19: The Hitchhiker’s Guide to the Galaxy

First, I want to thank all you listeners, old and new, for staying tuned in. Last month the show’s audience doubled in size- I honestly can’t believe it. Folks have been sending in fan mail, and our Spotify reviews are staying high at 4.9 out of 5 stars. For those who have reviewed, your effort is paying off. If you haven’t clicked the stars yet, it really does help the show grow. Speaking of growth.

For the past three episodes, we’ve been slowly looking at the origins of life, brick by brick. We haven’t seen the first life yet, but we’re getting closer. Last episode, we saw the first and smallest building blocks forming in outer space, bits of carbon and hydrogen assembled by exploding stars. These tiny organic molecules are impressive in the bleak dark void, but they’re not as sophisticated as bacteria, or even the butter in your fridge. We’re still a long way off from anything we could call alive. 

At this point in the story, 4 billion years ago, we’re also long ways off from the infant Earth. But don’t panic, there’s an easy way to get back. Today, we’ll hitch a ride home with some old friends from earlier this season, and just maybe we’ll meet an alien or two along the way.   

Part 1: Gifts from the Stars

Last episode, we peered into the depths of space to search for organic molecules, the basic ingredients of life. Some of these molecules are very simple, just one carbon atom stuck to a hydrogen. Some are more complex, forming daisy chains with strange names. But what about the actual building blocks of life, the proteins, carbs, and fats in our bodies and food? Are they floating around in space?

Yes and no. The best answer is this: we have found the pieces of proteins, carbs, and fats in space.

In Episode 17, we learned that complex carbs like bread can be stripped into smaller sugars. We also split proteins into amino acids like the infamous tryptophan in turkey meat. When we sift through starlight, we see the same amino acids, sugars, and the building blocks of fat in deep space. You can’t make a sandwich out of them, but we’re getting there.

How do we bring these little guys down to Earth? Like everything else in the Hadean chapter, more than 4 billion years ago, the best delivery service is by asteroid.

Since the beginning of this season, we keep running into special meteorites called carbonaceous chondrites. These chondrites helped build the earth in Episode 4, and Earth’s oceans in Episode 14. Every time you drink a glass of water, you’re drinking meteorite juice.  

Without the gifts of stone and water, there would be no Earth, no blue planet. But there’s one final gift from these meteorites, the most precious of all. It’s been staring us in the face this whole time. These rocks are called carbonaceous chondrites. Without them, no carbon. Without carbon, no life as we know it.

The Allende Meteorite from Episode 4, a classic carbonaceous chondrite.

So what carbon can we find inside a carbonaceous chondrite? When we crack open these meteorites, we find the building blocks of life: the same characters bumming around space and our dinner tables. For example, the Murchison meteorite from Episode 4 has the amino acids glycine and glutamic acid. Don’t worry, they’re not on the exam, but they are on the menu. Glycine can be used as a sweetener, and glutamic acid gives meat, mushrooms, and soy sauce their savory umami flavor.

I wouldn’t recommend licking a meteorite, though. These ingredients are just barely sprinkled in. But as the Earth was slowly built by trillions of asteroids, this organic matter started to build up, enough to form all the carbon-rich life we see today. It’s the same slow process that built Earth’s oceans from tiny crystals of ice. From small things, big things grow.

To be crystal clear, none of these organic molecules are alive. They were not alive when they were forged in starlight, they weren’t alive when they hitched a ride on asteroids, and they weren’t suddenly made alive when they crashed down to Earth. At no point have we mentioned anything about aliens on this show.

So, let’s talk about aliens.

Part 2: The Martian

The story starts like an old science fiction movie. The year is 1984- Madonna has released “Like a Virgin”, and Apple has just made its’ first personal computer. The place, the Allan Hills of Antarctica, where a few researchers are walking around in bulky winter coats. This is one of the few spots where naked rock juts out of the frozen continent, but the research team here isn’t looking for Earth rocks- they’re looking for meteorites. It turns out, if you find a lonely rock sitting on top of an Antarctic glacier, it probably crashed from above. As a side note, the first person to discover this was Sir Douglas Mawson in 1911, on the same trip where he was stranded alone for months on the ice. To learn more, check out our miniseries episode on Antarctica.

On December 27, just two days after Christmas, the researchers get a belated present- a lumpy dark meteorite the size of a bread loaf, weighing 2 kilos or 4 pounds. The sample was simply labeled Allan Hills 84001. Unlike most of the meteorites we’ve met, this one wasn’t a chondrite from the early solar system. It wasn’t even an iron-rich meteorite like the ones from Episode 5. A quick look at the chemistry screamed that this sample came from Mars.

Now, Mars doesn’t just decide to spit out rocks in our direction, like sunflower seeds. They’re usually kicked out of their planet’s orbit by other meteor impacts. Sometimes, these pieces happen to crash land on Earth.

The Allan Hills meteorite was formed on Mars 4.1 billion years ago, February 8th on the Earth Calendar. That’s older than any rock found on Earth. It hasn’t been sitting on Antarctica for that long, it only landed on Earth a few thousand years ago. And that was all people knew for 12 years, until 1996. Madonna has released “Don’t Cry For Me, Argentina”, Apple has moved on to laptop computers, and in a NASA laboratory, David McKay is staring at an impossible picture. If what McKay sees is true, this picture would be perhaps the most important in history. If what he sees is true, he has found the first tangible evidence for life beyond Earth.

The structures on meteorite ALH8001: striking, but probably not fossils

The picture is black and white, and shows what looks like a rough, shadowy, boulder-strewn landscape. In fact, the image was taken by a microscope- a human hair would look like a fallen redwood tree. In the very middle of the picture sits a long, thin cylinder, divided by a series of thin rings at regular intervals. For all the world, it looks like a worm, a caterpillar with no legs, or a striped pool noodle. To McKay, it looks like bacteria, which can have very similar shapes. The noodle isn’t moving, it isn’t alive- but it could be an ancient Martian fossil.

The more he looks, the more he sees these tiny noodles- it’s not just a fluke. Now, finding bacteria on meteorites is nothing new. Remember, this rock had already been sitting on Earth for thousands of years, plenty of time for our own life to colonize it. But these noodles were embedded in the meteorite- they were part of it, not simply new growth. Finally, there was one other weird thing about the noodles- they were the same shape as Earth bacteria, but they were much smaller, which is really saying something. A human hair is around 100 microns wide. An average bacteria is 1 micron wide. The tiny Allan Hills noodles were 10 times smaller than that- no bacteria that size has ever been found.

The discovery was published in the top scientific journals, the news made international headlines, and even the U.S. President, Bill Clinton, made an announcement to the world.

So what happened? Why wasn’t this the smoking gun for aliens? Why are we still alone in the universe? Clinton was more right than he knew when he said this rock posed more questions than it answered.

In short, there are many ways to make tiny noodles that don’t require life at all. Minerals and other materials can make shapes that look exactly like bacterial cells. Under the microscope, they look like orbs, long stringy hairs, noodles, and even more complex shapes, like tiny leaves. These shapes are called biomorphs, since they look like living things, but aren’t living.

I can personally sympathize with this problem- the search for alien life is very similar to the search for ancient life. When I find a weird rock, I have to approach it with a very skeptical mindset, testing whether any shape was caused by life or a biomorph. To learn more about these “fossil imposters”, check out my interview with Dr. Joti Rouillard, a biomorph expert.

As of 2023, most scientists do not believe the Allan Hills meteorite contains fossils. I’m not telling you this story to ridicule McKay or his colleagues. Their findings are part of a much longer conversation about what life is, how it started, and where it started, a conversation that goes back to ancient Greece.

Part 3: The Truth is Down Here

Before Aristotle, before Plato, before Socrates, there were many other, less well-known philosophers. One of them was Anaxagoras, who lived in Athens around 460 BCE, after Greece had fought off Persia, and before it would tear itself apart with internal strife.

Anaxagoras, the father of the panspermia idea

Like many ancient Greek philosophers, Anaxagoras had some ideas that were well ahead of their time, and others that were very far off. They were using basic observations to answer very complicated questions. For example, Anaxagoras thought that earthquakes happened by disturbances beneath the earth (which is right), but that the Earth was flat (which is wrong).

Anaxagoras’ passion was astronomy, the man was fascinated by space and the chance that life was somewhere out there. Anaxagoras correctly interpreted the patterns on the moon as mountains and valleys. However, he went a little too far and said if there are mountains, there must also be life up there. In the end, the idea that the moon and sun were worlds and not gods was too much for ancient Athens to handle, and Anaxagoras was exiled across the sea.

While most of Anaxagoras’ ideas have been proven right or wrong, there is an interesting thought experiment that is still debated more than two thousand years later. This idea is called panspermia, and it goes something like this. According to Anaxagoras, life first evolved on another planet and was carried to Earth a long time ago. Even today, modern science-fiction stories weave epic tales of alien spaceships bringing life to Earth like seeds for a garden. In real science, there is absolutely no evidence for advanced aliens sowing the Earth.

But a slightly more plausible idea has cropped up in the last century. Just like sugars and amino acids, perhaps alien life hitchhiked on asteroids from on world to another, including our own. The idea isn’t completely out to lunch. Amazingly, Earth bacteria can survive at least three years in outer space- just add water, and they wake up again. Still, panspermia is much easier said than done- the journey requires life to survive being launched into space, spend many years on the edge of death, then make a boiling, supersonic crash landing on Earth. It’s a big universe out there with many strange things, but those are still not great odds.

If you can’t tell, I’m not a huge fan of panspermia. I’m not saying it’s impossible. I’m not saying we shouldn’t think about it. My biggest issue is panspermia tries to tell us where life started, but not how it started. Think of it like this: if you ask a mechanic how your car was built, how the pieces work together, and they say it was made in Detroit or Japan, that doesn’t answer your question, does it?

In the same way, panspermia just moves the question of the earliest life somewhere else, a long time ago in a galaxy far, far away. As we’ve seen, it’s hard enough to figure out what Earth was like billions of years ago, and we live here. I think our time is better spent learning more about our world first. There’s one way I think panspermia could win out- if, and only if, we prove that life could not have evolved on the early Earth. If there’s no way the spark of life was possible in the Hadean, then maybe we can turn to other worlds for support. As we move forward in the show, we will put these ideas to the test.

Summary:

Organic molecules were delivered to the early Earth by carbonaceous chondrites, the same asteroids that built our world and its’ oceans. Many of these molecules can be found in asteroids today, as well as our own bodies and food. In contrast, alien life has not been found clinging to asteroids, though it is theoretically possible, and many people have tried to find it. Until that day arrives, we are forced to search for life’s origins in our own backyard.

Next episode, we learn how Earth took its’ organic gifts from the stars, and started turning them into the first living things.

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