Degrees of Separation

When trying to determine the relationship between any two entities in the GPS, one often runs into undecidable propositions. For example, try to figure out the relationship between the Holocaust and the Japanese monster movie Rodan. They can both be classed as things that happened on Earth in the 20th century, but is that all? How do you know if that's the only way their genealogies interconnect? For a classification system can also be looked at as a sort of genealogy of things, not necessarily in the literal sense of which things gave rise to others, but in a metaphysical sense, i.e. what categories and sub-categories of things exist, and how the different categories descend from each other in a giant tree. Just like in a family tree, you can determine how you're related to someone by tracing your respective lineages back far enough until you find a common ancestor. I read once that it's mathematically impossible for any two people on earth to be more distantly related than 50th cousin. Or it's sort of like how everyone in Hollywood can be located within six degrees of separation from Kevin Bacon. I haven't yet figured out how many degrees of separation are possible between any two entities, but it has to be a finite number, right? At any rate, even if the number is incomprehensibly large, it still boggles the mind to contemplate the fact that two disparate things like the mass murder of millions of people and a crazy Japanese monster movie coexist in the same universe. What is their common ancestor? How far is each of them from Kevin Bacon?