by chelsea schuyler

I have this vague memory from high school Natural History of mitochondria being crazy amazing, so I looked into it and will now drag you with me.

Refresher course: Mitochondria are those little bean things inside all animal, plant, and fungi cells, providing truckloads of energy from oxygen. Chloroplasts are the little green blobs inside all plant cells that make energy from the sun.

Before we had mitochondria, we were just piddly little simple cells called prokaryotes. Then we got them and became complicated, multi-cellular stuffs called eukaryotes.  So how did we get us one of those?

Nautilis magazine says “The transition from the classic prokaryotic model to the deluxe eukaryotic one is arguably the most important event in the history of life on Earth.” Wow, the most important event in the history of life, and it isn’t LIFE? This better be good. Like sliced bread, but times at least 5.

So, IN THE BEGINNING, life happened!

Followed by a billion years of single-celled blah. What was there to eat at such a time?  Well, a few million of these pajama’d prokaryotes stumbled into the kitchen, held the primordial fridge door open, and stared at the emptiness. Recall that this was a desolate time, no oxygen in the air, no coffee in the pot.

Eventually they just stirred up a roux of carbon dioxide and hydrogen. And since everybody poops (even then), they pooped out methane. And because everything is funnier when named after poop, they were called methanogens.

An hour later, a million prokaryotic roommates rolled out of bed and were like, “Dude, way to eat all the half decent gases, methanogen, what am I supposed to do?” But then they went to the window and said, “Hey look, sun.”

And so they began photosynthesizing, eating the sun’s light and pooping out oxygen. (Scientists say “exhaling” and they have a right to their wrong opinion). They were called cyanobacteria.

Wait, you say, I thought there was no oxygen in the early atmosphere, but if cyanobacteria is spewing it out everywhere, what gives?

The thing is, it can be difficult to get free oxygen in any atmosphere because oxygen is the slut of the Periodic Table. It will get together with almost any element. It sees a little Carbon, bam, CO₂. Sees a little Hydrogen, bam, H₂O. So though Cyanobacteria was releasing oxygen, it ran off with that motorcycle-riding hooligan, methane (1 carbon, 4 hydrogen) and changed into carbon dioxide and water.

So all seems in balance as methane, like their dinosaur proteges, ruled the world. But then, (spoiler alert) the Earth shot up from .01% oxygen in the atmosphere to 21%, turned into a giant snowball and everybody died! Almost. It’s called the Great Oxygenation Event (GOE) or if you’re feeling more dramatic, the Oxygen Catastrophe!/Oxygen Crisis!/Oxygen Revolution!

There are a few theories to explain the GOE, because it wouldn’t be science if we didn’t argue furiously.

My favorite theory for no justifiable reason is the Nickel Famine Theory. One little detail I forgot to mention is that methanogens need nickel to help their metabolic process. This probably involves “organic chemistry,” which no one can do, so we won’t go into it here. Anyway the Earth was cooling, and there was a lull in volcanic activity, lessening the amount of available nickel. Hence the Nickel Famine (which particularly affected the methanogens off the Irish coast).

Man, if methaogens had a nickel for every time that happened…

As the amount of methane reduced, Oxygen increased,  but Oxygen is so slutty that it even binds with itself, to make O₃, ozone. I may have also neglected to mention that in order for oxygen to react with methane, it needs UV rays from the sun. An ozone layer would have blocked UV rays from getting through. No rays, no methane reactions.

There’s a lot of other ideas too, but i’m trying to talk about mitochondria here. Remember the most important event in the history of life? Tryin’ to get to that.

So, yadda yadda, the Great Oxygenation Event happens, spurring the atmosphere into toxicity and causing both the first major extinction and the first known ice age for 300 million years, whatever. To explain the epic moment when eukaryotes appeared, we need to introduce the three main characters.

1. Basic prokaryote – eats other prokaryotes. We’ll call it Pac-Man

2. New! – Aerobic Prokaryote – figures out how to take in oxygen and make energy. We’ll call it Inky.

3. Classic model – Cyanobacteria – still making oxygen from the sun. We’ll call it Blinky.

Trust me, this will end well.

So the basic prokaryote, Pac-Man, is chillin’, eating fruity, pellety prokaryotes, thinking nothing of being the ancestor to every eukaryote known to future Earth, when it comes across oxygen-eating Inky. It eats Inky, but Inky is badass and doesn’t die, it just sits inside the Pac-Man’s body. Eventually, Pac-Man realizes that it can use the extra energy that Inky makes to become awesome, like eating the mushroom and becoming big Mario. Oh god, double video game analogy.  I’ll stop.

In Science-Nerd terms, a prokaryote literally engulfed the aerobic prokaryote, and they eventually evolved to benefit each other. Soon the aerobic prokaryote became dependent on its host and gave up its free-moving lifestyle altogether to become, mitochondria. The result? Eukaryotes, i.e., animals. Not like, giraffes or anything yet but future animals.

Not long later, mutated Pac-Man ate Blinky too, and the little sun-eating cyanobacteria also became dependent on its host, gave up its free-moving lifestyle and became chloroplasts. i.e. plants. (again, future plants.)

This origin story is known as Pac-Man-itude, or more commonly, the Endosymbiotic Theory.

How do we know this? Scientist Lynn Margulis (and the first wife of Carl Sagan) worked tirelessly to prove this theory in the 1960s. We used to think that mitochondria just evolved from other parts of the cell, but she was all Free Love man, and wrote a radical paper arguing for cells engulfing each other and living in harmony.

Her idea was rejected, as it had been when others considered it decades earlier, but she persevered with evidence. Mitochondria and chloroplasts not only look like prokaryotes, but they reproduce independently of the cells they live in, dividing by fission instead of mitosis (just like prokaryotes). What really sealed the deal though was when they were discovered to have their own DNA, not the double helix kind like ours, but a wonky, circular kind, just like the ones prokaryotes have.  Colonel Mustard in the Library!

So why do eukaryotes get to be cool, multi-cellular animals n stuff and prokaryotes have to stay as tiny, simplified things? Well, to be big and complex you need instructions to tell you how to do it, in other words, genes. Prokaryotes have some DNA, but barely (up to 10,000 times fewer than euks). That’s because copying DNA and then making the corresponding proteins is friggin exhausting, and without mitochondria to pump, them up, they gotta stay as girlie men.

Back to early Earth. The thing is, though the atmosphere now had oxygen, and eukaryotes were kickin it, the ocean (where life lived) was still pretty oxygen-less. For a long, unexciting billion years, literally called “the boring billion” by scientists, evolution was pretty much at a standstill.

The ocean was full of iron, so any oxygen around saw it, and bam, Fe₂O₃ (iron oxide) leaving iron bands on the rock.  But eventually the iron ran out, other elements and minerals shifted around, and the ocean became oxygenated.

With oxygen to fuel the success of the mitochondrial mutates, the boring finally ended as a massive diversification of plants and animals took over the ocean and partied like it was going to be 1999.

The coolest thing? This magical engulfing happened only one time. Lots of things in evolution happened multiple times, called convergent evolution. In other words, different species evolve basically the same trait without having to be descended from the first one to do it. Eyes, wings, even purring evolved multiple times in different cat species.

But not the prokaryotic merger. That came down to a single cell able to protect itself from the total freakiness of engulfing another cell and neither of them being completely annihilated. Happy ultimate father’s day.