Eukaryotes: Life’s Biggest Leap

If beauty is simplicity, and complexity disease;
then bacteria win, the true beauty queens.

The problem with being human is that you get an exaggerated sense of your own self-importance. You see tiny things in microscopes and think they can’t possibly matter as much as you in your big fancy flesh suit. We tend to think that the most important bits of biological history, are the bits about how our big fancy flesh suit started to come together (how did we get from some crabs and worms to us?). And that is absolutely an interesting story. However, the more interesting process, the one that took us three times as long, was the journey from an organism without internal structure, to one with it. Refining the flesh suit is easy. Creating the underlying complexity which allowed the flesh suit to exist was much harder. But we got there eventually. Let’s dive in, to Eukaryotes.

What is a Eukaryote? Why do they matter?

Eukaryotes are organisms whose cells have internal pockets where they do special chemistry. Prokaryotes are simpler creatures (like bacteria) that have no internal pockets and therefore have only one chemical environment in which to do things. This limits how complex they can be and what they can do. This picture shows a paramecium (a Eukaryote) and all the little internal structures it has developed (often from swallowing other bacteria – other bacteria can survive in there and do helpful tasks for the paramecium. It’s a really beautiful picture. Credit to Motic for the wonderful microscopy.

Life on earth can be divided up into lots of different categories – animals and plants, carnivores and herbivores, blue animals and animals that are not blue. However, the most basic and important distinction is between prokaryotes (creatures without internal structure in their cells, i.e. bacteria, archaea) and eukaryotes (creatures with these internal structures, i.e. plants, animals).

The moment these Eukaryotes evolved for the first time is probably the most important moment in the evolution of life on Earth (even more than photosynthesis – see our post here).

Prokaryotes, because of their lack of any compartments in their cells, can only do very basic things. Imagine this like cooking in your kitchen, but you only have one bowl. You need to store the recipes in there. All the wet ingredients and all the dry ingredients. You can bake it or freeze it, and you just have the one environment in which you need to do everything to make a meal. This is what the inside of a prokaryote cell is like. Because their entire internal environment is undifferentiated, they cannot have any extreme environments and are therefore limited in the sorts of chemistry they can do. Their DNA just floats loosely around in their cell (better not do anything to mess that up!).

Now you can take this analogy too far. Bacteria can still be incredibly sophisticated. In fact, most people really probably underappreciate how complex bacteria are. They have little wavy arms that they use to propel themselves. They can shoot little disks of DNA and chemicals at each other to try and re-write each other’s genes. Bacterial combat is wild compared to boring animal combat. They aren’t just biting each other or running into each other with their horns.

Now consider a eukaryotic cell. They have lots of little containers in which to do their chemistry. They can have a little bucket where they keep everything really acidic to recycle their used parts. They can have a little bucket to do their energy generation, another one where they build their proteins, and perhaps most importantly, a final, really thick bucket where they keep their DNA protected from the rest of the crazy nonsense they get up to. Imagine that now your kitchen has a bunch of specialized environments to do things in. There’s an oven and a fridge. A recycle bin and a freezer. And perhaps most importantly, you can store your recipe books on a nice safe shelf where they aren’t going to catch on fire or get wet.

How did the first Eukaryotes evolve?

The first Eukaryotes evolved when one bacteria ate another smaller bacteria. The smaller bacteria continued to live inside the big bacteria. The smaller bacteria slowly became more and more reliant on the larger one, until the smaller bacteria became more like an organ of the bigger bacteria than a separate creature. This is an image of Volox – a colony of algae that live in big groups all inside one larger “cell”. It’s not actually a Eukaryote, but we are using its picture to show how different small creatures can combine together to form larger ones – the same way early Eukaryotes did. And it’s very pretty (Image: F Fox, Wikimedia).

The evolution of Eukaryotes is one of the better-understood bits of early life. Put simply one bacteria ‘went om nom’ and ate another. Then, instead of dying, it started to live happily inside the bacteria and assist by providing the bigger bacteria nutrients it needed, and feeding on the nutrients the larger bacteria consumed. Over time, like an organism evolving to survive in a new environment, it specialized in creating specific things for its larger host bacteria, and became more like an organ of the larger bacteria, than a separate organism.

Now, this sounds pretty ridiculous to us as large animals. If you ate a smaller animal, things would not work the same way. But bacteria are very different, and it’s surprisingly easy to “swallow” a compatriot. In fact, it happens all the time. Bacterial cell walls are more like liquids, so being consumed is more like popping in and out of a bubble, than actually being eaten. The process of one bacteria, popping into another bacteria’s bubble and living there happily ever after is called “endosymbiosis”. Generally, the smaller bacteria (the “endosymbiont”) pays rent in the form of doing helpful things for its larger host, so that both organisms benefit from their living situation.

The date for when this happens first is pretty easy to check. The earliest Eurkaryotes turned up in the fossil record about 2 billion years ago. And the internal structures of those Eukaryotes looked a lot like other smaller bacteria that were about at the time. Easy to see who was eating whom. We’ve also been able to recreate the event and see large bacteria absorb other Eukaryotes. It’s probably something that’s happened millions of times over history with different types of internal structures being created, depending on who the bigger folk ate. You are what you eat is never more true than when the thing you eat continues on living inside you.

So what does this all mean?

This is a picture of the internal machinery that exists inside a simple animal cell. You can see it is vaguely like the alga above – lots of balls with a few dots, but times 1000 in complexity and variety. We here at The Universal Story, think it is one of the most impressive pictures the human race has produced. This image was made by Evan Ingersoll and Gael McGill – amazing credit to them, further details here.

A fun fact that people often like to throw around is that “the genes of a very simple organism (bacteria/plant/whatever) overlaps 80% with the human genome”. This seems very hard to understand at first. Humans and bananas look very different. It doesn’t seem like an 80% overlap makes sense.

However, when you start realizing that so much of complex life happens at the cellular level, this starts to make a bit more sense. We like to think of ourselves and all other animals as single units, indivisible before the Universe. The problem is, we are not. Inside every living thing, are incredibly complex systems of the biological and molecular machinery that create lots of little environments to do chemistry to repair, build and arrange our cells. And yes, the overlap of this machinery between a banana and a human is almost 100%. The fact that our big meat suit has some bones does not make us that much more special or different.

All life is incredibly complex. We sometimes look down on the bugs and bacteria, as if they are simple, almost closer to dirt than they are to us. They are not. Even the most basic worm has incredibly complex internal machinery that took billions of years to form and does not exist anywhere else in our infinite Universe other than our planet. And if you looked at this machinery, close-up, most of it would be the same as ours. All life is fundamentally connected. Us humans should probably keep that in mind more.

This is an image of bacteria growing on algal plates. Scientist Tal Danino has perfected this art of growing particularly colourful bacteria and then staining them to create some incredible imagery – see more of these fantastic images from his Ted Talk here. We here at The Universal Story, think they are stunning.

Share this post: