Day lengths on Earth have varied a great deal. As per fine-tuning of Earth for life, the Moon is responsible for slowing Earth’s rotation down.
Researchers already knew, from modeling the Moon’s distance from Earth and the resulting atmospheric and oceanic tides, that the infant Earth turned much faster on its axis than it does today. Many agree that 4.5 billion years ago, a day was only about 6 hours long. By about 2.4 billion years ago, the models predict, the pull of the Moon had slowed that spin to about a 21-hour day. Earth’s rotational speed then stayed constant for about a billion years, as its gravitational pull countered the Moon’s drag. Those forces fell out of balance about 700 million years ago, because the resonance cycle between Earth and the Moon is not completely stable, and the planet’s spin slowed to its current speed, creating a 24-hour day, according to the modelsElizabeth Pennisi, “‘Totally new’ idea suggests longer days on early Earth set stage for complex life” at Science (August 2, 2021)
The longer day lengths were consistent with leaps in atmospheric oxygen, thought to help promote complex life.
Still, Lyons and others say, many factors likely contributed to the rise in oxygen. For example, Fischer suspects free-floating cyanobacteria, not just those in rock-affixed mats, were big players. Benjamin Mills, an Earth system modeler at the University of Leeds, thinks the release of oxygen-binding minerals by ancient volcanoes likely countered the early buildup of the gas at times and should be factored into oxygen calculations.Elizabeth Pennisi, “‘Totally new’ idea suggests longer days on early Earth set stage for complex life” at Science (August 2, 2021)
The paper is open access.
Of course, there is much more to complex life than just having oxygen but length of days is surely worth pursuing.
See also: What becomes of science when the evidence does not matter?