- The Earth formed about 4.5 billion years ago and life appeared on our planet about 3.5 billion years ago, and probably even earlier. That's a remarkably short amount of time for our world to go from dead to alive. And while we know that this process happened, we're not exactly sure how, and figuring it out involves stretching the very definition of life itself. [orchestral music] The early Earth was terrible for life. I mean, when it first formed, it was literally a ball of molten rock, which is the very definition of inhospitable. And the young Sun, don't get me started on the young Sun. It would constantly emit flares of radiation that would just irradiate the entire solar system. And even after things started to cool off, our Earth suffered countless collisions from asteroids and comets that would just bombard the surface again, and again, and again, making the place very difficult for life to gain a foothold. But eventually things started to settle down. The planet cooled off, the crust formed, the oceans rose to the surface, and the conditions for life were there. Now, life needed three things to get started. One, it needed a stable environment. Two, it needed a soup of organic molecules for its building blocks. And it needed a source of energy. So, where on the early Earth could these conditions have been met? Well, thankfully, our planet is a pretty big place and there are lots of options that meet this criteria. For example, there could be hydrothermal vents in the deep sea. This is a source of energy, it has the right chemical mix, and is a stable environment. Or it could be hot springs, shallow or deep, anywhere on the surface of the Earth. Or even tidal pools that are sloshing in and out. This could be the home for the first life to appear on Earth. There's even more radical suggestions, like maybe beaches that were struck by lightning, providing the right kind of chemical mix and the source of energy. Or maybe it all happened deep underground. We honestly don't know yet which one of these places is more likely than the other to be the home for the first kind of life on Earth. But was the earliest life on Earth even alive, man? Well, there's over 100 possible definitions of what life is. And perhaps the most useful one for this context is that life is a self-sustaining chemical reaction that is subject to Darwinian evolution. That means you, Charlie. Don't worry, we'll come back to you. This is an incredibly broad definition, but we need this broadness to explore the origins of life. Because today in the modern world, this is obvious that you, or me, or potatoes are alive, and that rocks are not. The origins of life by definition straddle the boundary between these two extremes. And so, we need a very broad definition to explore those origins. So, how do a bunch of random chemicals find themselves subject to Darwinian evolution? Well, to do that, they need to perform three things. One, they need to store information. They need to keep some sort of memory about who they are and what they're capable of doing. Next, they need to catalyze reactions. This is AKA metabolism, and what makes life so much fun. And lastly, they need to be able to self-replicate. They need to be able make copies of themselves so that they can remember who they are and what they're capable of doing, and pass all that onto a new generation. Modern life uses a set of three macromolecules to get all those jobs done. One, we have our DNA, which is our store of information. The DNA creates RNA, which transcribes that information and then manufactures proteins. And the proteins are the ones who do the job of catalyzing reactions, including replicating DNA so it can make copies of itself. And this is an extraordinarily complex interaction that honestly we don't fully understand. And it's so complex and interconnected that it's obvious that early life must have been simpler. And it's possible that early life didn't even use proteins or DNA. It's possible that early life only used RNA. This is called the RNA world hypothesis, and it works because RNA is capable of self-replicating. It's capable of catalyzing reactions. And it's capable of storing information, just not as efficiently as the full RNA DNA protein combo. And most critically, RNA is capable of something called autocatalysis. Now, that's a $5 word if I've ever heard one. Autocatalysis is the ability for a chemical to catalyze reactions that generate copies of the original molecule. This is what enables RNA to participate in the whole Darwinian evolution game. So, in this timeline, the Earth is saturated with a whole bunch of organic compounds. And it's a pretty gross place if you ask me. But eventually, short RNA strands appear. And then, these short RNA strands start participating in chemical reactions that get ever more complex. And then, slowly over time due to evolutionary pressure, eventually DNA and proteins emerge as more efficient versions of the same basic process. And then, boom, you've got modern life. While the RNA world hypothesis is appealing and intriguing, it does have its shortcomings, just like any scientific hypothesis really. For one, we're not exactly sure how RNA is supposed to do all this. And RNA is very fragile, a lot less robust than DNA. So, it's not clear that it could actually survive long enough in the early Earth. This model just assumes that all the interesting metabolism just sort of happens eventually. This is just one idea of many. We can use the branches of the evolutionary tree of life to figure out what our earliest common ancestor was like. Come on, Charlie. Let's hit the chalkboard. The entire tree of life, and we can trace its origins back to a single comet. Very funny, Charlie. Why don't you just hang out in the corner here, okay? That's nice, very funny. We can sequence all extant life on Earth. We can examine their genes and look for commonalities. From the eukaryotes, including the animals and Great Grandma Maude is somewhere up here, and the plants and the flagellates. We can look at the archaea. We can look at all the bacteria, and there are a lot of bacteria, to see what few genes we have in common. And this kinds of sequencing has revealed about 330 genes that all life on Earth shares. This is something we all have in common across the globe. And we can use that to reconstruct something we call LUCA, the last universal common ancestor. Now, this is not the earliest life to appear on Earth. But it is the common origin point for all modern day life. And by looking at those genes, we can figure out what LUCA looked like. And LUCA had DNA, had RNA, synthesized proteins to have metabolism, was very heat-tolerant, and probably lived in a deep sea hydrothermal vent. This is our common ancestor that shared our same biomechanical chemistry. But to push back to even further generations, we need to bring in an expert. [bomb exploding] Thanks so much for doing this. This is gonna be a lot of fun.

- No problem.

- What is the earliest evidence for life, and then the earliest debatable evidence for life?

- One of the earliest pieces of evidence for life is in rocks in Australia. There are clear deposits that are carbon-rich and physically shaped like bacterial formations we see today. So, we assume it was early bacteria living in a style that we can recognize still. It's called a stromatolite, a giant mound of bacteria that grow in layers on top of each other. It forms in watery environments. And when you've got a shallow pond, the bacteria just start growing up in a cluster and eventually build up layer-on-layer on top of each other until they form an entire mound.

- Similar to modern life, or do we think it was perhaps some chemically different?

- It was almost certainly very similar to modern life, in terms of the biochemistry of it all. DNA, RNA, and proteins were almost certainly there.

- What was the energy source for these kinds of bacteria?

- Geological processes will produce chemicals that bacteria can harvest energy from. We also see that today in the deep sea vents.

- Are you aware of these debates when it comes to the origin of life between replication-first scenarios and metabolism-first scenarios?

- There's two ideas about how life came about. One is to think that you needed a metabolism. Basically, some way of taking environmental energy and converting it into useful chemistry. If you have the metabolism, then the rest of the cell, the genetics, the membrane, the things that maintain that metabolism, can build up around a chemical process that's already happening. And it's possible that life just sort of built up around that. The alternative is that through processes that are energetically favorable, you created a genetic material. And the genetic material, in order to self-sustain, evolved a way to harvest energy later. I tend to favor genetics first. I think forming a self-replicating molecule is harder than having chemical processes that sort of extract a little energy from the environment. And so, I think the harder step had to come first.

- What is the most stable environment to allow these fragile chemicals to sustain themselves?

- Something like RNA is very fragile in our present world 'cause everything views it either as food or as a threat. So, there's lots of ways to digest RNA. But back when it was forming, none of those enzymes that would digest it existed. And it's probably more stable than we give it credit for.

- What matters is its ability to self-replicate before it decays, and maintain that genetic memory.

- Exactly. Some of the environments they're proposing, like deep sea vents, those exist in chains over hundreds of kilometers. The potential to produce the raw materials is huge. It's not one little test tube in a lab.

- Right, it's billions of test tubes over the course of potentially hundreds of millions of years. When did DNA happen?

- DNA chemically is only one atom different from RNA. Since DNA is more stable, there would be an evolutionary selection towards having a more stable repository for genetic material. And it's possible that there is an intermediate state, where the chemistry wasn't picky and used both DNA and RNA at the same time. Last thing that's exciting to me is that people have been working on minimizing the genome of a bacteria. How few genes can you get away with? And one of the things they're thinking of doing is once you have this fully minimized genome, can you start taking out a protein and replacing it with catalytic RNA? So, have a sort of hybrid system. It's DNA-based. There's lots of proteins around. But some of the key reactions are done by RNA instead of proteins. That would reflect what we think would look like a key intermediate step before the last common ancestor of all life.

- Would you even call these kinds of chemical reactions alive?

- Defining what life is is a really challenging thing. We tend to look for binary definitions. Is it alive or not? Is it a species or not? And life doesn't necessarily give us the clean answer.

- Is the origins of life potentially incredibly messy, as messy as life itself?

- Absolutely. And I think that's something that people don't necessarily appreciate about this sort of research. We're never gonna go back in time to figure out exactly how life came together. And so, what scientists are trying to do is put together a whole set of plausible pathways that could produce something that's living. And we won't know whether that is exactly how life started, but what we would know is that it is possible to form life with simple chemistry. That's gonna be a mix of different answers that are satisfying to different degrees.

- It happened here on Earth. Did it happen on our sibling planets too?

- We can make some inferences based on what we know about them today, about what they looked like in the past. That lets us make some guesses about what might be happening on these other planets. It's also plausible that something we would recognize as life could have significantly different chemistries. We've only got one example of life to work with. And so, our biases are towards what can we find that looks like what we know works.

- Thank you so much for joining us. It was a joy to have you and I can't wait to have you back. Our exploration for the origins of life have taken us all across the globe, into the stars themselves. And we've had to combine biology, chemistry, geology, and even astronomy to understand our own origins. And really, that search is just beginning, unlike this episode, which is over. Come on, Charlie. Let's get a drink. [orchestral music]