when did the first membrane form and which eon was it in?

The first biological membrane most likely formed in the Archean.

http://en.wikipedia.org/wiki/Archean

"No one knows. It is currently impossible to calculate."

I disagree. the oldest evidence of life on earth dates to about 3.5 billion years ago, so the first membranes must have formed before that. We aren't talking about unknown life on unknown planets.

No one knows. It is currently impossible to calculate. Bio and Geo science are two different things. Not to menchen the fact that there may be life on other planets. Although people are entitled to their own opinion, the fact is their are countless planets each planet belongs to a star, most stars have more than 1 planet. Now their are countless stars in a galaxy and their are countless galaxy's in a universe. And maybe even more than 1 universe and so on and so forth. So if each universe is infanite than there are infanite planets stare ect. So most likely there is / was life on other planets. But probably no in our galaxy and if there is life on other planets there most likley not smart enough for cross-galaxy travel. It would basically be teleportation.