About 75,000 years ago, a massive volcanic eruption in Indonesia, one of the most powerful known volcanic eruptions in the last two million years, came close to wiping out the human species. How have scientists come to this conclusion? Lava and DNA.
The Kilauea volcano on the Big Island of Hawaii has been active for decades. Since the first part of May, that eruption has increased, ejecting a steady supply of lava that has formed a river of molten rock winding its way to the Pacific Ocean. It has scorched the land over a vast area, destroying homes, roads and infrastructure. Yet no deaths have been reported as a direct result. Compare this to the 1815 eruption of Tambora, which killed about 100,000 people. How can one volcanic eruption be so deadly and another more of a scientific curiosity?
The difference between the two volcanoes’ explosive power is not a factor of the volume of lava, but rather the chemical composition of the lava of each. The lava that produced, and is producing, the Hawaiian Islands is “runny” and more or less pours out of the ground, producing a mountain-shaped something like a “shield.” However, the lava that drives volcanoes like Tambora (and Mt. St. Helens and other stratovolcanoes around the Pacific “Ring of Fire”) is “thicker.” The thicker magma retains dissolved gasses, which steadily build up pressure in the confined space of a lava chamber. Over time, the pressure steadily builds until the overlying layers of solid rock can no longer withstand the pressure. The eruption is sudden and catastrophic. Without modern scientific sensors, residents living on or near such mountains have little warning that an eruption is imminent.
The human genome project (identifying our DNA “blueprint”) produced an interesting result. At the genetic level, humans are not a very diverse species. Between the most “different” humans, there is less than one half of one percent difference. Genetically, we are far less diverse than most other animals. As an example, dogs (wolves) have far greater genetic diversity, which is why there is such a bewildering assortment of dog breeds. Humans have three main blood groups, yet dogs and horses have eight major systems, and cattle 11.
Cheetahs are an exception. The DNA of any two cheetahs is almost identical. This is because, at one time, probably at the end of the last glacial “Ice Age” about 12,000 years ago, cheetahs nearly went extinct. There were so few of them that they became “inbred”, i.e. mated with close relatives. Scientists refer to this phenomenon as a genetic “bottleneck.” The comparative lack of genetic diversity in the human genome indicates that humans went through something similar, though not as extreme. Using mutations as genetic “markers,” scientists estimate that the human “bottleneck” occurred about 75,000 years ago. At that time, the human population crashed from a few million to just a few thousand.
That the prehistoric Indonesian eruption occurred at about that time places it high on the list of likely culprits. The eruption, the remnants of which still exist, was not the run-of-the-mill volcanic eruption, but a “super eruption,” something like the Yellowstone volcanic system. It would have produced a vast and caustic dust cloud that would have killed vegetation, blocked the sun, and caused temperatures to drop 30 degrees worldwide. As a result, our early human ancestors came close to annihilation. But DNA evidence indicates early humans survived, albeit barely, in Eastern Africa. After the effects of the volcano subsided after a few years, these early Homo sapiens began expanding northward into Asia and Europe. And the rest, as they say, is history.
Well, not quite. Super volcanoes that lay waste to normal weather patterns are a fact of Earth’s geology. They have happened in the past and will happen in the future. Add them to the list of challenges future humans will have to overcome in order to avoid a bottleneck with no way out.
Email Terry Mejdrich at firstname.lastname@example.org.