00:00Can you imagine that all modern humans came from about a thousand people?
00:05Around 800,000 to 900,000 years ago, almost all of our ancestors disappeared, leaving
00:11about 1% of the population behind.
00:14So what the heck happened?
00:17The Earth often goes through glacial cycles, things that we call ice ages.
00:21Back in the past, they were pretty mild and happened regularly, about every 41,000 years.
00:27The ice ages happened because of the Earth's axis.
00:30Our planet is slightly tilted at an angle, which is why its axis isn't straight up.
00:35Over thousands of years, this tilt has changed a bit.
00:38This teeny-tiny shift completely changes how much sunlight we receive, especially at the
00:43poles, and leads to incredibly strong cold or warm periods.
00:48About 2.6 million years ago, when early humans were settling in on Earth, the Pleistocene
00:54epoch started.
00:55That's the era we traditionally call the Ice Age, the one from the movies.
00:59Large ice sheets completely covered parts of North America, Europe, and Asia.
01:04It wasn't constant ice everywhere all the time.
01:07These glacial cycles came and went, and sometimes ice sheets melted for a while.
01:11But overall, things were still much snowier and colder compared to now.
01:16We can still see the evidence of this from back in the day.
01:19Those glaciers carved out valleys and left behind some awesome things like moraines and
01:24fjords.
01:25And since sea levels were super low, a huge part of the world was land.
01:29This allowed people and animals to travel between continents and spread everywhere.
01:34It was the time of the megafauna.
01:36You all know about it from the movies.
01:38Wooly mammoths, saber-toothed cats, giant ground sloths, and mastodons.
01:43Our ancestors had some fun times hunting all those giants.
01:47Humans didn't come from one lineage.
01:49It wasn't like monkeys slowly turned into humans.
01:52We also consisted of many species – Homo erectus, Neanderthals, and so on.
01:57But only one species, Homo sapiens, survived and turned into modern humans.
02:02Harsh conditions forced them to get smarter.
02:05They learned how to make tools, clothing, and shelters to survive.
02:08They became hunter-gatherers and invented fire, which turned out to be super useful
02:13during the Ice Age, you think?
02:15But they had no idea what horrible change was coming.
02:19Something disastrous happened about 800,000 to 900,000 years ago.
02:24Humans faced a drastic population crash.
02:27Only 1,280 people survived, give or take.
02:31Some 99% of early humans were wiped out, and this tiny 1% was lucky enough to become our
02:37ancestors.
02:39This almost-extinction, unsurprisingly, was the climate's fault.
02:43It was the Middle Pleistocene transition.
02:46The Earth's orbit around the Sun changed its shape a bit.
02:49Ocean levels dropped.
02:50A lot of stuff was happening, and the Earth's climate was going wild, becoming incredibly
02:55cold and dry.
02:56The seas suddenly got chillier, and Africa and Eurasia experienced horrible droughts.
03:02Looking for food was almost impossible.
03:06Scientists learned this by taking the DNA of about 300,000 people from all around the
03:11world.
03:12They used a new method to help them map the history of human evolution over a million
03:16years and learn more about it.
03:18No wonder we can barely find any fossils from that time.
03:21There was almost no one around.
03:23What's interesting is that before that happened, most apes had 24 pairs of chromosomes.
03:29But because the population became so small, two sets got fused together, and we ended
03:34up with 23 pairs.
03:36This was the final straw that separated us from other apes.
03:40Chimpanzees, gorillas, and orangutans all have 24 pairs, but our fused one became known
03:46as the human chromosome.
03:48Like a ripple effect, this little thing influenced our brain side, helped us start walking on
03:52our two's, and even formed language.
03:55It took humans about 100,000 years to recover from this catastrophe.
04:00Only after all that time, they started spreading around again.
04:04Isn't that insane how we managed to go from 1,000 to 8 billion people?
04:09In any case, the Ice Age finally ended about 11,700 years ago.
04:14The warm period, called the Holocene, began.
04:17As things got warmer, human civilizations finally began to develop.
04:215,000 years ago, it was Ancient Egypt, then the Indus Valley Civilization, Ancient China,
04:27and others.
04:28And while we'd been thriving for a while, there's one catch.
04:31The Middle Pleistocene Transition changed the length and intensity of glacial cycles.
04:37Before the transition, sure, there was the Ice Age, but the cold was milder, and the
04:41glacial cycles were shorter.
04:44Ice got covered in ice sheets about every 41,000 years.
04:48But after the transition, the glacial periods became longer and more severe.
04:53These cycles started following a new pattern.
04:55Although they happened every 100,000 years, they were super intense.
04:59Ice sheets grew large, even extending to the south.
05:03So from now on, ice ages, that will continue to happen in the future, will be a bit different.
05:09The next one should be at least 50,000 years from now, maybe longer.
05:13That's good for us because it'll occur later.
05:16But at the same time, the consequences will be stronger.
05:19What our ancestors went through is called a population bottleneck.
05:23That's what we call a sudden and huge reduction in the size of a population.
05:28Usually it happens because of things like earthquakes, floods or droughts, fires, and
05:32even human actions.
05:34Just a few people survive, and only they can pass their genes to future generations.
05:39And we've got plenty of such bottlenecks in history.
05:42Our modern species, Homo sapiens, originally lived in Africa.
05:46Around 60,000 to 80,000 years ago, a small group of them left Africa and spread across
05:51the world.
05:52It wasn't on a whim, though.
05:54They had to move somewhere because of the droughts and huge climate problems in Africa
05:58at that time.
05:59They had to go look for food and find new places to live.
06:03They traveled through the Middle East, moving into Europe and Asia, and eventually reaching
06:08Australasia and the Americas.
06:11They quickly settled across different continents and started mixing with local human-like species
06:15like Denisovans and Neanderthals.
06:18But there was a problem.
06:19Originally, the African species were super genetically diverse.
06:24But since only a small part of them decided to move out, they carried just a bunch of
06:28these diverse genes with them.
06:30Unfortunately, this caused a bottleneck for everyone.
06:33Human-like species from other parts of the world became less genetically diverse.
06:38Since they had a smaller gene pool, they became more vulnerable to all the harmful
06:42stuff.
06:43The African population also had a drop in genetic diversity.
06:47But at least humans managed to survive.
06:49Although eventually, non-African populations really dropped, and there were no Neanderthals
06:55or Denisovans left.
06:56Only us, Homo sapiens.
06:58That's why we can say that all modern humans came from Africa.
07:03Early humans sure had it rough.
07:05About 74,000 years ago, they went through yet another disaster, the Toba Catastrophe.
07:11But keep in mind that all these things are just a hypothesis.
07:14It's not certain whether it actually happened.
07:17Now basically, scientists think that there might've been a super volcanic eruption at
07:21Lake Toba in Sumatra, Indonesia.
07:23This volcano, going crazy, was an absolute catastrophe for Earth's climate and humans.
07:29The eruption was one of the most powerful volcanic events in the last 2 million years.
07:34It spewed out from 670 to 1,000.5 cubic miles of lava and volcanic stuff.
07:41Just so you know, there are about 3,100 cubic miles of water in our planet's entire atmosphere,
07:47and this eruption had almost half as much lava.
07:50This horrendous disaster led to the formation of Lake Toba, a massive caldera lake in Sumatra.
07:56It also spewed out tons of ash and sulfur dioxide into the atmosphere.
08:00This caused the so-called volcanic winter, which means that while it's not actually winter,
08:05the temperatures drop and the planet is freezing because all these ash clouds are blocking
08:10the sunlight from reaching Europe.
08:12This event affected climate patterns for years to decades.
08:16This also most likely sped up the arrival of the next glacial cycle.
08:20Luckily, humans are crazy good at surviving.
08:23The global human population was reduced to a few thousand people yet again, and they
08:28still managed to overcome this.
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