For decades, scientists have tried to understand why oxygen, essential for complex life, only started accumulating in Earth’s atmosphere billions of years after photosynthesis began. A new study suggests that Earth’s slowing rotation—which lengthened the day—played a key role in this oxygen rise.
How Earth’s Day Got Longer
Billions of years ago, Earth spun much faster. A full day may have lasted only 6 hours. Over time, the gravitational pull of the Moon slowed the rotation, stretching days to 18 hours about 1.4 billion years ago and eventually to the 24-hour cycle we live with today. This change in day length may have unlocked the conditions needed for oxygen to build up.
Microbial Mats Hold the Clues
Researchers studied cyanobacteria mats in Lake Huron, Michigan, which mimic ancient Earth environments. These mats contain microbes that produce oxygen when exposed to sunlight. During short days, oxygen had little time to escape into the atmosphere before being consumed. But with longer daylight periods, cyanobacteria could release more oxygen, allowing it to accumulate instead of being lost.
Modeling Supports the Theory
Using computer models and microsensor data, scientists found that the length of daylight strongly affects oxygen release. Longer days provided a bigger “oxygen window,” making it easier for oxygen to escape into the atmosphere. This suggests that physical changes in Earth’s rotation—not just biology—helped shape the planet’s habitability.
Oxygenation Events and Rotation
Two major atmospheric changes—the Great Oxidation Event (about 2.4 billion years ago) and the Neoproterozoic Oxygenation Event (around 550–800 million years ago)—appear to coincide with longer days caused by Earth’s slowing spin. These events paved the way for more complex life forms, showing that planetary mechanics and biology worked hand in hand.
Why It Matters Today
This discovery connects planetary physics with evolution, highlighting how changes in Earth’s motion influenced the rise of life. It also offers clues for studying exoplanets: day length and rotation could determine whether other planets can build and sustain oxygen-rich atmospheres.
