Comb Jellies

Increasingly frilly edges, covered with sticky cells develop on your outer surfaces and enable you to gain a rapid advantage in water.  You experience a substantial increase in efficiency and are able to grow rapidly bigger than other animals similar to you.

Eventually, the frilly edges, known as cilia, become big enough that you can actually use them to swim with, instead of just floating with the current.  This enables you to become a predator, following and eating other animals, which are actually your close relatives!  This is a common feature of predation: the best animals to eat are ones which are fairly closely related, because they share the same chemical make-up and so are the best match you to your needs.

Eventually, the large variation of size means that you become many different species, each tuned to a particular form of hunt.  Some of the largest of you effectively form swimming mouths, with stiffened cilia acting as teeth.

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Solid animals

As the ball of cells gets bigger and bigger, it loses structural integrity, like a partially inflated ball.  Smaller balls are OK, but bigger ones are likely to dent.  That’s what happens to you.

Luckily, this turns out to have an advantage!  At the back of the dent, water is trapped, and food particles don’t just whoosh past, they stop, and can be eaten.  This is the beginning of some sort of digestive tract.  The dent becomes deeper and deeper, and begins to look more like a sac, edged with cells which specialise in pulling in food and lined with cells that specialise in eating food.  Anything that can’t be digested has to come out the same way.

Further improvements develop, giving you the ability to pull the ‘mouth’ closed.  It’s a rudimentary set of muscles and nerves - not a central nervous system yet, but something that might one day develop into one. 

The same types of specialised cells which line the mouth could occur in other parts of the body, creating a sort of fringed edge which would help - as the water goes around your edges, you can maximise your surface area with a frill, which will increase the proportion of useful food items you can grab out of the stream.

Get frilly?
Stay smooth?

Sponges

Improvements in your ability to take in nutrients are developed by mutations which cause first dips, then channels, then pores, in your surface.  Bit by bit, you become more efficient at filtering the water flowing past you - indeed, through you.

Your reproduction process is the release of eggs and sperm cells, which meet in the water and form larvae, which swim until they settle to a new site and grow there.  This is the simplest form of animal currently known, although there are hints that tube worms may be even older and simpler.

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Animalia

This is the beginning of the animals, although you would be hard put to realise it.  At present you are an amorphous blob of cells, with digestive capabilities spread over the external surfaces.  You survive only in sea water, where sufficient nutrients pass by you that you can live.  You are, oddly enough, rather like a giant, multicellular bacterium.  It's odd how certain approaches to life appear again and again, at different scales and with different mechanisms, but the same behaviours.

And like all forms of life, if you don't find a niche that no-one else can use and occupy it, you will be squeezed out by other forms which do.  You need to improve your ability to take in nutrients.  

The strategy so far has been bigger and bigger balls of cells, but that is reaching its limits.  Perhaps the balls could split into loosely connected clumps and allow fluids to flow between them?  Or just keep getting bigger and bigger?

Let the water flow through me.
Keep me strong and solid

Podocarpaceae

With fruit being carried hundred of metres away, lone trees are emerging which may never be able to reproduce and are therefore wasted effort.  As the seed must be grown on the female plant, it's up to the male to find a better technique to bridge the gap by sending its pollen further.

Adaptations emerge which are advantageous, and spread throughout the population.

The female cone has already become so highly adapted that it is hardly recognisable as a cone.  Now it is the male's turn.  Cones become longer, thinner and lighter, more able to wave in the wind and throw their pollen rather than simple drop it.  In effect, in an example of convergent evolution (where two or more species solve similar problems in similar ways, but using entirely different genes), the cone has become almost indistinguishable from a catkin.

Pollen grains become smaller and smaller, till they are as light as possible.  At the same time, they develop slight ridges along the sides which will help them catch the wind and spread further, hopefully encountering a female plant.

Reproduction is now possible over greater distances, and your species longevity is assured.  A conifer ('cone bearing') tree which no longer bears recognisable cones on either of its sexes!

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Yew trees

Yew trees live for very long times.  Some are know to be over a thousand years old, and many are hundreds of years old.

All trees can grow new shoots and branches which replace those damaged by wind.  Similarly all trees can grow new roots, but this still means that damage to the central trunk will be fatal.  Rotten, hollow, trunks mean that a tree is on its last legs.  Unless they are yew trees.

Yew trees, when they become rotten, grow branches which bend back down, through the rotten wood, and grow downwards until they burrow into the soil and become whole new rootstocks.  They can do this as often as needed.

They also express a toxin in all parts of them except their berries, which means almost nothing predates on yew trees.

This apparent 'eternal life' may have been responsible for the yew tree being venerated as a religious icon in western Europe.  Churches traditionally have yew trees in the grounds, and monks are known to have planted yews around their chapels, but recent research has demonstrated that in many cases, the church was built where the yew trees were already, possibly in a pre-Christian sacred grove.

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Small fruiting conifers

Gondwanaland may be breaking up, but you don't care. Carried away from the far south of the world inside the bodies of birds, you recolonise the remainder of the planet as the ice age ends.  

Your trees grow across the continents and up into the northern hemisphere, but you are still reliant on wind to spread your pollen, so any tree which grows a long way from another of the same kind may never be able to reproduce.  In mitigation, living a very long time makes it more likely that you will find a friend.

You could either live longer, or spread pollen further?

Methuselah.
Live fast, die young, spread my wild oats.

Cephalotaxaceae

You have developed into a wide grouping of fairly small conifers, which produce a few large berries as each of the female cones matures.  As the berries are large, the group has acquired the nickname the 'plum yews' - the fruit really does look remarkably like a plum, which is a wonderful example of convergent evolution.

They are found all across South East Asia, where they have mostly been used for logging.

Recently however, they have been found to produce some toxins (which presumably evolved to stop herbivorous animals predating upon them) which may have some use as an anti-cancer medicine.  Contrary to expectation, this has not led to farming of the plants (possibly because they take so long to grow), but instead to widespread exploitation of wild stock, resulting in the possibility of extinction precisely because they are useful to humans.

Fruiting conifers

Some random genetic mutation causes the scales of your cones to express sugar.  Birds come and nibble at them.  Sometimes they eat a seed too, but the seeds are tough enough to survive the birds gut, and they get deposited many miles away, in a small quanitity of bird muck, which is a good start for a plant.

As time passes, the scales of the cone become softer, and sweeter, and wrap more closely around the seed.  Eventually, they are clearly recognisable as fruit, and it is easy for a bird to eat a whole 'cone' - which is now a berry - and take one or more seeds along for the ride.

Your next decision is simply whether to make clusters of small berries, or take a little longer and make fewer, larger, fruit.  Smaller fruit will probably be eaten by smaller birds who travel further.  Larger seeds will cost more to make, but might get eaten by larger animals who will provide them with more, shall we say, compost.

Expensive big fruit for big beasts.
Cheap little fruit for little birds.

Araucariacea

You don't develop fruit.  You remain, a very tough, long lived, hardy tree.  Possibly due to the genetic drift caused by living in very cold climes for millenia, you have become a slow growing, long lasting, big seed making, very tall tree.

This successful approach to reproduction is exactly at odds with the flowering plants which are your main rivals at this time.  This means that your survival is reliant on not competing for resources with them.

As it happens, once established, you will be around a very long time, which means that you do well in places where intermittent disturbances knock the flowering plant back for a while, allowing your new plants to get to a sufficient size, whereupon they will continue for centuries.

In the new landmasses of New Zealand, for example, with their seismic activity and occasional forest fires, you do very well. Eventually you become the biggest tree in the area, and in fact, one of the biggest trees in the world.

Wingless Antarctic Conifers

Gondwanaland has begun to break up for good, and the antarctic continent is drifting to the bottom of the world.  It's getting colder and colder.

Everything remaining on Antartica lasts a while longer, until South America drifts far enough away to allow a cold current, driven by the winds, to encircle the entire continent.  Warm winds can't break through very often, and the entire continent gets colder and colder until it is entirely covered in ice.  Everything that lived there dies.  You can't evolve sufficiently to live in those conditions in that time frame.

Luckily, as South America and Australia broke off, some of you lived on.  As long as there is enough diversity within a species, unless out-competed for resources, it will live.

In order to improve your chances further, you might like to enlist the help of the new creatures on the block... birds.  Birds can eat your seeds and, if they survive digestion, drop them a distance away.  Obviously the birds will only do this if they get something out of the deal, and you will only benefit if the seeds survive, so you will need to provide tough seeds, in a sweet package.  To put that shortly: fruit.

I'm feeling fruity!
I'm not.

Fruiting Cypresses

Instead of thin, dry scales forming a cone to  protect the seeds, you now want something succulent and attractive to attract your new helpers - so long as it doesn't cost too much.

The scales fill out, the cones shrink, and take on the form of a berry containing a tough seed.

You are a Juniper tree.

Aided by the birds, you are carried around the world.  Your distinctive, slightly sweet, fresh taste is approved of by more than just birds - the berries are collected and used by humans over thousands of years.

The ancient Egyptians collected Juniper berries, and valued them highly enough to use them as burial gifts.  The Greeks used them for medicine.The Dutch use them to flavour gin.

Non-Fruiting Cypress

Slowly slowly, catchee monkey.

You don't need to hitch a ride to conquer the world.  You can take it slow and steady, be the tortoise, not the hare.

So long as you live a long time, don't encourage other creatures to eat you, and drop a few successful seeds every year, you will do very nicely.  And you do...

You are a very successful, famous family of plants.  Cypresses without fruit grow very slowly, but life for an extraordinarily long time, becoming very, very large in the process.

Your numbers include the tallest trees (Coast Redwood), the most massive (Giant Sequoia) and the second widest (the Montezuma Cypress).

Some of your number live for thousands of years.

Cypresses

With one small mutation, giving you two ridges, one down each side of your seeds, you gain a slightly better distribution from each plant than before.  This is enough to help you cross continents, one seed fall at a time, over tens of thousands of years.

But there might be a better way, or at least a faster way.

Time has passed in the animal kingdom too, and there are now creatures called birds.  They like seeds.  Sometimes they eat the seeds, fly a long way, and then poop out the seed still in good enough condition to grow and become a new plant.  That's a lot better than being blown an extra thirty or fifty feet!

You could encourage the birds to help...

Feathered friends.
Sky rats.

Antarctic Conifers

Gondwanaland has gone.  Broken up into two or three pieces, the species found on each section will split and change, and form new, distinct species.

Here, on the southern section, near the bottom of the world, you are fine, adapted for cold mountainous regions, but on an Antarctica which is still warm and not iced over.  It is heading for the bottom of the world, but still attached to what will become South America.

Nearer the south pole, the winds are stronger.  If only your seeds would catch the breeze a bit better, they would get spread further, and could provide you with a small advantage for a tiny cost...

Fly my pretties!
Fall.

Sciadopityacae

The only single seeded conifer present in the northern hemisphere, out competed by other plants, especially pines, you were stranded, around 230 million years ago.

Constant pressure on your numbers squeezed you out of almost every niche, until, in the present day, you are found only on the islands of Japan.

You are a 'living fossil'.

On the other hand, you are one of the five sacred trees of Japan, you are planted in temple gardens and palaces, and your slow growing, very dense wood is highly prized.  So, not all bad.

Single seed conifers

At this time in history, most of the land is present in one large landmass called Gondwanaland.  Conifers spread across it.

But eventually, under the influence of tectonic plate activity, Gondwanaland begins to break up.  This leads to an example of speciation by literally drifting apart.  Some single seeded conifers stay on the northern section, others on the southern.  Inevitably, these identical trees are going to split into two different species, as their continued adaptation to their environment will mean that by the time they meet again, in a few million years, they will no longer be able to fertilise each other.

North?
South?

Ordinary Pines

Pine trees sweep across most of the mountains of the northern hemisphere.  They are adapted with soft, springy branches which tolerate the weight of snow, and sap which will not freeze.

They are used for many kinds of interior wood work, and are associated with pagan religious observation in Europe and North America.

They can even be eaten - the Adirondack area of the United States takes its name from the Adirondack Indians.  This was a nickname given by the neighbouring tribes meaning 'tree eaters', due to their habit of eating and drinking parts of the pine tree, chewed, or as a tea.

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Fire Pines

Exemplified by the Bishop Pine of California, Fire Pines produce sealed cones containing mature seeds.

The whole cone drops to the ground and lies there, sometimes for years.  Squirrels can't get inside, they are dry and secure, until...

Fire!

When a forest fire sweeps through an area, the cones are weakened by the intense heat.  When they cool, they open, and the seeds fall out, ready to grow in an area newly cleared of rival plants, with no predating creatures trying to eat them.

In an era when the oxygen content of the atmosphere was higher, and forest fires were regular occurrences, this adaptation must have been very successful.  Now however, forest fires are rare, and getting rarer still as humans prevent them or put them out.  Fire pines, one of the most interesting of the plant adaptations, are rare and likely to go extinct.

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Pines

Pine trees are the predominant softwood of the Northern Hemisphere, found over all continents, especially in well drained, acidic soils, to which they are better adapted than other plants.

They have no hard wood, and very sticky sap.  The sap is also... quite flammable.  At some stage in history, they must have been having a problem with forest fires, because a rather interesting adaptation turns up: sealed cones.

Close my cones and keep my seeds inside.
Let my seeds fly free

Conifers

You have evolved into the well known group of trees which produce seeds in cones (in latin, cone-bearing is 'conifer').

As ever, the drive is to become more efficient and take over more of the world.  You're already tall, producing multiple seeds from a single cone, and dropping those over the widest possible area.

There's a limit to how big and heavy a cone can grow on your branches, but increases in efficiency mean that you might be able to provide more nutrients to each cone.  As the seeds themselves are only a tiny fraction of the weight of the cone, an ideal mutation to have now would be to produce two seeds behind each scale instead of one. Each seed will be a bit smaller, which might not be helpful, but there will be twice as many.  What do you think?

Pile it high and sell it cheap.
Quality over quantity.

Gnetophytes

The development of 'vessel elements' allows you to grow longer and longer.  You don't need to have as much wood now, as you can lean on other tall plants and still pump water up.  Slowly, most varieties  lose the wood and become creepers.

In the Cretaceous era, the Gnetophytes were a wide ranging and varied group of plants which must have been worthy of several entries on their own to describe their speciation.  However, since then, they have slowly declined until now there are only a few varieties remaining.  Most are creepers in the arid parts of the Americas.  One is a tree growing in the tropics.  One is a desert plant growing in Central Africa - which only ever grows two leaves, but they are metres long.

Who can guess how they came to be so distinct?

If you can, please let me know...

Gymnosperms with pollen tubes

Instead of the pollen cell swimming to the egg, the next best thing is for the egg to reach out to the pollen. Ovule cells inside the plant are more likely to be fertilised if they are close to the surface, but this makes them exposed.  Slowly, a change evolves that delivers the best of both worlds: the ovule moves further inside, but grows behind it a tube which the pollen cell is transported down.

While we're on the subject of tubes, you're full of them: little tubes along which sap flows providing water to the leaves.  You could improve upon that though, and transport water directly.  All you need are more, smaller, tubes which will 'suck' water up via capillary action.

You'll need to grow thicker walls in some long thin cells, then when the cells die, the walls will remain, giving you tubes of lignin.

I'll take it.
No thanks.

Ginkgo

A mutation, or a series of related mutations, eventually create a very hard wood which you can use to grow very tall indeed.  Ginkgo trees can reach as high as 100m - three times the size of the average house.

Once, back in the Jurassic era, there were many varieties of Ginkgo, but in the intervening millennia they have been outcompeted by other plants and now only one species remains.  The Ginkgo tree is only found in the wild in areas of China.

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Cycad

The Cycads look like someone glued a large fern to the top of a palm tree trunk.  In fact, when new leaves grow, they look remarkably like ferns, as they emerge rolled up and unfurl slowly.

They have remained unchanged since the Permian era, becoming long lived plants with water storage inside their fleshy trunks which enable them to survive during dry times, and when they reproduce they can produce enormous cones, up to 300mm high, containing dozens of large seeds.  Some of them have developed a sort of tap root, and can actually be larger under the ground than above.

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Gymnosperms with Motile Sperm

Re-enabling the flagellum works perfectly: pollen cells land on the ovule and mature into sperm cells, growing a flagellum in the process, and then swim to the egg cell, where they fuse together and become a seed.  The seed grows on the outside of an adapted part of the plant until it falls off, and hopefully grows into a new plant.

The problem is that there are lots of other plants out there, growing tall, and crowding you out.  Your little new plants don't get enough sun and not too many of your potential offspring survive to maturity, so you need to get higher up, live for a long time, and drop as many seeds as you can.

One way to do this, is to only put your leaves and seeds at the top of the plant like a crown.  The other way is to get tough.  Attempting to do both won't work.

Crown me

Tough guy

Gymnosperms

Without insects, relying mostly on the wind, pollen are lucky to land in the right place.  This means that plants of your type will inevitably be more likely to live together in large groups, and also to grow higher and higher, in order to drop the pollen out with the greatest possible time in the wind, which provides the greatest possible range.

You slowly evolve into tall trees, existing in great forests.  Your reproductive system works under these conditions and you produce pollen cells which mature into sperm cells on arrival at the female plant.

Your ovules are as exposed as possible to maximise the chance of a pollen cell landing on an ovule.

The sperm cells will still need to migrate down into the egg cell, to complete the job.  Your cells have long since lost their flagella, as they are not needed any more, however they still carry the genes, they just never get turned on.  Perhaps it would be useful if, under these very specific circumstances, the genes for growing a flagellum turned back on?  Or you might try something else?

Tried and tested: bring back the flagellum.

Let's try something new.

Spermatophytes

You have become the first seed-bearing plant.  Instead of a simple spore, when your reproductive cells form, they are accompanied by a partner cell or cells.  These cells are filled to bursting with nutrients which will give the new plant a head start, enabling them to mature in a much wider variety of environments.

They are also of great interest to humans and other animals because they have a very high energy density and are ideal as a food source.  Many major global foods derive from collecting plant seeds in great numbers and crushing or otherwise processing them.

Possibly the weakest link in the chain of reproduction is now the provision of pollen to create sexually reproduced seeds.  You have to make a lot of pollen which is simply cast away on the wind in the hope that it will land on another plant of the same type.  While pollen cells don't cost much to produce, there must be a better way.

The world has been moving on in the last few million years.  There are now insects buzzing around.  Unlike you, they can move around and visit different plants.  Sometimes they land on two different plants of the same type and, rather obligingly, carry the pollen across.  Perhaps you could try to encourage that?

Insects are my friends

Eww.  No flies on me.

Megaphylls

As your leaves grow, they now spread and cast a much larger shadow.  They become substantially more efficient and you can carry on growing despite the continually reducing carbon dioxide levels.


Your leaves now spread and look like what we would think of as a leaf.  They are called Megaphyll leaves.


You're still reproducing by spreading spores.  Spores are tiny cells which, if they happen to land somewhere where they can start growing, will become new plants.


There is a mutation which means that spores won't be sent out into the world alone... how about if, when two cells divide and combine to create new spores, the 'spare' cells comes along for the ride?  Then it could provide an extra degree of energy stores which would give the new plant a head start, perhaps make them able to grow enough that they can become full plants even if they land in less hospitable places.


This will mean that the cost of making each potential new plant will go up - whether or not it will be successful.


What you gonna do?


Full Fat
Skinny

Club Mosses

There's no need to make big branching leaves.  There's so much carbon available in the atmosphere that all you need is a clump of leaves at the top.


Club mosses grow into huge, tall trees, dropping their simple leaves as they are shadowed by new leaves growing out from the top.  They look somewhat like modern palm trees, for the same reason: get a bunch of leaves up as high as you can and let the rest die back rather than repair any damage.
Giant club mosses form the majority of what are now called 'coal forests'.

This period is the carboniferous period.  It is the time when all the coal reserves on our planet were laid down.  Massive forests spread across the globe, far bigger than our current rain forests.  They lay down huge, deep peat bogs, which eventually form into coal as they are crushed under the weight of more and more earth.  The forests are alive with simple insects, fish and reptiles, all of which grow bigger and bigger as the air becomes enriched with oxygen, left over from the carbon-fixing carried out by the forests.

As the oxygen level rises, possibly as high as 35%, many of the insects actually grow much bigger than we currently see.  Hornets the size of your hand fly over the landscape.  But, eventually, the atmosphere runs out of carbon dioxide, at any rate in the levels needed for most of the giant club mosses.  Without a megaphyll leaf, the giant club moss species die out.


All of your giant varieties become extinct, now visible only as fossils in coal.  Only a thousand or so much smaller varieties remain extant today.

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Ferns

Ferns, far from being the woodland undergrowth plant found in temperate climates, are actually found all over the world.  They are simple, spore-reproducing plants, with efficient photosynthesising leaves which can flourish in a wide variety of places, from desert to drainage ditch.

They are remarkably tough.  They can live on ground which is unsuitable for anything else; tolerate the presence of heavy metals; endure blazing sun and the cold of the desert night.

They are an important food plant for many smaller creatures, and have survived almost unchanged from the Carboniferous period to the present.

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Moss

With no vascular system, all-over photosynthesis, and spores for reproduction, you become a small, low-rise plant, which requires a wet environment to survive.  You are a moss.

Mosses are amazing plants.  Some can withstand dessication for months and then start photosynthesising again without hours of rainfall.  Some produce scents to attract micro-insects which spread their spores. Mosses grow across the world, wherever there is (sometimes) moisture.

Spagnum moss has even managed to colonise the acidic water found over the surface of peat bogs.  In order to do this it has developed another mutation - dead cells containing water supplies.

In growing quickly, wherever there is damp, mosses have become an important part of the process of greening.  Mosses grow over newly damp ground and keep moisture in, which enables other plants to follow.

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Vascular Land Plants

The structures are a huge success!  Finally, you have the recipe for massive expansion on land!

This period of development on Earth is known as the carboniferous period.  The air is around 55% carbon dioxide.  Effectively, you have all the carbon you can fix, you can grow and grow and grow and still photosynthesise because you can provide water to the farthest sections of you.

You expand and expand and become hugely successful.  The land of the Earth becomes green for the first time.  Jungles abound, spreading almost from pole to pole.

However… (isn't there always a however..?)

The amount of carbon dioxide in the atmosphere is dropping.  All of the plants of this era are changing the climate dramatically.  Within (??) million years, the carbon dioxide level has dropped to under 30%, and the least efficient collectors simply become extinct.

You need to become more efficient or it might be game over.

You currently photosynthesise all over, although more towards the tips because they get more light.  Each tip is a long thin green point with a single vascule running up the centre of it.  It's where most of your photosynthesis takes place, but it would be better if it were wider, able to collect more light.

One day, a single plant experiences a mutation which causes the vascular system to branch inside the leaf.

Will you take that mutation and have the opportunity to make larger leaves, or stick without it and make more leaves instead?

Fewer, broader, leaves
Lots of thin leaves

Apically growing land plants

This adaptation is called apical growth - the tip of each part of the plant is the only part which will continue to grow indefinitely.  It leads to long, branching structures which are well adapted for collecting as much light as possible, maximising the opportunities for photosynthesis.

However, this does lead to a problem.  Out on the land, in order to 'fix' carbon dioxide, you can't just use the gas dissolved in the water.  You aren't in the water any more.  You need to open parts of your photosynthesising structures to let the carbon dioxide in before it dissolves in your internal water, where you can use it.  But this means that you will lose water to the air through evaporation.

So far, as a small plant, it's been sufficient to allow water to migrate through you between your cells via a physical process called osmosis, in which water will migrate to the most concentrated part of the plant.

Unfortunately, osmosis is limited to fairly short distances and you have got a lot longer thanks to your apical growth adaptation.

One solution would be for some cells to reach maturity, thicken their cell walls with lignin and then die back, creating very fine tubes within you which can carry water by a combination of capillary action and osmosis.

You will sacrifice some cells, expend some energy making lignin, and be at risk of pumping any nasty chemicals throughout your system, but you will be able to photosynthesise more.

Tubes please.
No tubes.

Liverworts

You are a Liverwort.  A family of thousands of varieties of small green plants which mostly live in the wetlands of the world.

I can't find anything interesting to say about you.  Sorry!

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Land Plants

With your spores inside a small wall, you can reproduce on the drier areas of the tidal zones. This gives your offspring the opportunity to start rapidly photosynthesising as soon as they grow, and proves to be a very successful approach.

You are still a rather amorphous green plant, with most cells capable of growing and splitting if they get sufficient light input.  This means your structure becomes rather lobe-like, or blobby.

Annoyingly, this means that one lobe can shadow another lobe which is a complete waste of all the energy gone on making the lower lobe, which is now unable to collect light.  You are effectively in competition with your own body.

If you experience a mutation which turns off cell reproduction after a certain number of divisions, except for a few cells at the furthest tip of a lobe, then you will spread out further, and should be able to reduce the shadowing effect.  If you don't you will possibly spread out more widely.

Blobby?
Pointy?

Lichens

Some species of fungi will only grow to reproducing if they are in contact with a green algae.  The green algae is surrounded by the fungi, which provide it with a good environment for photosynthesis.

(The green algae can live without the fungi, but the fungi can't live without the green algae.)

What's really interesting is that some of the fungi can live with more than one variety of green algae - and will grow into different body shapes depending which algae they absorb.

We call these symbiotic systems 'Lichen'.

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Green algae

Green algae dominate the shallower seas of the world in their largest forms - green seaweed.  They are also found all over the land in moist environments.

Now, it's not quite evolution (yet) - but there are further opportunities to exploit... you can grown inside other things.

Fungi?
Coral?

Viridiplantae

After creating 'chlorophyll b', you can use more of the light spectrum and grow faster.  You're still in the sea, but some of you happen to be in tidal areas where you dry out when the water goes away.

This is mostly acceptable - the benefits of lots of light are balanced by the damage done by drying out - except for your reproductive cells.  Currently, when you release new cell clusters which will grow into new plants - the earliest form of seeds - they sink to the bottom and grow in the ocean.

Now that you are invading tidal zones, they get thoroughly dried out and irradiated by ultra-violet light before the sea comes back.  This is enough to damage the important DNA inside them and prevent the new plants from growing.

If you want to hang out in the tidal zone, you'll need to keep the sun off them.

I like the dry, hot, tasty sun.  Protect my eggs.
I'm happy hanging out a few feet down.  Leave them bare.

Hacrobia

Hacrobia are tiny algae, divided into two main groups, the cryptomonads and haptophytes.

They spread throughout the world's oceans and fresh waters, and are hardly noticed at all, being some of the tiniest life forms on the planet.

They are, however, food for some of the largest lifeforms on the planet, where they are better known as part of the grouping called plankton.

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Brown Algae

Accidentally creating a molecule similar to chlorophyll a, but absorbing a slightly different part of the spectrum enables you to grow slightly bigger, and colonise deeper water.  It also turns you brown.  We call it 'chlorophyll c'.

The best known example of a brown algae is kelp.  Huge forests of the biggest variety, giant kelp, grow in the polar and temperate oceans, up to fifty metres long, growing over half a metre a day.  They provide the ocean equivalent of rainforest, with different species of animals living at different depths.

There is one odd thing about brown algae: there are no known single celled brown algae.  Nobody knows why (yet).

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Red Seaweeds

Without the size restriction of a shell, red algae can form larger plants, and spread throughout the ocean.

Many are widely spread, small (tens of centimeters long) , red seaweeds, but some grow much larger over substantial areas and have become human food crops.

In Japan, nori is used to make sushi and related products.  It has been eaten for centuries and farmed for at least three hundred years.

In Britain and Ireland, Dulse and Laver have been used to make local foodstuffs for generations.

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Algal Coral

Yep, that's right: even algae can grow a shell if the right chance mutations happen.  In seawater, there's a certain amount of dissolved calcium.  Algal coral luckily evolved a mechanism for fixing calcium to carbon and oxygen to express calcium carbonate.

On any patch of exposed rock in shallow water, there's likely to be thin layer of crusty, reddish, stuff. That's an algae, hiding inside a shell.

It doesn't make huge reefs, like polyp coral, but given enough time, it probably would.  Two utterly different species, kingdoms apart, have evolved exactly the same mechanism to solve the same problem.

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Red algae

Chance mutations cause a useful chemical to be expressed by your genes: Phycobilin.

It doesn't have the sugar-creating part of chlorophyll, but both of them are chromophores (light capturing chemicals).  If a phycobilin is attached to chlorophyll by a phycobiliprotein, then light from a much larger part of the spectrum can be used.  In fact, green, yellow, orange and blue light can be used, leaving only red to be reflected.  That's very efficient, and enables red algea to grow high in nutrients and proteins.

So high, in fact, that other animals find them very nutritious.  Oh.

You might want to get serious about protection.

Grow a shell?
Stay in the open?

Chromista

You, a non-synthesizing-chromista, consume a rhodophyte, and to everyone's surprise, it doesn't die, but you live happily ever after.  You provide the rhodophyte with chemicals, and it converts those, via sunlight, to sugar for you.  Marvellous.  It's a little engine living right inside you.

Well, almost.  The chlorophyll gained from the rhodophyte helps a lot, but it could still be improved.  For example, it doesn't work in deep water, where the sunlight is reduced.

Another of those little copy errors could make a different kind of chlorophyll that works better in the deep ocean and gives you the opportunity to colonise areas where nothing lives at the moment.  But that will mean having less of the tried-and-tested chlorophyll that works so well near the surface.

Call me Nemo
I like paddling

Chromista Chloroplast

You, a rhodophyte, are swimming around quite happily, living on the sugars made by the blue-green algae you ate earlier, when a non-photosynthesising chromista eats you.  Thankfully, you survive this trauma and live on, inside the chromista.

We're sure that this happened, because the chloroplasts in chromista have three or four membranes instead of two. This strongly suggests that a blue-green bacterium was 'eaten' by a rhodophyte, which was in turn 'eaten' by a chromista.

You get given chemicals, and convert those, via sunlight, to sugar for the Chromista.  Marvellous.

Well, almost.  The chlorophyll you provide helps the team a lot, but it could still be improved.  For example, it doesn't work in deep water, where the sunlight is reduced.

Another of those little copy errors could make a different kind of chlorophyll that works better in the deep ocean and gives you the opportunity to colonise areas where nothing lives at the moment.  But that will mean having less of the tried-and-tested chlorophyll that works so well near the surface.

Call me Nemo
I like paddling

Rhodophyta

Hmm.  That may not have been such a good choice. Chlorophyll a is just not good enough on its own.  You're going to need help.

You can either improve your chlorophyll's effect, or get help from a friend.

Archaeplastidia

You eat a photosynthesising bacterium.  It’s tasty food.  However, over time, you have found that allowing the photosynthesising bacteria to live for a little while inside you makes them even better, as they can continue photosynthesising and making sugars inside you.

In fact, you have evolved to provide the chemicals the photosynthesising bacteria needs as food to extends its life inside you and maximise your meal

This adaptation has now reached the point where you can contain multiple photosynthesising bacteria and they can live on inside you indefinitely.

You have entered a symbiotic relationship, and will come to rely on your guest, providing food and protection, for the price of a few sugars.

Your guest photosynthesises using a chemical called 'chlorophyll a', which turns you a blue-green colour and gives you a supply of sugars all the time you are in sunlight.  It doesn't absorb all the sunlight though - only a part - the rest is wasted.

The usual copy-error effect - inside the guest living inside you - can give you the ability to make a slightly different version of chlorophyll which will work using a different part of the sunlight spectrum, but it means you'll make less of chlorophyll a.  Interested?

I'll go with two.
The one I have is just fine.

Multicellular Fungi

When single celled fungi began to work together, they must have evolved some means to pass food along from cell to cell, which enabled some cells to specialise in food intake, and others in micronutrients (for example).

Eventually something interesting happens: different parts of an organism appear, as individual genes are turned off in some parts and turned on in others.

We see, for the first time, structures of cells making up a single life form.  Parts grow and specialise depending on what other parts are already present, and what is around them.

Typically all this activity, for multicellular fungi, happens out of sight, as they consume dead matter from other sources, until eventually they have enough stored energy to reproduce.  Only then do they grow the most specialised cells of all, the spores, hanging down from a fruiting body which we recognise as mushrooms and toadstools.

Over time, the energy and nutrient rich spores become a target for other, larger creatures, and defences are found in the form of toxic compounds.  Some of these are mildy irritating, and will make a mammal sick.  Some will kill.  Some will affect the brain and cause visions.

Some become the basis of religions such as shamanism and the religions of the North American Indians and Alaskans.

You've come a long way: even humans worship you!

Oh... and one more thing.  Something interesting happens for some of you if you come in contact with green algae.  Want to give it a try?

Unicellular Fungi

Unicellular Fungi comprise a group of organisms of which the best known are the yeasts and penicillins.

There is a huge number of varieties of yeasts, and they are found on almost any external surface of any fruiting plant.  When making cider, for example, there is no need to add yeast as the crushed apples will have had yeasts on the surface.

Penicillins are part of a family of blue-green fungi.  They also grow everywhere, aided by their ability to kill bacteria which are trying to compete with them for food.

Both have been adapted for use by humans, providing us with our daily bread, and our best defence against infection.

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Fungi

That's working nicely.  Whenever you find a food source, you grow into it, bit by bit, eventually growing big enough that you split in two.

Do you and your new friend want to work together, or work apart?

Opisthokonts

Swimming around, looking for food, big tough Opisthokonts are becoming the John Wayne of the unicellular world.  There's not as many of them as there are bacteria, but they live longer.

(This next decision is a little arbitrary, because biologists aren't really sure yet what caused the speciation)

Will you ingest your food - pull lumps right into you - or will you absorb molecules directly through the cell wall?  To put it another way, will you pull your food into you, or will you grow into your food?

I'll stuff my face.
Ill rub my face in it.

Cyanobacteria

Free-living, quickly-reproducing, sunlight-eating cells rapidly become one of the most successful lifeforms on the planet.  They invade every niche, and are responsible for one of the first non-physics-driven forms of climate change: they reduce the carbon dioxide in the atmosphere hugely, and cause the extinction of many oxygen-intolerant early forms of life.

They are first clearly present around 2.5 billion years ago, and survive to the present day in great quantities in many locations, from the blue-green algae which blooms on the Mediterranean to the camouflaging bacteria which turns sloths fur dark green.

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Cabozoa

You're pretty tough, and you can eat what you find and look after yourself.  As time passes, and the soup dries up, you develop into a set of different unicellular species, many of which are parasites living in the guts or blood of animals.

You cause a host of problems, including sleeping sickness and Chagas disease.  Thanks a lot.  In many species, the arrival of the insects helped you out enormously as they became blood-suckers, because they transport you from host to host.

You survive into the present day, but only as long as the creatures you live on or in are around.

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Corticata

Life is good inside your new tougher cortex.  You swim around, eating things smaller than you.  But some of the things you eat have got big enough and tough enough to survive inside you, if they're lucky.  It could be to your advantage to have something living inside you, if it isn't in competition with your current chemistry.  If it is, of course, you'll probably die.

One thing that clearly isn't in competition with you is a green/blue photosynthesising bacterium.  It eats light and makes simple sugars.  If you took in one of those, it could share the sugar with you, and you could keep it safe from other predators.  Win-win.

The other choice is a rhodophyte, a primitive form of red algae.  It doesn't make sugar, so it's not quite as rich a meal, but it makes a lot of other things of use.

Green!
Red!

Amoebozoa

You are a member of the amoebozoa phylum.  You reached this exalted position about a billion years ago, and have carried on till today.  Some of you are well known, such as the slime mould family, or the amoeba proteus with its visible pseudopodia.

All of you have single cells, and can take on 'dry' forms when conditions are not good.  Some of you (slime moulds) create specialised sub-copies of yourself already in dry form, called spores.  These are used similarly to seeds, in the hope of eventually arriving somewhere better.

Some of you enjoy living in the guts of animal from time to time, which makes them pretty ill.  All in all, a good spread of approaches ensures that this group will survive for a long time yet.

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Unikonts

Sticking with one flagellum is a safe bet.  More might enable more complicated movements, but will cost more.  Anyway, now you've made the choice and been successful, it's really hard to change it back unless you or your environment changes in such a way that it would be even better.  It does happen sometimes, but rarely, and only when it becomes advantageous.  That's evolution for you.

At a microscopic scale, water is pretty viscous.  You can pull yourself through it like helicopter blades, or push through like a boat's propellor. Trying to keep the ability to do both won't be as successful as choosing one.

Will your flagellum pull or push?

Bikonts

You are one of the biggest toughest lifeforms around at the moment.  But sometimes, you're still just food. You're just a little single cell bag of takeaway proteins to a bigger bikont.

It's getting tougher out there...maybe you need to toughen up?  Would like to grow a cell cortex?

Cortex me up.
Keep me slim.

Free Living Bacteria

From here on in, it's a question of live fast, die young, leave a good looking cell.

Bacteria colonise everywhere on Earth, from the deepest ocean to the highest cloud forest, inside trees, outside trees, inside animals, on the skins of animals, everywhere.

Some get thicker outer walls, and become gram-positive, some thinner and become gram-positive.  Some become parasites that live on, or in, larger creatures.  Some become spiral shaped, and others learn to shape-shift dramatically as a means of locomotion.

They are arguably the most successful life form on the planet.

Chemical Eating Bacteria

You can afford to keep a lot of different genes around now, so you can survive pretty much wherever you go, on whatever you find.

However, you're one of the smallest, weakest life forms around, because at this time, the eukaryotes, with their big, multi-layered walls, have grown up.  They like eating you, like a little snack.

Wouldn't it be nice to be big and tough like them?  Well you might have the options: what if you get inside one in a failed snack attack, and stay in there, acting like a little chemistry factory, making food for them?  Better to feed than be food?  Or better to stay on the outside, maybe get faster, or hide away?

Join a gang?
Play your own game?

Photosythesising Prokaryote

Ah, that was clever.  By using the sun's rays to fix carbon from the atmosphere, you can grow and grow and reproduce and reproduce.  There's no competition for the sun's rays, so long as you stay in the light you are good to go.

But all this free food is making you a tasty target for various eukaryotes, which are swimming right up to you, engulfing you, and eating you whole! This is bad!  These guys are a lot tougher than you.  Well, if you can't beat 'em, join 'em, right?  With a bit of luck, one of these eukaryotic bad boys will engulf you, but then keep you around inside so that you can eat sunlight and make food and let them have some.  It's basically the evolutionary equivalent of the Mafia.

So, whaddya gonna tough, tough guy?  You got a nice little cell here, be a pity if something happened to it..

I'm toughing it out, on the outside.
Safer to become a made man.  I'm in.

Eukaryota

With your densely packed DNA, you can afford to express a lot more proteins.  There is some serious evolution (without splitting into species) in the next few million years.  In no particular order:

• your DNA stops being stored in a loop.  Instead, you develop end caps called telomeres which act like the bits on the end of shoelaces that stops them unravelling.
• You get bigger.  Much bigger.  
• Instead of 'stealing' genes in the process of eating other chaps similar to yourself, when a pair of you meet up, one 'gives up' some genetic material to the other.

If this last one sounds crazy, because the organism giving up material will lose out, you have to remember that genes are survival machines both in themselves, and in groups.  In this case, the gene may well get the chance to make many more copies of itself when it joins with a new set of partners.

To facilitate all of these things, one major change takes place: the cell wall becomes less thick, and eventually forms a cell membrane.

Additionally, you need to move around - your food demands are so high that you can't just wait around and hope.  To this end, any change which makes you able to move, no matter how small, will be copied around the community.  Eventually, those small changes add up to something called a flagellum.

However, this flagellum needs a lot of energy.  In order to make enough energy, you need to eat, which is done by engulfing smaller things, prokaryotes, until eventually something odd happens...  you eat one, and it survives.  Inside you.  Ewwwww.  The prokaryote inside you lives on, and, protected by you from most other threats, eventually loses the genes it isn't using.  What is left, is a pretty simple thing, no longer an organism, but an organelle.  It's called a mitochondrium.

After all this, finally there is a choice again:

Grow another flagellum?
No thanks, one is enough!

Life Starts Here

In the beginning was the word, and the word was...

No, only joking!  We're going to start a little later than that in the story.  We're not going to wonder at how on earth the universe started.  We're going to wonder about how, on Earth, life started.

The Earth coalesced out of a cloud of mostly iron based compounds, and formed a hot, rocky core.  Eventually, other elements and compounds rained down on it from the gravity well.

After about a billion years, comes the beginning.

In the beginning was the primordial soup.  A mixture of simple chemicals, some gaseous, some liquid, and quite possibly some lightning.  Nothing alive, in the sense that there was nothing that reproduced.  There were certainly some chemical reactions going on, but nothing that couldn't be described in a line or two on a school whiteboard.

Then some chance chemical interactions produced simple nucleobases (nitrogen containing compounds). This will have happened quite a lot.

Some more random chemical interactions caused several nucleobases to fuse together, creating a very short strand of ribonucleic acid.  This may have happened several times, until, at last, one very special molecule occurred.

Ribonucleic acid (RNA) is the simplest known class of self-replicating molecules.  Estimates of the minimum number of nucleobases needed for replication used to be over 200.  Then they were 50-60.  Finally it was demonstrated (in 1975) that under some conditions, a single RNA molecule can form of very, very few nucleobases, and still reproduce.

You may well be thinking that this is extremely unlikely.  You may well be right.  You may therefore prefer to think that some kind of direction must have been provided by a creator, whether terrestial, extra-terrestial or supernatural.  But it doesn't matter that it was unlikely.  What matters is that the conditions were there, and that they were there for millions of years.  If the spontaneous formation of RNA would only happen once in a million years on a planet in a given state, but that planet is in that state for hundreds of millions of years, then you don't need to conclude that.  Our planet was 400 million years in the Eoarchean era between the end of the Hadean (hell-like) eon and the start of the Paleoarchean era.  There was no life during the Hadean eon, and there were definitely bacteria in the Paleoarchean era. 400 million years is a lot of time to roll the chemical dice.

So, at some time in the Eoarchean era, the conditions were right, and the first RNA which could replicate was fused.

As it must have been near some more nucleobases, it immediately replicated.  Then each of the copies replicated. Each of those copies was a perfect replication of the first ever life.  Some of them will have been destroyed by simple chemical reactions such as meeting up with an alkali, but only some.

Soon it had replicated so many times that there was a shortage of the required nucleobases in the area.   Eventually, one could not replicate completely and an imperfect copy was formed.  Then another, and another, and another.  Many of these were not able to replicate, and simply stopped reacting, but instead of life ending just as soon as it had started, one or more of those imperfect copies was able to replicate itself despite the 'error' and use some other nucleobase, and that copy continued to replicate.

Eventually, a variety of replicating molecules was present.  Again, some will have been destroyed, but some will have remained.  Those which could replicate most quickly and withstand chemical attack will have been present in the highest numbers.

Later, perhaps a day, perhaps a million years, two very similar RNA strands found themselves in close proximity, and a new reaction occurred. They both lost oxygen atoms from down one side and fused alongside each other, forming deoxy-ribonucleic acid or DNA.  This molecule is slightly less able to replicate than RNA, but more stable and less likely to be damaged by other chemicals.  Only when the DNA is in the presence of the right chemicals for replication does it split into two RNA molecules and start reacting.

The benefits outweighed the costs, and after a while, more copies of the longer-lasting DNA existed than RNA.

When DNA encounters strands of RNA, it can cause it to partially reproduce, which results in the expression of amino acids.  If the right combination is present, it will result in a protein. A section of DNA which expresses a single protein is called a gene.

At some stage, random combinations of nucleobases occurred which caused proteins to be expressed that caused a bubble or membrane to form around the DNA.  Not that the membrane itself was created, but that the proteins present attracted lipids, and the lipids closed around the DNA, keeping it even safer and making it even longer lived.

Finally, each had to make the first genetic choice.  Not actively choosing - life never actively chooses - but simply that two successful variations lived when all other variations died.

Choices are mostly made by errors.  Copying isn't perfect, as we saw when the RNA ran out of nucleobases.  Eventually, small changes turn up in the DNA that change the proteins expressed in the presence of RNA.  Different proteins will have different effects.  Most of the errors make the DNA less likely to survive, but sometimes an 'error' will make survival more likely.

Because DNA is two sets of genes, an interesting effect occurs. If a cell has two copies of a given gene, it will express one protein.  If it has two different ones, it will express two proteins in half quantities.  Sometimes, having two proteins is best.  Sometimes, having twice as much of either of two proteins will better.

You are now the Last Universal Ancestor.  Time to start making choices...