Earth's rotation is slowing down

 Earth’s rotation is slowing and it could be why we have oxygen for life       

Ever since its formation around 4.5 billion years ago, Earth's rotation has been gradually slowing down, and its days have gotten progressively longer as a result. While Earth's slowdown is not noticeable on human timescales, it's enough to work significant changes over eons. One of those changes is perhaps the most significant of all, at least to us: lengthening days are linked to the oxygenation of Earth's atmosphere, according to a study earlier. Specifically, the blue-green algae (or cyanobacteria) which emerged and proliferated about 2.4 billion years ago would have been able to produce more oxygen as a metabolic by-product because Earth's days grew longer. Imagine a time when a full day on Earth lasted just 18 hours, a world where nightfall came racing faster than today’s steady 24-hour rhythm. Over billions of years, our planet’s spin has been gradually slowing down, and this subtle cosmic shift might actually explain why the air we breathe today is rich in oxygen. It’s not just an interesting quirk of physics, the lengthening of Earth’s days could have played a vital role in shaping life itself. Recent research reveals a fascinating connection between the slowdown of Earth’s rotation and the rise of breathable oxygen, showing how even the smallest changes in our planet’s spin influenced the evolution of life on a grand scale.

When Earth first formed nearly 4.5 billion years ago, it spun much faster than it does now. Thanks to the Moon’s gravitational tug, our planet has been gradually losing speed, stretching those youthful 18-hour days to the 24-hour day we know today. This happens because the Moon’s gravity pulls on Earth’s oceans, creating tides, a process that works like a subtle brake, adding about 2 milliseconds to the length of each day every century. "An enduring question in Earth sciences has been how did Earth's atmosphere get its oxygen, and what factors controlled when this oxygenation took place," microbiologist Gregory Dick of the University of Michigan explained. "Our research suggests that the rate at which Earth is spinning, in other words, its day length, may have had an important effect on the pattern and timing of Earth's oxygenation." There are two major components, at first glance, don't seem to have a lot to do with each other. The first is that Earth's spin is slowing down. The reason Earth's spin is slowing down is because the Moon exerts a gravitational pull on the planet, which causes a rotational deceleration since the Moon is gradually pulling away. We know, based on the fossil record, that days were just 18 hours long 1.4 billion years ago, and half an hour shorter than they are today 70 million years ago. Evidence suggests that we're gaining 1.8 milliseconds a century. The second component is something known as the Great Oxidation Event, when cyanobacteria emerged in such great quantities that Earth's atmosphere experienced a sharp, significant rise in oxygen. Without this oxidation, scientists think life as we know it could not have emerged; so, although cyanobacteria may cop a bit of side-eye today, we probably wouldn't be here without them.

You might wonder how this tiny change impacts something as essential as oxygen. The answer lies in the ancient microbes called cyanobacteria, tiny blue-green algae which first began turning sunlight into oxygen through photosynthesis approximately 2.4 billion years ago. This monumental event, known as the Great Oxidation Event, dramatically increased oxygen levels in the atmosphere and paved the way for complex life. Cyanobacteria depend heavily on sunlight to produce oxygen. When days were shorter, their window for oxygen production was limited. As the Earth’s days grew longer, these microbes had more time to soak up the sun and pump out oxygen, slowly but steadily enriching the atmosphere. There's still a lot we don't know about this event, including such burning questions as why it happened when it did and not sometime earlier in Earth's history. It took scientists working with cyanobacterial microbes to connect the dots. Scientists have found a modern-day reflection of ancient Earth’s microbial world beneath Lake Huron, at the Middle Island Sinkhole. Here, purple cyanobacteria, oxygen producers, compete with white sulfur-consuming microbes in microbial mats. These communities shift their dominance between day and night, revealing the delicate balance which influenced early oxygen dynamics. In the early morning, sulfur-eating microbes top the mats, feeding vigorously. As sunlight intensifies, the purple cyanobacteria take over, starting their photosynthetic oxygen production. But there’s a catch, the cyanobacteria don’t immediately jump into action. They need a few hours to “wake up” and reach their full oxygen-producing potential. This delay means shorter days limit how much oxygen they can release.

Oceanographer Brian Arbic and his team were intrigued by the question: Would the gradual lengthening of Earth’s days allow cyanobacteria to maximize oxygen production? By combining field studies, lab experiments, and computer modeling, they confirmed that longer, uninterrupted periods of sunlight let these microbes work more efficiently. Interestingly, two quick 12-hour days don’t equal one long 24-hour day in oxygen output. The reason is molecular diffusion, a slow process that governs how oxygen leaves microbial cells. When daylight cycles switch too fast, oxygen can’t diffuse away efficiently—prolonged sunlight lets microbes release oxygen steadily and more abundantly. Purple cyanobacteria that produce oxygen via photosynthesis and white microbes which metabolize sulfur, compete in a microbial mat on the lakebed. At night, the white microbes rise to the top of the microbial mat and do their sulfur-munching thing. When day breaks, and the Sun rises high enough in the sky, the white microbes retreat and the purple cyanobacteria rise to the top. "Now they can start to photosynthesize and produce oxygen," said geomicrobiologist Judith Klatt of the Max Planck Institute for Marine Microbiology in Germany.

This research didn’t only connect Earth’s spin slowdown to the first oxygen surge billions of years ago. It also linked the lengthening of days to a second oxygen rise during the Neoproterozoic era, between about 550 and 800 million years ago. This second burst of oxygen coincided with the emergence of multicellular life, making the slow deceleration of Earth’s spin a key factor in shaping the conditions which allowed diverse and complex life forms to flourish. What this means is truly profound: the gradual increase in day length wasn’t just a matter of timekeeping. It fundamentally influenced the composition of Earth’s atmosphere and the development of life as we know it. Our very breath, rich in oxygen, owes something to the long, slow drag of the Moon’s gravity. This means the window of daytime in which the cyanobacteria can pump out oxygen is very limited, and it was this fact that caught the attention of oceanographer Brian Arbic of the University of Michigan. He wondered if changing day length over Earth's history had had an impact on photosynthesis. "It's possible that a similar type of competition between microbes contributed to the delay in oxygen production on the early Earth," Klatt explained. Reflecting personally, it’s amazing to realize how minute changes, like milliseconds added to a day every century, can lead to monumental shifts over time. It reminds us that the small, steady choices we make daily can quietly shape our lives in ways we don’t always notice right away. Just as Earth’s rotation nudges life toward complexity, our habits nudge us toward growth.

To demonstrate this hypothesis, the team performed experiments and measurements on the microbes, both in their natural environment and a laboratory setting. They also performed detailed modelling studies based on their results to link sunlight to microbial oxygen production, and microbial oxygen production to Earth's history. "Intuition suggests that two 12-hour days should be similar to one 24-hour day. The sunlight rises and falls twice as fast, and the oxygen production follows in lockstep," explained marine scientist Arjun Chennu of the Leibniz Centre for Tropical Marine Research in Germany. "But the release of oxygen from bacterial mats does not, because it is limited by the speed of molecular diffusion. This subtle uncoupling of oxygen release from sunlight is at the heart of the mechanism." This intimate link between Earth’s rotation and atmospheric oxygen highlights the incredible interconnectedness of cosmic forces with life’s evolution. It’s a vivid example of how planetary mechanics ripple through biological systems to create the world we inhabit. It also sparks a deeper question: Had Earth’s rotation stayed fast with shorter days, would oxygen have risen enough to support the spectacular variety of life we see today? Or might our planet have remained a simpler, less hospitable place? By understanding this delicate balance between day length and oxygen production, we gain new appreciation for how finely tuned the conditions for life really are. 

These results were incorporated into global models of oxygen levels, and the team found that lengthening days were linked to the increase in Earth's oxygen, not just the Great Oxidation Event, but another, second atmospheric oxygenation called the Neoproterozoic Oxygenation Event around 550 to 800 million years ago. "We tie together laws of physics operating at vastly different scales, from molecular diffusion to planetary mechanics. We show that there is a fundamental link between day length and how much oxygen can be released by ground-dwelling microbes," Chennu said. "It's pretty exciting. This way we link the dance of the molecules in the microbial mat to the dance of our planet and its Moon."