20: Primordial Soup

Episode 20: Primordial Soup

Since Episode 16, we’ve tackled one of the most important topics in Earth history: the origins of life. We started by introducing life’s raw ingredients: carbon and hydrogen. These elements were mixed together in the cosmic kitchen into sugars and organic acids, slightly more complicated shapes, but definitely not alive. These sweet and savory ingredients were then delivered by asteroids to the Earth’s surface.

Today, we return back to our Hadean homeworld 4 billion years ago, January to February on the Earth Calendar. We haven’t made life yet, but we’ve finally gathered the right materials together. All we need to do is to stir the pot.

Part 1: Darwin’s Dilemma

One of my goals on this podcast is to introduce you to unsung science heroes- folks who might not be household names. Folks like Inge Lehmann discovering the inner core or Marie Tharp mapping the seafloor. But there are some names that are famous for a reason, and our next guest is someone who needs no introduction- Charles Darwin.

To make a long well-known story short, as Darwin traveled around the world, he looked at many different animals, living and fossilized. Over time, Darwin developed a revolutionary idea.

In any group of living things like finches, turtles, or moths, there are differences between individuals- just like there are differences between you and your friends. In nature, such differences mean life or death- a warm furry coat in the winter camouflage to hide from predators. Creatures that live longer are more likely to have offspring with similar traits. Over time, changes start to pile up, turning animals and plants into new forms unrecognizable to their ancestors.

Darwin observed these trends in nature and in our backyards. If wolves could be shaped into Chihuahuas and Beagles over a few centuries, imagine what could happen over millions of years. Many ancient fossils like mammoths or saber-tooths were similar to modern animals, but still different and distinctly dead, supporting Darwin’s ideas.

This idea is evolution in a nutshell, the great dance between life and its’ surrounding habitats. Darwin sat on his thoughts for a long time, then published his classic book in 1859: On the Origin of Species. It’s hard to overstate the book’s importance in its’ time, and today.

If you read the book cover to cover, you’ll notice one big missing link. Darwin never says how life started in the first place. He explains how life changes over time, and discusses many fossil animals, but he can only go back in time so far. To see why, let’s zoom out and look at the entire Earth Calendar, something we haven’t done since Episode 1.

Earth is 4.6 billion years old. If we squeeze that massive timeframe into one Calendar Year, when do Darwin’s fossils show up? Let’s start young and work our way back. Ice age critters like mammoths, sabre-tooths, and ancient humans don’t show up until December 31st, just a few million years ago.

Well, dinosaurs must be older than that, right? Yes, but not by much, only December 14th. Most of Earth history is far older than the dinos. Hmm, how about those weird sea fossils, trilobites, sea scorpions, and armored squids? Darwin examined many oceanic oddities, and the oldest were around 500 million years old, a time called the Cambrian. That’s nothing to sneeze at, but the Cambrian still only late November on the Calendar.

The word “Cambrian” comes from Latin for “Welsh”, since Wales was where British geologists found what they considered the oldest fossils.

When Darwin and other scientists dug older and deeper down, below the Cambrian, they found… nothing. There were no bones, no leaves, no shells, nothing they could identify as a fossil. In their eyes, it looked like the ancient Earth was dead, then suddenly exploded with life all at once. The vast blank period before was hardly worth a name at all. Since the rocks were older than the Cambrian, everyone just called them the Precambrian, a name which has stuck to this day.

We now know that there are many Precambrian fossils, they just look nothing like life that Darwin would recognize. But at the time, this huge gap was a head-scratcher. In his book, Darwin guessed there were older creatures, but he hadn’t found the right rocks yet, and he completely avoided the topic how life started. This omission did not go un-noticed by readers at the time, and it was a big criticism- both from folks who opposed evolution, and from other scientists who were hungry for more answers.

Darwin had thoughts about the origins of life, but we only see them in personal letters. He guessed that in Earth’s earliest days, there was a “warm little pond”, where all the right materials and energy combined to kickstart life- water, heat, chemicals. The idea of a “warm little pond” hung around for nearly a century, evolving into the now-famous phrase “primordial soup”.

Eventually, people grew tired of talking about the soup and decided to make some for themselves. Our next story starts with one man and a laboratory filled with boiling beakers, a labyrinth of glass tubes, and an electric spark. The man is Stanley Miller, and his goal is to create organic molecules from scratch.

Part 2: Lightning in a Bottle

Stanley Miller was born in 1930, in Oakland, California. His mother was a teacher, and Miller quickly developed a knack for chemistry. But as a PhD student in Chicago, Miller was adrift. He bounced between advisers, trying to find an interesting thesis topic. Eventually in 1951, he heard a talk by Harold Urey, a world-famous chemist. Urey was a Nobel Prize-winner and a member of the Manhattan Project, developing the first nuclear weapons. By 1951, Urey was interested in organic molecules, and how they formed on the early Earth. In essence, he was searching for Darwin’s little pond.

Miller was hooked, and persuaded Urey to advise his PhD. Instead of relying on old rocks, Miller wanted to make an experiment recreating a Hadean habitat in the lab. His new adviser Urey wasn’t convinced. People had tried that before and failed.

Stanley Miller standing next to his experiment

First, Miller created an environment completely sealed off from the outside world. The early Earth had very, very little oxygen, so the smallest leak in your beakers would ruin the experiment. Next, you had to just guess what was in the Hadean atmosphere. Miller used a witch’s brew of gases: hydrogen, ammonia, methane, and of course, water. These gases provide the raw materials needed to build organic molecules: carbon, hydrogen, and some nitrogen for flavor. It wasn’t perfect, but it was better than not knowing.

Scientists had mixed the same recipe before without any big success. So Miller added a new ingredient: lightning. Or more accurately, a spark between two electrodes inside the sealed flask. The electricity simulated lightning in the Hadean atmosphere.

This sounds more and more like a mad scientist’s experiment, but there was a method to Miller’s madness. It takes energy to make complex things from simple ones. A pile of flour and sugar can’t become a cake without heat and stirring. On the ancient Earth, Miller proposed lightning as the literal spark to jumpstart life.

Inside the sealed flask, the spark flashed, the strange gases glowed, and a small pool of water boiled below. After one day, the water turned from clear to light pink. After a week, it was blood red and murky. Clearly something new had been made from air, water, and lightning.

Miller’s new potion was not something you would want to drink. The cocktail included toxic organic molecules like formaldehyde and cyanide. But hidden within were also amino acids, the building blocks of meat and your muscles. To put it another way, Miller’s experiment took gases that were only two to four atoms large and welded them together into Frankenstein molecules more than a dozen atoms long. From small things, big things grow.

The Miller-Urey experiment was an instant success and is still very famous- Carl Sagan discussed it in the original Cosmos series. Other scientists quickly tweaked the original recipe to make a buffet of sugars, acids, and fats from thin Hadean air.

But remember, Miller had to guess what that air was made of. 70 years later, did he get it right?

In other words, did Miller stumble on the right recipe with the wrong ingredients?

Part 3: Out of Thin Air

We don’t know what was in Earth’s atmosphere 4 billion years ago, but we can make some educated guesses, and the story has changed since Miller’s experiment.

To be fair, Miller wasn’t completely off base- his mixture of methane, ammonia, and water does make sense for an infant planet. However, the mixture probably didn’t last very long- only a day or two on the Earth Calendar. Methane and ammonia are fragile gases, easily broken apart by ultraviolet light- the same UV that gives you a nasty sunburn. Without a protective ozone layer, UV light quickly fried Earth’s first atmosphere away.

At the same time, new gases were pumped in from an unlikely source: volcanos. Every eruption makes flashy lava and ash, but also releases an invisible gas cloud. One volcanic gas is pure nitrogen, N2. Nitrogen is the most common gas in the modern atmosphere- every breath you take is 80% nitrogen, but it doesn’t affect your body at all.  

Another important volcanic gas is far more famous and infamous: carbon dioxide, CO2. CO2 is a simple gas with a long and complicated history. Since we’re meeting CO2 for the first time, let’s learn how it comes and goes. In a later episode, we’ll cover how CO2 warms the planet.

You, the listener, breathe out CO2 every minute, every day. You’re doing it right now! Don’t try to hide it from me. Other living things breathe in CO2 to build their bodies, like plants, plankton, and some bacteria. They breathe in, we breathe out. Consider this the meditative balance of carbon dioxide, gently rising and falling like waves on a beach.

In addition to the visible ash, this volcano is also belching out lots of CO2

Now volcanos also “breathe out” lots of CO2. Some do it in massive belches, others in slow quiet leaks. Every year, volcanos release 300 million tons of CO2 into the air. Something that big and invisible is hard to visualize, so imagine this gas filling a party balloon four miles wide. If it sat on the ground before you, it would stretch from horizon to horizon, as tall as a mountain. That’s the yearly budget of volcanic CO2.

Now you might be thinking: “Wait a minute, if volcanos fart out that much CO2 every year, why the heck are we worried about what humans make? CO2’s a natural part of the world, stop whining!”

Well, that’s not the whole story. Take a deep breath- I’m not telling you to calm down, I’m reminding you that CO2 is all about balance. Just like plants breathe in what we breathe out, the Earth does the same thing. As volcanos breathe out CO2, rocks around the world breathe it back in. More accurately, minerals mingle and react with carbon dioxide, slowly removing it from the atmosphere. It’s a very different process from what plants do. The best crystals for the job are, ironically, green. It’s our old friend from Episode 4: olivine. If you want to learn more about olivine as a CO2 sponge, check out my interview with Dr. Ella Holme.

To recap: plants breathe in what animals breathe out. Olivine breathes in what volcanos breathe out. The balance does shift, but it takes a long time. For example, the world has slowly cooled as olivine breathes in more CO2 than volcanos breathe out. That change took 4 days on the Earth Calendar, 50 million years. Now let’s talk about humans.

When humans burn coal or oil, they release CO2 into the air. That’s a simple fact. How much do we fart out compared with volcanos? Let’s return to our giant balloons. Every year, volcanos breathe out a CO2 balloon 4 miles wide and tall. The Earth breathes all this gas back in, no problem. The balloon that humans make every year would be 20 miles wide, bigger than Manhattan, much taller than Everest. In other words, each year, we make 100x more CO2 than all volcanos combined.

Well, why doesn’t the Earth just take care of our mess? It breathes in everything else, right? It does, but we’re pushing its’ limits. Each year, the Earth breathes in almost all the CO2 we release, but there’s a little extra leftover that lingers. We’re simply been burning it way too fast. In the past 200 years, we’ve pushed carbon dioxide to levels the Earth hasn’t seen in 5 million years, when Antarctica had no ice sheets. In other words, we’ve undone two days of the Earth Calendar in one second. Even if you don’t know what CO2 does to our world, that’s an unsettling idea.

I know that was a sobering side-trip, but I’ve heard folks say human activity is just a drop in the bucket compared to volcanos. Now you know that’s not true, and we should really give ourselves more credit. Whether we like it or not, humans are a geologic force, the only one that can think for itself. But that only works when we have the right information.

To finish, let’s return a simpler time: the Hadean. The strange atmosphere in Miller’s experiment was quickly replaced by CO2 and nitrogen from volcanos. Does this mean that everything he did was wrong? Fear not, the new atmosphere still held the ingredients to make organic molecules. The carbon comes from CO2, and hydrogen from our old friend water, H2O.

With this knowledge in hand, scientists have tweaked and repeated the Miller-Urey experiment many times. In most runs, simple gases are transformed into the stuff of life: sugars and organic acids. In other words, Miller’s recipe might have been off, but there are many ways to make primordial soup. And yet, I can’t stress enough: the soup is still not alive, just like sugar or butter or actual soup isn’t alive. To make life as we know it, we need to add a new ingredient, one that makes fats and proteins look like tinker toys. It’s time to start building DNA.

Summary:

To build life on Earth, you need raw materials and energy. The materials were supplied by asteroids and volcanoes, while the energy came from lightning strikes and chemical reactions. More than 4 billion years ago, all these conditions came together in oceans and “warm little ponds”. Scientists can recreate these ancient environments in laboratories, tiny windows on an ancient world. Inside, we see simple molecules energized into more complicated shapes: proteins, fats, and carbs. With each spark, we’re getting a little closer to a living organism.

Next episode, we start making even more complex materials from this soup, the DNA inside every living thing on Earth. It’s time to crack life’s code.