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!
