For more than four billion years, Earth has been gradually slowing down. Each passing century sees our planet’s rotation decelerate by roughly 1.8 milliseconds, thanks largely to the gravitational pull of the Moon.
In a 2021 study, researchers linked Earth’s lengthening days to the oxygenation of its atmosphere, a monumental shift that occurred in at least two known waves. The first, called the Great Oxidation Event about 2.4 billion years ago, marked a point when cyanobacteria—microscopic blue-green algae—began producing oxygen through photosynthesis in large enough quantities to transform the atmosphere. The second, the Neoproterozoic Oxygenation Event, happened hundreds of millions of years later.
But why did these events occur when they did? Scientists have long debated what triggered the timing of these bursts of oxygen. It turns out, the answer might be written in Earth’s spinning skies.
As Earth’s rotation slows, the length of its days increases, giving sunlight more time to interact with photosynthetic organisms like cyanobacteria. In a lakebed sinkhole in Lake Huron, researchers studied microbial mats containing modern analogs of these ancient cyanobacteria. During the night, sulfur-metabolizing microbes rise to the top of the mats. When daylight returns, they retreat, and oxygen-producing cyanobacteria take their place. However, these cyanobacteria are slow starters—it takes hours before they ramp up their photosynthetic activity.
This daily microbial turnover highlighted a crucial dynamic: the longer the day, the more time cyanobacteria have to photosynthesize and produce oxygen. Oceanographer Brian Arbic and geomicrobiologist Judith Klatt wondered if Earth’s gradually lengthening days gave these microbes more time to produce oxygen in the past, ultimately triggering the planet’s atmospheric shifts.
To test the theory, the team conducted both field experiments and lab simulations, pairing their findings with computer models. The results showed a clear link: as day length increased over geological time, so did the potential for oxygen accumulation. Marine scientist Arjun Chennu explained that while it might seem two 12-hour days equal one 24-hour day in terms of sunlight, the rate of oxygen diffusion breaks this symmetry. Oxygen release from microbial mats is slow, and longer days give it more time to accumulate in the atmosphere.
This subtle but critical disconnect between sunlight and oxygen diffusion could explain why Earth’s atmosphere oxygenated when it did—not sooner, not later. The researchers believe the same mechanism may have played a role in both major oxygenation events in Earth’s history.
As Chennu put it, this research ties together “the dance of molecules in microbial mats to the dance of our planet and its Moon.” In essence, Earth’s gradually slowing spin didn’t just shape tides or calendars—it helped shape life itself.
