Around 2 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 landmasses.
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, 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. Today, Earth’s plate tectonics 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 at the end of the Archean
Eon, almost 3 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 Archean 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 3 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 Archean. You see, geologists can look at minerals in
a rock to figure out how deep a piece of land went into Earth’s interior, and how hot
it got. And they found that starting around 2.78 billion
years ago, in the late Archean, there is solid evidence that Earth’s lithosphere was busy. There were supercontinents 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
Archean, and has been slowly cooling since then. 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. 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. 2 billion years ago, it’s 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 scifi 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,
including 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 subduction 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’ve found metamorphic minerals
that could only have formed at high pressure, deep in Earth’s mantle. And that would’ve 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 that there was even older evidence of tectonic movement, as far back as 4 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 - 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 occuring in several
ocean basins, and oceanic ridges are some of the best places to find hydrothermal activity,
where 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 the 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’ve 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. If you’re a fan of Eons, then you’ll love
the new PBS three-part natural history series: Prehistoric Road Trip! Get ready for an epic adventure through dinosaur
country to discover the mysterious creatures and bizarre ecosystems that have shaped Earth
as we know it. With popular YouTube personality Emily Graslie as host and guide, you’ll
travel thousands of miles to visit some of the most active and dynamic fossil sites in
the world. Prehistoric Road Trip premieres Wednesdays,
June 17th-July 1st at 10/9c. Streaming is available across platforms, including pbs.org
and the video app – find out more at the link in the description. No-so-boring high fives to this month’s
Eontologists: Lucan Curtis-Mahoney, Sean Dennis, Jake Hart, Jon Davison Ng, Patrick Seifert,
and Steve! Become an Eonite at pateron.com/eons and get exclusive access to our Discord and
the Eonites Only Podcast - hosted by me! And thank you for joining me today in the
Konstantin Haase studio. Subscribe at youtube.com/eons for more adventures in deep time.
I'm doing my Masters right now on literally exactly this. Watching this video was sort of surreal. I'll be sending this video to friends and family who ask what I'm studying.
That said, I disagree with modern-style plate tectonics starting in the Neoproterozoic for a bunch of reasons.
My dad was a geology professor and he met J Tuzo Wilson in the late 1960s. With that said I have met J Harlen Bretz who was awarded the Penrose metal. Eastern Washington Badlands Etc?
I see PBS still has their director of gesticulation. Once you see it, you can't unsee it. This is why I really can't watch these videos.
I wrote a review essay on this very subject just a couple of months ago, arguing that plate tectonics began in the Neoproterozoic. This is a fairly controversial idea, so it's cool to see PBS agreeing