Every Human Alive Descends From a Group That Almost Went Extinct

[00:00:00]Right now, there are 8 billion people on this  planet. But every single one of them, you, me, everyone you've ever met, came from a group  so small you could fit all of them into one small town. A group that came this close to  vanishing completely. If they had failed, you would never have been born. This isn't a theory  someone made up. It's written inside your body, inside your DNA. And when scientists first read  it, they didn't believe what it was telling them.

[00:00:30]Because human DNA has a strange problem. A problem  no one could explain. We are genetically one of the least varied large animals on Earth. Here's  how strange that is. Take two chimpanzees from two neighboring groups in the same forest. Not  different continents, the same forest. Those two chimps are more genetically different from each  other than a person from Norway is from a person in Nigeria. Think about that. Every difference  you can see in humanity, every skin tone, every height, every face on Earth, all of it fits  inside a smaller genetic range than two monkeys living a few miles apart. So why are 8 billion  humans so nearly identical on the inside? There's only one thing that does this to a species. When a  population crashes down to almost nothing and then slowly grows back, every descendant inherits the  DNA of only those few survivors. All the variety carried by everyone else dies with them. It never  comes back. Scientists call it a bottleneck. And we know exactly what it looks like because we've  seen it happen to other animals. Cheetahs went through one. Today, cheetahs are so genetically  similar that you can take a skin graft from a wild cheetah in one country and transplant it onto a  completely unrelated cheetah in another and the body won't reject it. Their immune systems can't  tell each other apart. That's what a bottleneck does to a species. And human DNA is carrying the  fingerprints of one. Because buried in that DNA is an actual number, a number for exactly how  many of us were left. And when you hear it, you'll understand why some scientists call it a  miracle that anyone is here at all. I'll show you that number in a moment. But first, you need  to see what almost killed us. Around 74,000 years ago, on the island we now call Sumatra, a  volcano named Toba did not just erupt. It was one of the largest eruptions in the last 2 million  years, thousands of times more powerful than any eruption in recorded history. It threw so much  rock and ash into the sky that geologists can still find the layer today, buried in the ground  on the other side of the world. Ash blanketed the sky, sunlight dimmed, the temperature dropped,  plants died, the animals that ate the plants died, and the animals that ate those animals started  to starve. And the small bands of early humans scattered across Africa were suddenly hungry in  a world turning cold and dark. They had no farms, no stored food, no way to travel somewhere  better. If the land around you stopped producing, you did not have a backup plan. You had days.  Some scientists believe this is the moment we nearly ended. So, how many were left? By measuring  how little variation exists in modern human DNA, researchers can work backwards and estimate the  size of the group we all descend from. And the answer is terrifyingly small. Some studies suggest  that at one point the number of humans able to have children may have dropped to just over 1,000.  1,000. That's fewer people than fit in a single small school. Fewer than the number of people  you'd pass on a busy street in an afternoon.

[00:03:34]Every human being who has ever lived since came  from that tiny group. And one recent study pushed it even further, suggesting that at an earlier  point in our history, our ancestors may have been reduced to around 1300 individuals and stayed  that low not for a year, not for a decade, but for something like a 100,000 years. 100,000 years of  being one bad century away from gone. And here's the part that makes it worse. We weren't alone  out there. At various points, there were other kinds of humans on this planet. Neanderthalss,  Denisven, others were still discovering. Most of them didn't make it. Extinction wasn't a distant  possibility for a species like ours. It was the normal outcome. Now, think about what surviving  actually required. Every one of those thousand people had to live long enough to have children.  And their children had to live long enough to have children. And theirs for thousands of generations  in a row without a single break. No hospitals, no antibiotics, no way to store water. A bad  infection was fatal. A broken leg was often fatal. A hard winter could take half a group.  And not one link in that chain could ever break.

[00:04:45]If a single one of your direct ancestors had  died before having children from a predator, a winter, a wound, a bad season, you would not  exist. Not you, not anyone who looks like you.

[00:04:57]That entire branch simply stops. You are the end  of an unbroken chain that runs back through every one of those survivors. And it never failed.  Not once. And there's one more twist that makes this even stranger, because we can actually  trace part of that chain back to one woman.

[00:05:14]Every person alive today carries a tiny piece of  DNA separate from the rest that gets passed down almost unchanged from a mother to her children and  from her daughters to their children generation after generation. And when you follow those lines  back far enough, all of them meet at a single woman in Africa. Scientists call her mitochondrial  Eve. She wasn't the only woman alive at the time.

[00:05:37]She wasn't the first human. She didn't know she  was special. And by any measure, she wasn't. not to anyone around her. She was just one person in  a struggling group fighting to see another winter.

[00:05:49]And yet, a piece of her is inside every single  human on Earth right now, including you, including everyone you have ever loved. And there's a mirror  to her. Follow the Y chromosome, the piece passed down from fathers to sons, back through time,  and those lines converge too on a single man.

[00:06:08]Scientists call him Y chromosomeal atom. They  almost certainly never met. They may have lived thousands of years apart, but between the two of  them, they are the narrow point that all 8 billion of us are threaded through. Then somehow it turned  around. The climate steadied. The group held on and that tiny population, the one that should  have disappeared, began to grow. They walked out of Africa and into every continent on this  planet, deserts, ice, mountains, oceans. Within a few tens of thousands of years, the species  that almost ended was everywhere. So the next time the world feels crowded, 8 billion people,  cities that never sleep, traffic that never ends, remember how it started. A handful of people,  cold, hungry, outnumbered by everything trying to kill them with no idea that anyone would ever come  after them. They should not have made it, but they did. And because they did, you're here watching  this tens of thousands of years later on the other side of the most improbable survival story  there has ever been. You are the proof that they survived. This channel is all about the impossible  things that had to go right for you to be alive to see them. If that's the kind of thing you want  more of, subscribe and I'll show you the next one.