"I'm going to deviate from all the Aeslin obsession above and admit to a cuckoo obsession (much less healthy). When I see something like this—a species that has almost all gone mad around their basic biological set-up—my assumption is that at one point they were more sane and adaptive, and something changed in their environment so that once-adaptive characteristics led to problems. And yes, I realize that they are still adaptive in the purely biological sense in spite of being sociopaths. But it still seems likely that they haven't always been that way. So, question—what's the original environment in which their parasitic telepathy evolved? What changed? Were they ever less completely destructive to their hosts? Symbiotic? Have they ended up, as parasites often do, in biological/telepathic arms races with other cryptid species?"
YAY I GET TO TALK ABOUT JOHRLAR YOU'RE ALL GOING TO BE SORRY!!!!
So the cuckoos are more properly called "Johrlac" (species name Johrlac psychidolos), and they are not from around here. I would call this a spoiler, since it hasn't come out in the series proper yet, but it's something that I talk about on panels, and it's something that anyone who's performed any sort of physiological examination of a cuckoo has probably guessed. Still, your personal perspective on spoilers may make this sensitive information, so I'm going to cut-tag.
The big thing to remember about cuckoos is that they are utterly alien. They're mammals, because they fit the basic standards for the current definition of "mammal": they have hair, they have three bones in their inner ears, and they're capable of lactation, although very few cuckoo mothers raise their own children. They do not have hearts. They do not have red blood cells, or actually, blood at all; instead, they have something very close to the hemolymph found in the circulatory systems of insects. Their evolutionary path began not with monkeys, but with parasitic wasps. Basically, the biological term for cuckoos is "fucked up." They make perfect sense, but not for this planet. Not for this evolutionary model.
They are also telepaths, and that's where the trouble really starts. See, for cuckoos, thoughts—and modes of thought—are contagious in a way that they aren't for humans. They're actually born with a certain level of language and historical perspective, because their mothers are literally telepathically conditioning them for the duration of the pregnancy (cuckoos have a gestation period of approximately forty-four weeks). Imagine being bombarded by your mother's thoughts for the entire time that you were in her womb. Does she smoke? You'll be craving cigarettes before you know what they are. Does she hate onions? So will you. Likes, dislikes, irrational hatreds, they're all contagious to infant cuckoos, and they have no defenses unless they, like Angela, are non-receptive telepaths (she can project, but she can't receive under normal circumstances). Which brings us to the next point:
All cuckoos currently residing in this dimension are, with the exception of the non-receptive telepaths like Angela, or those who have undergone psychic conditioning like Sarah, dangerously amoral sociopaths. They do not have a sense of empathy. They do not view humans, other species of sapient cryptid, or other cuckoos as "people," and they have no qualms about breaking their toys. But how did they get this way? They can't work together under normal circumstances. They abandon their babies as soon as they're born. They have no interest in building, just destruction. How can they function as a species?
Short answer: they can't.
Long answer: the cuckoos originally evolved in a dimension called Johrlar, where they were one of a multitude of large, insect-like species that we're going to call "wasps," for the ease of dealing with an alien biology. The proto-cuckoos looked a lot like Earth paper wasps, and they practiced herd parasitism on a species of small, non-intelligent mammals. They had already started to develop the seeds of telepathy, and used it to communicate within the hive, although it was difficult, and was mostly reserved for emergencies. If they had lived in a relatively friendly environment, like say, Earth, this is probably where they would have stopped. They had a stable form, a good niche in the ecosystem, and a steady food supply. They were happy.
Unfortunately, evolution doesn't select for happiness, and the ecology of Johrlar is sort of Murderland. Virtually everything parasitizes something; there are no pure carnivores, and the very few "true" mammals or reptiles which managed to evolve either entered into an immediate symbiosis with something else (like the small mammals and the proto-cuckoos) or became quickly extinct. The proto-cuckoos found themselves in the awkward position of becoming the preferred food source of another species of parasitic wasp, one which was larger, faster, and more dangerous than they were. Stronger telepaths had a tendency to live longer, and started breeding more. Larger individuals were preferred as hosts for the larvae of the second wasp species, which could have resulted in selection toward smaller individuals, but bigger bugs = bigger heads = bigger brains = stronger telepathy, so the smaller individuals began sacrificing themselves to help the larger ones escape. Eventually, they got so big they couldn't fly. Their wings atrophied. They got so big they couldn't safely and reliably gestate their young in their preferred prey species. The longer they could keep their eggs inside themselves without actually ovipositing, the safer the process became. Somehow, the jump was made to bearing live young. Evolution marched on, and on, and on, and down some really weird roads, until you had two species of what had previously been parasitic wasps:
1. The Johrlac. Humanoid, pseudo-mammalian bipeds with strong telepathic powers.
2. The Apraxis. Insectoid, hive-mind parasitic wasps whose "telepathy" is limited to mining the memories of the creatures in which they gestate their young.
Obviously, this evolutionary process was a lot more involved than this, but I'm skipping over lots of things to get to having Johrlac, because they're where the fun starts. Now, this is a very tight-knit culture, the culture of the Johlac. They're all telepathic, which not only means a certain element of Stepford fun-times, since any purely telepathic culture that doesn't encourage individuality is going to trend toward hive-mind, but leads to a certain amount of "what you will be is pre-determined." If the hive needs more workers and your mother is a seamstress, she'll be encouraged to read worker literature and think only about worker activities during the later months of her pregnancy. If the hive needs scientists and your mother is deemed not intelligent enough, she's likely to find herself telepathically muffled while smart people sit and think science thoughts at her stomach. And this is just how things are.
It's very stressful, in Johrlar. It's very dangerous to have independent thoughts, or to dream non-standard dreams. In the early eras of their civilization, they killed the mentally ill, the mentally handicapped, and the inconveniently independent, because they were too disruptive (and while it's very easy to condemn them for that, they were in a precarious position at the time; I don't condone what their culture did, but I understand them well enough to see why they did it). As they became more "civilized," they decided to be "kinder" to the people who were disruptive to the hive-mind, and exiled them to a nearby dimension that had no indigenous lifeforms they felt were worth worrying about. They sort of forgot that this was also the dimension they'd exiled the Apraxis to. Oopsie.
So now you have a colony consisting of people the Johrlac found disruptive to the hive-mind. And, out of fear and habit and loneliness, they form a new hive-mind. One where the kind, reasonable, gentle people are out-numbered by the ones who were exiled because they liked to break things just to watch the smashing. Ideas are contagious; they infected each other with the traits that are now the hallmarks of the cuckoo race.
Eventually, through a mechanism that I won't explain right now because it's going to be important later, the cuckoos—and they were cuckoos now; the brood parasitism had already started between individuals within the hive—found a way out of that dimension and into ours, and started their ongoing murder-party.
At this point, "cuckoos are sociopaths" is a self-fulfilling prophecy. Non-receptive telepaths avoid being conditioned in the womb, and so do the children of female non-projecting telepaths, assuming they're able to stand the company of another cuckoo long enough to have children. Their social issues and lack of empathy have nothing to do with human mental illness or brain chemistry; most of them would probably have been perfectly nice people if not for circumstances beyond their control. The same is true of their parents, and their grandparents, right back to the point where the Johrlac said "Wow, this hunter who got attacked by the giant millipedes that live by the shore is really depressed and unstable and needs help, LET'S PITCH HIM THROUGH THE DIMENSIONAL WALL AND FORGET ABOUT HIM."
...wow, this got rambly. TL;DR. So:
1. The original environment in which the cuckoos evolved sort of sucked. MURDERWORLD.
2. What changed: they left (not of their own free will).
3. They used to be reasonable people, and can be again, if deprogrammed early.
4. They were never symbiotic, but used to be pretty good caretakers for their little mammals.
5. Ooooooh, yeah, Arms race, thy name is Johrlar.
I love the cuckoos.
They're going to break everything.