Toba Supervolcano Probably Didn’t Kill Off Most Humans 74,000 Years Ago

Credit: Oliver Spalt/CC BY-SA 3.0

One of the oddities of human evolution is that we’re generally a lot more related to each other than you’d expect in a population as far-flung as ours. While anatomically modern humans are thought to have evolved 200,000 – 300,000 years ago, the most recent ancestors of the population of humans who remained in Africa versus those who migrated away from it dates to much younger, or roughly 70,000 years ago.

In fact, while researchers used to refer to the “Out of Africa” model as a single event, it’s now thought that this migration occurred in at least two large pulses — one beginning roughly two million years ago involving archaic species like Homo erectus and a more recent wave.

The fact that the last common ancestor between African and non-African populations appear to have lived roughly 74,000 years ago suggests that a common event might have shaped our own existence, killing off all previous human lineages outside of Africa extant at that time. There are only a handful of natural disasters capable of killing off an already far-flung species at that kind of scale, but there was one event in particular that stood out — the eruption of the Lake Toba supervolcano, some 74,000 years ago. New data published in Nature suggests the theory isn’t accurate, meaning we’ll have to find a different explanation for what caused the bottleneck.

Proposed migration routes taken by ancient humans across the Middle East, Asia, and into Europe

Lake Toba is what’s left of the crater on top of Mount Toba in Sumatra, Indonesia. The Toba eruption had an estimated VEI (Volcanic Explosivity Index) of 8. To give you some perspective, the AD 79 eruption of Vesuvius is considered a VEI 5 event, the 1883 explosion of Krakatoa was a VEI 6, and the Mount Tambora explosion of 1815 that created the “Year Without a Summer in 1816” was a VEI 7.

Each number up the VEI scale refers to the total amount of ejected volume. VEI 0 events eject less than 10,000m3, while a VEI 8 event would eject 1×1012m3 of tephra, or 240 cubic miles of material. Total cloud height from a VEI 8 would be roughly 66,000 feet.

VEI 8, in other words, officially qualifies as “a lot.” Toba’s estimated explosion volume was 100x larger than Tambora, and the Year Without a Summer caused massive climate upheavals. Snow fell in the middle of the summer across the United States, with river ice observed as far south as Pennsylvania in July and August and massive temperature changes from sub-freezing to 95F or hotter within a matter of hours. The price of oats rose from $1.68 per bushel in 1815 to $13.86 in 1816 (both values given in modern values, historic was from $0.12 to $0.92).

The fact that we have good historical data on how ugly a VEI 7 can be made the idea of an even more calamitous VEI 8 pretty easy to buy, especially given the primitive state of human technology 74,000 years ago, the relatively small number of people who had left Africa in absolute terms, and the fact that ice cores retrieved from Greenland show the Toba eruption could have depressed worldwide temperatures for up to two hundred years simply due to the amount of particulate injected into the atmosphere. The idea that the Toba Eruption could have nearly wiped out the human race by reducing us to as few as 1,000 breeding pairs has been researched for the past few decades, but new finds at Dhaba, India, suggest Toba didn’t hit us that hard.

Dhaba offers a critical window into how humans were spreading across the Indian subcontinent some 40,000 to 80,000 years ago. By this point, humans were making extensive use of stone tools and had advanced beyond the Oldowan and Acheulian eras. More advanced forms of stone-crafting technology, including the Levallois technique, were in-use by this time. Given that these people left an extensive record of stone tools, a major die-off would appear in the archaeological record. We commonly date human habitation in specific areas by analyzing which dirt layers an item appears in.

In this case, artifact analysis at Dhaba shows that the humans living in the area had been using the same technology since as long as 82,000 years ago, and they kept using it straight through the critical 74,000 year period and beyond. This is an important metric of continuity. Even if a new group of humans had settled quickly enough to leave no break in the geological record, there’s no reason they’d be using exactly the same techniques for flint knapping. In this case, however, the Lavellois flakes are only gradually replaced by microblades, most likely reflecting the general replacement of one technology with another rather than a sudden, catastrophic wipeout. Lavellois flakes are found in African sites up to 280,000 years ago, Arabian sites 100,000 years ago, and in northern Australia by 65,000 years ago. This fits well with the idea that humans traveled across Asia in waves, and that they settled in places like Dhaba as they did. The theory that humanity was wiped out across most of the globe by a single volcanic eruption doesn’t fit with what we’re learning about the timeline of settlement in India and other locations.

Feature image credit: Oliver Spalt/CC BY-SA 3.0

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