, says , who studies human evolution at Harvard University – and that makes this sort of speciation unlikely. Regardless of the apocalyptic scenario, unless humans lost the urge to explore, isolated populations would encounter each other and breed before speciation could occur.

Ultimately, H. sapiens might have to colonise other planets to provide the long-term isolation required for speciation. So if there is ever to be a new form of human, it will be shaped by the alien environment of a strange new world in outer space. Christopher Kemp

Read more about how everything you care about will end – and what comes after

This article appeared in print under the headlines “The end of ice”, “The end of you”, “The end of ice” and “The end of Homo sapiens”