Life on Earth will not last forever.

At the very least, the death of the Sun in 5 billion years will wipe out anything still living here. But life will almost certainly  be gone long before then.

Volcanoes, asteroids, and extreme climate change have nearly wiped out all  living things in the past. And even worse things could be coming. Like a nearby supernova or nuclear war would be … let’s say, not great … for life on Earth.

So, what will be the last  living thing in these scenarios? Scientists have actually given  a lot of thought to this. And despite what you might have heard, no, it will definitely not be the cockroaches. [Intro music] Most of the time, Earth is a pretty great place to live.

At least compared to, literally, any other rock we’ve found in the universe. We have a watery planet with balmy temperatures, and life has been chugging along here for the better part of 3 and a half billion years. But more than once, all that life has come close to disappearing.

The most famous of these mass extinctions was caused by a 10 km wide asteroid that slammed into the Gulf of  Mexico 66 million years ago, killing the non-avian dinosaurs and three-quarters of everything else. But that was definitely not the first time, or most serious time, that life almost died out. About 200 million years earlier, a completely different set of  life forms was flourishing, when something — maybe a burst of volcanic activity — took out 90 percent of them.

This was the deadliest mass extinction ever, also known as the Great Dying. But it also wasn’t the first. Another hundred million years before that, at least 70 percent of living things died in a slow series of extinctions that unfolded during the Devonian Period, as the climate changed over thousands of years.

We can see these die-offs in the fossil record, and we can see what sorts of  events lined up with them. Asteroid dust has been found  in the massive Chicxulub crater that dates back to the  disappearance of the dinosaurs. And millions of square-kilometers of hardened lava in what is now Siberia tell a  story of massive volcanic activity that lined up with the Great Dying.

But all these millions of years later, scientists are still trying to connect the dots and figure out what chain of  events turned these moments into all-out doomsday scenarios. Like, did the dinosaurs die of starvation as the dust cloud blotted out the Sun  and cut off photosynthesis for years, killing plants and toppling food chains? Or did they die because temperatures plunged and global climate systems went haywire?

Or… did they die because the  asteroid impact acidified the oceans in a flash, triggering the collapse of ocean ecosystems? It’s hard to get a clear  picture of past doomsday events. But as scientists have worked on it, they’ve gotten some pretty clear ideas  about what could cause extinctions.

And it turns out some of the  most disastrous scenarios could still be to come. Strangely enough, one of the biggest threats to life might come from light-years away. If a massive star explodes into  a supernova too close to Earth, that would almost certainly  end life on the planet.

Supernovas are not rare. In our galaxy alone, a star  blows up every few decades. And it sends a blast of radiation and a violent shockwave through space.

When we’re far enough away, that’s no big deal. Earth’s magnetic field and  atmosphere make a pretty good shield against a regular dose of cosmic radiation. But if the explosion happened within  what’s known as the “kill zone”... that would be a different story.

A shower of high-energy particles launched by a nearby supernova could strip off Earth’s ozone layer, leaving life on Earth exposed to  deadly radiation from the Sun. As for what counts as “nearby”... it depends on what kind of star explodes. But a 2023 estimate put the kill zone boundary at around 160 light-years away.

And this isn’t purely hypothetical. Supernova debris has rained down on our planet at least twice in the last 10 million years. The main clue is layers of  rock containing iron-60, a type of iron that’s produced  by supernovas but not on Earth.

So far, scientists have found two  of these layers in oceanic crust. One was deposited between  2 and 3 million years ago, and another around 6 million years ago. But there are probably more.

One theory even suggests that one of these blasts from space actually triggered that slow mass extinction at the end of the Devonian period. Fortunately, no giant stars  within that 160 light-year radius seem to be on the brink of explosion today. One star called IK Pegasi B is currently around the edge of the kill zone, and it will eventually go supernova.

But it has a good 2 billion years to go, and it’ll be far from Earth by then. As for Betelgeuse, a star that gets a lot of  attention for its potential to go supernova in the near future, it’s a safe 700 light-years away. So it’ll be nothing but a great  light show when it explodes.

But that doesn’t mean a supernova  will never threaten Earth. Right now, we’re in a sparse  part of the Milky Way, so nearby supernova rates are low. That will change over time as our solar system moves into one of our galaxy’s denser arms.

And supernovas are not the  only thing threatening Earth from outside the solar system. Every once in a while, another kind of powerful  explosion rocks the universe. They’re called gamma ray bursts, and astronomers think they  come from violent events like the merger of two neutron stars.

In a fraction of a second, a gamma ray burst can release more energy than the Sun would release in 10 billion years. And since they’re so powerful, we can actually detect gamma rays  from distant events hitting Earth pretty much every day. Fortunately, their source  is usually far enough away that they’re harmless.

The most powerful gamma ray  scientists ever detected struck Earth in 2022, from about 2 billion light-years away. It didn’t do any damage, but it disturbed Earth’s atmosphere about as much as a big solar flare. And it was a reminder that if we ever found ourselves directly in the line of fire, a close-up gamma ray burst would be .. like very scary.

The burst itself would be over quickly, but it would destroy the ozone  layer in just a few seconds. The radiation that followed would  kill a lot of living things, collapse food webs, and could  lead to a mass extinction. The good news is, unlike a supernova, which explodes like a bomb, gamma ray bursts tend to just shoot two jets in opposite directions.

Which means it’s less likely for Earth to find itself in the line of fire, even if one does happen nearby. A blast on par with the one  that struck Earth in 2022 is only expected every 10,000 years, on average. And a blast that could cause a mass extinction would have to come from much closer.

Some researchers suggest the source would need to be within about  6,500 light-years of Earth. But that’s less comforting than it might sound. As a rough estimate , that kind of event is expected to happen once every 100 million years.

And as you may know, the Earth has been around a lot longer than that. Like, It could happen. There are some rumors that the biggest and brightest star in our galaxy, Eta Carinae, could collapse into a black hole and produce a deadly gamma ray burst in the near future.

But even if that takes place, research shows that this blast  wouldn’t be aimed at Earth. So Gamma ray bursts probably aren’t something that should keep you up at night. But on a planet with a long life ahead of it, they’re not irrelevant either.

Of course, these days, one of the biggest threats to life  on Earth is much closer to home. If we ever use the nuclear weapons we’ve stockpiled by the thousands, a mass extinction would almost certainly follow. It would take more than a couple blasts to trigger a climate catastrophe, but it wouldn’t take an all-out world war either.

Even a relatively small-scale nuclear war, like a war between India and Pakistan, could be catastrophic. These countries have hundreds of nuclear warheads, and even the smaller ones have explosive yields similar to the bomb dropped on Hiroshima. And yes, the radiation from  the fallout would be bad.

But the soot would be even worse. By some estimates, a war involving dozens of these nuclear warheads could blast around 50 million  tons of soot into the atmosphere. That soot could stay there for years, partially blocking the Sun and  sending temperatures plunging to lows Earth hasn’t experienced since  its last Ice Age 18,000 years ago.

And, It would leave plants with less sunlight for photosynthesis. The climate disruption would last years, leading to food shortages, ecosystem breakdown, and starvation. And that’s just a relatively small war.

A larger war between Russia and NATO could send up triple the amount of soot, causing an even more severe nuclear winter. Meanwhile, nuclear fireballs  would heat the atmosphere, creating chemicals that destroy ozone. Once again, a depleted ozone layer would leave everything on Earth’s surface exposed to deadly levels of radiation from the Sun.

Any of these scenarios would be spectacularly bad for living things. But even if the worst of them did come to pass, it’s likely that some living things would manage to eke out an existence. Because we have some real  survivors on this planet.

Let’s start with the cockroaches, who have developed a reputation  for being hard to kill. And to be fair, they did survive the Great Dying and the asteroid that killed the dinosaurs, so they’ve earned their label. One of their greatest strengths is that they’re ruthless scavengers.

They’ll eat anything. If the food’s gone bad, even better. And if they have to, some can go months eating nothing.

But they have some big weaknesses, too. For one, they’re cold-blooded insects, so they don’t handle extreme temperatures well. And despite the widespread myth that they’d survive a nuclear war, they’re actually not especially  resistant to radiation.

But there are some other  survivors that might outdo them. Like ginkgo trees. Gingkos have survived three mass extinctions since they first evolved more than 270 million years ago.

They were also among the few living things to survive the bombing of Hiroshima. These trees have evolved lots of protections during their long stint on this planet. For instance, they produce compounds like  methanol and chloroform that protect them against bacteria.

They also produce new roots from non-root cells, which allows them to recover from severe damage. Some ancient ginkgos have  developed whole new trunks after being damaged by storms.. But a big part of their secret is that they just don’t age.

Their genes don’t have the  programming that tells cells to die … so the cells just keep going. That means once a tree is established, all its defense mechanisms  can stay in tip-top shape for thousands of years. Scorpions are another extremely  hardy group of species.

Another especially hardy group  of species are the scorpions. They’ve been around in some form since animals first ventured onto land, more than 400 million years ago. And unlike cockroaches, they can  survive a hefty dose of radiation.

Scientists don’t completely understand why, but it might have to do with an amino acid in their blood that acts as a kind of shield. It’s called taurine, and while research shows that it seems to have a protective effect against radiation, the exact mechanism is still a bit of a mystery. In part, it may protect organisms by  acting as an antioxidant.

One problem   with radiation is that it knocks electrons  loose, creating molecules that react very   easily with the molecules around it, and  that can cause a lot of damage in cells. But antioxidants bind to these  super reactive molecules, keeping them from wreaking havoc. And the list of survivors goes on.

A prehistoric fish called the coelacanth survived the asteroid that killed the dinosaurs, possibly by moving from shallow waters into the deep sea. Small, ground-dwelling mammals escaped the mayhem and survived underground. Bottom line, as hard as it seems to be  to get life to arise in the universe, it’s also hard to completely  stamp it out once it exists.

But even if none of the doomsday scenarios we’ve covered so far come to pass, there’s no question that life on Earth has an expiration date. Every second, the Sun is burning through its vast store of hydrogen and slowly getting brighter. In around a billion years, it will be 10 percent brighter than it is now.

At that point, temperatures on Earth will be so high that the  oceans will start to boil off. All the evaporated water will hang in the air and trap heat, leading to runaway warming that will turn the planet into a deadly inferno. Several studies have pointed out that as the brightening Sun heats up Earth, certain rocks will wear down faster.

When that happens, the newly exposed surfaces will absorb carbon dioxide and trap it in Earth’s crust. As CO2 disappears from the atmosphere, plants will run out of fuel for photosynthesis, and they’ll stop pumping out oxygen. Oxygen levels will drop to about a millionth what they are now, choking much of life on Earth.

The exact timelines for all  these events are a little fuzzy because there’s a lot we can’t predict about Earth a billion years from now. But it’s likely that one of these scenarios will be the final nail in the coffin for any life that’s still around. And in the final test of survival, whatever it is, the contenders probably won’t be cockroaches or coelacanths or ginkgos.

They’ll almost certainly be microbes. They will be microbes. All over our planet, some microbes are already living in conditions that would kill most living things in a flash.

They’re known as extremophiles. Some extremophiles can endure extreme heat. Like this little guy whose name really translates to “furious fireball.” It thrives in hydrothermal vents  that are literally boiling.

For most living things, that would be impossible. When cells get too hot, their molecules are moving around more, and it gets harder for cells to  fold their proteins correctly. A protein structure is key to proper functioning, so when proteins get disfigured, the things die.

But our furious fireball here has a whole crew of unique molecules overlooking its proteins to keep them from getting disfigured and to clean up any that do. Some of those molecules are  called heat shock proteins, which help fold and assemble others. They’re sort of like assembly line  workers making sure everything gets built according to plan.

Then there are molecules called proteases, which are like the custodial crew. They’re a kind of enzyme that breaks down and clears out any defective proteins. Meanwhile, some extremophiles are fine-tuned to survive the opposite extreme… like this Antarctic bacterium that has proteins that act as antifreeze.

The proteins have a special structure that lets them bind to ice crystals right when they begin forming, preventing them from growing. Other extremophiles can tolerate  extreme radiation exposure, sometimes by absorbing it  and protecting their DNA. This bacterium can’t protect  itself from all radiation damage, but it has a system for repairing  the damage once it’s done.

It takes advantage of the fact that it has several copies of its genome. When it has broken DNA to repair, repair proteins called RecA and RadA use other copies of the bacterium’s genome as templates to reassemble broken fragments, kind of like solving a puzzle by looking at the picture on the box.. There are lots of different  kinds of extremophiles.

They live in acidic hot springs, extremely salty lakes, at the bottom of the ocean, and deep in the Earth’s crust. But if life ends with the heating up of  the Sun and the suffocation of our planet, the ultimate survivors might be … anaerobic thermophiles. These are microbes that not only  thrive at high temperatures, but can do so without oxygen.

For instance, one kind of microbe, called Pyrolobus fumarii, lives in hydrothermal vents at boiling temperatures. Most extremophiles at least depend on oxygen in the air or water to turn food into energy.. But instead of breathing oxygen, Pyrolobus fumarii pulls nitrate from the vents where it lives, and uses that to produce its energy.

We can only speculate, but  if life ends on a boiling, suffocating planet, this little microbe might be able to hang on longer than just about anything else. Now, we have no idea what kinds of living things will exist in the next billion years. Maybe a mass extinction will give way to a whole new array of lifeforms.

Maybe some of them will even outdo today’s most extreme extremophiles. But what we do know is that the first living things on Earth likely fell into this category, and the last ones just might too. Life will probably go out a lot like it started.

A planet once teeming with lush forests and frolicking animals will return to a place with pockets of single-celled life, eking out a quiet existence. And a microbe in a boiling pool may be the one contemplating it all at the end. Thanks to our patrons for  supporting this Scishow video.

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