Bulbasaur: I got your back

Bulba-, Ivy-, and Venusaur introduced a generation to the world of Pokémon, so as the song says, “Let’s start at the very beginning, a very good place to start.”
The Pokédex entry in the Red and Blue versions has this to say on the subject of

Bulbasaur:
(Red Dex) A strange seed was planted on its back at birth. The plant sprouts and grows with this Pokémon.

So the 'Saurs aren’t really a single creature, but two creatures living in tandem. According to Bulbapedia, it’s the creature’s mother that initially plants the seed on its back. There are two ways to view this phenomenon: symbiosis and parasitism.

The Background
Symbiosis is close and often long-term interaction between different biological species, whereas parasitism is a type of symbiotic relationship between organisms of different species where one organism (the parasite) benefits at the expense of the other (the host). (Wikipedia)

While there are some fungal and flatworm parasites that are known to affect the minds of their hosts, causing them to act in ways that injure or kill the host for the betterment of the parasite (for example the Euhaplorchis californiensis, which travels to the brain of the Californian killifish and makes it 30-times as likely to be eaten, so the parasite can lay eggs in the stomach of the bird that ate it), the relationship between the animal- and plant-parts of the Saur-family don’t exactly fit this bill, so we’ll stick with symbiosis.

One of the more famous examples of mutually-beneficial symbiosis occurs between the clown fish and the sea anemone: the clownfish feeds on small invertebrates which otherwise potentially could harm the sea anemone, and the clownfish’s poop provides nutrients to the sea anemone. The clownfish is additionally protected from predators by the anemone's stinging cells, to which it is immune.

So, now that all the technical garbage is out of the way, let’s break these concepts down for the "too long; didn't read" crowd:

Symbiosis: everybody’s happy
Parasitism: somebody always gets hurt

The Partnership
In Pokédex entries of Bulbasaur’s evolutions, we learn more about the relationship between these two organisms:

Ivysaur (Gold Version): Exposure to sunlight adds to its strength. Sunlight also makes the bud on its back grow larger.

Venusaur (Silver Version): It is able to convert sunlight into energy. As a result, it is more
powerful in the summertime.

So the Saurs can absorb energy from the sun through the plant on their back. In hard times, this can probably mean the difference between life and death, but what does the plant get out of this arrangement?

Plants need more than sunlight to survive: they also need water and nutrients, and it can be assumed that they can get these from the Saur. They also have the opportunity to travel, increasing their reproductive range exponentially.

Plus, you know it's time to leave the neighborhood

when the trees take to thievin'.

The Breakdown:
The plant benefits because it’s attached to something that will actively seek out water and nutrients for it instead of just sitting around waiting for something to die at its roots, and because the nature of living creatures is to wander, it has a large territory in which to spread its genetics.

The animal benefits by gaining the ability to absorb energy from the most powerful source available: the Sun. Also, most of their most powerful offensive capabilities (Solarbeam, Leech Seed, all the various powders it has access to) come from the plant. In defending each other, they help insure both of their continued existences.

I got your back, bro!
 

The History
What kind of environment could foster such a uniquely beneficial, and perfectly balanced, partnership between plant and animal? Let’s hop in the way-back machine and check out a theory:

Living in close proximity to each other were a type of reptile/amphibian, and a type of flowering plant. One day, the plant releases a batch of seeds into the air, as usual. Nothing odd about this: they’ve probably done the same thing for millions of years without incident.

Same old, same old.
(Also, I am aware that this isn't actually a dinosaur, but it IS adorable.)

Except this time, a seed lands on the back of one of the rep-phibians. It’s probably a little moist back there, and the critter is just minding its own business and enjoying the sun. The seed gets stuck, finds some delicious dead skin cells and thirst quenching water, and takes root in the craggy back of this unsuspecting animal.

What follows was probably immensely painful for the creature. This makes me think that the first Bulbasaur was probably a female: everyone keeps telling me that girls are better with pain, and if you’ve ever seen any movie dealing with pregnancy ever, you’re probably more likely to believe it. As the plant begins to take root, the creature’s pain threshold would have to be astronomical, lest it would have died a long, lingering death and we would probably have only had an original 147 Pokémon (Mew doesn’t count because I never got one, and I’m the only person who matters here, so there).

But it didn’t die. BulbEve survived, and probably had some little Bulbabies of her own, and while she was looking out for them, the plant probably released some seeds, which took hold on the Bulbabies. Thus began a cycle.

Over the millennia, some fine tuning probably took place: continuing generations of Bulbabies were probably born with higher and higher pain tolerances, or their nervous systems got really good at putting out some totally far-out drugs to combat the unpleasantness.

 Another earmark in this story would have been the first time that a root hit the creature’s spinal cord. The creature, by this time, probably wouldn’t have noticed it… until it got scared. The ensuing rush of adrenaline and fight-or-flight hormones could have caused a chemical reaction within the plant, and guided by the signals shooting through the creature’s spinal cord, creating the first consciously directed Vine Whip, or Razor Leaf, or Solarbeam.

Again, this is only a theory: correct speculation is made all the more difficult when its subject is only a collection 1’s and 0’s inside a child’s game-a-jig, but it’s a theory that lets my over-active imagination sleep at night… unlike creatures like Jynx and Mr. Mime, which persist in haunting my nightmares.

On that happy note, I leave you, dear readers. Do you have a suggestion? Is there some Pokémon out there that you’d like me to throw my brain at for a bit and try to explain? If so, leave a comment and I’ll see what I can do.