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Here at SciShow, we are passionate about telling stories that highlight how fascinating the history of our planet is. So it might surprise you that this video is dedicated to a span of years known as the "Boring Billion." This was a time where, at least at first glance, not much changed on Earth: There were no huge shifts in climate; the continents, more or less, stayed in the same spots; even the number of hours in a day, which had been steadily increasing over time, stalled out. According to some scientists, life itself might be partially to blame. While some geologists call the Boring Billion the dullest period of Earth's history, we can tell a great story if we look a little closer.

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The boring billion started around 1.8 billion years ago. For context, we're so far back in time, life on Earth was dominated by single-celled prokaryotes; these are organisms that, like bacteria, don't have any internal cell membranes. But there were also a few single-celled eukaryotes, creatures that do have those internal membranes, that had managed to evolve as well. And that life was living the shadow of an event that was anything but boring.

About 600 million years prior, some single-celled critters known as "cyanobacteria" had managed to evolve themselves into using sunlight to make their own food and producing oxygen gas a a byproduct. They started pumping tons and tons of that oxygen into the oceans they called home, and eventually, there was so much oxygen to go around, some of it started accumulating in the air. This is called the "great oxygenation event," and before oxygen levels mysteriously hit pause for a billion years, it caused all sorts of dramatic changes all around the globe.

For example, by reacting with and removing greenhouse gases from the air, all this oxygen caused temperatures to drop and brought about a global ice age that lasted for over 200 million years. But all this extra oxygen is also how the Earth got its ozone layer, and while an ozone layer is great for things like shielding life from the Sun's harmful ultraviolet rays, it might have helped cause the Boring Billion, too.

We'll come back to the "how" later, but besides oxygen levels, what else hit pause during the Boring Billion? Well, around 2 billion years ago, the Earth's continents had merged into a supercontinent called "Nuna," and it took about a billion years for the tectonic plates to shift enough that Nuna broke up and reformed into a new supercontinent called "Rodinia." With tectonic rearrangement that slow, there were basically no new mountain ranges during the Boring Billion. Think about the Himalayas today; they were pushed up as India collided into Asia. If you don't have these kinds of big tectonic plate collisions, you won't have major mountain ranges growing.

This likely had big implications for life at the time. With fewer mountains to erode, fewer minerals were slipping down into rivers and eventually out to the oceans. And a lot of those minerals had been providing nutrients to the teeny tiny critters living in those oceans. A pause in mountain formation may be why we observe such a slow development of complex life around this time.

Having said that, I should take moment to point out that evolution did not stop entirely during the Boring Billion. In fact, there were a number of pretty huge milestones. For example, this is when the first multicellular life started cropping up, and that's, like, a pretty big deal. It's also when sexual reproduction started, which was also a pretty big deal for all the complex life that led to you and me.

In fact, one 2018 study suggested that the oceans having less access to nutrients may have put just the right kind of pressure on early eukaryotes to evolve these adaptations. The team of scientists behind this hypothesis described the Boring Billion not as the "dullest part of Earth's history," but as the slow pulling-back of a slingshot that would lead to complex life later shooting forward.

But for the most part, we've got a bunch of things hitting pause, at least compared to parts of Earth's history on either side of this billion-year timespan. And in a 2023 paper, researchers announced a new pause that may help tie all of them together - a pause in the number of hours in a day.

But first, let's take a step back and explain how such a pause can even happen. The length of the day is set by how fast the Earth rotates, and over the Earth's history, the days have gradually gotten longer and longer because the Moon is slowly stealing some of the Earth's momentum. When you've got two bodies in space - like the Earth and the Moon - they both exert a gravitational pull on one another, and this pull physically deforms them, creating bulges on both bodies. It's most noticeable on Earth in the form of our ocean's tides.

As the the Earth spins, different parts of the Earth's surface face the Moon, so the sea level goes up and down. The reason you can stay in one place and watch the tides come in and out is that the Earth is rotating faster than the moon orbits, so our plant's tidal bulges aren't pointed straight at the Moon; they're always a little bit ahead. The Moon is still pulling on them though, and that offset creates an effect called "torque" that slows the Earth's rotation. But there is a second side effect; that momentum created by the Earth spinning at a certain speed cannot just vanish - the laws of physics frown upon that. So, it goes into the Moon, making it try to orbit the Earth faster. But in order to stay in a stable orbit, it has to do so further away. In other words, Earth's day gets longer and a lunar month gets longer, too.

We've known about this relationship since the 1700s, but as it turns out, the Sun also has a little something to say about it. During the day, sunlight heats the Earth's atmosphere, and that change in temperature changes the density. This causes an atmospheric tidal wave that travels around the earth as it rotates, and that wave is also offset just a little bit. Once again, this creates a torque, but because the bulge is a little bit behind the Sun, the Sun's torque tries to make the Earth spin faster. Which, you might have noticed, is the opposite of what the Moon is trying to do. If one torque is stronger than the other, well, Earth's spin either speeds up or slows down. And for most of the planet's history, that has been the case; the Moon has won out, making our days get longer and longer.

But what if something happened that made the lunar and solar torques cancel each other out? Well then, the change in lengths of the day would hit pause. This idea was first proposed back in the 1980s, but scientists have generally assumed it hasn't happened, and that's partially because proving such a pause was easier said than done. Annual growth rings of trees and coral can be used to figure out how many hours were in a day, but that'll only work for the years after they managed to evolve - hundreds of millions of years after the Boring Billion ended. So instead, the scientists behind that 2023 paper sought the help of layers in sedimentary rocks.

For example, a new layer of sediment can get deposited every time the tide goes in and out, and when the conditions are just right, that can get preserved in the geologic record. And by counting the number of layers, you can figure out how many daily tides are in each lunar month, and from there, how long each day used to be. Unfortunately, finding well-preserved samples from a billion-plus years ago is very rare; the team behind this study only found three examples they could use. So instead, they mostly relied on a method called "cyclostratigraphy," which takes advantage of the fact that certain characteristics of Earth's orbit and rotation change over time. Those characteristics include how elliptical the Earth's orbit is, as well as how much the Earth's axis is tilted and what direction that axis is actually pointing in space.

Importantly, these changes are cyclical just like the ocean tides are, except way slower. The Earth's tilt has shifted between 22.1 and 24.5 degrees for the past million years or so, but it takes about 41,000 years to complete one cycle. All three of these cycles affect the climate on Earth, which changes the conditions under which sedimentary rocks form. For example, they can influence the amount of magnetic minerals in a given layer of sediment; so by measuring the magnetic properties of different sedimentary layers, we can get a record of these ancient orbital cycles. And if you want to calculate the length of a day, you just need a record of Earth's shifting tilt and wobbling direction, because how quickly those values change depends on how fast the planet is spinning.

The researchers collected results from a bunch of previous studies, and when they put them all together, they found the length of a day didn't lengthen continuously like some had assumed. In fact, for a time that almost perfectly overlaps with the Boring Billion, it hit pause at 19 hours long. And the team doesn't actually think this is a coincidence; in fact, their paper proposes that a lot of these pauses are all connected, and that life set it all off - like more specifically, the evolution of those photosynthetic cyanobacteria that indirectly created our ozone layer.

Thanks to all those ozone molecules floating around, the atmosphere suddenly became a lot better at absorbing heat from the sun. That increased the Sun's torque to the point where it balanced out the torque from the moon. So the Earth's spin, because of cyanobacteria, stopped slowing down, holding steady at 19 hours a day. By maintaining that steady spin rate, that could have helped put the Earth's tectonic plates into their own sort of kinetic stasis, not moving very much relative to one another. Which, as I mentioned before, may have affected how fast life evolved.

As for what triggered the end of the Boring Billion, the team isn't sure. However it happened, we know that Earth's days started lengthening again, and that would mean that there would be more and more minutes of sunlight for photosynthetic organisms to produce oxygen in a given day. So the fact that Earth escaped a billion year period of 19-hour days could explain the increase in oxygen at the end of the Boring Billion. And having access to even more oxygen may have been what stopped evolution from twiddling its proverbial thumbs and creating more complex multicellular life.

Of course, this is just one hypothesis; the various pauses that occurred during the Boring Billion could have had multiple causes, not just one that kick-started all the rest. It may have ultimately been a combination of life on Earth and the physics of our Solar System that was behind the supposed dullest billion years in our planet's history. But it seem like, maybe, we're gonna need geologists to come up with a better nickname than the "Boring Billion."

I'm sure that this video's sponsor wouldn't mind the "Brilliant Billion," because this SciShow video is supported by Brilliant, an online learning platform with thousands of interactive lessons filled with science puzzles. The Brilliant course explaining variation can teach you how to find the correlations and relationships across all sorts of things happening during that billion years that seemed to have nothing going on. The course even explores making predictions about the future, so you could see how likely we are to have another billion years like that.

To check it all out, head to brilliant.org/scishow or the link in the description down below. That link also gives you a free 30-day trial and 20% off an annual premium Brilliant subscription. Thanks to Brilliant for supporting, and thanks to you for watching this video and learning with us.

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