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As a SciShow viewer, you can keep building your STEM  skills with a 30 day free trial and 20% off an annual premium  subscription at Brilliant.org/SciShow. [♪ INTRO] In 1982, scientists set sail aboard a ship  with a giant drill stuck on the back of it. Their goal?

To extract columns of seafloor,  and let layers of sediment  tell a geologic bedtime story. But nestled within a layer  around 66 million years old,  they found something completely unexpected. They found a fossil, just two  and a half millimeters across.

But it wasn’t one of the last dinosaurs,   or any of the other life forms wiped out  when a massive space rock hit the Earth. It was a fossil of a rock!  A rock that came from space.  It could be a fragment of that deadly meteorite. And as one of the first fossilized  meteorites ever discovered,   it has shown us a new way to  learn about our planet’s past.

Now I know what you're thinking,  because it’s what I was thinking:   How can a rock become a fossil? Aren't fossils  things that were once alive and became rocks? Well, that is one definition.  But fossil meteorites go through   basically the same process as dinosaur bones.

After getting stuck somewhere  wet, like the bottom of an ocean,   they get buried under layer  after layer of sediment. Then, water can wear away and react  with the meteorite to remove some,   if not all of the original, primary minerals and  deposit new, secondary minerals in their place. Now, if you weren’t a rock at the beginning of that process, congratulations!

You are a rock now. And if you started out as  a space rock, well, now you   might seem indistinguishable from an Earth rock. When scientists dig up a bone-shaped rock,   it’s a bit obvious that it once  belonged to a living thing.

But how can they tell a rock-shaped rock  is a fossil that came from outer space? Well, it turns out there  are several kinds of clues. First off, there are some primary minerals  that are both only found in meteorites,   and the fossilization process has  a really hard time clearing out.

Scientists tend to hunt for certain  kinds of spinel in particular. And if you started off as an iron meteorite,   your extraterrestrial origins can be revealed by  a unique texture called a widmanstätten pattern. This pattern is created by  cooling iron exceptionally   slowly in the core of an ancient planet.

And while the iron in fossil  meteorites can be pretty badly rusted,   this telltale pattern is sometimes still visible. But to test that fragment of rock  pulled up from the seafloor…the one   that could have killed the non-avian dinosaurs…  scientists compared the amount of individual  elements against known space and Earth rocks. According to their measurements,  the amount of chromium, iron,   and iridium were very close  to modern meteorite samples.

Meanwhile, it had a lot less cobalt and nickel,   but about a thousand times more gold  than they would expect in a meteorite. These were all probably changes that  occurred during the fossilization process. So we can be pretty sure that this rock came from  space, and we know roughly when it arrived   on Earth, but we can’t say for certain it was  part of The Rock that caused a mass extinction.

It could have come from a much smaller impactor,   and just happened to hit our  planet around the same time. To draw any real conclusions, we’ll have to find  more fragments with a similar age and composition,   and preferably in completely different locations. That would indicate that an extreme  impact had flung them far apart.

But finding fossil meteorites  is a lot easier said than done.  When we say fossil meteorites  are rare, we mean really rare. In fact, in the early 20th century, some  geologists debated if they even existed. They thought meteorites were only a recent thing,   because they just hadn’t found any  that fell millions of years ago.   The first confirmed fossil  meteorite was found in 1952,   but it wasn’t until 1979 that scientists actually  took a closer look and figured out what it was.

As of 2017, we have found 115 fossil meteorites,   and the majority have come from a single place  in

Sweden: the Thorsberg limestone quarry. The limestone here is a treasure trove of fossil  meteorites because it formed unusually slowly. Beginning as an ocean floor over 470 million  years ago, layers of sediment built up about   10 times slower than usual…only a  few millimeters every thousand years. That meant there was more time for  meteorites to fall into the ocean   and get caught in the growing limestone layers.

But that doesn’t just make Thorsberg quarry  great for finding cool, unusual rocks. Together, these fossil meteorites tell the story  of a massive collision that took place in our   solar system’s asteroid belt between Mars  and Jupiter roughly 470 million years ago. And by massive, I mean the largest asteroid belt   crash that we know of from  the past 3 billion years.

Some of the debris quickly made its way to  Earth, causing a storm of space rocks to   rain down on the planet over 200 million  years before the dinosaurs showed up. Animal life on Earth was still  sticking to the oceans at that point,   including an ocean that would become  a Swedish limestone quarry one day. But that space rock storm  actually hasn’t stopped, yet.

Around a quarter of the meteorites  falling today have the same composition,   so we’re pretty sure they  are remnants of that event. And based on the number of fossil  meteorites we’ve found so far,   the impact rates started  off around 100 times higher. In fact, some scientists believe this ancient   barrage could actually have  given evolution a helping hand.

According to the fossil record, we  know there was a big acceleration   in the evolution of marine  life right around that time. It’s called the Great Ordovician  Biodiversification Event So one hypothesis is that after  this massive outer space collision,   the resulting space dust could have changed  Earth’s climate by blocking the sun. Or, a surge in impacts could have caused some  species to go extinct a bit more…directly.

This could have opened up space in the ancient  ocean ecosystem for new creatures to emerge. You know, let nature test out a bunch of new  evolutionary forms to fill some empty niches. Not everyone agrees about this, though.

One 2017 study suggests the time  between the barrage of impacts   and the burst of evolution isn’t close  enough for the two events to be related. So science is still working out the kinks, here. Now, because of the wealth of scientific  knowledge that fossil meteorites can bring,   Thorsberg’s quarry workers are  trained to identify potential fossils.

They’ve been methodically searching  in their worksite since 1995. But that’s not all. One scientist even made it a habit of crawling around in public buildings with limestone floors in Sweden, searching for  ones that slipped out of the quarry unnoticed.

In years of searching, he  claims to have found only one,   embedded in the floor beneath  the staircase of a train station. He will not, however, tell  anyone exactly where it is. So while your chances of finding  a fossil meteorite might be low, you might want to keep an eye  out the next time you're walking   through a limestone building,  especially if you’re Swedish.

Now, if you did want to calculate your  probability of finding a fossil meteorite,   you might want to start with Brilliant’s  course “Introduction to Probability.” Brilliant is an interactive online  learning platform with thousands of   lessons to choose from in math,  science, and computer science. Their “Introduction to Probability” course teaches   you the rules of probability and how to apply  them without getting bogged down in calculus. You’ll even develop skills that you can  use in your life, like risk management,   apartment hunting, and fraud identification.

You might even find yourself swept   up in the lessons and moving on to casino  probability and more perplexing probability. It all starts when you go to Brilliant.org/SciShow  or click the link in the description down below. You’ll get a free 30 day trial and 20% off  an annual premium Brilliant subscription.

Thank you to Brilliant for supporting   this SciShow video and thanks to you  for watching all the way to the end. Gosh, it was a good one! I want to go to Sweden now! [♪ OUTRO]