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Around 74,000 years ago, in what’s now Sumatra, a volcano called Toba exploded. It was the largest known eruption in at least the past two million years, a whole order of magnitude bigger than the 19th-century eruption of Tambora, which is the largest volcanic eruption in recorded history.

The Toba eruption produced 2800 cubic kilometers of magma, deposited meters-thick layers of ash, and spewed thousands of tons of sulfuric acid and sulfur dioxide into the atmosphere. It may have caused global temperatures to dip by as much as 10 degrees Celsius for the next decade — with some cooling lasting nearly a thousand years. This was the Middle Paleolithic Period, when the height of human technology was stone tools and fire.

So you get why scientists would think the giant explosion had a serious impact on the human population. But the evidence seems to show that humanity was … mostly fine. And we’re not totally sure why.

The main climate effects from volcanoes come from the ash and sulfurous gases they belch out. After a big eruption, this stuff can circulate in the atmosphere for years, reflecting sunlight and causing global cooling — hence that possible decade-long volcanic winter caused by Toba. It makes sense that an endless global winter would be bad news for the planet’s inhabitants.

For comparison, the eruption of nearby Tambora turned 1816 into what became known as “the year without a summer” and led to crop failure and famine around the world. And Tambora only put out 175 cubic kilometers of stuff, compared to Toba’s thousands. So in the 1990s a scientist named Stanley Ambrose proposed what’s been called the Toba catastrophe theory.

His idea was that the eruption might have nearly wiped out humanity, reducing the global population from around 100,000 people to around 10,000. People of African ancestry are more genetically diverse than other humans, which seems to suggest that the rest of humanity experienced a genetic bottleneck at some point — a dramatic drop in population that resulted in a loss of genetic diversity. According to the Toba catastrophe theory, the giant volcanic eruption and the global winter that followed were the culprits.

Africans’ tropical home, the theory goes, helped buffer them against the effects of the volcanic winter and prevented the huge population declines that happened in other areas. Which sounds plausible. But as we’ve continued to look for evidence of the so-called Toba catastrophe, things have become a lot less clear.

For one thing, scientists don’t all agree on just how bad the eruption’s effects on the climate actually were. In 2010, researchers created a mathematical model of the eruption’s possible climate effects, based on how the particles it shot into the atmosphere would have reflected solar energy. Their results showed that the eruption’s global effects may have been milder and shorter-lived than originally theorized — like a three to five degree drop in temperature for two or three years, instead of a ten degree drop that lasted a decade.

That’s still a really big deal — a drop of just one degree was enough to cause the year without a summer. But maybe it wasn’t a big enough deal to kill off 90% of the world’s population. Recent research has also found that charcoal and plant particles in sediment samples from Africa’s Lake Malawi don’t show any big change in plant life before and after the eruption.

Which is something you’d definitely expect to see if there had been a ten-year-long winter. And, the more archaeologists look for evidence of what actually happened to humans around this time, the more it looks like the answer is … nothing too devastating. A recent study from the coast of South Africa didn’t find any interruption in human activity there.

There is a layer of tiny volcanic glass shards from the eruption in archaeological deposits, but the human artifacts from before and after are pretty much the same. Some of the researchers involved suggested that living in a warm, coastal area with lots of resources might have helped people there thrive despite the eruption. But archaeological studies from much closer to the blast zone in India — don’t show much change in human communities there around the time of the eruption, either.

Humans probably were affected by Toba — the largest volcanic eruption in our history would be hard to ignore. But the latest evidence makes it seem pretty unlikely that 90% of the global population was killed off, and today most scientists consider the Toba catastrophe theory to be debunked. But that still leaves the question of what caused the genetic bottleneck as humans were expanding out of Africa.

The most widely accepted explanation today is that it was a simple case of what’s called the founder effect. Only relatively small groups of humans ever made the trek out of Africa, limiting the genetic diversity of their descendants as they populated the rest of the world. Perhaps the closest parallel to Toba threatening the world today is the massive volcano underneath Yellowstone National Park.

One of the Yellowstone supervolcano’s past eruptions, around two million years ago, was almost on the scale of Toba, spewing about 2500 cubic kilometers of magma. And today, humans rely on way more technology that would be negatively affected than we did at the time of Toba, from agriculture to airplanes. In some ways, our society is more fragile than it was back then.

So a modern Yellowstone eruption would be really bad news. But volcanologists say the chances of Yellowstone blowing up any time soon are incredibly small, so don’t lose too much sleep over it. Ultimately, though, even an eruption as large as Toba’s didn’t really alter the course of humanity’s genetic history.

Mind-bogglingly huge volcanic eruptions like Toba are a reminder of just how violent our planet’s geology can be — but in this case, it also showed that our species can be pretty resilient. Go humans! Thanks to Brilliant.org for sponsoring this episode, and this whole week, of SciShow.

Trying to unpack natural disasters from the past, or make predictions about the future is a lot more interdisciplinary than we tend to think. Scientists need to know about geology and weather, and in this case anthropology, but also probability. All this week we’re pointing out fun, interactive courses on Brilliant.org, and one of my favorites is Probability in Science.

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Right now, the first 200 SciShow viewers to sign up will get 20% off of the annual premium subscription. And let me know how you do on the Probability quiz! Or if you prove the existence of aliens! [ ♪ OUTRO ].