Scientists think all life shares a common ancestor about 4 billion years ago, but that common ancestor was already somewhat complex. It had hundreds of genes at a DNA, RNA, ribosomes. It did a version of the Krebs cycle. So the question that we have here is how do we go from no life to something like that? Well, the question really then is, how do we get to things that natural selection can act on?
Because, well, at least for me, if natural selection can act on something, I can imagine how things get more complex from there. And, you know, that train leaves the station and it develops into the diversity of life that we see today. But how do we get to that point? Well, scientists don't know and we probably will never know for sure, but we can come up with what we'll say here are potential steps. And we can test those potential steps and sort of see how likely they are.
So let's think about what had to have happened. Well, first now in the early earth, there weren't biological molecules. There weren't things like protein. So how do we get there? Well, we're going to do something called abiotic synthesis.
Now abiotic just means not alive and synthesis means to make something. So, we are going to make organic molecules, things like amino acids, from simple precursors that were around on the earlier things like what we see there: methane, water, ammonia, carbon dioxide. How do we get those to form? Well, this amino acid. Well, scientists think that this can happen in places like alkaline hydrothermal vents.
These sorts of geologic vents in the ocean that have alkaline or basic chemistry. They think it could happen there or in other places with such unique chemistry. For example, perhaps near volcanoes have just the right chemistry for this to happen. Now evidence for this comes from what we call the Miller-Urey experiment. Now this experiment, Miller, the graduate student, Urey, was his advisor and they showed in the 1950s that under appropriate conditions, amino acids can form abiotically.
They had this sort of real complex apparatus where they tried to model what they thought early Earth chemistry was like, and they shot in electricity to mimic lightning and they sort of made this sludge of organic molecules. But in those organic molecules were amino acids, so they showed it could happen. Now we now know that the chemistry they used probably wasn't quite right, but people have updated the experiment and repeated it with what they think conditions were more likely like on that early earth, and they get very similar results. Alright. But life isn't made of amino acids.
It's made of bigger molecules, things like proteins, right? Amino acids are just those building blocks that build those proteins. Well, so how do we get macromolecules? Well, we're going to say that macromolecules, well, they come when small molecules join into larger molecules. Again, this is going to happen abiotically outside of life.
So scientists have shown that nucleotides, the building blocks of nucleic acids, things like DNA and RNA and amino acids, they will polymerize abiotically. They'll form these larger molecules in the right condition. Specifically, they've shown it can happen in sort of hot drying sand, clay, or hot rocks. Now that may sound like a very specific environment, but yes, it is. But, right, the Earth is a big place, and there are a lot of environments where this could have happened.
And that is an environment that was present on the early Earth, so maybe it happened there. Alright. Well, life isn't just macromolecules though. Life is cells. So how do we get cells?
Well, we're going to say protocells, sort of the precursors of the cells. They can form, but we're going to say that membrane-bound vesicles can form spontaneously. Now a vesicle is just sort of a membrane-bound, a lipid bilayer sort of bubble on its own and made as phospholipids, and there are vesicles used in cells today. They're sort of used to transport things inside cells.
But if you get phospholipids together in the right place, enough of them in the right conditions, they'll form these on their own. Now vesicles aren't alive, but they do some interesting things. Right? Once you form a vesicle, they might capture macromolecules.
There could be those early proteins or RNAs inside there. Now vesicles will also reproduce. Now, not reproduce for saying in, you know, controlled reproduction like life, but if we have this vesicle here, if it gets big enough, it might break. And when it breaks, now you have 2 smaller vesicles. Now other phospholipids could join those membranes.
They'll grow larger. Those could break and make new vesicles. So it's a very sort of basic form of reproduction. Now inside the vesicle, you have this bilayer that separates it from the outside. So you can also have in there some sort of unique chemistry, chemistry that's going on that's different than what's going on around it.
Now when you put all that together, again, it's not alive, but it starts to sort of look like something that reminds me of life. Right. But what's missing from what we described there, though, are self-replicating molecules. Right? Specifically, when we think of life, we often think of DNA.
So how do we get that? Well, we actually think we don't start with DNA. We actually think we start with RNA, and we've shown that RNA could form abiotically. And RNA is really interesting because RNA can act as a catalyst. It can speed up chemical reactions.
Normally in life, we think of enzymes doing that, but RNA can do it too. RNA can do that, and it can also store genetic information. So these RNAs that work as catalysts, we call them ribozymes. And this is a really key idea because if RNA can self-replicate and it can catalyze reactions, maybe that was sort of the first biological molecule that sort of got everything going. That leads to this idea that we call the RNA world.
And this is just this hypothesis that early life was RNA-based. And it's based on that idea that RNA can catalyze reactions and it can self-replicate. That means that DNA actually came later. DNA probably evolved because it's a much more stable molecule. So if you want to store genetic information for a long time, you put it in DNA.
All right. So again, these are some potential steps. We don't know for sure how it happened, but these steps at least have been tested in different ways and on their own, they do work. So we could see how these kind of add together to get to that point where natural selection can take over. And once that happens, life is on its way.
Alright. We'll look at this more in an example and practice problem. I'll see you there.
