Oxygen: The Molecule that Made the World

Imagine a world where dragonflies have the wingspan of seagulls and giant spiders roam the earth. This wasn't a scene from a fantasy novel but reality on Earth 300 million years ago, made possible by an atmospheric oxygen level of 35%—far higher than today's 21% ¹ . Yet this life-giving molecule is also a toxic substance that damages our cells and contributes to aging ¹ .

This paradoxical nature of oxygen—both essential and destructive—makes it one of the most fascinating molecules in the universe. From its dramatic rise in Earth's atmosphere to its role in shaping complex life, oxygen has created the world we know today, acting as what biochemist Nick Lane describes as "the first great pollutant" ⁶ .

Earth's Atmospheric Revolution

For the first half of Earth's history, our planet's atmosphere contained virtually no free oxygen, with levels less than 0.001% of what they are today ³ . Early life consisted primarily of anaerobic microorganisms for whom oxygen was toxic poison ⁵ .

The transformation began with one of nature's most remarkable innovations: photosynthesis. Cyanobacteria, blue-green microorganisms, evolved the ability to harness sunlight to split water into hydrogen and oxygen ⁵ . The chemical equation for this revolutionary process appears elegantly simple:

For these cyanobacteria, however, oxygen was merely an unwanted by-product that had to be discarded to survive ⁵ . Little did they know they were about to transform the planet forever.

The Great Oxygenation Event

Approximately 2.4 billion years ago, atmospheric oxygen levels dramatically increased during what scientists now call the Great Oxygenation Event (GOE) ³ . The partial pressure of oxygen skyrocketed from extremely low values to around 150 mmHg, approaching modern levels ⁵ .

This oxygen surge had catastrophic consequences for the existing anaerobic life forms, causing one of the most disastrous mass extinctions in Earth's history ⁵ . Yet it simultaneously paved the way for oxygen-dependent life to evolve, creating new evolutionary pathways that would eventually lead to the incredible biodiversity we see today.

A Second Surge and Fluctuations

Earth's oxygen journey continued with a second major rise called the Neoproterozoic Oxygenation Event around 550-850 million years ago ³ . During the Cambrian period, atmospheric oxygen reached 5-10% of current values, then rose above 15% during the Devonian "Age of Fishes," and peaked at 25%—higher than today's 21%—during the Permo-Carboniferous period around 300 million years ago ³ .

This Carboniferous peak explains the existence of those enormous insects and lush vegetation that eventually formed the coal deposits we use today as fossil fuels ¹ .

The Oxygen Paradox: Life-Giver and Destroyer

Oxygen presents biology with a fundamental contradiction. As Nick Lane explains in Oxygen: The Molecule that Made the World, oxygen's toxicity stems from reactive oxygen species (ROS), sometimes called free radicals, which are formed as byproducts of oxygen metabolism ⁶ . These reactive molecules can damage proteins, lipids, and DNA, contributing to aging and various diseases ⁶ .

Yet aerobic organisms—including humans—have evolved sophisticated antioxidant systems to harness oxygen's power while managing its dangers ⁶ . We use oxygen as the terminal electron acceptor in cellular respiration, allowing us to efficiently extract energy from food. This paradoxical relationship means that, as Lane notes, "we cannot live without molecular oxygen, but we also have to work hard to live with it" ⁶ .

Scientific Spotlight: Edward Morley and the Precise Weight of Oxygen

While Earth's oxygen history spans billions of years, crucial details emerged through meticulous laboratory work. In the late 19th century, American chemist Edward Williams Morley conducted groundbreaking research to determine the atomic weight of oxygen with unprecedented precision ² .

Experimental Methodology

Morley's approach was remarkable for its thoroughness and innovation. While other researchers weighed only two of the three quantities involved in water formation, Morley correctly believed that highest accuracy required weighing all three: hydrogen consumed, oxygen consumed, and water generated ² .

His technical innovations allowed him to obtain oxygen and hydrogen in purer states than previously possible. He developed increasingly elaborate apparatus and methods to eliminate impurities such as water vapor, mercury vapor, and carbon dioxide from gas samples ² .

Results and Legacy

In 1895, Morley published his magnum opus, reporting the relative atomic weight of oxygen (on the scale hydrogen = 1 exactly) as 15.879 ² . The consistency of his results across different methods was extraordinary, with his physical and chemical determinations differing only in the fourth decimal place ² .

Modern Discoveries and Ongoing Mysteries

Despite centuries of research, oxygen continues to surprise scientists. Recent discoveries have revealed that Prochlorococcus—a tiny marine microorganism—is now recognized as a major source of atmospheric oxygen, possibly producing even more than cyanobacteria ⁵ . With a diameter of just 0.4 to 0.6 microns and a population estimated at 3 × 10²⁷ cells, Prochlorococcus represents an enormous oxygen source previously unknown to science ⁵ .

Meanwhile, researchers are employing cutting-edge techniques to refine our understanding of oxygen's history. A 2022 study used machine learning with global mafic igneous geochemistry big data to reconstruct atmospheric oxygen levels over the past 4 billion years . This approach confirmed the overall two-step rise of atmospheric O₂ but with additional shorter-term fluctuations superimposed on the broader pattern .

Conclusion: The Molecule That Made Our World

From its beginnings as a toxic waste product of ancient bacteria to its role as the essential energy source for complex life, oxygen has truly made our world. It has driven evolutionary innovation, shaped planetary chemistry, and continues to both sustain and challenge living organisms.

As we continue to probe oxygen's secrets, from the microscopic mechanisms of reactive oxygen species in our cells to the grand history of our planet's atmosphere, we uncover not just the story of a simple molecule, but the story of life itself and the unique conditions that make our living planet possible.