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Oxygen is crucial for life as we know it today. But it took a long time for enough of it to build up in Earth’s atmosphere for those modern forms of life to evolve.

And an international team of scientists have proposed a new potential mechanism for how that oxygen accumulated. In a new study published in Nature Geoscience, they propose that oxygenation may actually depend on how fast the Earth rotates. So Billions of years ago, Earth’s atmosphere contained very little oxygen.

About 2.4 billion years ago, there was a relatively sudden increase. And then, somewhere around 600 to 800 million years ago, it happened again. Scientists have struggled for years to explain why oxygenation happened in this series of distinct steps.

They believe that oxygen was produced by photosynthetic cyanobacteria that took carbon dioxide out of the environment and dumped oxygen back in. But for photosynthesis to work, you really need sunlight. And if you go back billions of years, the days were shorter.

Way shorter. As short as 6 hours. Because Earth was actually spinning faster.

It’s been gradually slowing down over time thanks to the. Sun and Moon pulling on the Earth. And that creates slight bulges in the shape of the planet that cause the Earth to spin more slowly.

So now it takes four times as long to rotate on its axis as it did a few billion years ago. And shorter days mean less time with maximal sunlight, and less time for cyanobacteria to do their photosynthetic thing. So the team first used computer models to determine whether the length of the day might affect how much oxygen photosynthesis could produce.

Then, they headed into the field to verify the models’ predictions at the Middle. Island Sinkhole in Lake Huron in Michigan. This is a sinkhole that contains a lot of sulfur and not a lot of oxygen, which makes it and the microbes that live there useful models for studying what Earth’s ecology might have been like billions of years ago.

The cyanobacteria in the Middle Island Sinkhole use sunlight as their main energy source, and form mat-like colonies alongside other bacteria that rely on sulfur for fuel. In the morning and evening, the sulfur-eating bacteria cover the cyanobacteria, blocking the sunlight and preventing the cyanobacteria from performing photosynthesis. But when sunlight increases in the afternoon, the sulfur-eating bacteria move downwards, giving the cyanobacteria the ability to bask in the sun and start photosynthesizing.

And that is why the length of a day would have mattered. Because there’s a delay before the cyanobacteria are fully productive. So they’d get less use out of a shorter day.

That delay means that when a day is less than 12 hours, the bacterial mats should actually absorb more oxygen than they release. In fact, it’s not until you get to a 16 hour day that the researchers predicted any net release of oxygen. And a 24 hour day allows three times as much oxygen production as a 16 hour day.

So, according to this model, as the Earth’s rotation slowed, the days got longer, and cyanobacteria were able to produce more oxygen. And that cleared the way for more oxygen-dependent organisms to evolve. Sometimes evolution can lead species into problems, though, as seems to be the case with young sea turtles.

New research in Frontiers in Marine Science reports that these sea turtles may have fallen into what’s called an evolutionary trap. An evolutionary trap occurs when a previously adaptive behavior actually starts to hurt a species’ odds of survival. In the case of sea turtles, newly hatched youngins have adapted to head out to the open ocean in order to avoid predators.

But in recent decades, the open ocean is full of plastic. And since juvenile sea turtles eat pretty much anything, they end up eating the plastic, too. So that has created the trap: the turtles head somewhere they are less likely to be eaten, which has been beneficial in the past.

But now the same behavior means that they are ingesting more potentially dangerous plastic. The study looked at juvenile sea turtles that either washed up on shore or were accidentally caught by fishers on the Pacific and Indian coasts of Australia. On the Pacific coast, 86% of loggerhead turtles, 83% of green turtles, 80% of flatback turtles, and 29% of olive ridleys had plastic in their digestive systems.

While on the Indian Ocean side, 28% of flatbacks, 21% of loggerheads, and 9% of green turtles had plastic in their guts. Now, the sample sizes here were small. So, for example, the study only looked at seven hawksbill turtles, none of which had plastic in their stomachs.

But this study only looked for relatively large chunks of plastic. Previous studies have found microplastics, tiny pieces less than 1 millimeter in size, in virtually all sea turtles. Which is a problem, because researchers still don’t know exactly how plastic affects the health and survival of the turtles.

We know that getting tangled in plastics can suffocate sea turtles, and there are some reported cases of turtles dying after larger hard plastic fragments blocked their gastrointestinal system. But we aren’t totally sure what happens when they eat plastic, so that’s the next thing on the research team’s agenda. And we’re also still not exactly sure how the turtles are ingesting the plastic in the first place.

Sure, they may just be chomping on plastic bags, but they may also be accidentally eating plastic that has contaminated the sediment on the sea floor, or eating other organisms that ate the plastic first. This study shows that evolution is a process, and doesn’t have any way of knowing where it’s going. These turtles adapted to head to the open ocean to survive, but now that adaptation is putting them in harm’s way.

Which means if they don’t evolve new behaviors to course-correct, they could be in danger. Of course, we humans probably need to change our behavior too. Thanks for watching this episode of SciShow News.

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