(host) Around two billion years ago, Earth was a very different place. Pretty much everything was more extreme than it is today, from ocean chemistry to slimy life on land to the position of land masses. Things were generally a little weird. The oceans would have been inhospitable to most animal life today because they were very low in oxygen but really high in sulfur. Meanwhile, on land, there wasn't much life to speak of except for microbial organisms like cyanobacteria and possibly lichens. Even the temperature inside the Earth was more extreme than it is today. But the world for these early living things was also kind of boring. The climate was remarkably stable. There wasn't much glacial activity. And the Sun was 5% to 18% less powerful than it is today. Even the rocks under the microbes were fairly stable. Modern plate tectonics, which is the movement of rock plates on top of Earth's mantle, hadn't gotten started yet. The mantle was just too hot, so it bound the continental plates together into a supercontinent. And the plates were flimsy and thin. Because this time in Earth's history was so stable, geologists once called it the Boring Billion-- the billion years from about 1.8 billion to 800 million years ago. But the fact is, this period was anything but boring. In fact, it set the stage for our modern version of plate tectonics and probably for the rise of life as we know it.

(Describer) Title: Eons.

Today, Earth's plate tectonic cycle is active. The plates move around, forming mountains as they smash together, shallow seas when they rift apart, and even volcanoes when they slide under each other. Plate movement is responsible for some of Earth's most noticeable features, including the Himalayas, the East African Rift Valley, and the Pacific Ring of Fire. They're so important that it's almost hard to imagine what the planet looked like before plate tectonics. To figure that out, we have to go back to when Earth was first settling down from its formation, back to the end of the Archaean Eon, almost three billion years ago. Unfortunately for geologists, a lot of the rock record has been continuously recycled and destroyed through subduction, where one slab of rock slides under another and dives down into the mantle, where it melts. That means evidence of the oldest rocks is really limited because they've been ripped apart, smashed together, and eroded away. But the Archaean rocks that did manage to survive on Earth's surface are spread out all over the world in places like North America, Australia, Africa, and Eastern Europe. And some are more than three billion years old. And that presented a puzzle to geologists who wanted to find out when modern plate tectonics started, and what it looked like. Since subduction is one of the main drivers behind today's plate tectonics, they looked for evidence of subduction as a sign of the beginning of ancient plate tectonics. So scientists slowly pieced together evidence from the few surviving rock slabs as well as computer models to start to understand the Archaean. You see, geologists can look at minerals in a rock to figure out how deep a piece of land went into the Earth's interior, and how hot it got. And they found that starting around 2.78 billion years ago in the late Archaean, there is solid evidence that the lithosphere was busy. There were super continents breaking apart and forming, mountain-building episodes, and metamorphism, where rocks are transformed by high heat and pressure. Now, that certainly sounds a lot like plate tectonics. But does it really count as the first example of plate tectonics? Well, geologists have opinions about that. It didn't work the same way as the modern version of plate tectonics because Earth's mantle was hotter than it is today-- about 250 degrees Celsius hotter. The mantle reached peak temperatures in the Archaean and has been slowly cooling since then. So 2.8 billion years ago, the mantle was still holding on to more heat than it is today. Those high mantle temperatures made the crust thin, weak, and easy to deform, like cookies straight out of the oven. The metamorphic rock record shows that if there was subduction into the mantle, the sinking crust stayed pretty shallow, unlike today. It was a different flavor of subduction. That's why some geologists don't think this really counts as the first evidence of plate tectonics. But these early movements of the lithosphere helped separate the crust into plates. Those plates crammed together in one spot, leading to the formation of the supercontinent Nuna by 1.8 billion years ago. Welcome to the so-called Boring Billion.

(Describer) Nuna is shown on a globe.

Remember, all of this was over a billion years before the Cambrian explosion. So, with the exception of some microbes, there was no life on land. It was confined to the ocean. And the ocean was very different than it is now. Two billion years ago, it was likely that most of the ocean was very low in oxygen. but what it did have in abundance was hydrogen sulfide. And by 1.6 billion years ago, that combination created a condition known as euxinia, which is toxic to most eukaryotes-- organisms that have an enclosed nucleus in each cell. But the prokaryotes-- simpler life forms that don't have an enclosed nucleus-- made the best of their strange environment. Microscopic life forms like archaea were perfectly happy, and so were bacteria like cyanobacteria, which were photosynthetic and could metabolize the abundant sulfur in the ocean. Also, there were purple and green sulfur bacteria, which are brightly-colored photosynthetic microbes that can form squishy mats in aquatic environments. They sound like they belong in a sci-fi movie, but they're very real and still around today. So most of the life found in the Boring Billion was prokaryotic. And with all this sulfur in the environment, researchers think this period was the stinkiest time on Earth. But to geobiologists-- scientists who study the interactions between the biosphere and Earth's physical processes-- this period was never boring because it marked the beginning of complex eukaryotic life. For example, in China, there are rock formations that contain fossils of eukaryotes that date back 1.7 billion to 1.4 billion years ago. The majority of these organisms lived in water, like protists and other early eukaryotes. So life on the early supercontinent with limited plate movement was doing okay, but it was still all microbes and slimy mats trying to survive in sulfur-rich water. So how did we get from there to here or even to the Cambrian explosion? Well, life needed Earth to shake things up, and that didn't happen in the Boring Billion-- at least not enough to make a huge difference. The old flavor of softer, squishier plate tectonics continued with minor shallow subductions around Nuna and the next supercontinent, Rodinia. Plates on the outside of the supercontinent were mostly stagnant but started sinking into the mantle, which was beginning to cool. Then probably no later than 750 million years ago, during the breakup of Rodinia, the cooler mantle meant that the plates weren't continuously melting and sticking together. Separate slabs of rock could interact, forming rift valleys and subduction zones. Some geologists say this was the beginning of modern plate tectonics because they have clear evidence of deep subduction. For example, they found metamorphic minerals that could only have formed at high pressure deep in Earth's mantle. And that would have been impossible under the earlier type of plate tectonics, when the plates stayed shallower and softer. So experts are still debating whether the earlier period of subduction really counts as the beginning of plate tectonics. And some say there was even older evidence of tectonic movement as far back as four billion years ago. But there's one thing that they all agree on-- plate tectonics helped shape the planet into the habitable world we know today. As supercontinents like Rodinia broke up into separate plates, the slabs jostled each other, smashing together or moving apart. When two plates separate from each other, oceanic ridges form, which are underwater mountain ranges where hot magma constantly comes out and cools, becoming part of the plates. This process is called seafloor spreading. Today seafloor spreading is occurring in several ocean basins. And oceanic ridges are some of the best places to find hydrothermal activity, where the water interacts with hot, fresh ocean crust. These vents are hotspots for biodiversity, especially because they are so rich in iron and silica-- important fertilizers for many life forms. And moving plates can constantly create new habitats and destroy others, which promotes rapid diversification of life. Researchers have shown that biodiversity increases really fast when there's more continental fragmentation. Plus, the arrangement of continents can impact ocean circulation, climate, carbon cycling, and many of the other processes that help shape life on Earth. So it turns out that the Boring Billion wasn't really boring at all. Earth was just settling down and getting ready for its next big move. And today we're the only planet known to have this type of plate tectonics. So, while the movement of continental plates can be destructive, it might have also been pretty important for pushing life past the squishy microbe stage to create the lush, complex diversity of living things that we know today. Accessibility provided by the U.S. Department of Education.

(Describer) Titles: Kallie Moore: Content Consultant, Host PBS