How a Supervolcano Ignited an Evolutionary Debate

About 74,000 years ago, something was stirring  on the island of Sumatra, in Indonesia.

It was a sleeping giant of  a supervolcano now known as   Toba - and it was getting ready to erupt again.

And this time would be the biggest of all,  an eruption so massive that scientists have   ranked it an 8 on the Volcanic Explosivity  Index - the highest ranking possible.

The eruption lasted between 9 and 14 days,   spewing out ash and rock at a rate  of a million cubic meters per second.

It was so huge that it covered 20,000  square kilometers around the caldera   with a layer of pumice and ash  deeper than 100 meters thick.

Ash from this eruption has been found as  far away as eastern and southern Africa.

It was the biggest explosive eruption  of the last 2.5 million years.

And humans were around to see it - or, at least,   to feel its effects.

But what exactly  those were is still being debated.

Either the Toba eruption was an  apocalypse for ancient humans... One that caused a global volcanic  winter leading to a new ice age and   climatic shifts so extreme that human  populations around the world died off,   leaving only small groups able to survive  in the forested areas of Africa... Or, it wasn’t.

But linking cause and effect in the distant  past takes pinning down precise dates for   the eruption, changes in climate,  and our own evolutionary history.

And lately, scientists are starting  to think that maybe you can have a   supereruption without an apocalypse.

The catastrophe hypothesis  got started in the 1990s,   when geologists, anthropologists, and  paleoclimatologists were all looking   at the different pieces of the Toba  puzzle from their respective fields.

As more ash from the eruption was discovered,   geologists realized that the volcano had been  even larger than they’d previously believed.

At the same time, researchers studying the  human genome pointed out that modern humans   within Africa have much higher genetic  diversity than humans outside of Africa.

Based on that observation and other clues in our  DNA, they suggested that human population size   had drastically decreased sometime between  100,000 years ago and 50,000 years ago... ...A time period when we know humans and   our ancient relatives had already  spread across Africa and Eurasia.

In other words, there was a bottleneck  before our population grew larger again.

And while all of this was going on, the  climate had been fluctuating between   periods of warming and cooling  for tens of thousands of years.

So, in a paper published in 1998,   an anthropologist pulled all of these  different strands of evidence together.

He proposed that the Toba eruption  initiated a long volcanic winter   that killed off lots of people across the planet,   and only those in relatively unaffected areas like  the warm, forested parts of Africa could survive.

This became the very popular  Toba human bottleneck hypothesis.

But to really prove the connection  between the volcano and human evolution,   scientists had to get a lot more precise  with their dates - and that’s where the   evidence started to paint a different picture.

First off, geologists had to precisely date  the material from the Toba eruption.

Without   a precise date for the volcano, there’d be no way  to say that it caused a bottleneck or an ice age.

Using a method called Argon-Argon  dating, they got a pretty narrow   possible date range for the eruption,  placing it right around 74,000 years ago.

And we’ve found lots of tools and fossils  that belonged to ancient Homo sapiens,   spread across much of Africa and Asia  dating to around 74,000 years ago… So we know our ancestors were in the area that  could’ve been affected by a huge eruption.

Next, researchers had to compare the  more refined date for the Toba eruption   with what was happening more  broadly with the climate.

Scientists have reconstructed the climate and how  much of the world was covered by plants through   the use of marine and lake sediments,  ice cores, and stalagmites from caves.

And they’ve divided the different climates  into categories called Marine Isotope Stages,   with the boundary between Marine Isotope Stages  4 and 5 between 75,000 and 70,000 years ago   marking a transition to a much colder  period of increased glaciation.

But the precise date of that transition  between Marine Isotope Stage 5   and the climate conditions that came  afterwards, is extremely important.

Because it could tell us whether the Toba  eruption actually kicked off that ice age,   or if the climate impacts from the eruption  were more of an unpleasant, decades-long blip.

For a human ancestor or relative living  somewhere on the Indian subcontinent,   that distinction really matters.

That area wasn’t close enough to  the eruption for its inhabitants   to be immediately wiped out by debris, but  it was definitely close enough that they   might all starve if plants stopped  growing and other animals died off.

So the question was, did Toba cause  a long-term climate disruption,   or did the world enter the cooler part of  the climate cycle without the volcano’s help?

Scientists looked at ice cores from Greenland,  pollen in marine cores near the Bay of Bengal,   and analyzed oxygen isotopes  to understand the climate.

Their data suggested that the world did  cool drastically right after the eruption,   and that there were several centuries of  lower temperatures and reduced precipitation.

But there are also some reasons to  think that the volcanic winter might   not have been devastating - and that  it didn’t kick off a new ice age.

Ocean cores collected near Sumatra that  contain pollen grains from plants around   the region show that some vegetation, like pine  forests, was strongly affected by the eruption,   but other types of vegetation  actually recovered pretty quickly.

Then there’s lake sediment that researchers have   collected from Monticchio Lake in Italy  and Bear Lake in the Rocky Mountains.

Those sediment cores have precisely dated  pollen samples from the last 100,000 years,   and they show that both regions  were transitioning from warm to   cold vegetation around 80,000 years  ago— before the Toba eruption.

Researchers also looked at sediment from  Lake Malawi, one of the largest lakes in   Eastern Africa.

There’s actually Toba ash in that  sediment, but remnants of the nearby vegetation   show no significant changes from year to  year or decade to decade after the eruption.

This would seem to suggest that  however cold it got after the eruption,   it wasn’t cold enough to  impact plant communities there.

And if plants weathered the eruption just fine,  maybe animals - including humans - did, too.

Another group of researchers looked  at fossil evidence from different   animals around Sumatra at  the time of the eruption.

While they couldn’t look specifically at  Homo sapiens, because they didn’t have   those fossils, they did find fossils of  one of our great ape relatives—orangutans.

Orangutans lived all around  Southeast Asia at the time,   and their population wasn’t affected by the  eruption, despite how close they were to it.

Considering that orangutans don’t  make tools the way ancient humans did,   and that they need the rainforest to survive, the  fact that they stuck around after the eruption   makes it seem unlikely that we wouldn’t have been  able to adapt to changes in the climate, too.

Researchers have also pointed to different  types of stone tools found around India and   other parts of Asia, which were essentially  the same before and after the eruption.

Archaeologists interpret that to mean that the  people living in those areas never died off, since   they couldn’t have passed on their knowledge of  tool making unless it was a continuous community.

So there is definitely evidence that even  though the Toba eruption cooled the planet down,   it didn’t kill off all the ancient humans  and our relatives living around the world.

Like, Neandertals survived just fine  in Eurasia, where the cooling effects   might’ve been the most extreme.

They didn’t  disappear until around 40,000 years ago.

But, I kinda have to ask, why wasn’t  the impact of the Toba eruption bigger?

Well, scientists have suggested  that Toba might not have released   as much sulfuric acid aerosols  as other types of volcanoes.

And that would mean the temperature didn’t cool   as much as it might’ve if the volcano  had a different chemical composition.

There’s also the chance that the  eruption released a ton of water vapor,   which might’ve counteracted the cooling  by trapping more warmth around the planet.

Really, it’s hard to say.

And the debate isn’t over.

There are still scientists who argue that the  eruption did kill off a lot of ancient humans.

For example, a study published in 2021  looked at how the eruption might have   affected the ozone layer.

And those researchers  concluded that the volcano would’ve allowed   a lot more dangerous UV light to reach the  surface of Earth—especially around the equator.

This surge of solar radiation  could’ve killed off a number of   people and had an impact on the population.

And while our understanding of the patterns of   human genetics have improved significantly since  1998, there are still signs of a bottleneck that   happened around 50,000 years ago -- though  this seems too late to be the result of Toba.

A lot of the debate comes down to the difficulty  of measuring impacts happening at different   time-scales in the deep past, like did an event  affect the world for decades or for centuries?

Linking specific causes to specific  effects is no walk in the park, either.

We know that the Toba eruption was massive, that  it did have an impact on the climate for a while,   and that the human population did  get smaller at one point in history.

But at this point, most researchers agree that  we can’t blame the bottleneck on the volcano.

What we can say for sure is that some humans  lived through that particular apocalypse,