That Time Our Ancestors Almost Went Extinct

There are over 8 billion people  on Earth. We are everywhere. In our short 300,000 year history, we have  spread to practically every land mass, and lived in every environment imaginable.

We’ve even been in space! As far as species go, we are extremely successful. But it has not always been that way.

New genetic evidence suggests  that our ancestors went through a severe population collapse that left  just a thousand individuals standing. So let’s talk about the time  that we nearly went extinct, in order to appreciate how unlikely  it is that we even got here. [♪ INTRO] Now if you think you know what  event we’re talking about, you’re actually, probably wrong.

You might have heard that humans nearly  went extinct about 70,000 years ago, but that is not the subject of this video. Because it probably is not true. Researchers do have evidence  of a genetic bottleneck that could have happened around that time.

Basically, the amount of diversity  in our genome was a lot smaller than it should have been,  particularly in humans living in Asia. For a while, researchers thought  this meant that some catastrophe had wiped out a chunk of the human population. They blamed the Toba volcano, a  supervolcano in present-day Indonesia that erupted around 74,000 years ago.

Because when you’re looking for  something that can kill a lot of people, a volcano is usually a solid culprit. But new research suggests that this  genetic bottleneck had nothing to do with a catastrophe at all, and that it is the result  of something called the founder effect. Basically, when some members of a population  go split off to find a new territory, they end up with less genetic  diversity than the larger group did, since moving away means majorly  reducing your gene pool.

So because only some humans  migrated out of Africa, those small, isolated populations  were less genetically diverse than the ones they left behind, resulting  in an apparent drop in genetic diversity without an actual population dip. But that’s not what we’re talking about. What we’re actually talking  about a totally different time when our ancestor species  may have nearly vanished.

So let’s set the stage. We are in Africa, 930,000 years ago. Our species, Homo sapiens, did not exist yet.

But our ancestors were there. The resident hominin species were  Homo erectus in Africa and Asia, and Homo antecessor in Spain. Now Homo antecessor probably  isn’t our direct ancestor, it’s more like our great aunt or something,  so we’re not going to talk about them.

But Homo erectus is, and we inherited at  least some of our genetic material from them. And their fossil record is pretty consistent. We can track them reliably  from when they first appeared just under 2 million years ago, right  up until about 900,000 years ago.

But then… nothing. We basically don’t find any Homo erectus fossils that date to between 900,000  years old and 650,000 years old. And that was, like, a pretty pivotal time for hominins.

Like, this is right around when the  lineages split between our species and Neanderthals, which we would  all really like to know more about. So you can imagine that this  fossil gap would be annoying, and that paleoanthropologists would  want as much information as possible to figure out where they went. The first guess was that this  was just random, bad luck.

Not every living thing gets to become  a fossil, and the factors that increase or decrease that likelihood could cause bias in the fossil record. Anything from where it lived, where it  died, what its body was made out of, or how big it was, can all affect an  organism’s odds of becoming a fossil. So maybe all the hominins were  living in places that didn’t have the right conditions to form fossils,  so there weren’t any for us to find.

After all, fossil formation  is actually pretty rare. Other than a few exceptions, fossils almost  exclusively form in sedimentary rock. And for a fossil to actually preserve, it has  to get buried really quickly, before it decays.

And that’s why a lot of fossils  form in ocean environments, because water plus erosion equals  sedimentation, and stuff gets buried fast. And it’s also why on land, fossils mainly  form in river valleys, especially where there are floodplains, because all that  flooding means lots of moving sediment. But one big problem with the  bias theory is that hominins have pretty much always liked  to live in river valleys.

River valleys means rivers, which  means good sources of freshwater, so they’ve been prime real estate  for our ancestors for forever. So it’s a bit of a stretch to imagine  that there weren’t enough hominins living close enough to a  river to leave fossils behind. Unless, of course, there weren’t  enough hominins living, period.

It turns out the evidence  for this population crash has been hiding right under  our noses this entire time. In fact, right under all of the parts  of our body, because it’s in our DNA. This SciShow video is supported  by Porkbun: the domain people.

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com/SciShow24  or click the link in the description. So let’s talk about DNA. DNA is a fairly fragile molecule that  tends to break down pretty easily, so we don’t have a lot of  it from the ancient past.

The oldest DNA samples we’ve ever found  come from about 2 million years ago, but those are from some plants and  animals in Greenland, not from hominins. Our oldest hominin DNA  sample is 400,000 years old, from a Neanderthal specimen unearthed in Spain. And our modern DNA has our  genetic history locked into it.

You just need to be able to read the code. Which is why researchers  decided to try using modern DNA to estimate the population size of much  older generations within the gene pool. They calculated something called  a site frequency spectrum, or SFS.

To get it, you calculate  the distribution of alleles, or different forms of a gene, across the genome. See, there are these parts  of the genome called SNPs, or single-nucleotide polymorphisms, that  are just one-letter differences in your DNA, and the vast majority of them are neutral. They don’t do anything, they don’t break anything, they're just a variation in which  letter base pair went in that spot.

And because they’re neutral, natural  selection isn’t acting on them, so lots of them can pop up over the generations. When you’ve got a population  that’s been stable for a while, you’re likely to see more and more  of these SNPs show up over time, which you can quantify by calculating  the site frequency spectrum. But if you have a population that goes  from big and bustling to itty bitty, you knock out a lot of that variation, which  you can detect when you calculate the SFS.

As time goes on and the population recovers,  they will start to have new SNPs show up. But if you know what you’re looking for, you’ll still be able to see the  effects of that old bottleneck. It’s sorta like if you smudge your nail polish  on the first coat but then keep painting over it.

That bump is going to get smoother the  more coats you put on, but you’ll probably still be able to see it was there, especially  if you know what you’re looking for. So these researchers used the  SFS of modern humans to estimate what was going on with the hominin  population from about a million years ago. The model showed that  hominin populations went from nearly 100,000 individuals pre-bottleneck, to fewer than 1,300 individuals,  drastically lowering our genetic diversity.

Obviously, this was not great. And if you look at what was going  on at that time geologically, it starts to make sense why it happened. See, this all went down right  around the same time as a huge shift in the climatic cycles called  the Mid-Pleistocene Transition, which basically made glacial periods  less frequent, but way, way colder.

And we think there were two big  things at play that caused this change. One was a change in ocean currents, and the other was that there was a lot  of erosion in the Northern Hemisphere. Yeah, we almost got extincted by erosion.

Around a million years ago, we started  to see the weakening of an ocean phenomenon called the Atlantic  Meridional Overturning Circulation. This circulation happens when warm  water from the Gulf Stream is brought up to the North Atlantic, where it cools  down and sinks down into the deep ocean. And when that cycle slowed down, it  brought less warm water to the far north, bringing down temperatures  by Greenland and the Arctic.

At the same time, erosion  in the Northern Hemisphere exposed a massive layer of  bedrock to the atmosphere. And since bedrock is rougher  and stickier than soil, that created a place for these large  ice sheets to grab onto and grow. The ice sheets got really thick, and  cooled down basically the whole hemisphere.

And because cold seawater absorbs more  carbon dioxide than warm seawater, the ocean started sucking up  the carbon from the atmosphere. And we know that carbon dioxide  in the atmosphere helps trap heat, because, you know, the climate crisis  that we are currently experiencing. But the inverse is also true, and having less  of it around causes temperatures to go down.

In this case, they went way down. So basically it kicked off a  daisy chain of events that means that once things start to get  cold, they got really, really cold. And that meant that the Ice Age  after the Mid-Pleistocene Transition was colder than anything our  ancestors would have ever felt before.

Like, think Game Of Thrones  levels of harsh winter. And our ancestors, just, were  not equipped to deal with that, which could explain why the population dwindled. Now, this population dip didn’t last forever.

Otherwise, our view counts at  SciShow would be, like, really low. But it took more than 100,000 years of  recovery before Homo erectus populations began to recover, reaching 21,000  individuals by about 800,000 years ago. And that date actually lines up with our estimates for when hominins first started controlling fire.

So figuring out how to make fire could be part of what led to our ancestors being  able to come back from the brink. It still took a couple hundred  thousand years for their fossils to start popping up again, but  we’ll take what we can get. And I don’t even think it’s a stretch to say  that this event made us who we are today.

Because, like, it did, on a chromosomal level. See, that genetic bottleneck happened  at the same time that two ancestral chromosomes fused together and  make what we now call chromosome 2. Our ancestors had 24 pairs of  chromosomes instead of our 23, and our closest living relatives,  chimps and bonobos, have 24 too.

That major of a genetic change  could have been a speciation event, leading to the emergence of  a new species of hominin. And that new hominin could have  been the last common ancestor of other hominin species, like the  Denisovans, the Neanderthals, and us. It’s pretty cool to think that  the mystery of the missing fossils might’ve been solved just by looking  at a few random letters in our DNA.

An entire population collapse  was hiding in some A’s and T’s. And knowing how low our numbers dipped, I think, can give us a new perspective on  how many of us there are today. Talk about a comeback!