This episode, we focus on just one special boulder from the Greenland tundra, a rock that might have Earth's oldest fossils. Over the past 10 years, scientists from around the world have used the latest technology to reveal this rock's secrets, including research published this year. What have they found?

Extra Credit: Make a time capsule or play a record.

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P.S. Thanks to a bad cold, both my voice and the audio quality are not on par today.

P.P.S. The cold has also delayed Episode 51, which will be up later this week.

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Finally, 3.7 billion years ago, we reach some of the strongest evidence for life on Earth yet. It takes a while to find, and it only looks like a few dark crystals, but it's better than anything else we've seen so far. Along the way, we'll meet three scientists who helped define the quest for Earth's oldest fossils, including one of my old bosses.

Extra Credit: Sharpen your pencils, spackle some cracks, or thank a mentor in your life.

In this casual chat, Dylan talks about the show's growth, the upcoming end of Season 2, a potential career shift to science communication, and other podcast recommendations. Thank you all for bringing this show to 50 episodes- here's to 50 more!

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Many folks have claimed to find Earth's oldest fossils in the Greenland rocks, 3.7 billion years old. Before we meet and judge these contenders, we need to learn what counts as a fossil. This early in Earth history, nothing has any bones or teeth or shells, so researchers need to search for other clues. Today, I'll show you four ways to look for fossils in Earth's oldest rocks, including my personal field of study: fossilized bacteria.

Extra Credit: Find different types of organic carbon in your day, or eat a burger.

What is the hardest rock to make?

Today's rock is very common in Earth's ancient past, and it should be everywhere today. However, it's nearly impossible to form in nature or the laboratory. Some experiments have tried for 32 years with no success! This rock is dolomite. Today, we address The Dolomite Problem, one of geology's longest-running mysteries. Along the way, we'll meet some smelly but helpful bacteria, sort a fruit stand, and learn about the cutting edge of geology research.

Extra Credit: Eat some citrus, and check your home for natural gas leaks.

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Today is a tale of two sisters: limestone and dolomite. Limestone is far more popular and well-behaved, one of the most common stones in nature and in architecture. Dolomite, though similar in many ways, is one of the most difficult rocks to make on Earth. Even dolomite’s name is a bone of contention. Today, we introduce both siblings to the show as we move forward in time, 3.75 billion years ago. Along the way, I’ll give you a real experiment to try in the kitchen and we’ll smash some stones together.

Extra Credit: Try today’s kitchen experiment, find some limestone around you, or get some calcium in your diet (dairy, leafy greens, or tofu).

What types of volcanoes were erupting 3.8 billion years ago?

Today, we meet a new type of rock and a new type of volcano, far more powerful and dangerous than those we've met so far. The old volcano is long dead, but the scars it left behind can still be seen across the Greenland landscape. Along the way, we'll learn how different types of lavas are made, why a few rocks can float in water, and learn about a field trip Dylan took to Iceland in 2010.

Extra Credit: Eat a grilled cheese sandwich, blow a bubble in gum, or make campfire.