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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.
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with the generous support of this month’s President of Science, Matthew Brant!
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