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CONTEXT/EVOLUTIONARY HISTORY

A book that I'm currently writing called Surge features an enemy faction called the Degenerates that are heavily inspired by the Scythians (Indo-Iranian horse nomads that ruled the Eurasian Steppes and Central Asia from the 10th century BCE to the 3rd century CE) and consists mostly of humans parasitized by a prehistoric endoparasite called Echidna (Aengapentastomumn corruptor), which parasitizes and radically alters the entire physiology of a wide variety of organisms from the phylum Chordata.

An excerpt from an in-universe field manual that was given to the protagonist (written in the vein of a scientific dossier or military briefing) implies that Echidna is a pentastomid (an endoparasitic crustacean that feeds on vertebrates) that first appeared during the tail end of the Cambrian period. While most pentastomids evolved to infect and enter a vertebrate’s lungs via an intermediate host, Echidna forewent this reproductive process by tunnelling into an animal’s brain and hijacking their body for the purpose of giving birth to thousands of larval Echidnae.This was a simple, yet ingenious survival strategy since Echidnae possessed no natural defences and an organism controlled by an Echidna would be used as a living suit of armour that allowed the parasite to sustain itself in a hostile environment until it reached sexual maturity.

Echidna originally evolved to parasitize marine arthropods inhabiting Cambrian seas. Upon locating a suitable host, Echidna would use its razor-sharp teeth to cut through the hard exoskeleton without damaging any internal organs and burrow into the host's brain. Upon locating the frontal lobe, Echidna would spread root-like tendrils that fused with the host's central and peripheral nervous systems, bringing the creature under the parasite's control. Once this was done, the host's physiology would undergo a series of changes as the Echidna "edited" the host's genetic structure by adding, removing or substituting parts of the genome with genetic material acquired from previous hosts (e.g the excerpt mentions that the fossil of a parasitized trilobite was discovered possessing raptorial appendages like those of Kodymirus vagans and extremely long, hollow spines that possibly contained venom.) It was also not uncommon for Echidna to absorb genetic components from their hosts using Horizontal Gene Transfer, which would've been incorporated into their offspring's genome.

It wasn't until vertebrates migrated onto land during the late Devonian era that Echidna made the jump from arthropods to vertebrates and truly began to thrive. As countless species came and went, Echidna evolved so rapidly that it was almost impossible for the fossil record to keep up and its genome became a vast catalogue of genetic information compiled from countless extinct species. Over time, Echidna gradually evolved to infect humans that settled on the Australian continent, some 65,000 years ago before the events of Surge.

But for reasons unknown, Echidna suddenly vanished from the fossil record. It wasn't until the early 21st century that Echidna returned from the brink of extinction, when human activity awakened a colony of Echidnae from their millennia-long slumber. The Echidnae wasted no time in undergoing an abrupt re-evolution in order to parasitize not just anatomically modern humans, but also Australian vertebrates that evolved in the wake of the Quaternary extinction event.

ADAPTIONS

Now, to the main event. The stuff I need to know what sort of adaptions would Echidnae need to:

  • To enter through a vertebrate's ear canal and tunnel into their frontal lobe, without killing the host

  • Successfully avoid triggering an immune system response

  • Assume full control their hosts' behaviour and bodily functions

  • Alter the host's physiology and genome using Horizontal Gene Transfer

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    $\begingroup$ As I understand it we're to take it as a given that the "context" bit is true and then your question really just boils down to the last four questions. If that's true you might be better off asking those four questions as separate posts and without so much "context" at the start. $\endgroup$ – Lio Elbammalf Apr 14 '18 at 16:10
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    $\begingroup$ Pentastomids are already successful at not triggering immune responses. Entering through the ear canal does not seem reasonable given their lifecycle. However, they do migrate from the esophagus to the lungs normally, so entry through the spinal column may make better sense. Once in the brain they only need to interface with certain areas to gain control. I will think about this and maybe turn it into an answer later today. $\endgroup$ – pojo-guy Apr 15 '18 at 9:57
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Background

Pentastomids are a good candidate, but the secret to their success in this case is in their lifecycle. The ear canal is a possible entry point, but only as a means to get to the spinal column and cervical joint at the base of the skull. However, it is extremely unlikely that an adult Echidnae would ever attempt such a thing - like other pentastomids, they are obligate parasites, unable to survive long outside of a host.

They are already good at masking their presence from the immune system - that is part of their basic abilities as successful pan-species parasites, like all other pentastomid species.

Brain control

The frontal lobe is not a good place to control a creature from. While problem solving happens in the frontal lobes, behaviors are not governed by rational thought. Contrary to popular belief, emotions are the core of sound decision making, not rationality. Without experiencing emotions, people have great difficulty prioritizing, forming memories, or learning.

In my own case, I have problem solving capacity in the 98th percentile of the population (great frontal lobes). However, I am very slow to learn new information, and under pressure would be organically incapable of prioritizing the house over my head being on fire versus a sudden desire for a peanut butter and jelly sandwich (lack of emotional connection).

With the research into my own brain injury, I know that a relatively small section of the brain, comparable in size to the pentastomid itself, is responsible for carrying information between the region of the brain that "has" emotions and the portion that "experiences" emotions. Disruption of this section of tissue in a creature effectively gives the parasite full control of the creature, regardless of its rationality.

Lifecycle

Pentastomid eggs are consumed by the candidate host, and hatch in their digestive tract. The larval Echidnae migrates through the espohagus, but unlike its more common cousins, it ignores the lungs and moves higher before burrowing into the back of the the throat, seeking out the spinal column, and then migrating into the brain stem through the cervical joint. Following this route, it has no need to penetrate bone.

Once in the brain stem, the parasite latches on to a capillary bed and feeds much like its more common cousins do in the lungs. Once it is fully mature, it begins to grow neural tissue outside of its shell which forms connections with the host's nervous system.

Genetic Changes - Horizontal Gene Transfer

The second part of the question involves genetic manipulation. A certain portion of all animal DNA is actually viral DNA that has become embedded in the genome. The Echidnae has a retrovirus that carries with it the genetic encoding for the physiological traits desired. The virus is transmitted by exchange of fluids - as the parasite consumes blood it excretes into the cerebro-spinal fluid, releasing the virus into the host's body.

Since the virus affects the genome of the host, the changes will breed true. Even if they don't breed true in the first generation, the infant will be infected in-utero and will breed true in the second generation. Furthermore, the infant will be infected with Echidnae eggs, which will hatch and parasitise the young.

A side effect of this potential multi-infection is a higher than normal cancer rate amongst Degenerates.

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  • $\begingroup$ I hate saying this because I generally like your answers but this looks more like a comment to me. Can I assume that you're going to flesh this out some? $\endgroup$ – Tim B II Apr 16 '18 at 4:04
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    $\begingroup$ @TimBII I accidentally posted a comment as a response earlier, then undeleted and edited it when I answered. Thanks! $\endgroup$ – pojo-guy Apr 16 '18 at 4:53
  • $\begingroup$ Always a pleasure Sir. Good answer, BTW. $\endgroup$ – Tim B II Apr 16 '18 at 4:56
  • $\begingroup$ Another way for the parasite to avoid bone is to target babies and infants and get in through the soft cranial sutures before the skull has fused closed. That way the parasite can also integrate itself with the growing and developing brain. And 65,000 years ago, the people who noticed odd behaviour in the host may have been reluctant to kill or abandon their 'sick' toddlers. (After all they didn't have brain scans to reveal the parasite back then). $\endgroup$ – DrBob Apr 16 '18 at 18:27
  • $\begingroup$ I've got a personal stake in knowing the limits of today's brain scans. Pentastomids are too small to be seen directly in in current technology brain scans. $\endgroup$ – pojo-guy Apr 16 '18 at 18:45
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I'm mostly focusing on the genetic/immune system questions - I don't think you can provide a scientifically coherent explanation for the other ones without a lot of handwavium. In my opinion it would make most sense for the parasite to enter host body close to the spine and burrow from there to the stem brain. From there its much easier to take control of the central nervous system (and therefore the body) and then slowly the brain. This is also much more similar to the prehistoric parasite you describe.

Successfully avoid triggering an immune system response

The immune system attacks anything that looks foreign to the body. Therefore, in order to not get attacked, your parasites need to look 'similar' on a molecular level. This is actually a routine procedure for it, since "its genome became a vast catalogue of genetic information compiled from countless extinct species" - it has a couple of mammalian (amongst other) looking surface markers laying around which it can quickly adapt to mask it against the human immune system.

Alter the host's physiology and genome using Horizontal Gene Transfer

This is a bit tricky and also you don't actually mean "Horizontal Gene Transfer" - this only describes the process of one species taking up DNA from another species over the course of evolution. What you seem to want is genetic engineering of the host body (which as a single organism is not subject to evolution).
Since you can't control the DNA of cells remotely your parasite need to spread throughout the while host body (or at least to all areas it ants to modify). Then in order to change the host bodies DNA it needs a mechanism similar to the one that retroviruses use. Actually it probably uses the exact same one - retroviral elements are clustered throughout the genomes of almost all species and during the evoltuion of the parasite it didn't only pick the up, it also managed to activate them in order to use them on his host body. Since it also has an arbitrary library of genes from various species in its genome it can introduce whatever features it wants into the host.
Please not that just changing the host bodies DNA doesn't automatically mean it will grow new appendages or organs. It's much more likely that the skin (or nails) change or that pores in the skin start producing new molecules. Actually starting the growth of new organs/limbs/... will wreak havoc with the body, likely lead to cancer and in the long term kill the host body.

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Logic says that basic characteristics would be: -octopus-like physiology (to be able to slip through the ear cannal and maybe even very thin cuts) -Something like disguising with e.g. antibodies copied from not-yet-host's cells. -Creating carefully analized hormones and neurones with axons touching host axons I don't know answer to rest of questions, and that's just how I would get out of this trouble.

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  • $\begingroup$ Octopus are cephalopods, and mollusks under some taxonomies, not crustaceans. Crustaceans are arthropods. $\endgroup$ – pojo-guy Apr 15 '18 at 9:38
  • $\begingroup$ Even with energy through the ear canal, the parasite still has to drill through bone to get to the brain. $\endgroup$ – pojo-guy Apr 15 '18 at 9:42
  • $\begingroup$ @pojo-guy I think that evolved version of the organ allowing for breaching exoskeleton would work for skull if packed with harder "teeth" and stronger muscles. $\endgroup$ – KarolOfGutovo Apr 15 '18 at 14:57

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