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How Earth's Gravity Caused Life on Earth

Day length has increased dramatically over Earth’s history. More than three billion years ago, entire days may have been just six hours long. And around 2.4 to 2.2 billion years ago, geological records indicate that the amount of oxygen in the atmosphere shot up while the volume of carbon dioxide shrank. That rapid increase in oxygen is generally credited to the proliferation of marine cyanobacteria, some of which absorb energy from sunlight and produce oxygen.

Judith Klatt had been studying microbes called cyanobacteria for years, and she was initially skeptical when some colleagues came to her with an idea: Could the length of a day on early Earth have mattered to the rise of life as we know it?

To investigate that pattern, the team turned to a unique ecosystem on the bottom of Lake Huron called Middle Island Sinkhole. They paired oxygen concentration measurements in the cyanobacteria-rich sinkhole and experiments in the lab with computer models of Earth’s rotation.

The results don’t entirely solve the mystery. But as the team reports, the data do open up exciting possibilities for how day length and biology could have co-evolved on Earth—and beyond.

Our current roughly 24-hour day is the result of Earth’s spin slowing down over its 4.5 billion years, and much of that change can be linked to the tides.

If you’ve spent a day near the ocean, you’ve probably watched the tides rise and fall along the shore. That seemingly gentle motion is due to a massive give-and-take of energy between Earth, its ocean, and the moon. As it orbits Earth, the moon gravitationally pulls on the ocean, and the water pulls back. The ocean responds to this tugging in the form of tides, and that introduces friction between the water and the rocky seabed beneath it

That friction saps Earth’s rotational energy, slowing its spin and lengthening the day. This process happens very, very slowly over hundreds of millions of years, so changing day length is not something we can readily observe, and it’s been tough to track in the deep geologic record.

While many models for Earth’s rotation rate exist, one that has been used since the late 1980s proposes that days got progressively longer until about 2.5 billion years ago, when day length stabilized at around 21 hours and remained relatively unchanged for millions of years.

Around that time, the tides, Earth, and the moon could have reached what modelers call resonance. There are really two rotational forces that affect Earth’s spin. The tides drag on the planet, slowing it down. But the sun also heats one side more during the day, causing the oceans and atmosphere to expand, which pulls Earth forward a tiny bit in its rotational path.

Tides were winning this tug-of-war until the system hit resonance, when those two opposing forces cancelled each other out. And once it reached that “magic” frequency, the spin rate would have been unlikely to budge for quite a while.

Klatt and her team used this model as the basis of their new work.Intriguingly, they found that the resonant 21-hour day set in around the same time that geologic records reflect a burst of oxygen in the atmosphere.

The water in the 75-foot-deep sinkhole has high concentrations of sulfur and not much oxygen. Scientists think those conditions could be similar to the ancient ocean billions of years ago. By studying that environment, Klatt’s team can get an approximate idea for how similar ancient ecosystems could have behaved.

The answer is a physical mechanism called diffusion, in which oxygen gas in spots of high concentration tends to move to areas with lower concentrations. It’s like when you open a bottle of soda and the bubbles of carbon dioxide rush out.

While short days have cyanobacteria constantly switching on and off, longer days let them photosynthesize for longer stretches of time, building up oxygen concentrations around them until some is driven out and up into the atmosphere.

Laboratory studies show the reality of this process. Microbial mats, when exposed to daylight for a long time, exuded more oxygen in production. This suggests that cyanobacteria have an integral role to play in ensuring longevity on earth, rather than trying to adjust to already changed limits. The team's research is presented in the journal Nature Geosciences Article source: National Geographic

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