I prefer to think of a family tree.
If you check a few part numbers, the 'Grandfather' is the early FZR600, the 'father' is the 2nd Gen FZR600R engine and the FZS600 and Thundercat engines are it's two 'sons', sharing a lot of mechanical DNA but also having quite a few differences. The Fazer engine has the most recessive genes as the other three are from sporty designs with beam frames, downdraft carbs and air box hoses going to the front fairing.
JonesT27: No, the R6 engine was a completely new engine compared to the above 'family tree', although it has itself gone on to spawn the FZ6 engine