How come nobody here did the Apoc aliens? I will start.
As X-COM Apocalypse revealed, there is more than one dimension/universe. In fact, there are likely more - the aliens would originally have multiple pseudo-random dimensions. Let's call it... Dimension X.
The Micronoid dimension is a parallel universe not unlike our own. The main divergence, is that our sun and solar system never formed. Instead, another solar system formed in its place, with a different star and planets.
If we assume both our stars developed at the same time, the Dimension X Not!Sun is a larger, brighter, shorter-lived star. Stars like the sun last... Ten billion years, I think? I don't remember. Our sun is a middle-aged star, their sun was already a bloated, leaking, dying stellar object when X-COM first formed in '99.
This sun eventually wrought a very much Earthlike planet, where life took its own path.
The Micronoids: Bacteria in another dimension which could develop neuron-like structures in their colonies, each individual Micronoid being a "neuron" inside the greater whole. An emergent intelligence.
In our world, there was a time when the surface was dominated by huge bacterial mats. Perhaps Dimension X is a world where the bacteria won out.
Other Aliens:
Larger life developed originally from omnivorous worm-like creatures. Those were the original Multiworms and Hyperworms. Possibly water dwellers originally.
Eventually, Multiworms started fitting their environments, by creating new life to serve them. They are natural collectors of bio-matter, allowing them to create Chrysalis capable of working not unlike a cloning chamber. The Chrysalis then creates a more developed individual.
The original creatures were probably just different types of worms, but eventually they assimilated the genetics of other creatures in their planet, and new forms started evolving.
The Hyperworms were so voracious, they eventually devoured and assimilated everything that could compete.
Eventually, the proto-Multi-Worms ate and assimilated most or all macroscopic life in their planet.
Micronoids discovered they could thrive inside the cerebral structures of other beings. Eventually, the Micronoids acquired psionic power - perhaps through another species. Psionics allowed Micronoids to establish larger links, not just between individuals in colonies, but between separate colonies. This lead to an emerging intelligence.
There may have been multiple micronoid-like bacteria, competing and mixing with each other, but eventually one won out. Maybe the Micronoids are the last one left when everything went super-nova.
The Micronoids eventually realized the Multi-Worms and their life cycle were a thing. So, they infected those creatures, and through them, gained the ability to create new life, shaping their entire environment.
Eventually, the entire ecology was dominated by the Micronoid-Multi Worm symbiosis.
Two sentient creatures developed from this Alien Life Cycle: The Anthropods and the Skelletoids. Beings like Psimorphs might have come from there, too. Alternatively, those beings evolved independently, but were assimilated.
(Spitters are humanoid but not sentient. They don't even have brains)
The "Alien Life Cycle" allowed for the creation of biological technology. Rather than develop tools, create life to be tools. There's no need to create a gun, when you can create a handheld monster which spits acid at high speeds upon pressing a protuberance.
With this, they could create and grown any sort of creature, tool or building. They grew buildings, they grew ships, they grew tools. They created Queenspaws and such to increase the rate of breeding and growing.
Best way to understand those beings, are as a self-evolving alien ecology.
Micronoid Non-Organic Technology:
A good question. Where this came from? Apocalypse states rather clearly that some Micronoid non-organic tech doens't look like their organic technology and comes from another alien intelligence.
One possible theory is that the Micronoids were already dimension-hopping before the events of Apocalypse. After all, the original concept included multiple Alien Dimensions. They may have come into contact with a fourth unknown alien species, and taken technology from them.
Another theory, is that this technology is actually from the Sectoids who got literally blasted into another dimension at the end of Interceptor. This might include the dimension-portal tech. We know from Apocalypse that the Micronoids had enslaved Sectoids in their dimension.
In fact, this may also be the origin of their dimension portal tech, and their knowledge of Planet Earth.
The Real Alien Threat:
The Apocalypse aliens seek to take over humans, because the Alien Life Cycle is not sustainable on Earth. Given enough time, the aliens die in our atmosphere. They cannot simply invade and turn Earth into their world.
Their technology is almost entirely biological, so they can't even terraform the world. Hell, even the Earth circa 2080 is not terraformed, it looks more like an alien hellscape then.
Given time, those strange beings could adapt. But they don't have time, not any longer. Their world is dying. A super nova blasted their world. Their world is baked, and eventually it will turn into a ball of ice when things cool down.
The Micronoids discover something: They can survive Earth's conditions within the bodies of humans, both pure and hybrid.
To be infected by Micronoids is akin to toxoplasmosis: You are still you, but your biological instincts are hijacked to serve a parasite. They will sacrifice your life to save theirs, if needed be.
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