Life on Earth figured out how to use oxygen long before the gas actually filled the sky. Researchers at MIT have traced a crucial oxygen-processing enzyme back hundreds of millions of years earlier than anyone expected — to a time when the atmosphere was still mostly devoid of the element.
The discovery rewrites a fundamental chapter in Earth's history. For most of the planet's early existence, oxygen barely existed in the air. That changed around 2.3 billion years ago during the Great Oxidation Event, when oxygen finally became a stable part of the atmosphere and unlocked the possibility for complex life to evolve. But the new research suggests some microbes were already equipped to use oxygen centuries of millions of years before that pivotal moment.
How early life learned to breathe
The story starts with cyanobacteria, the first organisms to produce oxygen through photosynthesis. Scientists estimate these microbes emerged around 2.9 billion years ago, which means they were generating oxygen for hundreds of millions of years before it ever accumulated in the atmosphere. So what happened to all that early oxygen? Some of it reacted with rocks. But the MIT team's research suggests living organisms were actively consuming it too.
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Start Your News DetoxThe researchers focused on heme copper oxygen reductases — enzymes that convert oxygen into water and are essential for aerobic respiration. These enzymes exist in most oxygen-breathing organisms today, from bacteria to humans. By mapping the genetic sequences of this enzyme across thousands of modern species and placing them on an evolutionary tree, the team estimated when the enzyme first appeared: the Mesoarchean era, between 3.2 and 2.8 billion years ago.
That timing is striking. It means that soon after cyanobacteria began producing oxygen, other microbes nearby evolved the machinery to rapidly consume it. Early life may have essentially been slowing down oxygen's rise in the atmosphere for hundreds of millions of years, using it up as quickly as it was made.
"This does dramatically change the story of aerobic respiration," says Fatima Husain, a postdoc at MIT's Department of Earth, Atmospheric and Planetary Sciences. "Our study adds to this very recently emerging story that life may have used oxygen much earlier than previously thought."
The findings suggest that life was far more adaptable and creative in Earth's early days than scientists once believed. Rather than waiting passively for conditions to become suitable, these ancient microbes were actively responding to their changing environment, developing new biochemical tools to exploit resources as they became available. When the Great Oxidation Event finally arrived hundreds of millions of years later, life was already primed for an oxygen-rich world.
