Basket Stars

Each of your five arms develops arms of its own.  Those arms develop even smaller arms.  This repeats until your arms fill the gaps between them like a filigree. When you hold them out, they look like a fine mesh basket, which gives you your name: a Basket Star.

Beautiful, but mostly unseen, living in the deep cold waters of the northern hemisphere of the planet,  you crawl across the ocean depths, hiding during the day, and feeding during the night.  You are a pitiless hunter, grabbing krill - small swimming creatures -  in your basket and eating them.

Brittle Stars

A neat, small central body with serpentine arms reaching out from it enables you to move rapidly across the ocean floor, grabbing on to whatever you can to pull yourself around.  However, your relatively small body prevents you from eating anything sizeable, and so you remain a detritus feeder, scavenging your way around the deep seas of the world.  You are a Brittle Star.

Holothuroidea

Getting bigger, longer, musclier, and thicker, you have a long, efficient gut, and can get all the nutrients out of your food.  You live through several stages in your life now: tiny worm, little star, bigger star, until you become so thick and muscled that you tilt back and begin to look like a worm again, but a huge, thick worm, the size and shape of a common vegetable: you are a sea cucumber.

You still have radial symmetry, but unlike your sister species, your axis reverts back to horizontal, and you crawl or float or swim along with two 'legs' down, two to the sides. and one on top.  You secret a poison called holothurin which keeps you safe from many predators.  You mostly eat whatever detritus you find on the bottom, or whatever floats past you in the current.  Different species range from a few millimetres long to a whole metre long, and you live throughout the seas, although you are at your most dominant in the deep sea.

Asteroidea

The centre of your body is wide, giving you a large stomach, and your 'legs' are more triangular to fit around it. This shape gives rise to your popular name: the starfish.

You walk around on the ocean floor, in depths ranging from sunlit coral reefs down to abysmal depths, with your lightly armoured back protecting you from most species, your mouth facing downward, and no particular direction being 'forward', although some legs may be preferentially used.  Your legs have simple small eyes at each end.

With your powerful tube feet able to pry open bivalves by tiring out the muscles holding them closed, you become a predatory species, eating not just tiny bits of food like your recent ancestors, but whole small shellfish.

Some of you develop a neat trick of turning your mouth and stomach inside out and digesting whatever you press it against.  This enables you to consume even larger prey as your mouth no longer needs to be bigger than your prey!

Many of you develop a fascinating life cycle, where the young are male, but then turn into females after a certain age.  Some reproduce by budding, and some can regenerate their entire bodies from a single arm.  Truly, you are one of the most amazing creatures on the planet.

Echinoidea

You simply can't stop big jaws from crushing you.  But you can make it so unpleasant that these newly evolved predators will choose someone else.  The flat plates you already have become lumpy, and bumpy, and unpleasant to put in your mouth.  As time passes, those lumps and bumps get pointier and pointier, until you look like an explosion in a needle factory.

You are a sea urchin - a round-ish body covered in pointy spines.  You roam around the ocean from the tidal zone down to around three miles down, and very few creatures will eat you.  Those that do - crabs and lobsters, triggerfish, eels, humans -  avoid the need to bite down with soft mouthparts on spines by having special adaptations such as strong claws or bony mouths.  Or hammers.

Medium Sized Gut Echinoderms

Your gut fills up all the space inside you, but you don't grow to create space for a really really large gut.  You can efficiently move around, find food, eat it, and digest it.  But while you've been evolving, so has everything else:  The Mesozoic Marine Revolution has arrived...

Elsewhere, some fairly close relatives of yours have just developed something called a 'jaw'.  It is a large, leveraged, crushing device, which means that even relatively tough creatures like yourself are vulnerable.  These new-shaped creatures roam the oceans, spotting slow moving, armoured individuals of your species, pick them up and crush them to bits.

You have a thick, armoured skin, but it's simply not enough to protect you against the big creatures who are evolving now.  You can't really get tougher fast enough.  In this arms race, increased toughness will lose to a bigger and bigger jaw, and more and more armour, until you look like a concrete block and you can't move at all.  Some other approach may work, or you can just run away.

What would make you really unpleasant to eat?

Spikes.  All over.
Save the weight and effort of growing spines, and just run away!

Crinoid

You make lightweight armour from the calcium carbonate dissolved in sea water, but concentrate your energy on reproducing as fast as possible.

You float on the end of your 'stalk' or 'holdfast', a cup shaped flower, with both mouth and bum on top.  Many of you are eaten by creatures who have evolved jaws, but you grow fast enough to spread far and wide.  You spread like weeds across the seas of the prehistoric world, shedding so many thin calcium carbonate armour plates that entire chalk deposits, tens of metres thick, are entirely made of the broken up bodies of Crinoids.

Around 252 million years ago: the planet either gets hit by a huge space rock, or super-volcanoes explode, and the climate changes rapidly.  The seas change, becoming a little more acidic, and you can't make the plates of calcium-carbonate armour any more. Luckily, that's not crucial for your survival, and many of your species live on, adapting to become more flexible again.  Some of you become free-swimming as adults, with many flexible arms which float in the currents like grass in the wind, and inhabit both deep waters and reefs, where you are admired for your beautiful colours.

Blastozoa

You make thicker and thicker armour from the calcium carbonate dissolved in sea water.  It keeps you safe - for now.

You float on the end of your 'stalk' or 'holdfast', a small, five-sided ball, with a mouth on the top, and a bum on one side as far down as possible to take advantage of gravity.  On each of your sides, you grow an armour plate, and you filter feed in the shallower waters of the prehistoric world.  Many of you are eaten by creatures who have evolved jaws, but you grow fast enough to spread far and wide.  It's a good balance between the cost of armour and speed of reproduction.

We know of you now as one of the most common fossils of the Cambrian era, showing that there were uncountable billions of you.

Sadly, we can't know of you as a living species, because something quite bad happens around 252 million years ago: the planet either gets hit by a huge space rock, or super-volcanoes explode, and the climate changes rapidly.  The seas change, becoming a little more acidic, and you can't make the plates of calcium-carbonate armour any more.

You all die.  All the uncounted billions of you.

On the other hand, you're not alone, because around half of all life forms on the planet also die.  This is the Permian-Triassic extinction event, commonly known as The Great Dying.  It will take millions of years for the planet to recover.

Echinodermata with U-shaped gut

A flower shape, which collects passing food, with a stem, sometimes short, sometimes long, and a gut that loops back, is a pretty good shape.

Your 'stem' grows longer and longer, so you can reach more food, passing it to your central mouth via many tiny 'tube feet'.  Some of you flip over, abandon your stem altogether and roam around, using your tube feet to move. You develop armoured plates which protect you from predators.

However, we have now reached the Mesozoic era, and some seriously large animals are now roaming the oxygen-rich Earth.  They have recently evolved jaws, which are good for crushing your armour.  Do you want to get into an arms race, and get bigger and harder armour plating, or live fast, eat well, and die young, having reproduced as fast as possible?

Armour me up!
Feed me up!

Echinodermata with I-shaped gut

A flower shape, which collects passing food, with a stem, sometimes short, sometimes long, and a gut that goes right through you, is a pretty good shape.

Your 'stem' grows longer and longer, so you can reach more food, and eventually you flip right over like a wilting flower, and end up 'face down' on the sea bed.  You can use your tube feet to move around now, as well as eat.  You already have hard scales on the 'back' of the flower, so you are reasonably well protected, and you grow bigger, and bigger.  Except during your larval phase, you no longer look even remotely like the worm you were, just a few choices ago, and you don't even need the stem any more.

In evolution, anything which is not needed is just extra cost, and so the genes which create the stem are selected against, and you become a free-roaming little star-shaped animal, with an armoured back, eating whatever you walk over, and excreting the waste through an anus on top of you.

Now that your mouth is at the bottom, and your bum is at the top, it might even be more efficient to just dispense with that too, and let any waste fall back out of your mouth.  Sounds awful, but  efficiency is often king in evolution.

Leave my bum alone!
Don't need a bum.

Last Universal Ancestor

You're at the beginning of life on Earth... you are a bubble of fats, held in place by proteins, containing some salty water and the amazing molecule, DNA.

You're at the beginning of life on Earth... you are a bubble of fats, held in place by proteins, containing some salty water and the amazing molecule, DNA.

You have genes within your tiny, circular, single chromosome (called a ‘plasmid’) which can process the simple sugars which move easily through your fatty wall and turn the energy into copies of themselves.  

At this stage, reproduction is still very much at the gene level.  There are so few genes in existence, with such general functions, and such a thin cell membrane, that it doesn’t really make sense to describe you as a reproducing set of genes, an organism.

Sometimes your bubble splits and a selection of those copies gets separated, creating a new ‘you’.  Sometimes your bubble bounces into another and you merge.  Sometimes the copy process is disturbed, and a new gene is created.

Eventually, one bubble accidentally creates a gene which makes a simple cell wall form, and although it reduces the rate at which sugars can enter, it means that this particular set of genes get isolated and can create copy after copy of themselves without merging in any other set.  This is the point at which we can start thinking of a single organism…  it is called a prokaryote*.

Slowly sugars enter your cell, and slowly you utilise the energy they contain, make copies of your dna and reproduce.  It’s a relatively long-lived form of life for this era.  You get to make many copies of yourself before eventually some bad luck takes you out.  You still, on occasion, swap genes with others around you, and sometimes there are errors in copying, both of which create new organisms.

In order to create the most copies of yourself, you need to make efficiencies in the way you process the available energy.  There are other energy sources which can improve you: the Sun, and other chemicals in the soup.  If you use the Sun’s energy, you can create the sugars you need right inside you.  Otherwise you can be more of a generalist about which chemicals you can consume.  After all those chemical pathways are close to yours, so only a small error might be enough.

More sugars
Sun

* This isn't entirely true.  There's another family called Archea which has a different type of cell wall.  However, it's only a few years since we realised they aren't bacteria, and so their evolution is incredibly poorly understood.

For a good discussion of the current state of Archea, try Wikipedia.  If some research comes up that changes what's been written here, then I'll bring it up to date.

Echinodermata

Once your larval form finds itself in a good location, it decides to stick around.  Literally.  It holds on to the bedrock, in the deep ocean, and waits for food to pass by.

Many species behave like this, but they all started off with a body shape which was suitable to filter feeding.  The most efficient shape to filter food from seawater, is a fan shape.  You are a tubular little worm, and it doesn't really work, although if you curve your body and look at it from one side, it'll do.  But this is evolution: you can't go backward, you have to do the best with what you have.  And when you do your best, something amazing can happen.  In this case, something really amazing.  So amazing that the great Charles Darwin described this as "the great mystery", although, like everything in evolution, it's simple when you know the answer.

One side is preferentially used to hold on to the rock.  The other reaches up to search for food.  Once you are in position, or 'sessile', therefore, there is an advantage in one side growing bigger and bigger, but the other can stay pretty much the same size.

There is now an evolutionary advantage in genes which make one side grow bigger than the other.  It happens to be the left side which holds on, and the right side which undergoes metamorphosis.

The right side grows, and as it grows, it adapts into five large sections, each of which grows outward to reach food, until it becomes something that looks like an arm.  Your arms are covered with tiny 'tube feet' which grip food by suction, and pass it to your mouth, now located at the centre.

Your left side is almost entirely re-absorbed during this process.  The result is an animal which no longer has bilateral symmetry, but radial.  You are a flower shaped eating machine, on top of a small stem.

There's a plumbing problem here though: your expanding right side curls right around your mouth, and the little tube feet on the upper surface pass the food to it, but where is your bottom going to go?!

If your right side grows around below your anus, then you'll end up with a U shaped gut, and mouth and anus on the same side of the body. If it grows around above your anus, you'll get an I shaped gut, and mouth and anus on opposite sides.  What's it to be?

U?
I?

Cycliophoria, or Symbions

Hidden away, deep in the cold oceans of the world, you stick close to the source of food you have adopted and adapted to: tiny fragments of food dropped by lobsters.  You have reduced in size, presumably to make sticking close to a lobster easier, and are now less than a millimetre in length.

Yes, you eat the crumbs from lobsters' dinners, by gluing your sac-like body to the mouthparts of a lobster, and waiting for something to fall your way.  Luckily, lobsters, like most relatively large carnivores, are messy eaters, and very few species actually eat them, so you have managed to get food and safety sorted at one step.

Soft Unsegmented Lophotrochozoa without a Lophophore

OK.  You don't want to run away.  You don't want to eat and reproduce as fast as possible.  We're running out of options here.  You can either get help, or fight back, by eating other things, the bigger the better.  The improved nutrition from eating other, bigger, organisms will enable you to get bigger and stronger, but there is another choice - you can hide away somewhere with a decent food source and keep going on what you can eat in secret.

Big and bold.
Secret snacker.

Soft Lophotrocozoa

If you don't want the cost of a shell, then as simply a small worm with an efficient central nervous system and a frilly mouth, then you'll need to find some other way to survive as a species.  There are lots of ways to achieve this really - you can move faster, eat and reproduce faster, or fight back.

The main way that you move is by squeezing the muscles in your abdomen to wriggle along.  Your head gets pushed in front of you, and your gut just drags along behind.  If you increased the number of muscles you have, that might work.

Muscle bound hunk?
Slim and elegant?

Nemertea

Fight back!  Alright!  Let's do this.

You hide in the mud at the bottom of the sea, and wait.... when something yummy passes by, you turn your entire face inside out and stab them with a small, venom bearing, sting.  To eat them, you wrap your face around them and turn it back in again, drawing them inside, then go back to hiding in the mud.

Some of you develop long tails, anchoring you deep into the mud, where you stay for a lifetime.  Others swim or crawl on a slime trail between meals.  Hardly any of you are food for other animals.

It's a successful approach... if you like that sort of thing.

Annelida

Your body plan, so far, has been simple, like all worms.  Head, containing brain, mouth and sensory organs.  Thorax, containing lungs and other organs.  Abdomen, or tail, containing the gut.  The best muscles are around the thorax.  If you want more of those, there's only one effective way to do it: make another abdomen.  Then another.  And another.

Yes, you make segments of your body duplicate in their entirety: you become a segmented worm.  This turns out to be incredibly good.  You have redundancy in your organs, speed, and a nice big gut to get the most out of what you eat.

It's so successful that you develop into well over twenty thousand separate species (we just can't cover them all) including the earthworm and leech families.  You can swim, by undulating, wriggle through the soil or crawl across the top, using protrusions from the sides of your segments.  You are an enormously important aspect of soil health, and you are found just about everywhere that is not frozen or completely dry on the planet.

Soft Unsegmented Lophotrochozoa

Slim and elegant, eh?  That's nice, but something's still got to change.  If you speed up the rate at which you get food, you will be able to reproduce faster, and your species will survive even though you get eaten a lot.

Stuff my face as fast as possible.
No!  I said slim and elegant.

Lophophorata

That frilly mouth of yours we just mentioned?  It's pretty good.  One of the easiest ways to evolve is to do something you've already done - but even more.

You grow frills on your frills!

The super-frilly mouth is called a 'lophophore'.  You can hide away in the mud, and slide out your frilly frills and quietly eat whatever passes by.  As time passes you adopt many shapes, but all basically mud-hiding, filter-feeding, hard little worms.