A meteor hurtled through the dark, cold universe like a silver bullet. It had been traveling in space for quite some time now; at least a few billion years. Its rugged surface was unusually dotted with thousands of white blotches, each blotch was several feet wide and had irregular borders, as if they were paint stains.

Upon closer inspection, these spots were colonies of hibernating microscopic life.

Minuscule, cream-white and incredibly resistant to environmental hazards that other forms of organic life would find anywhere between unpleasant and unlivable, the Limba –singular; Limbus– clung to the meteor's rugged surface for as long as it had existed, holding on for dear life. Each Limbus had a tiny tail sprouting out of its unicellular body. They were in a dormant state, requiring almost zero nutrition, which they could scrape off the meteor's surface, or even each others' corpses, should the need arise. Their cellular membranes were capable of withstanding extremes of temperature, pressure and radiation.

What were these curiously resilient creatures doing on a meteor in the middle of the literal nowhere? How did they even "board" a meteor that could have been traveling as fast as 200,000 kilometers per hour, flying through countless, highly irradiated nebulas and star systems, and still were capable of survival? Those questions did not matter then. What mattered was what happened to be in that meteor's current course; an ocean-blue speck of dust somewhere in the Orion arm of the Milky Way.

Before long, the gaseous atmosphere of the blue planet was resisting the meteor's advance, almost igniting the great rock midair, but the meteor survived the friction, and so did its tiny, stirring inhabitants. In a colossal splash, the meteorite fell into somewhere in the middle of a giant body of clear liquid that covered most of the planet's surface.

As soon as the meteorite began sinking, the Limba had awoken. Compared to the harsh extremes of space's environment, this place was heaven! Warm, wet, spacious, and very little gamma rays.

As though with sheer curiosity, they swam across the clear medium in all directions, exploring their new surroundings. Unfortunately, it was all water, and nothing but. The Limba were not easily disheartened, however, and this new habitat had nothing for them to complain about, but they had hoped to find something, anything. For years they continued searching, eventually spreading across the entire planet.

One fateful day, a colony of Limba finally located the natives! Trillions of protobionts, primordial lifeforms, were scrounging around the ocean's bottom. True, the primitive unicellulars were much bigger than Limba, but nowhere as evolved when it came to internal biochemical infrastructures. A single one of those native microorganisms, compared to a Limbus, was akin to a sundial next to a digital wristwatch with a built-in calculator.

That was when the Limba began doing what they do best: Parasitism.

Each Limbus, hurriedly and forcefully, ingrained itself into one of the protobionts, merging with it while remaining partially separated from its gooey inner structure. It took several decades, but eventually, there was not a single Limbus that was not implanted in a protobiont's innards. The remaining, "clean" natives were quickly fed upon by their Limba-controlled relatives and eventually eradicated.

In less than three thousand years, the Limba unequivocally ruled supreme. Earth was successfully invaded by extraterrestrial life 3 billion years ago.

Limba, however, were not entirely malevolent, per se. In return for hosting a Limbus, a protobiont was rewarded by faint electrical impulses that gave it a rudimentary pleasurable sensation. The natives were enjoying being parasitized. In fact, the protobionts did not feel subservient to their parasitic masters, but still felt as free individual beings, with what little primitive forms of "feeling" these Earth-native creatures could muster. It's not subordination if you still "feel" in charge... right?

Each time a protobiont underwent a reproductive cycle, a new Limbus was also born inside the new individual. It was now impossible to be Limbus-free. But why would a protobiont want to be free? They were being continually recompensed with new improvements in exchange for hosting Limba. As the millennia went by, protobionts became prokaryotes. Some grew flagella (tails), others even became cyanobacteria and gained the ability of photosynthesis. The natives were evolving at a steady rate, all thanks to their cosmic benefactors.

It took millions of years–there's only so much a Limbus could do with such shabby, last-eon equipment–but it was an inevitable eventuality: Multicellular lifeforms came to be. They were reared by the Limba, and with them, the Limba became multicellular themselves. They even grew gardens of green organisms that served as perfect feeding areas.

The natives, who had forgotten the Limba entirely–let alone their responsibility for such accelerated evolution–were granted even more biological upgrades as the epochs passed by, like crude vision and sexual reproduction. What these poor, primal natives could not begin to realize was that the Limba were using them to evolve and survive, while giving them the impression that they were surviving and evolving on their own.

The Limba developed vision so they could see, while granting their hosts the illusion of vision. The Limba developed sexual reproduction so they could further diversify the gene pool, while giving their hosts one rewarding surge of pleasure after another during copulation so they would willingly– nay, gladly, do it again. The water-dwelling natives were used by the parasites in the development of survival properties that best suited them while being tricked into believing in their "free will." It was the perfect plan.

On the other hand, the Limba were slowly losing their own survival capabilities, which helped them survive on their long journey to Earth in the first place. They became so dependent on their hosts that they grew weak, fragile and would almost instantly die if they were exposed to any outer environment. So the Limba found the most suitable solution: Bones. The Limba developed bones in their hosts to reinforce the hosts' defenses and chances of survival, while encapsulating themselves in boxes made entirely of these sturdy, calcium-laden "hulls" for their own protection.

Fish. Insects. Amphibians. Reptiles. Mammals. They all carried Limba. They all lived because of them. They all came to be thanks to them.

Humans, the latest byproduct of Limba's evolutionary efforts, first discovered the Limba by opening a skull and finding one neatly fitted within it. They called it a cerebrum. Brain.

With phrases such as "human brain," "my brain," "my thoughts," "I saw," "I tasted," "I heard," "I felt," "I imagined," it was apparent the humans were entirely oblivious to the parasites that spent billions of years sculpting one primordial generation after another into what they were today that they considered them an essential part of themselves; a vital organ.

Humans have not yet learned the truth as to why the "brain" selfishly encased itself in bone, leaving the rest of the body comparatively defenseless. Humans have not yet figured out that their senses, their feelings, their very sentience are not their own. Humans have not yet realized that they are mere containers of flesh and nutrients, serving as hosts to the ultimate parasite.

And they never will.