Lophotrochozoa

In order to catch more food, you develop a ring of tiny tentacles around your mouth which pull food in.  This is called a lophophore.

In a marine environment, it is very useful to be able to pull in food as it drifts past, or as you move through the water.  There is plenty of organic matter in the water, some of which is alive until you grab it.

Unfortunately, some of the things being grabbed, are your young.  D'oh!  Maybe it's time to toughen up again?

Grow a shell?
Stay shiny?

Platyzoa

You remain a small worm, with an incomplete gut, which makes a living in the safe crevices of the world.

Many of you become flattened in order to maximise the rate at which oxygen travels through your skin, as you have no complex internal organs to pump blood or exchange gases.  Some of you acquire a common name of 'tape worm' as a result of this shape.

Some of the crevices you hide in are inside other animals, and you become a parasite, causing disease and slow death, as well as damaging the brains and reproductive capabilities of your host.  But what do you care?  You've evolved to survive, and this tactic is working well for you.

Soft Protostomia

Your muscles develop, and you become a faster and faster moving little worm.  You remain small though, and are still ideal food.

You spread throughout the oceans, probably burrowing into the soft mud at the bottom and coming out to feed.

There are lots of other little worms around, with slightly different lineages since you were bilateria.  They will be competing with you, and you will need to out-compete them.  You can either get better at detecting and consuming food, or you can find a way to hang out where there is plenty of food and very little danger.

Bravely go out and find food.
Safely hide where the food is.

Enteropneusta

As you crawl along, you run the edge of your mouth along the bottom, taking in sand and dirt, and digesting anything useful which goes in with it.  Your long gut becomes longer and longer, to extract as much as possible from the detritus you find on the ocean floor.  Eventually, some of you become more than two metres long, all comprised of gut behind a muscular 'head' section.

For safety, you tend to burrow into the sea bottom, and then up again, making a U shape.  You stick your head out to feed, or stick your tail out to defecate.  When you live at the sea's edge, this makes makes characteristic 'casts' of sand at low tide.

The muscular head gives you a characteristic shape, which looks familiar and gives you your common name.  You are an acorn worm.

Pterobranchia

As you crawl along, you sometimes stop and rear your little wormy head up into the passing water.  A nice little plankton occasionally drifts into your mouth as a result.  A bigger mouth develops, which makes this more likely, and the edges of the mouth become frilly with cilia edgings, which wave in passing food.

You crawl less and less, and while you rear up your head, you dig your tail down to help you stay anchored.

When you create eggs, your larvae are free swimming until they find a good place to live, at which point they start to change shape, and excrete a glue which helps them stay still.  There you will stay for life, often in a small colony of similar, possibly even related, filter feeding worms called Pterobrachia.

Hemichordata

You develop a fold in your gut layer which differentiates to become a simple neural cord.  Now you can grow a longer body without one end losing touch with what is happening at the other.

You are an ocean living worm with a 'head' end which contains your organs, a mouth which sits behind that, and a 'tail' end which contains a long gut for extracting as much as possible from your food.  For food, you will eat anything you find dead on the floor as you crawl along, or anything living which drifts in as you swallow water.  Your nice long gut doesn't care.

Competition is tough, as always, and you need to collect more food to win.  You can specialise in crawling, finding dead stuff on the floor, or stick your bum end in a hole and stick your head up to catch whatever is swimming past.

Head up, bum down.
Crawling along.

Tardigrada

Instead of staying long and smooth like a worm, you develop bands around the body which give you a segmented shape.  Each segment develops a thicker part like a claw on each side.  Four segments are found to give the best results, three with two 'claws' each for walking, slowly, and one at the back which can be used to hold on to whatever you are living on.

You spread around the world, living in all sorts of conditions from the deep ocean to the high mountains.  You are almost unbelievably tough!  You can endure dehydration, radiation, temperatures only one degree above absolute zero and well above boiling point.

That 'slow walk' gives you your name: Tardigrades, commonly known as water bears.

That's as far as you can evolve here!  
You can go back to the start with the link above, or share what you became with the icons below.

Nematoida

You stick with your worm shape, which is pretty well adapted, and spread around the world in massive numbers.  Your family, the Nematoid worms, eventually turn into thousands of different, but similar little worms, each adapted for a specific part of the global ecosystem.  Some live among the roots of trees, others are parasitic on a variety of hosts.

Possibly the most amazing thing about you is that you are everywhere.  There is no location on Earth where life is found where nematode worms are not found.

That's as far as you can evolve here!  

You can go back to the start with the link above, or share what you became with the icons below.

Chordate animals

Now that you've developed into a tube, some parts of your body are a long way from some other parts.  You already have nerves and sensory apparatus, but it would be useful to have some decent plumbing to get signals from one end to another.

As you develop, you are comprised of three layers of cells - the endoderm, mesoderm and ectoderm, which become, roughly, skin, muscle, and gut respectively.  You need to make a new kind of tissue to become a communications hub.  It can develop from either the mesoderm (muscle layer) or the endoderm (gut layer).

Muscle.
Gut.

Spine-less, unsegmented, non-retractable-headed cuticle worm

You’re getting bigger, which makes you a bit faster, but you’re still a slow little worm.  Are you sure that segmented body plan was a bad idea?

Oh go on then, segment me up.
I like being a worm.  Leave me alone

Deuterostome

As each individual in your species develops, its cells divide, grow and differentiate, forming a tube shape with a front and a back, symmetric sides and different kinds of cells.  Cells which will form the sensory apparatus which the animal uses to hunt and eat appear on the opposite side to the initial dent, which lengthens until it eventually bursts through in the centre, creating a tubular gut with sensory apparatus at one end, and an anus at the other.

The sensory apparatus is currently quite simple, detecting food and closing the mouth, then opening it again once the food has been consumed.  Over time, it will become more sensitive, enabling you to choose between two options: detect traces of larger food animals which have passed nearby, which you can expend energy to go and fetch, or stay still and wait until you detect smaller particles.


I'll let the food come to me.
I'll go to the food.

Scalidophora

A small change to the way the muscles are arranged, and you gain the ability to pull your entire head down inside your body.  This keeps you safe when you are on the surface, although you spend most of your time down in the mud at the bottom of the sea.  You eat a variety of foods, from diatoms to other worms, and are effectively a wriggling mouth into the present day.

You have divided into three main types. Priapulida and Kinorhyncha, commonly known as penis worms and mud dragons, and Loricifera, which have only recently been discovered as they spend their entire lives buried deep in the mud at the bottom of deep seas.  This latter group has one very interesting characteristic: they are the first multicellular life form which can live entirely without oxygen.

Way back on our evolutionary tree, a life form acquired a special ‘organelle’ called a mitochondrium, which was once a captured bacterium.  They have their own DNA, and their own, continuing evolutionary story inside this one.  In Locifera, the mitochondria have mutated slightly, and no longer combine carbon with oxygen to create energy.  Instead they are able to metabolise carbon compounds by removing hydrogen.  As a result, some locifera species live their entire lives without needing oxygen.  In the event of a catastrophic change in our climate, they might be the start of the next phase of evolution.

That's as far as you can evolve here!  
You can go back to the start with the link above, or share what you became with the icons below.

Eukaryota-engulfee

Around you are lots of early eukaryotes, which engulf smaller things, such as you, a prokaryote, to steal their energy until eventually something odd happens...  one eats one, and it survives.  You're inside another organism.  Ewwwww.  You live on inside the prokaryote, and, protected by you from most other threats, eventually lose the genes you aren't using.  What is left, is a pretty simple thing, no longer an organism, but an organelle.  You are now called a mitochondrium, and you are part of a Eukaryote.

At this time, the Eukaryotes are large, sexual, organisms with a tail for swimming called a flagellum. Two are called flagella.  One or two?

Two, for luck?
One is enough!

Chemical Prokaryote

Chemical prokaryotes tend to be bigger.  They have a larger plasmid (circular chromosome) to carry all the genes they need to express several different chemical pathways.  This is actually becoming a bit of a problem, as groups of genes which work well together are getting longer and longer as well as more numerous.  You need some way to pack them in better.  Genes are often thousands of times longer than they are wide, resembling massive threads, and one well known way to tidy away threads is to wind them on a spool.  There are several different chemicals which can act as a spool, but two in particular stand out with the lower cost for a given functionality.  One costs more, but allows more efficient packing, and is called histone.  The other is called HU.

Histone
HU