[busyant]

Why do you say "apart from the brain"?

Also, as others have noted, your idea is not necessarily wild. Certainly, at the sub-cellular level, there is tremendous evidence that symbiosis played a part in creating "higher level" organisms (i.e., eukaryotes).

Many genomes are like a junk-yard with fossilized relics of infectious agent nucleic acid (e.g., viruses), etc. Apologies for the junk-yard / fossil mixed metaphor.