[However, if the Sun formed initially as a part of a star cluster, as is commonly assumed, we cannot rule out the possibility of transfer of life from one of the sister systems to us. Likewise, there is a possibility that some extra-solar planets carry life that originated in our solar system.]

Granted about the transfer system, but basically during that transfer there were the usual noble and basic elements that were being transfered which led the way towards our 'protoplanetary' system and maybe even many others.

What I'm trying to say is that those noble gases in star birth are everywhere (darn that I said that probably) and that we were lucky enough to have such rocky rocks to have harboured or even have been in such a solar system's atmosphere to have melted such organics.

That makes a world of difference.