Animals may have evolved because the Earth’s rotation slowed, resulting in longer days and higher oxygen, according to new research published in Nature Geosciences.
The rise of oxygen was necessary for animals to evolve, but for decades researchers struggled to explain the factors that controlled this nearly two-billion-year process.
Now, an international team of researchers, led by Judith Klatt and Arjun Chennu of the Max Planck Institute for Marine Microbiology and the Leibniz Centre for Tropical Marine Research in Germany, has theorised that oxygenation may be linked to day length, a previously undescribed connection.
When the Earth was young, it spun faster than it does now, and the days were as short as six hours. Eventually, though, the tug of the Moon’s gravity and friction from tides slowed Earth’s rotation.
The team hypothesised that, as Earth slowed, the days became longer and allowed bacteria that photosynthesised to thrive. In turn, these bacteria may have released lots of oxygen into the air, which allowed oxygen-breathing critters to evolve.
The researchers came up with this hypothesis by studying microorganisms that live at the bottom of a sinkhole, accessible only by scuba diving. There were purple oxygen-producing cyanobacteria and white sulphur-oxidizing bacteria that use sulphur, not sunlight, as their main energy source.
“We do not fully understand why it took so long and what factors controlled Earth’s oxygenation,” says Klatt. “But when studying mats of cyanobacteria in the Middle Island Sinkhole in Lake Huron in Michigan, which live under conditions resembling early Earth, I had an idea.”
Almost all of the breathable oxygen on Earth comes from products of photosynthesis, which is released by plants or bacteria, and if the working day was long, thought Klatt, maybe they had more time to produce oxygen.
The team found that the white bacteria would form a mat on top of the purple bacteria at night-time, when it could eat sulphur and the purple bacteria couldn’t make oxygen because there was no sunlight.
When the sun came up, the purple bacteria rose to the top, ready to work, and oxygen was released into the atmosphere following photosynthesis – but the productivity depended on the length of the day.
“The idea is that with a shorter day length and shorter window for high light conditions in the afternoon, those white sulphur-eating bacteria would be on top of the photosynthetic bacteria for larger portions of the day, limiting oxygen production,” says geomicrobiologist Gregory Dick of the University of Michigan, US.
“Simply speaking, there is just less time for the oxygen to leave the mat in shorter days,” explains Klatt.
“Our research suggests that the rate at which the Earth is spinning – in other words, its day length – may have had an important effect on the pattern and timing of Earth’s oxygenation,” continues Dick.
This may have been further exacerbated because the purple bacteria aren’t morning people – there was a lag between when they rose to the top and when they started to really get going with oxygen production.
So, as the days became longer and the sun was most accessible in the afternoon, the later risers had longer to really ramp up oxygen production, leading to atmospheric changes.
The team modelled how oxygen could diffuse into the atmosphere depending on day length – did the bacteria work more efficiently when there was one long day over 24 hours, or when there were two shorter 12-hour days?
“Intuition suggests that two 12-hour days should be similar to one 24-hour day,” says Chennu.
“The sunlight rises and falls twice as fast, and the oxygen production follows in lockstep.
“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 means the bacteria were making oxygen in the morning, but it wasn’t diffusing into the atmosphere until later in the day. The models showed that the bacteria could pump more oxygen into the air over one single long day, because they had time to reach, and work at, their most efficient levels for longer before the sun went down.
The team incorporated these findings into global models of oxygenation to see whether the oxygen released by the bacteria could affect the whole planet’s atmosphere.
Their simulations showed that the extra daylight brought about by the Earth slowing down could have affected the bacteria enough to boost oxygen levels globally and lead to an atmospheric shift.
“We tie together laws of physics operating at vastly different scales, from molecular diffusion to planetary mechanics,” says Chennu. “We show that there is a fundamental link between day length and how much oxygen can be released by ground-dwelling microbes.
“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.”
With these models, the researchers showed that the two major jumps in oxygen levels over Earth’s history – the Great Oxidation Event two billion years ago, and the later Neoproterozoic Oxygenation Event – may have been linked to increasing daylight. If so, it could mean that complex life was the result of long days at work for oxygen-producing bacteria.
“Juggling with this wide range of temporal and spatial scales was mind-boggling – and lots of fun,” concludes Klatt.