[♪ INTRO] Some people might remember a time  when one of the biggest mysteries of Earth’s history was what killed the dinosaurs.

Nowadays, we’re pretty confident that  dinosaurs were killed by a comet or similar space impactor, but it took many years  of research to come to that conclusion. And Earth’s history has many more secrets.

The dinosaurs weren’t the  only species that went extinct under mysterious circumstances. There are many extinctions in Earth’s  history. While some of them only involve one species going extinct, others  affect many other species.

Each one affects the course of evolutionary  history, and the more we understand about them, the more we learn  about evolution overall. But big or small, some  extinctions remain unsolved. Here are a few, and what we’ll need to figure out before we can learn the consequences of each one.

First up are our close cousins the Neanderthals, whose extinction set the course of  our own history as Homo sapiens. Neanderthals lived in what is now  Eurasia until about 35,000 years ago. They eventually died out and were  replaced by modern humans, but although we have many theories for why they  died out, we don’t know for certain.

One of the oldest theories for Neanderthal  extinction proposes that they were wiped out by Homo sapiens in direct  conflict, possibly over resources. Another theory says that Neanderthals  could have been wiped out by diseases carried by Homo sapiens when  they entered Europe from Africa. Since warmer environments tend to host  more diseases than colder environments, modern humans could have had more  robust immune systems than Neanderthals, and might have passed illnesses on to them.

There’s also the idea that Homo sapiens won out over Neanderthals through interbreeding. In fact, many researchers contend that the Neanderthal extinction  wasn’t really an extinction. They just were absorbed into the human  population through interbreeding.

But the evidence doesn’t support this theory. Some people have a small amount of Neanderthal DNA represented in their genomes,  especially modern Europeans and Asians. But it’s not really enough to suggest  their populations simply combined.

One of the most interesting theories  about what happened to the Neanderthals is in a weird way also the saddest:  It may have just been bad luck. A study from 2019 suggests that the  Neanderthals might have always been on the verge of extinction. They had a small population to begin with, which means that they didn’t have  a whole lot of genetic diversity.

Genetic diversity is important for  the overall health of a population. It helps keep their defense  against diseases more robust, and tends to water down harmful recessive genes. The study proposed that the size of the  Neanderthal population could have meant that they were always on the cusp of extinction.

They were just large enough, with  just enough genetic diversity, to keep going for some time. But a few particularly bad years could  have been enough to drive them extinct without having to consider the  competition with Homo sapiens at all. That said, we still don’t know for sure  what caused the Neanderthal extinction.

It’s even possible it was a  combination of these and other factors. For example, a small population  wouldn’t have helped them recover from new diseases brought by migrating humans! The other extinctions on this list  at least have proposed explanations.

But there is one extinction event  that was discovered so recently, we still don’t have very many  theories about it at all. And although only one group of animals were  affected, it’s likely this extinction will change the way we think about the  members of that group that survived. This extinction happened to sharks.

In a study published in 2021, researchers  looking at fossils in the Pacific Ocean found something dramatic  that they hadn’t seen before. They found a gap in the fossil record  suggesting that 19 million years ago, 90% of all open-ocean sharks died out. This was surprising, because  sharks are not delicate creatures.

Sharks have been around in some form  or another for 400 million years. They survived a major climate  swing around 56 million years ago. And they’ve faced reductions in numbers before.

After the event that wiped out the  dinosaurs 66 million years ago, about 30 to 40% of all shark species went extinct. But although that impact also caused the  extinction of species all over the world, it actually had a smaller  effect on sharks than this new, mysterious shark extinction 19 million years ago. And while we still don’t know what could  have caused it, we do know a few things.

But to explain them, we first have  to talk a bit about shark biology. Sharks don’t have bones the way we do. Their skeletons are made up mostly of  cartilage, the lightweight material that makes up your nose, and  cartilage doesn’t fossilize well.

So to look at sharks in the fossil  record, you can look for their teeth or their dermal denticles, the toothlike  structures that make up their scales. Both of these are made of a mineral called  bioapatite, which does fossilize well. And because sharks have so many more  dermal denticles than they have teeth, it’s easier and more efficient to  search the fossil record for these.

That’s what the researchers were doing  when they discovered the extinction. They were searching layers of marine  sediments for these dermal denticles. Because layers are deposited  roughly in chronological order, they can give you a good sense  for when things happened.

And they found that the  fossils declined by about 90% in sediments from 19 million years ago. What’s more, there was a sharp contrast  between the denticles of species that went extinct, and the  10% of species that survived. The shape of the denticles was  different between the two groups.

The surviving sharks had linear striations  on their denticles, while the shark species that died out had denticles  in a variety of geometric shapes. Linear striations are what we  also see on most sharks today. Geometric-shaped denticles never  really came back to the prominence they had before this event.

That means that whatever happened  to the sharks was selective, affecting sharks with geometric scales  more than sharks with linear scales. But we still don’t know what  could have happened to them, only that it probably wasn’t climate change. 19 million years ago was a relatively  stable period in Earth’s history, all things considered, at least as far as we know. We’ll probably have more theories  about this in the years to come.

The Pleistocene megafauna  were large mammals that lived between 1.6 million to 10,000 years ago. Some of their most famous members were the woolly mammoths, giant sloths,  and saber-toothed cats. Some megafauna species, like  elephants, rhinos, and giraffes, are still around today, but most of them  went extinct about 10,000 years ago.

There are two competing theories  for why the megafauna died out. The first, the overkill theory,  states that the megafauna extinction happened because of humans. As humans became better hunters, they  started hunting the megafauna and eventually killed off so many of them  that the species weren’t able to survive.

Proponents of this theory point out that  megafauna decline on a lot of continents also coincides with the arrival of humans,  particularly in North and South America. But opponents argue that there isn’t  a lot of archeological evidence that humans actually did this. The other major theory is that  climate change caused the extinction.

After all, 10,000 years ago is  when the last glacial period ended and things got a little bit toastier. The idea is that megafauna were unable  to cope with the changes and died off. Opponents of the theory, however,  point out that there were other periods of climate change when megafauna were around, and they were able to weather those.

To complicate matters, the  evidence is different depending on where in the world you look. While climate change could have  affected megafauna populations in North America, some investigations  into South American megafauna show that the decline began right  around when human hunting technologies in South America were improving. It’s possible that the real explanation  is a combination of the two, and which was more prominent varies  in different parts of the world.

When one species goes extinct, even  one as significant as the Neanderthals, it’s a fairly small event on a planetary scale. When a group of closely-related  species, like a group of sharks, go extinct at the same time,  it’s a bit more interesting. An entire category of species like the megafauna is a much more significant extinction.

But there are extinctions in Earth’s history  that are even more impactful than that, where extinctions happen across  category and species lines. We call these mass extinctions. The end-Guadalupian extinction is a bit  different from the others on this list, because we actually know what caused it.

But what makes this extinction  mysterious is that for decades, scientists kind of overlooked it. That’s because this extinction happened  just eight million years before the Permian mass extinction, an apocalyptic event around 252 million years ago that  radically reshaped life on Earth. That mass extinction wiped  out 95% of marine species, and came dangerously close to  wiping out all life on Earth.

Eight million years is a blip  by geologic time standards, so the Guadalupian extinction kind  of butts up against this other one. Because of this, scientists didn’t even notice the Guadalupian extinction for a long time. There are usually five mass extinctions  cited in Earth’s geologic history, and while the Permian is one  of them, the Guadalupian isn’t.

In 2015, researchers looking at the fossil  record in South Africa’s Karoo Basin found some of the most significant evidence  for the Guadalupian mass extinction. 74 to 80% of the land vertebrate species  that they were looking at were wiped out. Since then, further study has  shown that 60% of marine species were wiped out during the Guadalupian extinction. This number pales in comparison to the Permian, but the Guadalupian extinction still had  a large impact on global biodiversity.

This is why some researchers suggest  that the Guadalupian should be included in the Big Five mass extinction events,  making the Big Five the Big Six. As for the cause, it’s thought  to be volcanic activity. When the Emeishan Traps in  southwestern China erupted, the impact was enough to change  the chemistry of the ocean, leading to widespread ocean  acidification and lack of oxygen.

The Guadalupian extinction shows how  disruptive powerful volcanic events can be to life on Earth. And the fact that it went  undetected for so long also shows that there are a lot of mysteries  left in the geologic record. It’s often claimed that we are currently  living through another mass extinction, this one caused by humans and  their effect on the environment.

Understanding the factors that led  to species going extinct in the past can help us predict how  species might go extinct today. Who knows? We might even learn to spot the warning signs, to prevent extinctions from  happening in the future.

Thanks for watching this episode of  SciShow, which was brought to you with the generous support of this month’s  President of Science, Matthew Brant! We seriously appreciate you, and thank you for helping us bring  everyone the year 2021 in science. If you’d like to support SciShow on Patreon, and maybe run for the office yourself, you  can get started at patreon.com/scishow. [♪ OUTRO]