This is a video about the origins of cells, A2.1, and it is for higher level. In order to talk about the origin of cells, we first need to understand the origin of all the complex molecules that make up cells, like proteins and nucleic acids, those kinds of things. And in order to understand how those were formed, we need to travel back in time to what we call prebiotic Earth.
So that's the Earth. during the time period before life existed. And early Earth or prebiotic Earth is very different than it is now.
So there was very little oxygen, lots of methane. It was really hot. There was a lot of carbon dioxide.
There was no atmosphere. So the UV radiation from the sun was very intense, and there was just lots of lightning. And this is literally a perfect environment for the spontaneous generation of some of those complex carbon compounds.
Now, once living things arose, they started to change Earth's environment. They started producing things like oxygen and we started getting an ozone layer. So those conditions, which were perfect for the spontaneous generation of these complex compounds, no longer exists. So we don't see any more spontaneous generation of those compounds. But I do want to travel back and talk about how that might have first happened.
Now if we're going to talk about the first cells, we need to have a clear understanding of what a cell is. And a cell is self-sustaining, and that means that it's using energy to keep this highly ordered state, that things aren't just falling apart. Living things can pass this highly ordered state onto their offspring, so it's a very unique quality of living things. Organisms can either be unicellular or multicellular. We know whole organisms are living, but how do we know, especially multicellular organisms, about their smaller components?
Well, we have some evidence to suggest that cells are the smallest unit of life. First of all, they're using energy to maintain order, to maintain compartmentalization. When cells stop doing this, they tend to die.
New cells can produce more cells. That's great evidence that something is living. And cells can actually live outside of the body.
So I can actually take some of your cells outside of your tissue and put them into culture. And as long as they have all the nutrients that they need, they will continue to live. We cannot do that with smaller cell components.
So I cannot, for example, take some organelles outside of a cell and put them in a petri dish and expect them to live. I can't do that. So that is how we arise.
at this cell being the basic component of a living thing. Now, we've mentioned spontaneous generation a couple of times. Pasteur was a scientist that did some very famous experiments to falsify this idea of spontaneous generation. And spontaneous generation was a thought that living things could just generate out of nowhere.
So for example, if I left my dirty laundry on the floor, mice could just spontaneously generate. We now know that that is not true, that new cells only come from existing cells. And that was a great learning from past year's experiments. But it doesn't help us explain where the first cell came from.
So if we think about this very first cell, here's what had to have happened in order for it to form. It first needed to have some kind of control over the chemical reactions. metabolism is very complex, there needs to be some semblance of control over what reactions are and are not happening.
There needs to be some kind of self-assembly of polymers. So we need to be able to take amino acids and make proteins or nucleotides and making DNA. And it needs to be compartmentalized. So the cell needs to have a way of separating itself from the outside environment.
And then of course, having self-replicating molecules. So all of these must have come together in order to form that first cell. Now, if we think about these polymers, complex molecules like amino acids, or even the proteins made of those amino acids, we have to have a way of explaining how those might have formed spontaneously in order to support that hypothesis about the very first cells.
And that was the whole goal of this Miller-Urey experiment. And so basically, this apparatus... sets up an environment very similar to that prebiotic earth.
So water vapor was added into a chamber full of ammonia, methane, hydrogen, all those things that were really prevalent in the atmosphere of prebiotic earth. And then they applied energy. So that's what this is here. So that could be in the form of like lightning strikes or volcanic eruptions, all those things that were happening quite frequently back then.
And then these samples condense in this part of the apparatus and samples were taken. And they actually found that this process produced amino acids. And that was so important at the time. When this was happening, the prevailing thought was that only living things could produce compounds like amino acids. But what the Miller-Urey experiment showed was that carbon compounds actually arose on Earth before life evolved, that it was due to the conditions on prebiotic Earth.
So it gave a lot of support for the hypothesis that all of these components could have originated outside of a living cell, right? So independent of a living cell. But then when that cell came together, once the conditions on prebiotic Earth started to change, this process was no longer possible. So it's just a great piece of evidence and a great experimentation model for setting up something really similar to the environment of prebiotic earth.
Now we talked about compartmentalization being necessary for the formation of this first cell. It has to separate itself from the environment. And to do that, it needed to form what we call vesicles. Those are structures surrounded by membranes.
Now in that primordial soup, right, that liquidy state in prebiotic earth, we found a lot of phospholipids. Well, I wasn't there. I'm not that old, but there were a lot of phospholipids that existed. And phospholipids are very cool because they have both a hydrophilic end and a hydrophobic end.
And when you have a lot of them together, they spontaneously form bilayers. So that means two layers. And the reason that they do that is because... those little hydrophobic tails, these guys in here, they really hate water.
And what is on each side of a vesicle? Yep, you guessed it, water on the outside and water on the inside. And so what I'm basically looking at up here is a zoomed in portion of maybe this part of a vesicle. And so those phospholipid tails come together to get away from the water. And so That happens spontaneously, and this helps the first cell form these compartmentalized regions.
And the other advantage here is that these phospholipid bilayers are what we call partially permeable or semi-permeable. And that means they are selective about what things enter or exit the cell. All these things that are important for the origin of the first cell. We now know that all living things on earth use DNA as the storage molecule for their genetic material, but it's presumed that RNA was actually the first genetic material in that first cell, and here's the evidence that we have to support that idea. First of all, RNA is self-replicating, so that was an important thing for those self-replicating molecules of the first cell.
It can act as its own catalyst, so it doesn't need an enzyme in order to start replicating. And some viruses have only RNA. Now viruses aren't themselves a living organism, but the way that that RNA functions within the virus tells us that that RNA is able to do some of those functions that would have been necessary for the first cell. Now what's really interesting here is that RNA mutates at a much higher rate. The reason for that being is that there's no complementary strand.
It's really hard to proofread for any errors in replication. But what makes this so interesting is that it is mutations that drive genetic variation in a population. So the faster you have mutations accumulating, the more genetic diversity you'll see in a shorter period of time.
And when we think about how evolution may have really gotten started after those first cells were formed. this starts to make a lot of sense. Now let's consider three very different organisms, a tree, a dog, and a horse.
They may seem super different, like maybe they don't have anything in common, but actually they have quite a bit in common if you start to look at the inner workings of not only their bodies, but their cells and the molecules within their cells, particularly their DNA. So what that shows us is that at some point, These organisms shared a common ancestor. And if I go back in time far enough, I can start to see common ancestral links between all of the different organisms on earth, even the really weird ones like bacteria.
And if I go far enough back in time and I think about these common ancestors, I need to come up with the original common ancestor. And that's... Something that we call LUCA. So LUCA stands for Last Universal Common Ancestor.
And it's part of the idea that all of the living things that arose or that are alive or were once alive on Earth all descended from a single common ancestor. The reason why we think that is because there is universality in our genetic code. So the only way of explaining why all organisms use DNA and all of that DNA works in the same way is if they came from the same common ancestor. We also have some similarities in terms of ribosome structure and how DNA and RNA are synthesized.
And again, the only way to explain how all the organisms on earth have that same structure or mechanism is if they came from the same common ancestor. Now it's thought that... life probably evolved separately several different times.
Like this first cell looked like this, and this other first cell looked like that. But all of those other life forms or early cells must have died out. And all of the life forms currently on earth today must have come from Luca.
That's the only way of explaining how such a diverse array of organisms could all have so much in common. So again, in thinking about this theme of unity and diversity. There's some great stuff to unpack here.
Now, as soon as you tell me all living things come from a common ancestor, one of the things that I want to know is when. When did that common ancestor arrive on Earth? And there's some really cool techniques for trying to figure this out, and there are a couple that we will compare. One of which is called carbon dating. So carbon dating uses carbon isotopes or looks at carbon isotopes that have known half-lives.
And if you know how much carbon is left, you can backtrack and figure out how much time has passed. So for that, we really need fossils. And there's some really cool fossils from the Streli pool that date from about 3.42 billion years ago.
And they look like this. There are some early life fossils in there. Now, we know for sure that there are older rocks and older fossils, but not necessarily older life forms.
And that's just because... Rocks go through cycles of their own. They may have metamorphosed into something else.
So what we're doing is we're looking at certain fragments from sedimentary rocks and carbon dating those. And using this method, it's estimated that life arose about 4.1 billion years ago. Okay.
So the important learning from here is that using carbon dating and the oldest known fossils, we can estimate that Luca arose about 4.1 billion years ago. And that is in contrast to a different method that comes up with, you guessed it, a different answer because biology. So if I go back to this concept of common ancestry, a dog and a horse are different, but they're not so different.
I bet they have more genes and more base sequences in common than let's say a dog or a horse. Okay. And so those are driven by mutation.
So I'm going to go back to the first one. A dog and a horse, I don't have to go very far back in terms of like the whole timeline of Earth, but their common ancestor is much more recent than the common ancestor that a horse and a tree might share. So I would expect more genetic differences between a horse and a tree because I have to go further back in time to find their common ancestor. Now, the great part about using some of these living species, or maybe even fossils, is that we can quantify, we can count the number of genomic differences between different organisms.
And it tends to follow a pattern. So you can actually write an algorithm, you can write a formula that says, okay, if I know I have this many genetic differences, that is equivalent to this amount of time that has passed since those organisms had a common ancestor. And if I keep doing that and I go all the way back to LUCA, then that data is showing me that LUCA actually formed 4.5 billion years ago. So a different answer than we're getting from carbon dating.
Now that just happens to be around the same time as Earth's formation. So maybe we didn't have like a prebiotic Earth for very long. The thing about these timelines is that they're only falsifiable. So carbon dating has its merits using genomic Genomic differences also has its merits. We cannot prove either one of these.
All we can do is falsify them. And so these are really highly contestable ideas, but that's what makes biology so great is that we're always looking for new data to either support a theory or to falsify one. Now, we may not know exactly when Luca formed, but we have a pretty good idea of where.
So the beauty of the idea of common ancestry is that all of these current living species, if they descended from a common ancestor, they will share common genes. Again, unity and diversity. So some of those common genes that we share are genes that code for protein for living in really weird environments, like places that are anaerobic or have lots of hydrogen or carbon dioxide or iron.
And so if all of these organisms that don't live in those places, have those genes, the best explanation is that they got them from their common ancestor. And so that tells us that the common ancestor probably arose in an environment that had those conditions. And so one of the places that has conditions that match the genes from that last universal common ancestor are these hydrothermal vents. called white smokers.
So these are underneath of the sea. They contain minerals and temperatures that could have led to the spontaneous generation of carbon-based life forms. And they also make sense with what we know about the genes that all living things have in common.
So just, again, a really great way of providing evidence for that leuka.