Okay, I think that a lot of us tend  to think about, and talk about, evolution in the wrong way -and hear me out because I know I've done this for a long time too Like, we think of the aquatic creature climbing  out of the water onto land for the first time, with the benefit of bigger, stronger forelimbs.

We think of that terrible and  misleading the so-called March of Progress, which gets so many things wrong, not the  least of which is the idea that humans are somehow the pinnacle – the ultimate  culmination – of the evolutionary process.

We think that evolution only goes  in one direction — toward getting bigger, more advanced, and more complex.

But I’m here to tell you that’s not the  case.

Things turn out to be much more nuanced than that.

And the proof is what’s perhaps the  most bizarre animal in the world.

Yes, this is an animal.

It’s among the smallest and  simplest animals ever found.

It has no mouth and no nervous system, and at  least one species can’t even breathe oxygen.

How did this thing come to be?

Well, it used to  be … something else.

Something a lot more complex.

Until it evolved … but in a way that’s  the opposite of the way we’re used to thinking.

The name of this strange little animal is myxozoan, which means “slime animal,” I mean honestly I've been called worse... and there are more than 2000  known species of them today.

And they all make their living  by being parasites - they rely on infecting hosts in order to survive.

The vast majority of them have life  cycles that involve alternating between infecting invertebrates - mostly annelid  worms - and vertebrates - mostly fish.

And while their life cycles are kind of complex,  that's really the only thing about them that is.

Like I said, they’re incredibly small, both in terms of their physical size  and in the size of their genome.

For most of their lifecycle, they're  smaller than the width of a human hair.

And one species has a genome that’s a contender  for the smallest animal genome ever known.

And they’re missing some of the most basic  features that we associate with animals, like a mouth, a gut, a nervous system,  and true gametes - egg or sperm cells.

This level of simplicity is why myxozoans were  grouped together with single-celled protozoa for more than a century after  they were discovered in 1838.

But, over time, we began to realize that  this classification just couldn't be right.

For one thing, myxozoans aren't single-celled - at least, not all the time.

At certain  points in their complex lifecycle, they’re multicellular - although just barely,  consisting of only a few cells, at most.

And as researchers dove deeper  and deeper into myxozoan biology, they came to a bizarre, but  inescapable, conclusion: Myxozoans started out much larger  and more complex – specifically, they used to look something like modern jellyfish.

But then, over time, they shed many of  their biological components to become much smaller, simpler animals.

Experts were able to figure this out, because these animals still hold some  clues about their evolutionary past.

For example, they still have a few genes  that are found only in the animals known as Cnidarians, which  include jellyfish, sea anemones, and corals.

And an analysis of their DNA has found  that myxozoans are closely related to the subgroup of cnidarians called  Medusozoans - aka the jellyfish.

And beyond the genetic evidence, they’ve also kept  at least one physical feature from their ancient jellyfish-like past: they have nematocysts, or stinging structures.

Nematocysts are only found in cnidarians, and they use them to do things like  capture prey or defend against predators.

And myxozoans still have them, along  with the genes and proteins needed to build and use them.

But they seem to have  adapted them for their parasitic lifestyle.

I think it's funny that we call it a lifestyle- because it's kind of a charitable way of putting it don't you think.

like keto diet is a lifestyle, stand up paddling is a lifestyle, being a parasite is like....

I would classify that differently- but.

They’ve lost the toxins that the  nematocysts would normally inject, so they’re no longer used as stingers.

Instead, they’re used as anchors to  help the animal attach to its host.

It’s one of the few physical  features that they kept and adapted, rather than just losing altogether.

This process of simplification is  sometimes known as regressive evolution, and it’s pretty common among parasites.

I mean, after all when species can exploit  some of the features of their hosts many of their own features just become … redundant  like– nothing more than evolutionary baggage.

These unnecessary features  can then be lost over time, which means the animal can expend  less energy and resources on them.

For example, parasites often become  less mobile, because their hosts can move around and get them from place to place.

And  they have reduced sense organs because, again, the host is keeping track of  their surroundings so they don’t have to.

But, we’ve never seen any other  group of animals go through such an extreme process of regressive  evolution as the myxozoans have.

I mean, at least one species of myxozoan has  even lost its mitochondrial DNA, and with it, the ability to carry out aerobic respiration  – that's the process of generating energy in the presence of oxygen – because the genes that make  respiration possible are in the mitochondria!

This makes it the only known animal that  doesn't, and physically can't, use oxygen!

So how do we know that these  parasites are actually animals, even though they’re missing so many  hallmarks of the animal kingdom?

Well, they trace their ancestry to the animal  kingdom, which makes them still animals.

It’s kind of like how birds  are still considered dinosaurs, even though they don't look much like what  people picture when they hear the word dinosaur.

It all comes down to a  group’s evolutionary history.

Sowhen did myxozoans begin  their weird evolutionary journey?

Well, the fossil record of myxozoans is basically  non-existent, because they’re tiny and … squishy.

But by comparing the genes of myxozoans  with the genes of other animals, researchers could roughly calculate how long  it would have taken the differences - aka mutations - to take place, after they  diverged from each other.

And this analysis, called the molecular  clock, places the origins of the group at around 650 million years ago when they last shared a common ancestor with the Medusozoans.

These ancient roots make them the  oldest known parasitic animals.

And while they were probably more anatomically complex than they are now,  their lifecycles were simpler back then.

Today, most myxozoans alternate between  using invertebrates and fish as hosts, but 600 million years ago, fish didn’t exist yet.

So scientists think that their original hosts were aquatic invertebrates, like  bryozoans and annelid worms.

Then, when fish arrived on the scene  over a hundred million years later, myxozoans began infecting them  too, and they wound up with the two-host, invertebrate-to-fish lifecycle  that most of them still have today… Which probably happened independently  multiple times, as new groups of fish evolved.

As their hosts diverged and  radiated, so did the myxozoans, in a co-evolutionary dance  that continues to this day.

And this is probably the key to the  incredible diversity of living myxozoans.

The 2000-or-so species that have been described so far represent around 20%  of all known cnidarians.

The story of the myxozoans makes  us fundamentally reconsider what it means to be an ‘animal,’ by showing  just how unrecognizable one can become, given enough time and the  right selective pressures.

And it shows us that evolving doesn’t have  to mean becoming bigger, or more complex.

Sometimes, less really is more.