On the second planet of a generously bright yellow sun, a mixture of chemicals resolved into microscopic self-replicating processes. Their metabolism was driven by photosynthetisis. The unicellular organisms found organization as multicellulars. Fuzzy black sheets of them covered large swaths of the terrain.
On this planet, slow-moving goliath storms mutilated and burned everything in their paths. The organisms were brought near to extinction hundreds of times before a form of locomotion evolved. The sheets grew small, rootlike appendages that could be controlled like muscles. The brainless collections of photosynthetic organisms learned to evade storms.
With locomotion came competition. The first predators were patches that possessed roots like straws, which would bleed the nutrients from other organisms. The defenseless adapted with quicker locomotion. The predators adapted by developing social tendencies and a rudimentary ability to predict where their prey would run to. Groups of predators became able to catch prey in abundance. Their social interactions grew more complex and they learned a form of agriculture, farming the prey they ate.
This social intellect and an abundance of food led to art and superstition. The individual photosynthetic organs had come to function like neurons, and the flexible roots of the creatures made the usage of tools a natural advancement. After a few millennia, the species developed a primitive terraforming technology which allowed them to control the path of the storms so that their settlements could be permanent, ever beyond the reach of their destructive arms. Civilization occurred, and scientific inquiry followed.
The species was identified as the Moss, the various species they bred and fed on as Vines.
Cultures rose and fell but scientific knowledge steadily grew. Roughly three millennia transpired between the advent of written communication and the first Moss' escape from the atmosphere.
The star system possessed five planets. The closest to the sun was a hunk of minerals, as were the fourth and fifth. The third planet, beyond the Moss', was a massive gas giant. It hosted two moons: one nearly as massive as the Moss planet itself with a thin atmosphere almost entirely comprised of nitrogen and hydrogen, the other a smaller rock with a saturated atmosphere and frozen oceans of water.
A manned mission landed safely on their moon. Resources were poured into terraforming technologies and it became naturally habitable by the Moss and their Vines.
There was a key factor in the psychology of the Moss species. Photosynthetic organs could be regrown, and roots could be regenerated. The Moss and Vines were both biologically immortal so long as nourishment could be had. In addition to this, reproduction was involuntary. The photosynthetic patches were constantly open to receive pollen-like emissions from others of their same species, and would eject spores when the analogues to ovum and sperm could be combined. The spores would grow quickly into patches of photosynthetic organisms and those patches would develop roots. It was good for the Moss that their food replenished so quickly and easily, but their own numbers multiplied mercilessly and exponentially.
Through this, the primary motivation for the Moss to reach into space was land to graze Vines to support the ever-growing population.
The moon was relatively limited in the amount of real estate it had to provide. The Moss terraformed and colonized the smaller, waterlogged moon of the third planet. It became filled, and the Moss looked to its other moon. They determined that though the atmosphere lacked carbon dioxide, they could, like on the two moons they'd already settled upon, manufacture chemical processes on a vast enough scale to remedy it within a matter of years, and that although the surface was turbulent sufficient atmospheric modifications would render conditions tolerable.
When they landed expeditionary parties on the moon they were inexplicably demolished.
The larger moon of the third planet had a history of its own. Its thin atmosphere provided little protection against the light of the sun. The surface temperature of the moon oscillated wildly as it fell into and out of the shadow of the planet it orbited. Its mineral composition was rich in silicon and oxygen, and the hydrogen atmosphere would often be swallowed into subterranean pockets by tectonic activity. These elements combined in just the right way to produce unicellular life which thrived near the moon's molten depths. Unicellulars became multicellular wormlike creatures that bored through solid rock, consumed complex chemicals and the trace deposits of water, and excreted inert grind behind them. The ecology was stable due to tectonic movements which brought rock down toward the core where it was superheated and vomited back up to the surface via volcanic tubes; the inert materials left behind the worms were shuffled and enriched by the process.
The moon's low gravity and high availability of minerals caused the worms to grow gradually in size over millions of years. They developed sophisticated digestive tracts left plankton-like organisms in their wake. Several species evolved which tunneled through the rock and fed on these excretions.
One of them became dominant over the others by developing social, nearly hivelike behavior and an intellect which allowed them to predict and manipulate the paths of the behemoth worms. They created complex systems of tunnels and in them grew civilizations.
The planet's ecology only contained so many worms; in addition to the species which thrived on the excretions of the worms there had evolved predators which kept the population in check. Thus, only a limited population of those which required the worms for life could be supported. Out of this arose a core psychological instinct which the increasing control of predators and resulting increase in worms would not shake. The various species, especially the intelligent and social Ruth, learned conservation and the need to control reproductive impulse.
The moon's surface was characterized by violent eruptions, extreme temperature fluctuations, and a barren geography. It was many millennia of scientific progress before the Ruth had any chance at exploring the surface of their home. When they did, they observed the destruction on the surface and the points of light in the sky, and then retreated back into their cities.
And then, following thousands of years of cultural prosperity, alien probes landed on the Ruth's moon to survey and consider it for terraforming. The Ruth, with their monitoring of seismic activity, knew of the anomaly right away. They found the spacefaring vehicles with their alien pilots and immediately destroyed them.
The Moss discovered what had happened and launched an assault against the Ruth. Unlike the Moss, who were at the top of their food chain and had a pacifist hedonistic culture, the Ruth possessed weapons. They fought back the invaders and devised rockets to deliver world-threatening payloads of thermonuclear arms.
The Moss colony on the Ruth's sibling moon was depopulated with two enormous explosions. Their ignorance of the stellar geography prevented them from knowing of the Moss' home planet and moon. The Moss, not interested in submitting, labored to replicate the technologies used against them. They would not be withheld from expansion. They produced fission devices and launched them.
Explosions ravaged the already-torn surface of the Ruth moon. The Ruth discovered the origin of the offense and returned in kind.
Not to be fooled twice, the Moss were prepared with defenses against such ballistics. The projectile was detonated safely in extraplanetary space. They revised their aggression strategy and bioengineered a virus they judged would upset and collapse the biological systems of the Ruth. They sent capsules of infectious gas they prayed would find its way into Ruth civilization.
The unmanned probes they periodically sent stopped being destroyed.
They finally dared to investigate and found their subterranean cities littered with corpses.
The Moss gained domination of their solar system. They colonized almost every body with solid ground using well-developed transport and terraform technologies. They sent gigantic ships loaded with vast numbers of their population into the stars where they spread to other worlds in their galaxy.
A million years passed and Moss spilled from every habitable planet in their galaxy; they had eradicated every species which opposed them. Not a single populated world was spared gratuitous overpopulation.
The effect of overpopulation on Moss and Vines was profound. As an apparent result of the inescapable proximity to such numbers of others, they ceased to reproduce. The effect was permanent.
Every Moss and every Vine ceased to live. The Vines eaten, the Moss starved. The culture died, as though a bubble inflated until burst.