About 10 million years ago, an opportunistic semi-aquatic scavenger foraging along the shores of the oceans of a distant planet got sick. It was an infection common to its species, and one that it probably caught from a chance encounter with another of its kind, but it had profound consequences. This microbe had adapted especially well to a life cycle in this type of host. Instead of simply replicating as much as possible until defeated by the host immune system, it used special enzymes to avoid detection and selectively colonized the creature's brain. There it set about the subtle process of altering its chemistry.
This is a fairly common story in the history of parasitism. There are many well-know cases of Earthly parasites that change their host's behavior for their own benefit. Dicrocoelium dendriticum infection in ants causes them to climb to the tip of grass stalks where they are more readily eaten by sheep, the parasite's preferred host. Sacculina establishes a barnacle-like existence in a host crustacean and castrates it, effectively changing the host's self-interest from reproduction to survival, very much to the benefit of the parasite. Even viral infections like Rabies or rhinoviruses can alter behavior to enhance their own propagation.
Best models suggest that the first thing primordial Lexocyte infection did was make the normally solitary Crabs gregarious. Infected individuals would seek out other Crabs, better to infect them. At some point the behavior became counterproductive, however, as the Crabs gathering together would mostly be already infected and their higher density would cause them to deplete the available food and starve. It turns out, we believe, that the second Lexocyte innovation came to the rescue: trophallaxis.
The Crab esophagus runs basically right through its brain, so passing a brain parasite between individuals is easily accomplished by allowing one to feed from the stomach of the other. Trophallaxis, the sharing of food by mouth, allowed the gregarious, infected Crabs to survive. Originally a behavior induced to improve cross-infection, this "communal stomach" had the side-effect that concentrated populations were able to thrive by gathering food in dangerous places and sharing it in the safety of a social hub. What was parasitism evolved towards the virtuous cycle of symbiosis: host Crabs were far more successful than the uninfected, and so were the Lexocytes they harbored.
It's difficult to have a lot of confidence in evolutionary just-so stories like this one, but there is some paleontological and molecular evidence to back it up. Even if the exact details are more complicated, the Crabs and their symbiotes discovered a way of life that promoted their mutual survival. Over the intervening millenia their partnership transformed their home planet and took them into space.
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