The Medea Hypothesis: How Bacteria May Cause Mass Extinctions

Could the greatest threat to life on Earth come not from asteroids, but from the smallest of organisms? Peter Ward explores the intriguing, and somewhat alarming, possibility that bacteria have been the primary drivers of mass extinction events throughout our planet's history.

The Fragility of Earth's Habitable Zone

We often take Earth's life-sustaining conditions for granted. However, as Ward points out, the window of time in which Earth can support complex life is relatively short. Our planet is currently in its "golden summer age," but this is not permanent. Like all things, Earth has a life cycle, and it will eventually succumb to the increasing intensity of the sun.

• The sun will eventually consume the Earth.
• Earth's current "golden summer age" is temporary.

Beyond the inevitable fate of our planet, there's also the risk of accidental death – catastrophic events like asteroid impacts or nearby supernovas. However, Ward proposes a more insidious threat: the potential for life itself to trigger its own demise.

The Role of Microbes in Mass Extinctions

Traditionally, mass extinctions have been attributed to external factors like asteroid impacts or gradual climate change. The discovery of iridium in the geological record provided strong evidence for the asteroid impact that led to the extinction of the dinosaurs. However, Ward suggests that this is not the whole story. He argues that microbes, specifically those that produce hydrogen sulfide, may be the real culprits behind many of Earth's major extinction events.

Hydrogen Sulfide: A Microbial Weapon

Certain types of bacteria, particularly those found in oxygen-depleted environments, produce hydrogen sulfide as a byproduct of their metabolism. This compound is highly toxic to most animal life, even in small concentrations. Ward proposes that under certain conditions, these bacteria can proliferate, releasing massive amounts of hydrogen sulfide into the atmosphere and oceans, leading to widespread devastation.

• Hydrogen sulfide is toxic to most animal life.
• Certain bacteria produce hydrogen sulfide in oxygen-depleted environments.

Evidence from the Geological Record

Recent advances in paleontology have allowed scientists to identify biomarkers in ancient sediments, providing clues about the types of microbes that were present at the time of mass extinctions. These studies have revealed the presence of isorenieratane, a biomarker specific to bacteria that thrive in oxygen-free, hydrogen sulfide-rich environments, at several mass extinction boundaries. This suggests that these bacteria were indeed present and potentially played a significant role in these events.

Flood Basalts and the Rise of Hydrogen Sulfide

So, what triggers the proliferation of these hydrogen sulfide-producing bacteria? Ward points to flood basalts – massive volcanic eruptions that release vast amounts of carbon dioxide into the atmosphere. This increase in CO2 leads to global warming, which in turn can deoxygenate the oceans, creating ideal conditions for these bacteria to thrive.

• Flood basalts release large amounts of CO2.
• Increased CO2 leads to global warming.
• Global warming deoxygenates oceans.
• Deoxygenated oceans favor hydrogen sulfide-producing bacteria.

The Comp Hypothesis: A Chilling Scenario

Ward, along with Lee Kump and others, has proposed the "Comp Hypothesis," which suggests that many mass extinctions were caused by a combination of factors, including:

• Lowering oxygen levels due to high CO2.
• The production of hydrogen sulfide by bacteria.

This scenario paints a grim picture of a planet reverting to a Precambrian state, dominated by microbes and hostile to complex life.

A Medical Breakthrough?

Interestingly, Ward highlights a potential silver lining to this otherwise bleak scenario. Research has shown that mammals, including humans, possess a biochemical response to hydrogen sulfide. In small doses, hydrogen sulfide can induce a state of suspended animation, slowing down metabolism and protecting cells from damage. This discovery has led to promising research into the use of hydrogen sulfide in emergency medicine, potentially saving lives in cases of trauma or cardiac arrest.

A Warning for the Future

While the potential medical applications of hydrogen sulfide are exciting, Ward emphasizes the urgent need to address the underlying causes of these mass extinction events. The continued release of carbon dioxide into the atmosphere poses a significant threat to the stability of our planet's ecosystems. By understanding the role of microbes in past extinctions, we can take steps to prevent a similar catastrophe from happening again.

Italicized and bolded terms are for emphasis.