Bottleneck in Human Population May be Due to Ozone-Destroying Volcano

James Maynard
Jun 3 · 4 min read

An ancient bottleneck in human population could be explained by an ancient volcano, and a whole lot of sulfur.

An ancient volcanic eruption — and ensuing cloud of sulfur dioxide — may have caused a bottleneck in human population seen today in the human genetic code. Image credit: Taro Taylor/Editing by The Cosmic Companion and Richard Bartz

Reading the human genetic code reveals a bottleneck in human population took place between 100,000 and 60,000 years ago, suggesting our fledgling species suffered a massive loss of life at that time, significantly reducing human populations.

A new international study builds on earlier work, suggesting this ancient catastrophe may have been the result of a massive volcanic eruption, tearing apart the protective ozone layer surrounding the Earth. Early humans worldwide would have perished from famine and disease, potentially leading to the dip observed in human numbers during this time, researchers propose.

Sure, People Died, but Think of the Coffee…

“Toba has long been posited as a cause of the bottleneck, but initial investigations into the climate variables of temperature and precipitation provided no concrete evidence of a devastating effect on humankind,” explains Sergey Osipov from the Max Planck Institute for Chemistry.

This peaceful lake is the site of an ancient supervolcano that wiped out most of the human race 74,000 years ago. Image credit: Yudhapohan

Large volcanic eruptions send massive quantities of gas and ash into the air. At high enough concentrations, these can form a layer of aerosols, blocking the Sun, cooling the Earth. This cooling, similar to a “nuclear winter,” can lead to cooler oceans, crop failures, and increased incidence of disease, researchers explain.

“Supereruptions have been assumed to cause so-called volcanic winters that act as primary evolutionary factors through ecosystem disruption and famine, however, winter conditions alone may not be sufficient to cause such disruption,” researchers describe in an article detailing the study, published in Communications Earth & Environment.

It was like a Planetwide Tanning Bed…

“The ozone layer prevents high levels of harmful UV radiation reaching the surface. To generate ozone from oxygen in the atmosphere, photons are needed to break the O2 bond. When a volcano releases vast amounts of sulfur dioxide (SO2), the resulting volcanic plume absorbs UV radiation but blocks sunlight. This limits ozone formation, creating an ozone hole and heightening the chances of UV stress,” Osipov explains.

A Landsat 7 satellite image of Lake Toba. For scale, the tiny inlet on the southwest (looking like a silhouette of Snoopy) is seen in the “peaceful lake” image above. Image credit: NASA

The Mount Pinatubo eruption in the Philippines in 1991 released 20 million tons of sulfur dioxide (SO2) into the stratosphere, which oxidized into sulfate aerosols. This resulted in oceans around the globe cooling by an average of 0.3 °C and prolonging an El Nino event.

In April 1815, the Tambora eruption in Indonesia released three times that amount of SO2, driving more than twice the cooling, and producing “the year without a summer.”

“Following the Tambora eruption, anomalous cooling and reduced precipitation provoked crop failure, famine, and the outbreak of diseases such as cholera in North America, Europe and Asia,” researchers describe in their journal article.

The Toba eruption 74 thousand years ago likely released 100 times as much SO2 as the Pinatubo event, causing the greatest natural disaster of the last 2.5 million years. Temperatures dropped between 3.5 and nine degrees Celsius worldwide, and global precipitation decreased by 25 percent.

“People never believe in volcanoes until the lava actually overtakes them.”
— George Santayana

Researchers carried out computer simulations of the Toba supereruption, using the Shaheen 2 supercomputer running the ModelE climate model. They found this event could have wiped out up to half the ozone in the atmosphere of Earth.

Even small eruptions were found to have a devastating effect on Earth’s protective ozone layer. The effects on human life would have been severe.

“The UV stress effects could be similar to the aftermath of a nuclear war. For example, crop yields and marine productivity would drop due to UV sterilization effects. Going outside without UV protection would cause eye damage and sunburn in less than 15 minutes. Over time, skin cancers and general DNA damage would have led to population decline,” Osipov describes.

These discoveries reinforce the need to account for massive natural disasters when studying human history and genetics. And, as we learn more about exoplanets, this study will help us better understand how life on other worlds may be affected by volcanism.

James Maynard is the founder and publisher of The Cosmic Companion. He is a New England native turned desert rat in Tucson, where he lives with his lovely wife, Nicole, and Max the Cat.

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