Hello everyone, it's Nate The Ape here making his debut on the Books section! First, I must say that Animorphs is perhaps my favorite book series of all time. One of the many reasons why I love it so much is because of the evident inventiveness, thought, plausibility, and last but not least, respect that Applegate put into the various alien races she conceived over the course of the series. Too many sci-fi writers are unacceptably sloppy and careless when it comes to putting an alien race down on paper. Not Applegate. Not only did she take the time to give a detailed description of what her aliens looked like, but also to tell us at some point in the books how they used all those physical structures, how and what they ate and drank, what their original habitat and lifestyle was before they went into space, how they evolved, their motivations, and what sort of culture and values their race had. In short, whether Helamcrons or Pemalites, they were all three-dimensional, and laid out with integrity and dignity.
All the same, I personally wish she would've gone into somewhat more depth along the line about exactly HOW the Yeerks developed such a weird, amazingly complex relationship with the Gedds that served as their first hosts. Sometimes you just have to answer questions yourself though, and thus the idea for this fic was conceived.
This is written as a sort of mock pseudo-dissertation, written from the point of view of a scientist after the events in the series. In some cases, I've taken the respectful liberty here of extending Applegate's vision to make my speculations plausible without messing up or violating her original "plan" for how both species are portrayed in her books.
And yes, I know that both Yeerks and Gedds are fictional beings. I'm just doing this for fun and some intellectual exercise.
The Gedd and the Yeerk: An Natural History of Parasite and Host.
Near the beginning of his hirac delest, or final testament, Prince Elfangor-Sirinal-Shamtul gave an excellent description of the remorseless ambitions and iron-fisted aggression of the parasitic Yeerks as they conquered and infested an increasing number of sentient races.
"The Yeerks were loose. Like some terrifying disease, they spread their evil from planet to planet. They took species after species, crushing all resistance. Their spider-like Pool ships roamed throughout the galaxy, their armies of Hork-Bajir and Taxxons, all under the control of Yeerk slugs, rampaged, killing, butchering, enslaving. They were annihilating entire planets. Only we Andalites stood against them."
The Yeerks however, intelligent and capable as they may be, did not develop their space faring abilities and the indispensable portable Kandronas-which mimic the light of their original planet's sun-all on their own. In a terrible irony, this galactic frenzy of slavery and slaughter was the unwitting result of the Andalites themselves allowing the planet-bound Yeerks access to advanced technology, especially the well-meaning Prince Seerow, who wanted to help them achieve greater things.
It is well documented that although Yeerks are apparently capable of infesting any large animal with a suitably wide ear canal, their original hosts belonged to a species known as Gedds. About four to five feet in height, Gedds are medium brown in color, and bear something of a resemblance to an ape walking on its hind feet, with a hunched, awkward, hand-dragging style of locomotion, and a growling, dog-like manner of speech. They also share some similarities with terrestrial otters, such as webbed feet and hands, large eyes, small ears that hug the sides of the head, and sleek, short hair.
Despite being partly webbed and having only three fingers, Gedd hands still possess enough flexibility and strength to pick up and manipulate objects, use tools, drag or carry loads, hold and aim weapons, and even build simple structures. When left to their own devices, unlike most other species that Yeerks have succeeded in infesting, observations and experiments dealing with the cognitive abilities of Gedds indicate that at best, they are only semi-sentient, with an intelligence level lower than an ape's and probably on par with a horse or pig's. It is probably not surprising that most Yeerks consider a Gedd to be a "bottom-of-the-barrel" host, one to inhabit only if there is no other option and for as short a time as possible.
Yet, until only around fifty years ago, the Yeerks, ignorant of alternative, exciting new species to utilize as hosts and the greater universe around them, were restricted to the Gedds that stiffly loped and shambled across their planet as their living vehicles and way of gaining a richer, more varied sensory perspective of reality. Obviously, this indicates that both species evolved in tandem together. But how did this bizarre, formerly unheard of relationship between a parasite and its host species come to develop in the first place?
A close examination of Yeerk genetics, oral history, and comparisons with terrestrial parasites has yielded some important clues, which I shall attempt to sum up and draw conclusions from in this thesis.
First of all, it is obvious to even a casual observer, one who has little or no knowledge about the species, that the mark of the parasite is written all over a Yeerk in its natural state. Superficially slug-like, blind and deaf, with only an excellent sense of touch, its highly developed sonar, and a mild sense of smell to guide it, their skin is pale gray-green in color, coated in mucus and thin as wet tissue paper. Flaps of tissue along their sides propel them through the water in an undulating, up and down manner similar to an otter, only they are far slower and much less agile. On land, they can crawl remarkably well by writhing their bodies in a fashion something like an eel, but are at great risk of drying out. As a result, many people have compared Yeerks to terrestrial parasites like leeches or flatworms.
Studies of Yeerk physiology while the individual is controlling the behavior of its host have shown that soon after altering the consistency of its connective tissues and encompassing the host's brain with its now near-liquid body, a Yeerk fuses its circulatory system with the blood vessels running through the surface of the host's brain, absorbing supplementary oxygen and nutrients as desired. A host's blood however, can never supply the Yeerk with the Kandrona it requires, and it must take this up along with the bulk of its other nutrient requirements every third day in a Yeerk pool, where vital substances are absorbed through its delicate skin.
Despite this method of nutrient uptake, Yeerks still possess a crude gut and vestigial mouth, containing two raspy surfaces that are similar to fine sandpaper. Indeed, Yeerks have been shown to retain an ability to grab and ingest soft-bodied creatures like earthworms, ground meat, pieces of raw fish, and other soft, meaty foods in experiments with their degenerate mouths, using their rasping surfaces to mash the proffered food into a sort of fine puree before swallowing. Later, on being placed into a willing host or morphing into an animal and then communicating telepathically, the Yeerk would confirm that it had found the task of feeding orally to be a tedious and difficult one, on par with a human being attempting to manipulate objects with only their feet and toes. None of the Yeerks perceived a sensation of taste while they ingested food.
Yeerk oral history also states that at one time, before the Gedds appeared, they used to have fairly well developed, underslung mouths for active feeding. Their rasping plates were said to have been even larger and much coarser at this time as well, very like human wood files. Indeed, fragmentary fossil evidence on the Yeerk homeworld confirms this (Counciler FohyaboxSix-two-five, Sstram Controller, personal communication).
This would seem to indicate that the ancestral Yeerk was a species of invertebrate that originally lived and foraged in slow-moving creeks, swamps, and other shallow bodies of reasonably still, permanent water. In these habitats they performed the ecological roles of an active predator on smaller invertebrates, and an opportunistic scavenger on whatever corpses, softened by early decay, chance offered up to them.
Then, at some point in their evolutionary history, massive global changes affected the Yeerk's planet. Today, it is known that the Yeerks inhabit a hostile, highly volcanic world that only just manages to remain suitable for life, with a landscape similar to Wyoming's Yellowstone National Park or New Zealand's Rotorua area on Earth. Outpourings of lava and poisonous gases, earthquakes, mudflows, acidic rain strong enough to burn living tissues, and falls of ash are just some of the environmental hazards that native lifeforms and visitors alike must simply put up with whether they like it or not. The sky is a sickly green-black in color, often crackling with lightening.
Naturally of course, these profound planetary changes had a devastating effect on global biodiversity. Fossil evidence shows a thousandfold decrease in species diversity over this period. Today, as a recently completed yearlong assessment by human xenobiologists reveals, the Yeerk planet is home to a mere one hundred and fourteen wide ranging animal species, with only several dozen kinds of plants, all of them durable and opportunistic organisms.
Into this challenging, harsh new world evolved the Gedds. As noted before, Gedds are extremely lacking in intelligence, and so lack anything that could be called a culture, much less a shared history of their species.
Still, there are some clues to how they arose. Their remarkably ape-like appearance seems to indicate that at one time, their ancestors were probably at least semi-arboreal creatures, gathering fruit, seeds, bark, and small invertebrates in the treetops. Alternatively, they may have lived in rocky, mountainous environments, where grasping hands, long arms, and a low center of gravity would've been quite useful features.
DNA evidence tentatively suggests that the ancestors of Gedds may've either had a more advanced level of intelligence than they do today, or at least been well on their way towards gaining it. Studies of free Gedds show that they like to associate in pairs, trios, or quartets if possible. Yet they also seem to be accepting of solitude for weeks at a time if need be. It is likely though, that their ancestors had tight, permanent relationships, and lived in significantly larger groups.
Whatever their social arrangements may have been, the ancestral Gedds would've been highly pressured by these severe changes. Today, trees on the Yeerk world grow short and widely scattered, and if they had once been semi-arboreal, these Gedd ancestors then would've been forced to live on the ground.
Now, it would've been necessary to search for food either among the scrubby vegetation, or in shallow aquatic environments. In an evolutionary step that was to have unexpected, profound consequences for both their species and life throughout the galaxy on tens of different worlds, natural selection tailored the Gedds to be suited for both habitats alike.
Even if they had evolved to be mostly terrestrial creatures though, the Gedds still would've had to come down to the water to drink or get across. Contact with the creatures that would evolve into the Yeerks was inevitable.
And all this time, the Yeerks themselves were also adapting to their new circumstances. The overwhelming majority of parasites are super specialized in both their form and lifestyle, to the point that they are often adapted to live on or in only a certain part of one specific host's body. There is a species of copepod for instance, that lives only in the nostrils of blue sharks, feeding on mucus and flesh.
In the most curious of ironies though, it seems that the Yeerks first became parasites as a result of taking up a way of life that was as unspecialized as possible. In difficult environments, it is the generalist organisms, the unfussy opportunists who do best in the long-term. Already predators and scavengers, the Yeerks diversified their ecological portfolios even more, and became parasites as well, drinking blood from other animals that entered their pools.
Examination of larval Yeerks shows that there is a brief stage during their development when their vestigial, rasping mouth plates protrude out and almost resemble saw blades, covered in teeth. This strongly implies that at one time, they used similar structures to cut through the skin of animals wading or swimming in the water and then engorge themselves with blood, just like terrestrial leeches.
Leeches also produce an anesthetic chemical in their saliva when feeding from an animal, so as to prevent it from noticing and flinching during this time. The painkiller substances that Yeerks produce while forcing apart the structures of the inner ear en route to the brain are almost certainly rooted in a similar adaptation. Now the Yeerks had evolved to fit three different ecological roles at once, predator, scavenger, and parasite.
What happened next is unclear, seems to have happened in a very short period of evolutionary time, and we have only the vaguest clues from Yeerk oral history to go on. At some point though, these Yeerk ancestors began to rely on blood for their main source of food.
The semi-aquatic Gedds would've then become their most common targets, naturally enough. There was a problem with this though. Each Gedd would only be in the water for a limited period of time, and if it went back onto land while the Yeerk was still taking in blood, it would be forced to either let go or slowly die of desiccation.
But if the Yeerk could enter one of a Gedd's orifices and remain there, it would have access to a constant supply of blood, permanently moist environments kept at a stable, warm temperature, and dramatically increase its mobility.
When an animal lowers its head in order to drink, forage, or reach food in water, its nostrils, mouth, and ears are its most accessible cavities. Coincidentally, these are also some of the most blood-rich areas of the body as well. On earth, some species of leeches are known to exploit this fact by living in the nostrils of land animals like antelope, horses, ducks, and moose, waiting in shallow water and then suddenly slipping up into the sinuses before the host can react.
Less readily accessible, but still available as a feeding site, is the animal's ear canal. As semi-aquatic creatures, the Gedds would often be placing their entire heads under the surface for a brief time to grab food or bathe. This meant that the ear canal was readily accessible to an enterprising Yeerk out for blood.
Now there was a new challenge for the Yeerks to overcome. With their squat, cylindrical bodies, they could only enter so far into a Gedd's ear canal before being forced to stop. There was an even greater incentive placed before them at this point too, one that was literally just beyond reach. Of all the structures in a living creature's head, the one that demands the largest volume of blood and contains the most blood vessels, a thick, dense network swaddled in energizing fat-this is the brain.
To the ancestral Yeerks, this motherlode of blood, a refuge safe inside a bony skull and the chance to disperse their genes to pools miles away from their birthplace, was an evolutionary pot of gold at the end of the rainbow. So the Yeerks changed their bodies in three different new ways. First, they developed catch-connective tissues, just like echinoderms on earth. By altering the rigidity of the connective proteins in their bodies, they could now change their very flesh from a stiff, dense consistency to nearly liquid. Second, their skin became much thinner, like toilet paper, allowing them to slip and move through a Gedd's inner ear more efficiently and faster. Third and most bizarre, they evolved a drastic new method of reproduction.
If not killed by outside causes, Yeerks seem to have a lifespan approaching that of a human's. Invariably though, the act of breeding is a fatal attraction.
Although their oral history professes that once they had distinct male-female genders and spawned in a manner similar to fish, burying their eggs in sand, Yeerks no longer mate as we understand it. (Although Yeerks technically have no genders as we humans understand them, an individual Yeerk nevertheless has a tendency to strongly identify with the gender of its very first host and prefer hosts of that sex in the future. This may have something to do with exposure to the host's sex hormones.) As reported and observed, breeding Yeerks first form triads and swim in a stacked, vertical fashion, each barely brushing the one below. At this point, the bottommost Yeerk then flattens and curves its body downward to form a crude, upside-down bowl. The Yeerk in the middle performs a similar maneuver, spreading and flowing over the one below, with the third, topmost Yeerk doing likewise. Then all three Yeerks contract tightly, shriveling together like a massive raisin, and slowly sink to the bottom of the pool.
For 30-50 minutes, the triad is fused together as genetic information passes freely between all three individuals. Then, their catch-connective tissues abruptly and fatally disintegrate, with the now formless lump of Yeerk tissue falling apart into 150-410 smaller pieces or buds, each of which becomes a larval, genetically unique Yeerk through the activation of certain hormones. Some of these buds may divide like sea anemones before the larva fully develops, giving rise to identical twins. In just 14 to 18 days, the larval Yeerks develop into mature adults, ready to infest new hosts.
As drastic and unorthodox, perhaps even downright disturbing, as this method of reproduction might seem, it is also an efficient method of quickly producing large amounts of offspring. Indeed, the majority of parasites on Earth have evolved to increase the chances that their young will successfully take up residence in or on the proper host by producing staggering numbers of eggs, seeds, etc., as often as they can manage-of course though, the parents usually don't give their lives in the process like the Yeerks do!
Now the Yeerks were 100 percent parasitic. Tuned to a lifestyle that was already wildly unlike anything displayed by an earthly species, they now fed by crawling into a Gedd's ear canal and inserting themselves into the space between its skull and brain surface, where they tapped into a vessel and drank the blood until they were satisfied. Their Gedd hosts transported them great distances to other pools, where the Yeerks could leave the skull, slip into a whole new home to digest their blood meal, and perhaps breed with partners they never could've encountered on their own.
With their thin, membranous skins, they also could gain supplementary nutrition too by absorbing some nutrients right from the water they swam in between chances to infest a Gedd. Later this would become a far more important-and certainly more efficient-method of feeding.
There was the risk though, that whenever a Yeerk detached from a Gedd's brain tissues after feeding, the Gedd could die or suffer permanent brain damage from the resulting intercranial bleeding. This would quickly deplete the number of available hosts, and so was not in the long term interests of the Yeerks. Indeed, if a Gedd died from a brain bleed while away from a body of water, the Yeerk would soon perish as well.
As obligatory parasites, infesting hosts that were only vulnerable for a limited window of time, the Yeerks also needed a more reliable way of detecting when a Gedd's head was underwater and then pinpointing the ear opening when they got close.
It is thought that these two selective pressures were now responsible for the tandem evolution of sonar in Yeerks, and their method of taking in blood by fusing their circulatory system with the host's. Excellent navigation skills and the ability to communicate more effectively with their fellows would've been an added bonus of having a form of sonar, and Yeerks probably took a big step towards intelligence soon after in order to quickly process the large amounts of complex data constantly being sent and received through their biosonar.
The Yeerks and the Gedds weren't evolving in isolation on their planet. If there is a large source of prey available, whether the creatures are plant eaters or parasites, it is almost a certainty that some carnivore will evolve to take advantage of it. And thus the Vanarxes evolved around this time.
We shall now briefly digress from the main topic of this thesis to take a closer, cursory look at the biology of this extraordinary, poorly known creature, with its chillingly gruesome eating habits. An even more in-depth examination of this highly specialized predator will form an Appendix to this thesis.
Looking like an eight foot long combination between a seahorse and a bear, Vanarxes have partly translucent tissues, and are covered in a soft, pliable, pebbled armor, very like an armadillo's. Adult males are royal purple in color, while adult females, roughly 25 percent smaller and having a pouch for carrying several large eggs, are an emerald green, both sexes being solitary.
As reported and witnessed, a Vanarx will first locate a Gedd-Controller at a distance (often asleep, for Gedds are diurnal) by smell, confirming the presence of a Yeerk solely by the miniscule amounts of scent leaking out from the host's earholes. Drawing a bead on the Controller with its phenomenal night vision, it then makes a stalking approach on all fours, never taking its eyes off the victim. When close enough, it rushes the infested Gedd on all fours, the alien beast rearing up and jabbing its long, retractable claws deep into the Controller's upper torso, pinning and restraining it.
Engulfing one side of the Gedd's head with its yawning, sucker-lined mouth, the Vanarx then sucks in air over the host's ear canal, so hard that it rips out the eardrum, splits the inner ear bones, and eventually dislodges the Yeerk from its grip on the brain cortex. The doomed parasite has no way to go but down the exploded, shredded, ear canal-and into the stomach of the Vanarx.
Unfortunately, the suction force also inadvertently bursts and ruptures veins and arteries in the inner ear and brain, and pints of blood are drawn into the Vanarx's mouth along with the Yeerk, not to mention portions of the fat-rich brain tissue itself. Unsurprisingly, the host becomes collateral damage, dying a swift, nightmarish death.
In this brutal manner, the Vanarxes began to act as a control on the numbers of the ancestral Yeerks. But despite the threat posed by these new predators, they continued to press on and develop. All the same, this grim predator probably had its own role to play in Yeerk evolution.
The risk of being drawn out from a host and being devoured by a Vanarx may have induced Yeerks to begin to rely less on whatever blood they could actively take from a Gedd's brain and more on what nutrients they could absorb through their frail skins in their more typical aquatic habitats.
Then a profound new threshold was crossed. As mentioned before, it is very likely that selective pressure associated with the development of biosonar in Yeerks and the need to quickly process all the input had caused the Yeerks to become a semi-sentient species by this time, which Yeerk oral history seems to confirm.
Evolution is driven by mutations, changes, errors in DNA. If one of these changes either gives an individual organism any advantage, however humble it may seem, or at least doesn't harm it, it will be passed on first to its descendants, then spreads through out its population, and maybe even its whole species.
Somewhere, somehow, on the Yeerk's planet, one of these accidents of heredity resulted in a single Yeerk that was able to not just link its circulatory system with a Gedd's brain, but its nervous system as well!
The advantages were enormous. By tapping into a Gedd's neural networks, a Yeerk could get a picture of its world that sonar could never match. It could look through the Gedd's eyes and tell if the pool that it was going to move into was a small, shallow one at risk of drying up, or a large, deep one where it would be secure. It could passively spy on other Gedds, getting an idea of which pools they preferred to feed and bathe at, and at what time of the day they did so. It could get an idea of which individuals were the best to infest, and gain some clues as to where Vanarxes lurked.
Bombarded by sensations and information received through the Gedd's nervous system, the semi-sentient Yeerks would've been required to become fully intelligent, fully self-aware, in order to deal with and interpret it all. And that's exactly what happened to the fast-breeding Yeerks. They developed a real language of dolphin like clicks and were able to actively speak among themselves. They could ponder mysteries, assess their past experiences, learn from those experiences, and control instinctive impulses.
They could imagine, have an idea of the future, assimilate important knowledge, tricks picked up along the way, and pass it on to their descendants. Most importantly-and perhaps touchingly-of all, they could appreciate, marvel at, and find beauty in their world as they experienced it through the eyes, ears, skin, nostrils, and taste buds of their Gedd hosts-a true hallmark of self-awareness if there ever was one.
One law that holds true throughout the universe is that energy can never be created or destroyed, but only changes its form. Another is that so, so much of life, whether it be what happens to a person throughout their day, or the development of a species, boils down to sheer unsympathetic luck. Randomness rules.
Adolf Hitler could've lost his bid for chancellor of Germany. That Mount Everest sized asteroid that gave mammals dominion could never have plowed into Mexico 65 million years ago. A different species of hominid could've proved to be the superior competitor.
And the Yeerks could well have been tailored to a more passive lifestyle, one where they could look through a Gedd's eyes and enjoy the experience of expanding their perceptions while taking their pint of blood, but not interfere with their host's activities or enslave it. To the Gedds, the Yeerks would've been little more than an inconvenient nuisance, one that sometimes uncomfortably wriggled and squelched up your ear while you foraged or gamboled in the water and caused you to be somewhat lightheaded for a time, but eventually dropped back in the next deep pool that you visited, fast as a star twinkling.
But it was not to be.
Another accident of creation happened in a Yeerk's genes, one that would have horrible consequences for all of creation itself.
There are a few parasites on Earth that actively control the behavior of a host by sabotaging its nervous system. Gordian worms are an example of this. Adults lay eggs in the water that hatch into larvae which burrow into the larvae of mayflies, caddis flies, and other aquatic insects. When the larval insect changes into a winged adult, the larval worm is along for the ride. If the mayfly, stonefly, etc. is eaten by a predatory insect like a cricket, dragonfly, mantis, or beetle, the Gordian worm develops even more, growing and feeding on the predator's internal organs until it occupies the entire abdomen, curled up tight like a spring.
And then it sprays a chemical on its host's brain that makes the insect thirsty. Desperately thirsty. The worm's host craves water so badly in fact, that it leaps right in on finding it. As the insect drowns, the Gordian worm erupts from the abdomen, a long black creature, and swims away. In time, it will mate and lay eggs of its own.
All of these parasite species however, either use a drug of some sort they produce or damage the brain to make the host do their bidding, and only infest small invertebrates. The Yeerks though, didn't rely on chemical goads or wreck brains, and went for something more complex, and more impressive.
That one mortal mutation gave a Yeerk, its nervous system already linked with that of the Gedd it inhabited, the power to transmit the desires and thoughts generated in its own flattened brain through its body, and into the Gedd's own brain, where they acted on the Gedd's own voluntary muscles. The blood drinking parasite had transformed into a puppet master.
Now the Yeerks could actively choose which pools they moved to, or even ensure that they would always be able to return to a specific one. They could actively run away from or defend themselves against Vanarxes or predators that ate Gedds. Instead of having to wait for days on end for a Gedd to arrive at their pool and stick its head under, their fellow Yeerks could forcibly bring a free Gedd to them and put its head under the surface for their benefit.
They could make decisions for themselves about which sights, sounds, and tastes they wanted to experience, instead of having to accept whatever the Gedd happened to come across. Indeed, now a Yeerk could even be creative, and produce sights and sounds and tactile sensations all its own. It could engage in interesting and stimulating physical activities like running and walking.
We can never know for sure what that first Yeerk felt when it realized it was different, that it could exert at least some degree of control over the movements and activities of its Gedd host. Perhaps it was disbelief, a sense of surreal shock that it could actually do such a thing, like a person given the power to fly might feel. Maybe it was filled with a giddy, heady sense of excitement and power, the thrill that it could now force the Gedd to do whatever the Yeerk pleased, instead of just having to go along for the ride. It might have been frightened and nervous, as the ramifications of what this brand new development potentially meant for the soul, the moral makeup, the quality of life of both its species and the Gedds.
The first major change to the bodies of the ancestral Yeerks was to develop mouthparts that could cut through the skin of the ancestral Gedds in order to enjoy an opportunistic blood meal. Now, in an ironic evolutionary step, their last significant genetic act was to almost completely discard blood from their diet.
It is in a parasite's best interest to have its host be as healthy as possible. This is especially true for Yeerks, since the healthier their host is, the better they will be able to see and hear, to fight or flee in a battle situation, to get physical labor done and attend to their duties.
When less blood reaches the brain in any animal, they become sluggish, confused, and less alert. This would never do for the Yeerks. So, instead of feeding by absorbing blood from the host's brain into their bodies, they adapted to rely mostly on nutrients absorbed from the water and synthesized from the light of their planet's sun whenever swimming freely outside their hosts and in their natural state.
Nowadays, Yeerks only feed on an occasional, small blood meal from a host's brain as a sort of light snack, and only when the host is sound asleep.
We can never know exactly how intelligent the Gedds were when the Yeerks gained the ability to control and enslave them. What we do know though, is that heartbreakingly, genetic and fossil evidence suggest that over thousands of years of being infested by Yeerks, the Gedds became much less intelligent than they formerly were, almost certainly as a way of assisting the species in coping with the despairing helplessness and psychological stress endured during enslavement.
It is all too possible that if the Yeerks had not been defeated in such a crushing manner by the group of children and one young Andalite who dubbed themselves "Animorphs," and then offered an alternative to enslaving intelligent beings by being allowed to use Andalite morphing technology, this grim, slow decay of intellect, enjoyment of one's life, and free will itself would've been humanity's ultimate fate as well.
At any rate, the Yeerks were now the masters of their planet, and the Gedds their shambling horses, totally subservient to their will. This was the status quo for a long time, over countless centuries-until the day a strange new object appeared and then descended through the greenish sky. The rest, as they say, is history.
So, in conclusion, all available evidence suggests that Yeerks progressively evolved from opportunistic aquatic predators and scavengers with very crude mental capabilities to specialized, self-aware, parasitic beings that use their Gedd hosts not as a source of food or a place to reproduce, but as a form of transportation and a way of both experiencing their world and acting on it more completely, all by tailoring their bodies and brains to the space between a Gedd's ears.
These ten words say it all quite well:
"First they took Gedd blood. Then they took Gedd minds."
The Appendix will come later.