As well as being a regular on Firefly, Adam appeared in Stargate SG1 - which brings us neatly back to Richard Dean Anderson!
Six degrees of separation (also referred to as the "Human Web") refers to the idea that, if a person is one step away from each person they know and two steps away from each person who is known by one of the people they know, then everyone is no more than six "steps" away from each person on Earth.
It works, try it for yourself. Here is a list of people who have worked with someone else on a Sci-Fi TV show, but I've tried to make the list as long as possible, just to see how far the links could take us.
In doing so I found out two things:
1) Almost all roads can easily lead to Stargate SG1
2) Firefly roads don't go far