Where did life originally come from? How did we get from rocks and gas to living animals and human beings? No one really knows but today, we’ll get closer to the answer of the origins of life.
Where life comes from might be one of the biggest open questions of humankind. Thanks to a French chemist named Louis Pasteur (yes, he’s the reason pasteurized milk is a thing) proved that life isn’t created spontaneously, so life doesn’t magically appear. Sounds simple enough, right? Well, his groundbreaking experiment was even more simple: he used two flasks and sealed one while leaving the other open to the environment. Surprise, surprise, only the unsealed one got contaminated by bacteria, so life. Another white man ensured that his name never gets forgotten—though I’m pretty sure the pasteurization thing played more of a role in this particular case. I still wonder how much of a role the food storage knowledge of the housewives in his life played in this insight.
Anyway, a second experiment called the Miller-Urey experiment tried recreating the conditions of early earth. In a very flawed experiment, they showed that organic compounds can form spontaneously under the right conditions. The two American scientists Miller and Urey (yep, white males) first performed this experiment in the 1950ies. A few much less flawed recreations have since shown that they got to the correct results with their flawed procedures. Researchers have been able to generate more than 20 amino acids—that’s the stuff your body makes protein from—from inorganic compounds. But these amino acids needed quite a bit of help to get created. Essentially, the experiment recreated original atmospheric conditions on earth with gases in flasks and sparks to recreate lightning and such. And then everything settled into a wanna-be ocean in one of the flasks: the primordial soup equivalent.
There are a few different hypotheses as to where life was first created. Some people say life was created in the primordial soup, so the goo that covered earth after early meteorites. But saying that life arrived from another planet is really just pushing the issue from one place to another. Life has to have been created somewhere at some point.
So to return to Earth, where could life have been created? The two most likely places are also two ends of an extreme: the very hot or the very cold.
One commonly known option are hydrothermal vents, so cracks on the ocean’s floor where hot water seeps out from below. They usually exist near volcanic or tectonic (earth plate) activity. The water that seeps out is high in minerals, among them sulfur, which seems to be essential—or so those scientists who redid the Miller-Urey experiment said. And even now, millions and millions of years later, there is still life very similar to what we think this first life looked like there: bacteria that uses sulfur to chemosynthesize energy much like their near-surface counterparts photosynthesize the sun.
What most people don’t know is that first life could also have originated from ice. When seawater freezes, the salt gets pushed out of the ice. These so-called brine channels are very nutrient-rich, and bacteria thrive in them. Another likely contender for first life.
In reality, it’s probably both—and many other ways we haven’t even considered, too.
Our universe is 14 billion years old. Our planet Earth is 4.6 billion years old.
LUCA, the last common ancestor that connects all living beings on earth, not just animals but also plants and fungi and all the really small things humans tend not to count as life. Organisms started spreading out into the different branches on the tree of life “shortly” after. Shortly on a millions-of-years timescale, of course.
A lot of evolution lies between this LUCA and Homo sapiens, but evolution started even before. If you follow standard procedure, you need DNA to make proteins and proteins to make DNA, so to get from those amino acids shown to form from inorganic matter to actual living and reproducing organisms takes quite a few steps.
You know how much I prefer life large enough to see with my bare eyes, but this crab is so cool, I’ll go even smaller than a cell to our genetic material. Bear with me.
You’ve all heard of DNA before. RNA is pretty similar. Their chains are made up of sugar phosphates and one of four so-called nucleotides, with letters representing each (an abbreviation of the corresponding chemical compound). There’s adenine, for example, a chemical compound found in the As of our genetic code, but also in adenosine triphosphate, one of the major energy storage of our cells. Pretty important little compound. These little things are pretty selective, though. Guanine and Cytosine like each other but mistrust Adenine and Uracil (or in DNA’s case, Thymine), and vice versa. Kind of like sexual orientation, though they are very strict and binary about it.
RNA is usually involved in bossing around the protein synthesizers, storing genetic information, and such, but they are probably also what got life started.
I won’t go into the usual function of RNA here, but check out the video about protein synthesis by the Amoeba Sisters or their channel in general. I’m not one for anime eyes, but they are pretty good at their job.
So, what’s so special about this compound? Well, you know how DNA forms this helix and then unfolds to get read? RNA is only one of those strands, and it can fold up on itself. When it’s folded up like that, it can act like an enzyme and create, you guessed it, proteins. So, when I said earlier that you need DNA to make proteins, this is the unusual case.
So, we’ve got RNA folding up and making enzymes, but what this really means is that they are now able to replicate. And anything that replicates can undergo evolution. RNA was probably the first replicating “organism,” for lack of a better word, and the first whatever to undergo evolution. Pretty damn cool for such a small whatever-it-is.
So, the RNA World Hypothesis essentially says this. At some point in time, Earth’s chemistry lead to random RNA and this RNA performed it’s magic trick and replicated and evolved. Tadaa, life.
With a little help, scientists have been able to actually make this happen in labs. It’s quite possible this is actually real. I mean, life had all the time in the world to happen upon those perfect conditions.
There are some alternative hypotheses like the aforementioned primordial soup hypothesis and the metabolism-first hypothesis, which assumes that acetate spontaneously formed and life started there, so feel free to look into those and learn more. I babble enough without going down every rabbit hole.
If microevolution happens within species and macroevolution creates new species, should we call this chemical evolution teeny-tiny evolution? What comes below micro? Duck. What did we come up with? Right. The Giant Meaty King Henry Died While Drinking Chocolate Milk, Mmmh! Nasty! Poor Fatty! Ah, right, nano and pico and femto came after micro. I like teeny-tiny better.
But what even is evolution? I’ve been asked this question by quite a few teachers in quite a few classes. Each time, we get a different answer in response. But there is some consensus: evolution is some form of change in the gene frequencies. Mutation (so random changes to the genetic material) lead to variation. Some of the population is smaller or larger, faster or slower, darker or lighter.
And then: Survival of the fittest.
I’ll return to larger things because it’s just so much easier to wrap your head around. If the birds can see you a little less well against your surroundings, you might be the mouse that survives. So, as you are still alive and breathing, you’ll also be the one to, let’s stick to scientific terms, duck. As there are no condoms in the mouse world, that means tiny little baby mice with that survivor’s genes. (Side note: that was a completely non-gendered paragraph. What gender was your mouse? Your own? Male default? Female because of the gender mice have in your language?)
Well, if you’re the female, you also have to survive long enough for them to get born. If you’re the male, you gotta hope that the female survives, but there isn’t much you can do about that—except maybe get seen less by ‘em birds who hunt you.
So, whatever helps a little in survival gets passed on to the next generation. When people first came up with these ideas, there was this dude called Jean-Baptiste Lamarck (and potentially a lot of other people) who thought that evolution happened within a lifetime and that what you did changed your genes. So if you were a giraffe, and you stretched your neck a lot to reach the higher leaves, you’d pass a stretched neck onto your kids who would continue the stretching. Interesting concept, really.
So, we know how evolution happens. Evolution can happen when something in the surrounding changes or in response to other species evolving. If your food evolves, you have to, as well. And if your predators evolve, you better be the one with the right characteristic.
The same mechanisms also lead to speciation, so the breaking off of one species from another or one species breaking in two. However, you wanna see that.
But let’s return to our little cells for a moment. We left them in a single-celled state. Still a long way to go from there to the diversity of animals, plants, mushrooms, and protists out there. And that’s not even mentioning the multitude of single-celled things still alive now. And technically, there’s also kinda multicellular prokaryotes. But let’s not get into multicellular bacteria, okay? Though it might be worth mentioning that cyanobacteria are pretty major player anywhere, and they can only fix nitrogen and use oxygen because they have specialized cells.
Multicellularity evolved at least fifty times independently. I guess that means it’s pretty useful. And the essence of it is that multicellularity means multitasking. And I don’t mean watching Netflix while you do your dishes. No, I mean breathing while digesting. Multicellular organisms can distribute tasks between cells. They are also larger and can thus fill new ecological niches.
Oh, and then there’s the minor benefit of not dying every time a single cell dies, I guess.
Of course, it’s not all roses and sunshine. Multicellularity is also pretty energy-draining. Populations are smaller, and it takes longer to grow up (and thus duck and reproduce and all that evolution stuff).
Through fossil records, quite a bit of guesswork, and genetic analysis, we can now follow the branches on the tree of life to where we are now. Some organisms like the axolotl, Ambystoma mexicanum—did you know that axolotl can actually molt and convert to a terrestrial life? It apparently happens when they eat too much salmon and something builds up in their cells. They turn all dark and blue and move out of the water. There’s a guy talking about it on Reddit here. Wait, where was I?
Axolotl and other ancient species haven’t changed much in recent history, other species have adapted and adapted, split into species, adapted some more, and created the diversity we see around us now—well, the diversity that is vanishing before our eyes now.
With all that it took to get here, I’d really prefer not to duck it all up. And Homo sapiens is speeding up species extinction by 600 times. Great.
Sources:
I study Marine Ecology at the University of Hamburg, so a lot of this knowledge comes from hours of research and sitting through lecture after lecture.
Going through the lecture slides from school is a process that involves a crab-ton of fact-checking, as a lot of what we learn is pretty outdated. So, all semester, I google things to death, read papers and essays, ask a million questions, and discuss things with friends and classmates.
Where the source isn’t our lecture slides or unidentifiable sources from hours of late-night knowledge hunts, I have linked them in the text.