the typical steps to explain the evolutionary origin of sth

[ludwigye]

What are the typical steps to explain the evolutionary origin of some organism (e.g. eukaryote)? Is it right/logic that

1) I first point the time of the oldest eukaryotes known, then
2) compare them with the prokaryotes, which just evolved before them (maybe it sounds stupid, but how can I know there were no mediate organisms?), making out the common and different characteristics.
3)According to their relationship9mainly on difference),ask the question'how were this unique characteristics acquired?',then
4) give the explanation, then the evidence.

I've no idea whether I have expressed my meaning clearly, sorry if you find it confusing.

[mith]

endosymbiont theory.

Intermediate species? I don't see how, there's either a nuclei or not....what are the other choices? Symbiotic?