Origins of life | Biology | Khan Academy

We have many videos on Khan Academy on things like evolution and natural selection. We think we have a fairly solid understanding of how life can evolve to give us the variety, the diversity that we've seen, and the complexity that we've seen around us. But it still leaves unanswered a very fundamental question, and this might be the biggest question known to us, and that is the origins of life. How did life first emerge, at least on Earth? That even starts to lead to other questions about, is there life outside of this planet, and what could it be like?

And so let's start with what we actually know, and I'm going to start with a timeline. So let's go 1 billion years ago, let's go 2 billion years ago, 3 billion years ago, 4 billion years ago. So this is now, and once again we're talking about billion years ago. You'll sometimes see the abbreviation "bya" for billion years ago, which is an unfathomable amount of time going into the past. But we know that Earth, along with the rest of the solar system, was formed around 4.6, 4.6 billion years ago.

So that's when Earth was formed, and right at 4.6, or even, you know, a casual 100 million years after that, 4.5 billion years ago, we believe that Earth wasn't very suitable for even very simple life to form. And that's because the solar system was a crazy place. You had collisions of all scales happening all of the time. The moon itself was formed from the collision of two planet-sized objects. One would kind of call it the proto-Earth and another planet-sized object, and they collided, and then they started to spin around, and one part became the moon.

They became tidally linked with the Earth, but you could imagine that's not an environment where it would be easy for life to form. Even once the moon was formed, you had a heavy bombardment of things continuing in the solar system. The solar system was a messy place. It took a long time for the stability that we now observe out there, and so that continued, we believe, until about 3.9 billion years ago, which is the earliest that we currently think that Earth might have been suitable for life.

Before that, there might have been pockets where the bombardment stopped, and maybe some type of primitive life might have formed, but then they would have gone away with the heavy bombardment. But who knows? Maybe they could have survived that somehow, but that's the current mainstream belief. The other thing we know is that we see fossil evidence for life 3.5 billion years ago, and these are stromatolites, the fossil evidence microorganisms.

They form these structures that actually continue to be formed today. These types of structures continue to be formed today, and although it might not feel like microorganisms or complex life, when you think about what has to happen within a microorganism, they are actually incredibly complex, especially if you compare them to very simple non-living organisms. So our current belief is, well, someplace in this region, life must have arisen on Earth.

But that still doesn't, even if we were able to answer that question, "Oh, it was exactly 3.7 billion years ago," was the first time that some RNA decided to—well, not decided, or ended up getting in the right confirmation so it could replicate itself in some way. Even if we know that date, it still leaves unanswered maybe the most interesting question, which is the how. The how is really, at least to me, more interesting than the when.

To the how question, there's a couple of layers on it. The first is, well, let's just start with the most simple molecules that we would have expected to find on early Earth. Here are some examples of it right over here. This is H2O, or more commonly known as water. Right over here is CO2, more commonly known as carbon. That's a little hard to see; let me do a lighter color.

So we have carbon dioxide right over here; here we have molecular nitrogen, you have some ammonia, you have some phosphate. These are, and many other of the elements that we see on Earth today, they might have been available in that early Earth, but how do they form? At least even the next step up, which is the slightly more complex or actually a good bit more complex organic molecules.

When people talk about organic molecules, they might be talking about things like this: these are amino acids. These are the building blocks of proteins. Amino acids you see over here, nucleotides—these are the building blocks of RNA, DNA, and other things. And so the first question is, and these aren't the only simple organic molecules. You could think about sugars and all sorts of other things, but the question is, is it realistic? How do we at least understand how we can go from these very simple molecules up here to these more complex, often called organic molecules?

The simple answer is we now have a lot of evidence that this is doable, that you can go from these things to these things abiotically and without the presence of life. You'll hear that word abiotic a lot. Think about it: antibiotic—you're killing life, you're killing bacteria. Abiotic—that is without life. The points of evidence that we now have are we believe and we've seen evidence that there are amino acids and organic molecules related to them on comets, meteorites, and other planets, that they formed spontaneously in space, once again without the presence of life.

We've even been able to form amino acids and other molecules like this from these more simple elements in the laboratory. The most famous experiment there is the Miller-Urey experiment. This was in the 1950s, where they were able to show that with some energy—they provided a spark. You could imagine that in the early Earth it could have been from lightning—and they tried to set up a mix of gases that they believed was similar to the atmospheric mix in the early Earth, which didn't have much oxygen in the atmosphere. Then we needed life to actually start to produce some of that oxygen.

Even though today we think that they probably didn't have the mix of gases right, they did do something significant. They were able to show that with that mix of gases, at least they thought were in that atmosphere, and some energy being added to that system, that they were able to form some of these organic molecules. So we should feel pretty good that at least this first step is doable.

Now the next question is, well, these organic molecules by themselves—that's not life! In fact, these aren't even the most complex molecules that we believe are essential for life. Proteins are where things start to get really interesting. A protein or proteins can have thousands of amino acids—thousands of amino acids. Things like DNA and RNA also we believe are essential for life, or at least life as we know it, could be made up of tens of millions of nucleotides for one DNA molecule.

So for example, this is just a small part of a DNA molecule, but you can already see it's much, much more complex than what we see over here. And there too, we have evidence that you can go from the amino acids to the proteins, or you can go from the nucleotides to the DNA, without the presence of life—that these things can happen spontaneously if you have the right context, the right energy available. Some people believe, or it's been observed, that if you have the right surfaces, that these molecules can be organized in the right way to form these more complex things.

Now I know what you're thinking. All right, proteins are really cool. DNA and RNA are really cool. But then how does that become life? At what point would we start counting, "Well, that was a proto-life form?" And this is where we really get into the area of the unknown because we don't know. There's a couple of hypotheses out there. One of them is called the RNA world hypothesis. I'll write that down: RNA world hypothesis.

And this is the idea that the first proto-life was self-replicating RNA molecules. The reason why people tend to focus in on RNA a little bit more than DNA is that even in cells today, RNA doesn't just store information; it can actually play a role as a catalyst. When you think about things like tRNA and you think about ribosomal RNA, maybe some of that first proto-life was RNA that information replicated itself and catalyzed the replication of itself. Maybe it somehow got organized into membrane-bound structures so it could separate, so you had environments that were separated from the outside world.

But the simple answer is we don't know. Another mainstream hypothesis is the metabolism-first hypothesis—metabolism, metabolism first. And this is the idea that a lot of basic pathways that you might study in, say, a biochemistry book—that these were kind of first just happening in, well, all of this could have been happening in this primordial soup where you had these organic molecules.

In the right conditions, maybe around heat vents and whatever else, but the metabolism-first hypothesis is that some of these mechanisms that we now study in biochemistry—these might have happened outside of a cell or outside of life, and they just kept creating more and more complexity. But at some point, these things started happening in kind of self-organizing membrane-bound structures.

Maybe there's some kind of combination of the two. The simple answer is we just don't know, but there's some fascinating clues. Even if we observe current biology, in fact, even if we see the commonalities of things that happen—the central dogma of biology—if we see how proteins, which structures are common to all life as we know it, it might give us clues or hints at what some of that very earliest life or proto-life was actually like.