It's one of the biggest mysteries of recent human evolution. Roughly 70,000 years ago, Homo sapiens went through a genetic bottleneck, a period when our genetic diversity shrank dramatically. But why? In the late 1990s, some scientists argued that the culprit was a massive volcanic eruption from what is now Lake Toba, in Sumatra, about 74,000 years ago, whose deadly effects reduced our species to a few thousand hardy individuals. Now, new evidence suggests we were right about the volcano—but wrong about pretty much everything else.

The so-called Toba Catastrophe Theory was first proposed by University of Illinois anthropologist Stanley Ambrose and popularized by University of Utah anthropologist Henry Harpending, who was trying to understand what caused the genetic bottleneck. At the time, mounting evidence suggested that the volcano had had a global effect, because debris from it can be found throughout the world. Many scientists thought it was likely that airborne particles from Toba caused a "volcanic winter" that lowered Earth's temperatures. Harpending and his colleague Gregory Cochran suggested that it ushered in a millennium of frigid temperatures, driving humanity to near-extinction and pushing it out of Africa in search of better habitats.

Once the globe warmed up again, the theory goes, humanity started to recover its ranks. But the population crash meant that we had lost a lot of genetic diversity. This hypothesis sounded reasonable at first, but then scientists began to uncover intriguing new evidence that humans hadn't died out at all.

The catastrophe that wasn’t

A paper published last week in Nature explores much of that evidence and presents a coup de grâce discovery that debunks the Toba Catastrophe Theory. That discovery comes from two beautifully preserved ancient campsites at the very southern tip of South Africa. There, in a cozy rock shelter and an open campsite, humans lived through the Toba eruption and thrived afterwards. University of Nevada, Las Vegas, geoscientist Eugene Smith worked with an international team of researchers to analyze the sites, digging down through many layers of habitation to see exactly what happened in the years following the supposed catastrophe.

What allowed the team to get this incredible glimpse into human history was a lucky accident of geology. Humans lived for millennia in the open area and the rock shelter, which are known by the glamorous names Vleesbaai Area B (VBB) and Pinnacle Point Site 5-6 (PP5-6), respectively. Likely VBB and PP5-6 were used by the same groups of people, and they left behind ample evidence of their lives in both places in the form of refuse: ash from fires, bones and seeds from food, and discarded stone tools and weapons.

Over time, their trash accumulated in layers beneath the ever-present sands that blew across their campsite and into the rock shelter. When Toba erupted in Sumatra, it left a telltale layer of tiny glass shards in those layers, too. Smith and his team performed a variety of chemical analyses on these shards, comparing them to known examples of Toba shards. It was an almost perfect match. Now Smith and his team had a clear record of human activity at the sites both before and after the eruption.

What they found surprised them. There was actually more debris from human habitation directly after the eruption than before it. A massive population crash would have resulted in far fewer tools and food remains, and the site might even have been abandoned. But what the researchers saw was a thriving, growing community, inventing new tools and forms of artistic expression. If there had been a volcanic winter, it must have been relatively mild.

"If Toba had triggered a major global climate event, Africa probably would have been affected, and they see no evidence of that," University of Alberta geologist Britta Jensen told The Atlantic's Ed Yong.

Of course, it's always possible that this group was an outlier. Curtis Marean, an Arizona State University archaeologist who has worked for years on the Pinnacle Point rock shelter, suggested climate change might have driven people to the resource-rich coasts. "It is possible that people moved out of terrestrial locations and into this more productive coastal zone," he told Peter Hess at Inverse. "Think of it as a refuge."

Still, there is ample evidence from other studies that the planet didn't suffer a global die-off. Animal and plant populations remained robust, and there are no signs of Neanderthal populations shrinking in Europe. There's no doubt the volcano left debris across the planet, but it doesn't appear to have caused a human population bottleneck.

Beyond the bottleneck

And yet geneticists agree there's an unmistakable shrinkage in Homo sapiens' genetic diversity, which starts roughly 70,000 years ago. If there was no massive population crash, what caused that shift?

Some scientists believe it might not be a crash at all but instead an artifact of many founder effects as Homo sapiens spread across Eurasia, splitting off into small groups with less and less genetic diversity. This also would explain why there's a much greater genetic diversity in Africa than elsewhere. Founder effects became more pronounced the farther that Homo sapiens wandered into Europe, Asia, and the Pacific. People who stayed behind in Africa remained part of a much more genetically diverse group.

As evidence for the Toba Catastrophe Theory has fallen away, it has also become clear that there was a strong bias motivating Harpending's enthusiasm for the idea. The anthropologist, who died in 2016, believed the Toba Catastrophe Theory proved that there were fundamental evolutionary differences between racial groups. In books like The 10,000 Year Explosion: How Civilization Accelerated Human Evolution, he argued that the genetic bottleneck meant that Africans had undergone a "genetic break" from other races that made them fundamentally different from Europeans. Eventually, his research became so focused on this idea that the Southern Poverty Law Center, which tracks extremist groups, identified him as a white nationalist.

This is in some sense how all scientific hypotheses die. Researchers find new evidence that contradicts the original claim and validate that evidence over a period of years. Sometimes they discover a previously unknown bias in the hypothesis. In the case of this theory, it took nearly two decades to reach a new hypothesis. Or, as Wellcome Trust Sanger Institute geneticist Chris Tyler Smith put it to the BBC: "[The Toba Catastrophe Theory] was an exciting idea when it was first suggested, but it just hasn't really been borne out by subsequent advances."

Listing image by C.G. Newhall / Wikicommons