Evolution isn’t a scientific theory. In fact, it has no more to do with science than does a tree. Evolution is an observable reality, while science is just one of the tools to observe it. Unlucky for us though, it usually becomes apparent on a large streaks of time, so we have to connect the dots we observe. My personal favorite example of evolution at work is Cnidaria, the group of organisms of which you definitely know a couple, like jellyfish, sea anemone or corrals. There are some 10-000 species of cnidarians, and though quite diverse, they all share a couple of traits.

You see, some unfathomable 600 million years ago a lucky marine organism discovered two life hacks. First, that being a muscular bottle with tentacles at the opening is neat, second, that having venom-filled pneumatic harpoons on those tentacles is even neater. These two inventions, or apomorphies if you’re into jargon, propelled the evolution of cnidarians. All of them share the basic tentacled bottle body plan, and all of them use the same weaponry to sting their foes and prey. So what did they do with that? The short answer: everything.

First, lets start with the basics. The body plan of a tentacled bottle is called a polyp. A lot of cnidarians remain exactly that. A fresh-water Hydra being a great example. It sits on some algae and uses its tentacles to catch and bring prey to its mouth – the opening between the tentacles. If it needs to move, it stretches its body sideways, steps on its tentacled end, stretches again, steps on its bottom end, then tentacled end again and so on until it has arrived where it wanted to be. But a hydra is small, and quite simply built. Some solitary polyps go further and grow much larger, up to a meter in diameter, like Stichodactyla sea anemones. They are still essentially a tentacled bottle made of muscles, but there are way more tentacles and the inside has muscular cross-walls to account for their huge body. Being huge allows them to eat bigger prey, like fish. However, even that isn’t enough for them, so they have symbiotic algae that give them their vibrant colors.

Now, what if you don’t like to be alone? What if you like to hang around with some friends? Well, in that case you might want to try being a coral. A coral is a colony of polyps, that grow out of each other’s body. It probably evolved from cnidarians’ usual mean of reproduction, by just sprouting a new animal on the side of the body and then cutting it off to live a life of its own. Colonial polyps instead decide to stick together and hunt in pack, sharing their meals via connected guts. So instead of a simple muscular bottle, you have a branching one, like a nightmarish tree, with tentacles on every branch and a single giant stomach inside.

This coral idea proved to be so successful, that several groups of cnidarians have evolved it independently. Not only you can have more regular meals by relying on your buddies to catch some when you’re unlucky, but also you have more protection – by having other’s weaponry alongside yours (remember those venomous pneumatic harpoons?) and you can divide responsibilities. To realize how important that last one is we only have to look at the bombastic success of multicellularity, which only works because each cell is single-mindedly devoted to a single task, with different lineages busy with different tasks, like red blood cells distributing oxygen, bone cells making bones and neurons governing it all. In corals’ case though that manifests in some polyps specialising in hunting and growing long, armed tentacles to catch swimming prey, while others get rid of tentacles entirely and become larvae-producing factories.

But being large chunks of muscle – however well armed – usually isn’t the best idea. Venom can be tolerated and one only needs to have thick scales to protect oneself from harpoon stings. So you need armour. And boy cnidarians have evolved the hell out of it. There are two basic types of armour in animals. One is outside of you, another is inside. Evolving any of those usually means a great success. Thus, our bones allowed vertebrates to dominate the megafauna, while chitinous carapaces made insects’ the dominant group of land animals in their size. Well, the corals not only have evolved both, but they also tried as many versions of those as they could. Black corals have a thick rod inside their colony, and when threaten they flat themselves, so you have to try hard to scratch them out. Some other species prefer greater mobility and have small bits of skeleton distributed within their bodies akin to plate armour. Others build fortresses that their whole colony resides in, with each polyp having a small bastion of its own, that it can retract to in times of danger. These fortresses form the basis of the coral reefs. And yeah, all those skeletons are build of nothing else but calcium carbonate, the thing marble is made of, so breaking it is kinda difficult.

But what if you don’t like to be a sessile fortress? What if you like to be free and roam the oceans? Well, there are several options you can do exactly that while still being a tentacled bottle. The easiest solution is to become a jellyfish. All you need to do is flatten the bottle on the vertical axis, so that the sides become this big round fold, with the tentacles being on the rim of it. The thing you usually think of as jellyfish’s tentacles are in fact corners of its mouth, stretched that long to help with the movement and prey catching. What will probably come as a surprise to you, is that most jellyfish are tiny and only serve as a way for corals to disperse. That is right, some corals produce jellyfish, they swim around, mate, then settle down and become a new colony. The big jellyfish though are the opposite of this. Like harmless boreal Cyanea, a couple of meters in diameter with tentacles tens of meters long. They are produced by tiny corals, that form stacks of jellyfish plates, which separate one by one and begin their life. And obviously some jellyfish got rid of the polyp generation altogether and produce larvae that become a small jellyfish straightaway.

I hope by this point you’ve already got the idea that if something is possible, cnidarians have evolved it. So learning that they have eyes (called rhopaliums) won’t shock you. Or that they are good at regeneration, to the point that some of them reproduce mainly by leaving traces of flesh on their path, that turns into new polyps. Or that some jellyfish never swim at all, but sit on the bottom of the ocean with their tentacles up and filter water through small holes their mouths have evolved into. Or that they are so big on symbiotic algae, that most of the coral colours come from that and they can’t really survive without food that the algae produces for them, so when the algae leave the coral – which manifests in a reef bleaching – the corals die soon after. All good? OK, then, time to talk about Portuguese man o’war.

This little animal, also called blue bottle jellyfish, isn’t a jellyfish at all, but a floating colony of polyps. But here’s the twist. The members of the colony evolved to a point when their individuality is completely lost. Other corals usually can have as many members of the colony as they wish, and you can tell individuals apart by their tentacle-surrounded mouths. Not the case with man o’war. Those guys completely merged to create a superorganism, with some becoming a bubble that acts both as a buoy and a sail, some forming reproductive organs, some being nothing more than a long tentacle, riddled with batteries of venomous harpoons, some – nothing more than propulsion pumps. This animal is truly a new step in colonial polyp structure, akin to the change from a single cell to a multicellular organism. But at its basis lies the same principle – muscular bottle with harpoon-armed tentacles.

This brief introduction into the amazing world of cnidarians should illustrate one important point about how evolution works. When a group of animals stumbles upon an invention, that makes their life so much easier (multicellularity, spinal cord, legs, chitinous carapace), they then explore this invention to the very bottom and do all kinds of crazy stuff with it. Sometimes they will even lose all other traits and organs, but those “core” ones, to squeeze into ways of living no other can have access to. For life isn’t just made of water, it acts like water and will – with enough time – enter all the niche crevices and spaces there’s access to.