Did you know that the length of Earth’s day has been getting longer for billions of years? This gradual slowdown isn’t something you would notice in your lifetime, but over vast stretches of geological time, it has had profound effects on life itself. Scientists believe that this slowing spin played a crucial role in one of the most important events in Earth’s history: the oxygenation of the atmosphere.
Without our oxygen-rich environment, complex life—including humans—wouldn’t have evolved. But what’s truly fascinating is how something seemingly unrelated to the planet’s rotation could have triggered such a monumental change.
Why Is Earth’s Spin Slowing Down?
Earth’s rotation is slowing down because of the gravitational pull from the Moon. As the Moon’s gravity tugs on Earth’s oceans, it creates tidal friction, which gradually reduces the planet’s rotational speed. Over millions of years, this process has added hours to our day. Fossil evidence shows that around 1.4 billion years ago, a day lasted just 18 hours.
Currently, Earth’s rotation slows by about 1.8 milliseconds per century. This might seem insignificant, but over billions of years, it adds up.
The Connection Between Longer Days and Oxygen Levels
Around 2.4 billion years ago, a major event known as the Great Oxidation Event occurred, during which Earth’s atmosphere experienced a sharp increase in oxygen levels. This rise in oxygen is believed to have been driven by cyanobacteria—microbes that produce oxygen through photosynthesis.
Scientists have long wondered why this event happened when it did and not earlier in Earth’s history. The answer might lie in the length of Earth’s day. Recent research suggests that longer days allowed cyanobacteria to photosynthesize more efficiently, pumping more oxygen into the atmosphere.
According to Judith Klatt, a geomicrobiologist at the Max Planck Institute for Marine Microbiology, cyanobacteria need several hours of sunlight before they can start producing oxygen. With shorter days, the window for oxygen production was much smaller. As days lengthened, these microbes had more time to do their work, leading to a gradual oxygen buildup in the atmosphere.
Insights From Lake Huron’s Microbial Mats
Scientists studied modern analogs of ancient cyanobacteria in microbial mats found in Lake Huron’s Middle Island Sinkhole. These mats consist of purple cyanobacteria that produce oxygen and white microbes that metabolize sulfur. During the night, the sulfur-eating microbes dominate, but as the Sun rises, cyanobacteria take over and begin photosynthesis.
However, it takes hours for cyanobacteria to ramp up their oxygen production. This delay intrigued researchers, who wondered if longer days in Earth’s past might have given cyanobacteria the extra time they needed to produce significant amounts of oxygen.
The team’s experiments showed that even small changes in day length could have a substantial impact on oxygen levels. They created models linking day length to oxygen production and found that the slowing spin of Earth likely played a pivotal role in both the Great Oxidation Event and a second oxygenation event around 800 million years ago.
“The findings link something as cosmic as Earth’s rotation to the microscopic world of microbes,” said marine scientist Arjun Chennu of the Leibniz Centre for Tropical Marine Research. “This connection between planetary mechanics and microbial life helps explain how Earth’s atmosphere evolved.”
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