23: It’s Alive!

The story of humanity begins in the darkest depths of the ocean, more than 4 billion years ago. For countless leagues, the water is cold and barren above a flat, featureless seafloor. Finally, we find a crack in the seabed, with boiling water shooting through chimneys of black rock. The sea here is a rich soup of minerals, proteins, and even DNA’s older sister, RNA. It’s a bustling nursery in the lonely void. In one corner, nestled in a tight crack on the seafloor, are tiny bubbles of fat. Each bubble has its’ own blend of primordial soup- a few have RNA and proteins pushing on their enclosures. Many of these recipes are duds- the ingredients just don’t mesh.

Given enough time, one bubble, one recipe does something special. Proteins start to push the bubble apart from the inside. For a minute, it looks like a suicide mission, like the bubble will burst. Neighboring bubbles are trying similar experiments, with much grislier ends.

But then, just as all looks lost, the proteins squeeze the walls back in, pinching one long bubble into two. After a few minutes, this next generation splits again. One daughter explodes with an unstable recipe. But the other splits in two with ease. If it can keep this up, this bubble could do some incredible things.

It is brief, it is dark, and it is precarious, but this dance marks the first day of life on Earth.

Part 1: Order and Chaos

Welcome to the Season 1 Finale! We’ve finally made it through the Hadean. Since Episode 16, we’ve taken a long and winding road describing life’s origin, but never making life itself… until now. The question “Where did life come from?” takes a while to answer, and now you know why.

First, the conversation needs some background lingo- what is an organic molecule? What exactly are proteins, fats, and DNA? Hopefully, I guided you through without needing an entire dictionary.

Second, the origins of life took a long time with many complicated steps. As they say, Rome was not built in a day, and the same is true for life. The first cell did not just pop out from nothing.

Let’s follow up on that thought for a second. You might have heard that the universe tends towards chaos, entropy, decay, a lack of order over time. In other words, it’s easier to break objects down than build them up. If so, how did life come around, which is so complex and intricate?

Put simply: The energy released from the death of stars helps fuel growth elsewhere

I’m not a physicist, but here’s a simplified answer. While chaos and decay can increase over time, it doesn’t stop local pockets of order and growth for a limited time. Remember way back in Episode 4, we saw the simple force of gravity pulling atoms together to make the Sun. That growth is balanced by a release of heat and energy, decaying across the universe. Eventually, that decay will win, and the sun will die.

On a smaller scale, gravity built the Earth using asteroids, an order which is balanced as Earth releases its’ own heat. Eventually, Earth will also cool and die.

But as stars and planets decay and release energy out, that same energy can build complex objects. In Episode 18, the violent explosions of other stars shoved carbon and hydrogen atoms together, increasing order and building the first organic molecules. In Episode 22, Earth’s released heat into water, making complex proteins and DNA from simple building blocks- small pockets of order in a vast, chaotic universe.

Many people balk when they hear that the origin of life was a “random” event.  I can sympathize, because while this long process wasn’t planned or designed, it also wasn’t a simple coin flip, a straight roll of the dice. It was a logical series of events, cycles of chaos and order. Exploding stars pushed atoms into molecules, which volcanic heat shaped into complex forms. Given enough time, those materials began to react with each other in intricate reactions- the first cells. From small things, big things grow.

We have the broad strokes of this story down, but there are still many questions we could tackle. How did DNA form after RNA? Why didn’t the first cells simply pop away? When exactly did all these steps happen? However, it’s high time we move the story forward. As scientists make new discoveries, I’ll release update episodes to keep you filled in.

For now, let’s start in the modern day, and work our way back to life’s most ancient roots.

Part 2: The Tree of Life

Before we begin, take a second and find the closest living thing around you. Maybe it’s a tree, maybe it’s your dog, maybe it’s a relative.

Let’s start with your human relatives. If you have siblings, you probably share at least one biological parent. In that case, your mom and/or dad is your common ancestor- the most recent point where the family tree splits.

For your cousins, your common ancestors are your grandparents. In other words, you and your cousins have different parents but the same grandparents. When you’re at a family gathering, you’ll meet distant relatives who share your great-grandparents and beyond. If we expand this idea back in time, we see that every human is related to each other- we all share the same family tree. At this point, it’s difficult to pinpoint exactly one person who we all could call “grandmother”. It’s better to think of your ancient ancestors as communities separating from each other.

On the great Earth Calendar, everything I’ve just described, every human who’s ever lived, is crammed into the last 300,000 years, the last half-hour of December 31st. But we can go farther back. Let’s check out your pets, or your favorite animal.

You and your dog/cat also share a common ancestor. There are some family resemblances- you both have hair, can make milk, and store eggs inside the body. 60 million years ago, December 28th on the Calendar, two populations of mammals went their separate ways. One group would start living in trees, becoming primates and us, while the other became predators, hunting and killing us before eventually becoming our best friends.

Charles Darwin’s 1837 sketch of the Tree of Life

We can play the same family game between any two living things on Earth- tigers and tiger sharks, elm beetles and elm trees, bacteria and Bruce Springsteen. If we lay out all these connections, we would make a gigantic family tree stretching over billions of years. In fact, Charles Darwin did exactly this in The Origin of Species- it’s the only picture in the book.

As we rewind in time, there are fewer and fewer branches- fewer living things around. So how far back can we go- when is the beginning, and how do we know?

This brings us back to our new friends from Episode 21- DNA and RNA. As a quick refresher, DNA is shaped like a twisted zipper, while RNA looks like a zipper split in half- only one set of teeth. Yet RNA is arguably even more important than DNA- it helps read the code and turn it into proteins. But RNA isn’t perfect- sometimes it makes a mistake and changes the code. These changes are called mutations.

Mutations are a double-edged sword. Most mutations either harm a living thing or have no real effect- if it ain’t broke, don’t fix it. But sometimes RNA makes a “happy accident” and gives a creature better camouflage or sharper teeth. If the creature lives long enough to have children, the next generation could inherit that mutation. As RNA and DNA make more happy accidents over time, they make completely different living things- different branches on life’s great tree.

By comparing modern DNA with ancient fossils, we can learn when primates and dogs split apart, or when birds split off from dinosaurs. But when we follow the tree down to its’ roots, we run into problems. Earth’s oldest rocks are few and far between, and the rocks that do exist do not preserve fossils well. Finally, the critters that were around- bacteria and their cousins- do not make the best fossils in the first place.

But don’t despair! We can still make educated guesses without fossils. For an example, let’s revisit the split between dogs and primates. Let’s imagine that there are no fossils of dogs or wolves on Earth, none at all. We could still estimate when dogs and primates went their separate ways by comparing DNA. The more differences between codes, the farther back in time they split. This idea is called the molecular clock- as you can imagine, it’s not as precise as using fossils, but a rough window of time is better than having no idea at all.

The window of time for Earth’s earliest life is nearly the entire Hadean, from 4 to 4.5 billion years ago. We really can’t be more specific than January or February on the Earth Calendar. Most scientists place the Moon formation from Episode 8 as a safe early boundary- the planetary collision would likely have erased any earlier versions of life.

But while DNA can’t give us a specific date, it can tell us which genes the first life had. In other words, we can actually paint a decent portrait of our earliest ancestors.

Part 3: Your Greatest Grandmothers

Even though we don’t have fossils of our oldest ancestor, scientists have still given it a name. A human name, in fact, but probably not the ones you’re thinking of. It’s time to meet LUCA.

LUCA is actually an acronym- it means the Last Universal Common Ancestor- L.U.C.A. As we learned last section, a common ancestor is a great-grandmother, someone who gave birth to your family members. The word Universal just means that LUCA is the great-grandmother of all life on Earth today.

But what about that word “Last”? Does that imply that there was life before LUCA, a gap between the first cell and our first ancestor? Yes.

Like any first project, the first cell was a very rough draft, working just well enough to survive and pass DNA to the next generation. As one cell became millions, RNA began making mistakes, mutations that tweaked these daughters. One group of cells did very well for themselves, with just the right toolkit to grow, multiply, and drown out the competition. As far as we know, this is the only group that left survivors. So LUCA wasn’t really one cell, but a community, just like we saw with early humans. For now, I’ll keep referring to LUCA as a single unit.   

Finally, we’ve reached the portrait gallery, the family album for life on Earth. What did LUCA look like? Where did it live, and what did it do? We can answer these questions by looking at DNA and RNA. We don’t have DNA from LUCA preserved in amber like Jurassic Park, but we can compare modern critters that are the lowest on the tree of life and see what traits they share. Here’s what LUCAs great-granddaughters tell us:

Clostridium septicum, a bacteria “close” to the root of life’s tree, which also causes gangrene

LUCA probably looked like a bacterium swimming in water- a microscopic sausage only 1 micron wide. LUCA had a small ring of DNA floating inside, copied and translated by its’ sister RNA. LUCA harvested carbon dioxide and hydrogen from surrounding waters to make energy and grow. In this respect, LUCA behaved more like a plant than an animal- it took ingredients from its’ surroundings instead of eating other critters. Two key differences are that LUCA didn’t use sunlight for energy, and hated oxygen- any amount would have been lethal. Fortunately, oxygen wouldn’t be a problem for a long time.

LUCA did like hot water, hotter than 100 F, or 40 C. Just like last episode, scientists debate whether LUCA hung around deep ocean vents or hot springs on land like Yellowstone. As usual, there are plenty of debates about just what LUCA did or didn’t like, but the broad story is surprisingly similar.

You might be wondering, “If I wanted to look at my most distant cousin today, the closest thing on modern Earth to LUCA, where would I look?” You wouldn’t have to look too far, but be careful what you wish for.

One LUCA’s closest modern relatives is an infamous bacteria. You might not know its’ official name of Clostridium, but you’ve probably heard of its’ diseases: botulism and tetanus. I won’t get into gritty medical details, but like LUCA, Clostridium hates oxygen. This is why it grows in sheltered zones like food cans for botulism, dirt caked on a nail for tetanus, or inside your gut if it gets there. Clostridium isn’t as heat-loving as LUCA was, but after 4 billion years, a few things are going to change.

Before we wrap up this arc on the earliest life, one final note. You might ask, “If life started once on Earth, why didn’t it start again?” That’s not an unfair question- there’s still carbon and energy on the modern world, and it’s even more hospitable now. Why aren’t completely new critters walking out of the ocean, even on a microscopic scale?

In short, to the victor go the spoils. There are many ingredients for life on Earth, but they’re gobbled up by the life that’s already here, leaving very little for anything else. I’m not saying it’s impossible. As we speak, there might be a tiny bubble forming at the bottom of the sea, filled with RNA and proteins. But as soon as any bacteria comes along, that bubble is lunch. In the Hadean, the first life had the advantage of being the only life for millions of years- lots of time and space for practice.

Summary:

Life on Earth started sometime between 4.5 and 4 billion years ago, before February 15th on the Earth Calendar. The first life was a product of millions of years of slow growth from atoms to molecules to cells. There was a window of time when these first cells eked by, dividing and mutating. While many perished, some of these mutations stuck, evolving into LUCA, which would grow and multiply on the early Earth. LUCA lived in hot, oxygen-poor waters in hot springs or seafloors, turning carbon dioxide into new cells. Some of these cells survive relatively unchanged, while many others would find new paths, becoming every living thing on the planet, including me, including you.

And that’s the end of Season 1, the Hadean.

At the beginning, I called the Hadean the Invisible World. And while there are still many questions left, the fact that we know anything about this time with no rocks or fossils to work with, is incredible. Now, when you look at the moon, the ocean, or any living thing on Earth, you’ll know a bit more about how it got here.

Instead of recapping everything we’ve learned right now, I’ll post a summary episode soon. For the next few weeks, I’ll be taking a break for research and relaxation for before picking up where we left off.

I want to thank you all for listening this past year. The audience has exploded in the past month, and I’m excited to take the show in new directions. I also want to thank my collaborator, Resherle Verna of Be Giants Media, for taking a chance on this show. It’s been a pleasure making Bedrock, and we’re only just beginning. Stay tuned in a few weeks when we begin Season 2- The New Dawn, 4 billion years ago with Earth’s oldest rocks.