Could Yeerks Really Evolve?

[Kotetsu1442]

I don't see how they could really evolve, because like it was stated before, as soon as they left the host, the host would probably attack them or escape. But I don't know much about evolutionary theories, so I'm just going to say:
I blame Crayak. :| It's just the kind of thing he'd do.

Well, we already discussed a lot more of the plausible suggestions in more detail in this thread, but a short answer is no, the host wouldn't necessarily attack the Yeerk. If the Yeerk was in no way attractive as a source of food and was at an early point in the evolution the host wasn't even intelligent enough to understand infestation it could easily ignore the Yeerk that left its body entirely; and if the infestation mutually benefited both species, Gedds would evolve toward being more willing to accept infestation in exchange for more intelligent guidance as a survival trait.

There are plenty of examples in our nature of animals being perfectly comfortable with smaller creatures crawling around, on or even in them, it's not an evolutionarily unusual thing. And besides, "Ellimist/Crayak did it" takes all the fun and imagination out of looking over this series.

[Myitt]

My big question is: Yeerks appear to be photosynthetic organisms, or at least chemosynthetic autotrophs, sort of like multicellular algae or protists.  The benefit of infesting a host and removing yourself from access to critically vital sustenance would have to outweigh the risk, at least more often than not. 

Also, a Yeerk would have to be able to infest a number of evolutionarily diverse hosts while not killing the host or itself.  Whatever else the Yeerk excretes besides painkillers would have to be some kind of sterilization agent, and it would have to regulate its body temperature to match that of its host. 

The idea of a Yeerk burrowing under all those cerebral membranes and the cerebrospinal fluid cushion to wrap around the brain itself, without damaging it or creating deadly pressure, in a matter of minutes--that would imply that the Yeerks could really frigging move quickly while infesting, almost at a fluid level, before coagulating back into their sluggy form. 

Just throwing these ideas out there.

[donut]

I'm trying to think of what they said about yeerk reproduction, was it like 3 fuse together and produce a thousand offsrping?  If so that means that if only 1/100 survive to reproduce it would still have a growth rate of over 3x per generation so alot of the slugs could get killed without it being a problem


yeah, the mechanics of the yeerk infesting a host without injuring it would be the hard part in all this.  you're right, it would need some kind of antiseptic now that I think about it, and the puncture it made in the white matter would have to be small enough to heal rapidly.  I'm thinking the yeerks would have to be very thin when they surround the brain, just to keep the brain alive by moving nutrients through.  I think the yeerk would have to absorb at least some the csf to keep from putting too much pressure on the brain, or maybe the csf could escape through the hole the yeerk came through, although the brain can hold a lot of csf, at least in comparison to the volume of slugs (how big are yeerks?) 

I never thought of the yeerk having to match temperature with the host brain, but yeah it'd kill the host if it didn't, the easiest way I can think of to solve this is that the yeerks are prolly cold blooded

[Kotetsu1442]

My big question is: Yeerks appear to be photosynthetic organisms, or at least chemosynthetic autotrophs, sort of like multicellular algae or protists.  The benefit of infesting a host and removing yourself from access to critically vital sustenance would have to outweigh the risk, at least more often than not. 

Well, just to clarify, I would point out that they are not actually photosynthetic oranisms; that is to say they do not actually using energy from light to produce organic compounds for their bodies. Though the Anis frequently refer to the process with quips and phrases like "soaking up rays" it is explicitly stated that they are actually absorbing nutrients. Obviously the original sun of the Yeerk home planet, what the Kandronas simulate, can not possibly radiate nutrients through the vacuum of space yet its rays are necessary for the nutrients to be present in the pools, so even though it is expanding on explicit canon the only reasonable supposition is that some form of small (perhaps even microbial) life is present in the water-based mixture that makes up the Yeerk pool 'sludge' and it produces organic compounds from Yeerk homeworld sunlight that is useful to the Yeerks, but when out of the present of that light the compounds are stored in a form that is not useful to the Yeerks.

The reason this distinction is important is because in order to gain enough energy from photosynthesis an organism like the Yeerks would need to spend many hours a day in the Kandrona rays, but on the other hand if they are swimming over an area of millions of small plant or bacterial organisms that are all preforming the actual photosynthesis and feeding off of these organisms (in a manner not too different from one of our Mysticeti Cetaceans, what you would most likely think of simply as 'whales', feeding off of millions of small plankton) then they need not spend the vast majority of their time in the rays but only need to feed a couple of hours over a few days (as we see in the Yeerks). This being the case, it is certainly possible for their to be circumstances on the home planet where the risk of temporarily removing yourself from the medium you feed in is very worth-while,  in order to take shelter from natural predators for example; this is basically how amphibian creatures originated, during the Devonian Period they evolved the ability to flee water temporarily but had to return to feed and eventually mate and lay eggs, it was only in later periods that they gained the ability to feed on bugs from land move up the food-chain somewhat.

On the other hand, your points on being able to infest such a large variety of hosts is a very valid one in what makes the Yeerks seem much more improbable, particularly that they creatures from completely foreign planets turn out to be physiologically similar that they are just as controllable. However, many other incredibly 'unlikely' similarities between creatures from completely different planets (land creatures are capable of breathing in each other's atmopheres, Andalites can eat other planets' grass and Hork-Bajir can eat bark of other planets' trees without them turning out to be completely different in organic makeup, and perhaps most notably the morphing technology works on animals from any planet) forces one to assume that, for reasons beyond our incredibly limited knowledge of how life forms, life isn't just as likely to form off of many different bases and for some reason a carbon base and the early history of life on this planet is basically the only way life can form and therefore lifeforms on different planets are quite likely to be capable of biological interaction.

The intra-cranial pressure is one big issue that hasn't come up much yet, but in order for it to work the Yeerks would have to actually have remarkably little mass of their own. Though it is never noted in the series, perhaps the 'slime' that they are coated in is actually filling a great amount of them so that, if stuck away from a pool and outside of a host for a period of a few hours they are able to gradually secret enough of this slime from their stores to keep themselves moist on the surface while they try to reach a moist place or a host (it has been noted they can survive outside of their own 'sludge' in relatively moist areas for a few hours, but will gradually shrivel up). This being the case, they could squeeze out all the extra 'slime' stores as they prepare to enter the host and only actually being an extremely small mass inside of the infested host. This would explain what we see in the series, that they can only infest a host of sufficiently large cranial capacity but in such a case they can do it without causing undue damage to the host (except in the case of the modified Arn, who deliberately can not survive with even a slight increase in cranial pressure).

And yes, I would agree with donut's hypothesis that the Yeerks are likely cold 'blood'ed and capable of surviving in a decent range of temperatures rather than needing to needing to regulate their own internal temperatures.

I'm trying to think of what they said about yeerk reproduction, was it like 3 fuse together and produce a thousand offsrping?  If so that means that if only 1/100 survive to reproduce it would still have a growth rate of over 3x per generation so alot of the slugs could get killed without it being a problem

Well, it is hard to say, because the books are never actually specify on this issue. They usually only specify that 'many' grubs are produced from the biological parental tripartite, which has lead readers to visualize anywhere from a couple dozen to thousands. Esplin 9466 refers to the ofspring as 'several hundred' however, meaning the number will potentially reach into the low thousands, but not necessarily reach several thousand. The Yeerks named/numbered in the series are often only in the hundreds (remember, Esplin 9466 isn't nine-thousand, four-hundred and sixty-six, it is nine-four-double-six, meaning that the 946th grub split to form two twins). In #41, The Familiar, there are Yeerks with incredibly high numbers, but either the naming/numbering system could have changed in the future or it could have simply been one of the 'incorrect' details that was inserted into the odd dream-vision that Jake had that revealed it to not be the true future.

In any case. you are correct that about a thousand offspring per tripartite which, along with their aggressive nature, would imply that earlier in the Yeerk evolutionary history the Yeerks were hunted in large numbers by some form of larger prey and selected toward a high amount of competitive offspring in order to make very low survival rates acceptable; but when the Yeerks became the dominant life-form on the planet and gained a fairly high survival rate this would result in a population explosion and continued exponential growth; hence the need for an expanding Empire form of society.

[Myitt]

Interesting points on the Yeerks being more like mysticete whales (don't worry, you're talking to a biology major and a grad student in museum studies in paleontology, here--woot for bio-nerds! )--however my issue with the Yeerks being able to absorb microbial life is that there is very little to suggest the Yeerks had a method of doing so.  The Yeerks are described as having 'osmosis nodes', which are the equivalent of a "mouth" according to Ax; however their name suggests that they aren't phagocytizing or engulfing whole organisms with their very fluid cells.  Rather, the term osmosis makes me think that they are allowing a concentration of chemical nutrients and water to be absorbed by their hungry cells.  You can certainly draw any conclusions you want from this, feel free to make the Yeerks whatever you imagine them to be---but my opinion based on this description is that the Yeerks were simply absorbing amino acids or vitamin nutrients along with water to replenish their bodies.  I always just figured that while the nutrients in the pool are important, the one thing that Yeerks really needed to survive was Kandrona, interacting directly with their bodies, and not indirectly through facultative cyanobacteria or something similar. 

Also, practically speaking, how would Yeerks create portable pools when they needed not only a portable Kandrona generator, but also the microorganisms from their homeworld?  Is it ever stated that they needed a petri dish of fresh organisms to install in a portable pool?  No, which doesn't rule out the idea, but I think it's a little far fetched given how critical Kandrona is described as being to the Yeerks' lives, and not necessarily the nutrients themselves.

My defense for the Yeerks being autotrophic (again, in my opinion) is that firstly, these are aliens.  They may need only a small amount of time under the energy of what is described as a "Kandrona wave/particle generator" (light is both a wave and a particle) in order to replenish their cellular protein store, aided by the nutrients in their pool which I believe are secondary to their survival.  After all, wave-particles do indeed travel through the vacuum of space, evidenced by...well, all life on Earth depending on the photons from the sun exciting the chloroplast electrons in plants.  I figure that the Kandrona wave/particles have a radically different wavelength (or optimum wavelength reception inside Yeerk chloroplasts) than that of Earth sunlight, and that their "strange sun" is so unique in this production of its wavelengths that it is rarely found anywhere else but produced by the Kandrona itself, or the Kandrona wave/particle generators.  Just my two cents, and since I have plenty of Yeerk characters in RPs I had to figure out some nerdy way to consolidate their biological need for Kandrona

I'd forgotten about the Arn's self-destruction when there's a slight increase in cranial pressure!  That's awesome, it means that K.A./Michael Grant were thinking about this stuff at least a little, bwahaha.  Yes I agree that a particular suspension of disbelief and a reasonable assumption that life around the galaxy is more carbon-based than not are useful tools when discussing the biology of fictional alien species.

Another note that kind of throws the topic of "bloodedness" into question is that when Cassie morphs into Illim in #29, her heart stops beating, then shrivels up and disappears.  I always took that to mean that Yeerks didn't have a circulatory system, at least not one that is anything like we know.  Perhaps the slime that they're so dependent upon is the same makeup as their cytoplasm, and the fluidity with which their bodies can change and flatten both rules out a need for a circulatory system and protects the host brain from any conflicting movement or additional pressure from blood pumping and coursing in a tissue-thin Yeerk wrapped around it.

So...maybe they had their own version of cold-bloodedness, or perhaps they were able to adapt to any host body temperature (within reason--they still couldn't survive the heat of a jacuzzi) by having no need for internal circulatory temperature regulation within a certain buffer zone.

Interesting note, too, with the #41 mistake (?) or number change.  That always could've had something to do with there being a vast number of hosts to control, but I tend to think it was some kind of mistake by the puppetmaster behind Jake's vision...

I'd argue against the Yeerk's aggressive nature, however.  Yeerks describe themselves as peaceful by nature on their homeworld (Temrash, Aftran, even Edriss says they had only fought a handful of conflicts--possibly only after leaving their homeworld), only infesting what they were born to infest, and only after creating an Empire and being basically given the shaft by the Andalites ("look, stars!  No, you can't have them.") did the Imperialist and aggressive nature (power hungry, sadistic, and a little bit crazy bananas) of a few (but not all) Yeerks come to the surface.  Even the hierarchy of Yeerks wasn't nearly as competitive until ranks like Visser and Sub-visser came around, evidenced by the young Esplin 9466 in his newfound pride at having a title beyond his birth rank.  The Council always existed, but it seems as though social competition really ramped up after the Yeerks left the homeworld.  So, I guess I don't think the Empire form of society was necessary, but it was certainly desirable for many Yeerks who saw a way to be as dominant as the Andalites in the galaxy, in the only way they knew how--controlling other species.

Nevertheless, their biological need for exponential growth really does indicate they were preyed upon pretty severely over the millennia.  There is that good old Yeerkbane/Vanarx in the series, after all

[Kotetsu1442]

It certainly is interesting to look at completely different approaches to explanations, without canonical evidence there is enough room for assumptions and various diverse conclusions. I agree that the descriptions in the book, though again never explicit, sounds more autotrophic, I avoided it myself though because of the idea that photosynthesis doesn't allow for enough energy for a creature of that size to sustain itself through the conversion of inorganic molecules; but now that you bring it up, it is certainly possible that the waves/particles that make up what we know as Kandrona waves may be supplied in a sufficiently intensity and be more efficiently absorb-able by the Yeerks than we are used to in plants here on Earth; so there is certainly plenty more plausibility to that interpretation than I gave it credit for initially.

As far as the portable pools from my interpretation goes, I assumed that either the small organisms could survive in the water/sludge in an inert form for long periods of time before portable generator is activated and used, or they were simply stored in an inert form (kinda like Baker's yeast  ) and were released into the sludge as part of the process of activating the portable Kandrona. Again, there's certainly no details that indicate the the portable Kandrona function like this, but there really aren't details on how the portable Kandrona work in general; this is simply how they would function if the Yeerks feed in the method I previously described.

I also think that the Baker's Yeast organisms are supported by the "single use" portable Kandrona V1 had, if they simply didn't have enough energy stored to continue powering them then her survival wouldn't be limited to 'less than ten days' with three porta-pools if she could find an alternate energy source to power the generators but with only three supplies sets of an organism that is also necessary for the process then she really had only that much time before she could get back into an accepted place in the Yeerk Empire (I'm referring to the situation in #30, The Reunion). Her and her partner earlier in her time-line, as told in Visser could also have stayed on Earth indefinitely without rejoining the Yeerk Empire if they had the knowledge to repair their Kandrona generators or build new ones, but again if they had stores of organisms that they could use a limited number of times but could not create an environment to further produce them then that could be an explanation for why they 'ran out of uses' of their more long term Kandrona generators.

Again, just an explanation built off of assumptions with limited details, but worthwhile; with the thoughts you provided your perspective is just as plausible as well. In any case, the only actual relevant point that this made in the earlier post is what we actually do know: The Yeerks can survive for long periods of time outside of their feeding medium with only a little time in it, making infestation evolutionarily viable.

Interesting note, too, with the #41 mistake (?) or number change.  That always could've had something to do with there being a vast number of hosts to control, but I tend to think it was some kind of mistake by the puppetmaster behind Jake's vision...

Yeah, it could go either way, but I tended toward that assumption being more likely as well.

And yes, you are correct about the Yeerks not being explicitly aggressive by nature, now that I think about it I was certainly incorrect there; they are not necessarily so, at least not any more than humans are by nature; it is much more their current government/social situation and the drive to gain all the senses they lack in their natural form that produces their current aggressive behavior.

[Myitt]

Interesting idea with the Baker's yeast, I have my characters carry around packets of minerals but packets of some kind of inert fungus/bacterium could work.  I still think that Kandrona was the main source of energy for the Yeerks, simply because it's made out to be so important to the Yeerks.  I always took the situation with the Kandrona running out in Visser to be a miscalculation on Essam and Edriss' part, or a faulty generator.  The single use generators, to me, just meant that the generators weren't powerful enough to last for more than one full feeding cycle.

I always wondered what Yeerk society would have been like in its earliest days.  How did Yeerks develop their ultrasonic language, and how did it spread to all the pools?  Were there Yeerk dialects?  What did Yeerks teach the young just-out-of-grubhood Yeerks in training?  It seems based on Seerow's observations that the Yeerks were smart enough to grasp concepts far beyond their limited knowledge of the universe, and it seems likely that they had basic concepts of mathematics and perhaps physics based on their circular pools and currents.  This is just an assumption, however, because the Yeerks didn't have written language until the Andalites came along, so they'd have to do all the calculations in their heads. 

Then again, maybe Yeerks are just big squishy brains without much else to impede direct contact between their neurons and the host's neurons, and this intellectual development helped aid their evolutionary development of infestation?

[donut]

wow, I just got finished reading that book you guys wrote, interesting discussion.

for some reason a carbon base

We do have some idea of why this is, it certainly isn't concrete and I wouldn't be surprised if we eventually found exceptions to it.  Carbon is really the only atom that has the necessary properties to be useable as the basis of life. I can't quite remember what the reason that silicon probably wouldn't work but I think it was because of silicon's, what-was-the-term, squishiness(?) to be used. It's too large, it distributes charge too well and so it becomes too unreactive.  Again, I can't quite recall but I think that's at least part of it.  Boron and Nitrogen are also alternatives, like silicon I can't remember but I don't think they form enough complicated molecules to be suitable, plus with boron it carries a charge with a filled octet, I don't know but I'm guessing that this would be a significant problem in macromolecules to have thousands of positively charged atoms in every, whatever the equivalent of DNA is, molecule and protein in the organism.  Just remembered, one of the reasons nitrogen prolly isn't viable is that N₂ has too high of bond strength.  That and anything nitrogen based would have to have an extremely cold environment, so even if they existed in the series they wouldn't be mentioned since they wouldn't appear anywhere the anis could survive at.

land creatures are capable of breathing in each other's atmopheres

I'm thinking that all the creatures had nearly identical atmospheres on their homeworlds, since as you said they all can breathe on other planets, and they all see visible light.  I think this might be fairly likely if they're carbon based, since except for a very recently discovered exception which uses hydrogen, all known multicellular lifeforms use oxygen for respiration.  I really have no idea if there's a reason other elements can't be used or not, maybe myitt knows if there's one or not.  At vey least if they're carbon based they'd need a significant amount of oxygen available to make the various organic molecules important to life (maybe even if they weren't carbon based).  So I think that it's plausible that complex carbon based life would need an atmosphere similar to ours to develop.

I also thought it odd that the physiology was so similar between the species, more examples to show this: toxins are universally effective and thought speak works with all species suggesting that not only is the physiology of the neurons similar, but the way brains process and store information would have to be similar, unfortunately we know so little about how the human brain stores information that I can't make any guesses one way or the other as to whether this is plausible.

I guess a general and admittedly weak argument that all life would be similar or rather that the chemistry of all life would be similar is that chemical reactions have a tendency to form the lowest energy products (not precisely accurate, but close enough for our purposes, and the argument still holds either way) so it makes sense that on a basic level life would be similar because it is likely that the way we know life is the lowest energy way to have life. Although it could be argued that it's only the lowest energy way to have life here.

Now about the kandrona rays, I'm also thinking they're autotrophs.  Partly because they don't have the kandrona on constantly in the smaller pools, I would think that'd starve the microbes.  The yeast idea is interesting though, but they never mention having to replace anything other than the kandrona generators.  I think the sludge of the yeerk pool is necessary to the yeerks survival since they always use the sludge rather than water or any other liquid.  I'm guessing that the sludge in the pools acts as some sort of catalyst for the kandrona rays to be able to be used by the yeerks since they're always concerned with running out of kandrona and not the sludge itself, perhaps the kandrona also converts the waste products of the yeerks back into the sludge.   I think the generators would need some sort of special material to produce the kandrona rays, like making light at different frequencies than is normally produced at the temperature the source of light is at, and that the single use kandrona generators prolly consume this material in the process.

I'd actually be interested in hearing a possible explanation for how the yeerk's "brain" works since to be able to be squished and stretched like it was it'd have to be something completely different in structure than anything we know of, but to be able to interact with our brains they'd have to be similar in the chemistry to our brains.

   ok, so there's my chapter in the book

[Kotetsu1442]

   ok, so there's my chapter in the book

I hope that that facepalm isn't meant to imply that you're afraid your post is too long, because I am very much of the opinion that size restrictions in popular mediums of sharing information (like time-slots on shows or word-length restrictions in magazine articles) has caused our society to become concerned with restricting size in a manner that restricts the information. The feeling seems to be that "Any important issue can be summed up in a 7-minute segment on the news. Three minutes for one side of the argument, three minutes for the other (notice that there can only be two sides to an issue) and a minute to summarize it." If you can't fit the information into this, then it "must be extraneous or irrelevant information." Such thoughts, conscious or unconscious, restricts the sharing of information and ideas as well as any progress that may be made as a result of them; so talk on donut, as long and drawn out as you feel is necessary to share and support your thoughts (you've doubtless noticed that I don't hesitate to do so  ).

And yes, you are totally right about a lot of the findings that we have about why carbon-based life works so well, that it and only a handful (less than a dozen) different atoms makes up life as we know it is nothing short of astounding. We can study these basic building blocks and see each has specific properties of how they interact with each other and that this set of reactions applied in boundless different combinations creates an incredible diversity to what we know as life with only a few different individual parts; like a beautiful symphony with a handful of different instruments creating harmonies together that are much more than the sum of their parts.

Anyways, I appreciate your elucidating on why our 'carbon based' set of atoms works so well for life, but I can't help but be fascinated with wondering whether it is the only set that will work for life (which is certainly possible, but we are ridiculously far from knowing enough to prove that it is the case) or whether, as you say, there may be exceptions, that is other sets of atoms and interactions that can make up other forms of life; it may be that in our study of carbon-based life and having millions of examples of it we have become far to familiar with it as the basis of life that it will take a great leap of thought to be able to think outside of that box and find another way that works. I just think it would speak volumes for human intelligence and knowledge of life if we could predict or even artificially create examples of other possible forms of life and then discover a naturally occurring example of it in our universe.

As far as the Ani-verse goes, as I alluded to here and elaborated on in in the http://animorphsforum.com/forum/index.php?topic=4222.30 thread, in order for its universe to make sense you have to assume that within it either:
A. carbon-based life is the only life that will work
B. carbon-based life is much more probable to work, and therefore when traveling to new planets it is unsurprising to find that they are all made up of life that has carbon-based DNA. That is to say, that given a universe with energy in different forms and matter in different forms all interacting at random, different types of life will form according to different probabilities, but carbon-based life will form with a greater probability than others so these carbon-based lifeforms encountering each other is not unlikely.
OR
C. carbon-based life isn't necessarily more likely than other possible forms of life, and all of these different forms of inter-planetary life meeting each other due to relative proximity and turning out to all be carbon-based is just an extreme coincidence.

Then, you have to take either A or B further (or just go all out with the coincidence theory) and suppose that within the Animorphs universe not only life, but also intelligent life either has to form within very specific parameters or is just most likely to form within these parameters and less likely exceptions are possible but do not make the encounters we see unlikely. (donut, am I correct in my understanding that B. is what you mean when you refer to 'lowest energy' forming of life, that this type of life is assumed in this universe to be the 'easiest' way for life to randomly form, that is to say it has with the greatest probability of forming?)



And yes, my version of possible Yeerk life is admittedly speculation without support, it is possible but there certainly aren't details providing that it has to be this way. Still, the way that it works in fictional universes like this is you can't really subscribe to believing it doesn't work in a way until details are provided; if in the first handful of books you were to suppose how the morphing technology works you could make a lot of reasonable guesses about the stored DNA being used as a set of blueprints to 'grow' a replication of the animal, but based on the descriptions the person is always entirely changing into the animal with no addressing of how that is happening physically, there is no hint at mass being shifted into Z-space and a mental link between the person in Z-space and the animal brain; that is later established (there is still no canonical establishing of where extra mass comes from for larger morphs, only speculations). With the vague details provided of 'absorbing nutrients' the autotrophic route is in itself a speculation; because just as there is no mention of replacing lifeforms they also don't have any mention of needing to replace inorganic molecules in the artificial pools to simulate a natural cycle on their home planet; Ax does specify that the 'sludge' is mostly water, but what the remainder is that makes it fit that molten-lead description is left entirely to speculation. If they don't have to simulate an environment artificially other than providing energy from the rays then at least one another life-form creating a cycle is reasonable (although I'll admit that my earlier phrase of it being the "only reasonable supposition" is an exaggeration).

Man, wouldn't a canonical expanded universe be great if for no other reason than to satisfy all of us nerds who like to wonder about how it all works?

[Myitt]

Personally, I think I'd go with theory B, because I'm also fascinated with the idea that life can be built on other base molecules like silicon or something wildly different from carbon.  In order to make the Yeerks' spread across the universe plausible, and the fact that DNA is so common within the Animorphs universe, I like to think that carbon-based lifeforms with a deoxyribose helix of nucleic acids just happens to be one of the more common, evolutionarily feasible ways for life to develop around the universe.  It's good that there are a few examples of creatures that couldn't be infested by the Yeerks: the Ongachics (I think?), whose brains were spread out in nodes around their bodies; the Skrit Na, who need to metamorphose as a part of their life cycle; and even Edriss mentions that it would do no good if Earth-based life survived by breathing methane, so even the atmospheric makeup of Yeerk-controlled planets had to be similar enough for each species to survive on each invaded planet. 

As for Yeerk brains, I always just pictured them as having a brain stem of some sort (described in #17) but that their brains themselves were spread out across their whole bodies, just a big network of memory and motor/sensory neurons able to spread paper-thin.  It would make sense then that Yeerks are so greatly intelligent, if their encephalization quotient (the E.Q., or body-to-brain ratio) is great enough to make them almost all brain power and almost complete direct connection between their brain neurons and their hosts'.  They are described as having "few nerve endings" (#29 by Aftran) but they did have other senses.  Perhaps their senses of smell and hearing and echolocation were greatly enhanced by being in their natural liquid medium (sonar and sound travel better and faster through water than air), or these organs were so compact and fluid that they could also stretch cell-thin inside their hosts.  Basically it seems like the Yeerks had evolutionarily been stripped of everything but the most useful adaptations for high intelligence in such a small body, plus direct connection with their neural network to that of their hosts'. 

A long time ago I actually made a diagram of Yeerk anatomy, but again take it with a grain of salt--I imagined them to be autotrophs and also had a lot of free time during the summers while I was a bio undergrad XD

[Kotetsu1442]

OK... I don't normally do much in the way of emoting beyond a basic smiley, but...

So totally awesome Myitt!

[Myitt]

Haha, thanks, I told you I had too much free time! XD  Now if only we could make something similar for the other species...hmm....ou r own sort of Animorphs field guide   Dave/Pez001 has suggested a small anthology of short stories that explain the unknown (both biological and story-wise) in the series...but a visual representation of the biological stuff would also be kinda fun!

[Terenia]

Wow.....um....yeesh .

Okay, bear with me. I'm no bio major, and I haven't even had a biology class since I was a freshman in college, which was some time ago. What I do know is Animorphs, though, and I can find a couple of examples that, to me at least, help support Kotetsu's Theory B.

We see at least two examples of lifeforms that may not be entirely carbon based. The first is the Venber. We do not ever actually see a true Venber, but merely a cross-mutation of Venber and humans. Based on what we know of their physiology and home planet it may stand to reason that they come from a Nitrogen based world. The primary reasoning here is that they come from a planet that is incredibly cold (Ax says it is on average -200 degrees Fahrenheit). I do not know how the Yeerks would have been able to successfully mix carbon and nitrogen based lifeforms together to get one that could live in Earth's polar atmosphere, but I suppose that is a debate for a different topic. Regardless, here is a bit from #25 The Extreme that Ax says in regards to The Venber:

"Just what everyone knows," he said. "I mean, what any Andalite
knows. They were a primitive species with a highly unusual physiology.
Unique, actually. They do not seem to have required radiant energy of
any kind. Obviously they are not carbon-based."

The second example is even more odd. The living asteroids that we see in The Andalite Chronicles seem to be capable of living in the vacuum of space. Now, I'm fairly sure that this is an instance where we fully cross from the plausible to the inplausible in the realm of Science Fiction, but it still interesting to note.

As far as species evolving similarly, we can reasonably assume that many species do based on the evolutionary building blocks that donut already mentioned. However, this is not entirely exclusive to the functions that we recognize within our own human biology and it is important to remember that even Earth has a wide variety of ways to piece together the vital functions and systems that create multi-cellular life.

Within the Aniverse we are able to see that all lifeforms deemed appropriate as hosts meet several requirements:

• They are carbon based
• They have a central nervous system (note: in either HBC of Visser the Yeerks find a species that does not meet this requirement and is deemed uninfestible).
• They must maintain a certain sustainable internal temperature (Ax mentions in #25 that the Yeerks would freeze if they infested the Venber)
• The hosts natural habitat does not seem to be limited (for example, it does not matter to the Yeerks whether the hosts breathe oxygen through the air, or live primarily underwater as is the case with the Leerans)

The Yeerks have their own, extra rules as well. For example, they do not bother with the Skrit Na due to what Edriss refers to as their 'annoying' ability to phase, and the Yeerks are opposed to entering non-intelligent lifeforms as seen when they infest horses in #14; however, this seems to be more of a preferential decision than one based in necessity.

As far as the Yeerk brain is concerned, I had always imagined that the Yeerks themselves do not have a central nervous system, but rather something similar that would look similar to a 'net' of neurons spread throughout the body that is as elastic as the Yeerk body itself. The Yeerk would not have a circulatory system for reasons Myitt already mentioned and really, beyond the net-like neural system, would only need a system designed for absorbing nutrients and Kandrona as well as a reproductive system of some sort. Both of these could be incredibly small. Nutrients could be absorbed directly through the skin/slime and as far as reproduction is concerned they would really only need a means to 'join' with the other two Yeerks. The resulting process I imagine would be one of cellular division with already existing cells, which would result in the parental Yeerks deaths and the creation of so many grubs.


....let me know if I am WAY off on any of this. Like I said, I haven't looked at a bio book in about five years.

[Myitt]

No that sounds completely plausible, Jessi!  And you provided so many more examples than I could've remembered I always imagined Yeerk reproduction to not even need a reproductive system as it were, and just involve fission like you pretty much said (and Ax said )

[donut]

 
It seems oddly fitting that as I write this "through the wormhole" is on with an episode on alien life


@myitt
  To the ubernerds: may we never stop our pursuit of completely useless knowledge for absolutely no reason, or our pointless discussions of irrelevant issues for nothing more than the sake of discussion

I hope that that facepalm isn't meant to imply that you're afraid your post is too long

actually yeah, I was concerned that it was too long, not so much because it's too long, but because I always try to follow the kiss and bluf principles, and a long post is a red flag that I wasn't, although it doesn't necessarily mean I didn't.  It's not so much that I think that if it can't be said shortly that it isn't worth saying, it more like whats worth saying should be said as simply and concisely as possible, I think it helps aide (aid?) understanding.

Now it's my turn 

OK... I don't normally do much in the way of emoting beyond a basic smiley

I always thought smileys were one of the greatest additions to text, because a significant amount of information is communicated through body language and tone, smilies allow us to have a limited expression of emotions and gestures, although some people certaintly do overdo it

donut, am I correct in my understanding that B. is what you mean when you refer to 'lowest energy' forming of life, that this type of life is assumed in this universe to be the 'easiest' way for life to randomly form, that is to say it has with the greatest probability of forming

Pretty much

they also don't have any mention of needing to replace inorganic molecules in the artificial pools to simulate a natural cycle on their home planet

That's where my guess about the sludge being a catalyst came from, but yeah it might just be that replacing it was so much simpler than replacing the kandrona that it wasn't worth mentioning

Thanks T, there's not much point to this if we don't keep what was said in the books in mind

"Just what everyone knows," he said. "I mean, what any Andalite
knows. They were a primitive species with a highly unusual physiology.
Unique, actually. They do not seem to have required radiant energy of
any kind. Obviously they are not carbon-based."

That seems to confirm that almost all life in the aniverse was carbon based, I forgot about those asteroids, yeah they seem like the least plausible lifeform in the series, but without knowing anything else about them it's really hard to say

I"ve wondered about their reproduction, mostly about needing 3 parents.  Does this imply that yeerks are triploid?

@Myitt
Jessi?