When last we left our lovely Brucey, he was just getting settled into his new home on a freshly cooled off Earth. Next, we’ll dig into the first 4 billion years of life on Earth, uncover the origins, development and characteristics of early life. First things first, we need to define life. The problem is, no one is quite able to do that. So maybe the best way we can do this, is to take a sample of “things”, and decide whether or not they’re actually alive. And hopefully we can narrow in on when and how early life really began on Earth. Let’s start easy. Well, she certainly looks… Lively. Ugh, fine. Philistines. So let’s jump to the other side of the spectrum and take a look at this lovely Pixar-lamp. This, despite what Pixar will tell you, is very much not alive. At all. Let’s jump over to bacteria, which are essentially just super-basic cells. They’re alive! Simple as that. On the other side, we have simple chemicals, like ammonia, cyanide, and formaldehyde, which are very much not alive. I know this for a fact, because chemistry makes me feel dead inside. Back on team life, we’re edging towards the gray zone with viruses. On the one hand, these guys grow, carry genetic material, and reproduce, but they don’t have cellstructure, and they can’t reproduce on their own. Yes, much like high school language classes, they suck the life out of you and then take over your dead body to reproduce parasitically. Like I said, just like high school language classes. On the other side, we have self-replicating sheets of clay known as clay crystals. Much like viruses, clay crystals do some life-y things, like grow, replicate, and evolve by natural selection, but not so much the cells-business. Some scientists like to call clay crystals a pre-life scaffolding of sorts, and I mean, hey, you can’t disagree with those labcoats. Finally, in the middle-y-ist of the middle, we have RNA world, which some people think marks the beginning of life. Now, you may be asking yourself “why doesn’t Benedict Cumberbatch love me?”, and I’ll be totally honestl I can’t help you with that, but you also may be asking “what is RNA”, and than one I CAN help you with. But we’re gonna need to make a slight detour to explain it. So come with me kids, and I’ll explain the central dogma of molecular biology! The central dog-ma – no dogMA, yes, thank you – is DNA makes RNA, makes protein, and it’s super important to life. So, we start with DNA. DNA is the double helix-shaped chemical chain-thingy that contains all of the instructions for being you. It’s made up of a bunch of nucleotides, which you can think of like 0’s and 1’s in code, except there are four of them: A, C, T, and G. A goes with T and C goes with G, so if you have one string, you can always produce the opposite one. Two things can happen in DNA: 1) It can be replicated and be made into a second strand, or 2) It can be used as a template for RNA, which we’ll get to in a bit. As an example, when my cartoon haircells need to produce the parts that make my cartoon hair the way only my cartoon cells can, they dig into my DNA and do a command-F search for silly hair and unzip the DNA when they get to the right spot. From here they can make RNA. RNA is like DNA, except ribonucleic acids instead of deoxyribonucleic acid. It is single-, not doublestranded, and instead of a T-nucleotide it uses U. The most important part of RNA for our purposes is that it serves as an intermediary between DNA and proteins. So the DNA is transcribed into RNA and it gets sent to be translated into protein. Now you may be asking yourself “what am I doing with my life?”, to which I’ll respond that that got way too deep way too quick and I was not prepared for that, and I’llpretend like you asked me “why not just make protein from DNA and skip this RNA-nonsense?”, to which the answer is: it’s kind of like books. So, when you want to make a cake, you don’t go and kidnap Cake Boss so he’ll give you the original manuscript to his recipe book. You avoid commiting a felony and settle for a copy of his recipe book, which you take home with you and use to make a cake. It’s just easier that way. And if someone else is trying to kidnap Cake Boss too, than you’re kind of in trouble, and half a Cake Boss is as good as no Cake Boss. So the bottom line is: RNA is a very useful middleman here. This middleman runs to a ribosome so it can get translated into protein based off the specific code the RNA has. And then that protein goes where it’s needed, in this case, my silly cartoon hair. Proteins are made of chains of amino acids, which the genome code’s for. Every three consecutive nucleotides in a gene codes for one type of amino acid. Each set of three is a codon. Some proteins are made in the cytoplasm and remain inside the cell, while others are made on on what’s called the rough endoplasmic reticulum and are secreted out of the cell. So that right there is the central dogma. Say it with me again: DNA makes RNA, makes protein. It happens everywhere, all the time, because everything needs proteins to work. Back to RNA World. We now know the RNA-part, but we need to sort out the World-part. Tangent time. So I was simplifying it earlier, but RNA can preform lots of different functions. It’s so versatile that it could act as DNA, RNA, and protein too if it needed to. It’s like the Kevin Spacey of biology. RNA could be a usual suspect one minute and the president the next, and you would never question it. Thus is the magic of Kevin Spacey. So with our multi-talented RNA all ready to go, we need a place to put it. And that’s where we get cells. Cells are separated from the outside world by a phospholipid bilayer membrane, which is like double-sided tape. On the outside its hydrophilic, meaning it likes water, so water will stick to the surface but not pass through, because the middle is hydrophobic, meaning it hates water. The membrane is made of two rows of these suspiciously clothespin looking phospholipids, and the bottom line is that unless there are specific transport passages there, the outside stays on the outside, and the inside stays on the inside. Like a balloon. So like a balloon made of double-sided tape? Eh, crap, there go my metaphors. Well, I shouldn’t be too sad, they were really meta-threes at best. Anyway, this membrane here, yes this specific one, is where the RNA in the RNA World hangs out. So this theory is pretty good for a lot of reasons. If we have a bunch of cells with RNA acting as both the genome and the protein, it’s a great transition step between clay crystals and stuff like viruses and bacteria. The only problem is that there are some pretty sizable holes in the explanation of how such a set up even came to be. There’s a lot of debate as to how this would even work, and getting to that point of it even working is still a problem. Beyond that, is replicating RNA even alive? So, while we’re ruminating over all of this stuff in RNA World, let’s look back at the chart from earlier and observe that when we get to this middle range, the idea of what is alive gets tossed right out the window. But say we roll with RNA World, where do we go from here? The next stop is Luca, out last universal common ancestor. Luca has some features that should look familiar. It has a phospholipid bilayer membrane, a genome, and it metabolizes energy, because living things kind of have to turn fuel into energy if it hopes to, you know, actually do stuff. Looking good. Fair to say, our boy Luca here is alive because it looks a lot like a bacterium, and that’s great for our old friend Bruce, who has been hanging out for the past half billion years playing on his phone under rock at the bottom of the ocean. Not sure how he got down there, but don’t question the Bruce. Luca and Bruce were best buds. They did everything together. They watched DNA replicate, chased after RNA on its way to the ribosome, and talked about the newest Marvel movies for millions of years on end. Two billion years passed in the blink of an eye, and before poor Brucey knew it, Luca had changed. Bruce’s first hint was the nucleus. That certainly wasn’t there before. What happened was, Luca was practicing his stretching and his membrane kind of awkwardly folded in on itself in a really weird way, and the result is that it made a sort of ball around the genome that separated the rest of the cell from what is now the nucleus. Later, foreign blobs started coming in, poking through Lucas membrane and getting all cozy, and after a couple hundred million years, Luca had assembled quite an assortment of little organelles, which objectively had a positive impact on Lucas life, but Bruce felt betrayed when Luca was more concerned with hanging out with his cool new friends than hanging out with his old friend Bruce. That, kids, was a metaphor for how simple prokaryotic cells became complex eukaryotic cells that I got a little bit too invested in. So, that happened, and almost 1 billion lonely years later, Bruce saw something really crazy happen, multi-cellularity, which is basically like the Siamese twins of cells. Multi-cellularity had evolved several times, but it only stuck about 1 billion years ago, when cells decided to be a team. If you ask me, what I think did the trick was a bunch of duct tape and glue sticks. Regardless, the fact is that multi-cellularity did the TV interview circuit so it spread like wildfire. And soon enough all the cool kids are multi-cellular, even Brucey boy. Unlike those single-cell plebs. Video source: https://www.youtube.com/watch?v=7baC9DcrAz0
