[edwin hubble]

"fi = The fraction of life bearing planets on which intelligent life emerges

...is where the chances start to drop off significantly. "


Well, it's looking like life evolved here pretty soon after things cooled below boiling here on Earth.

And stayed at about the same microbial level for two billion years.

Recent conjecture (posted in this forum) speculates that the great Cambrian explosion of life forms could have occured instead a billion or two years earlier. In which case we would be as much as 2 billion years farther in evolution.

Of course, the implication is also there that things would more likely still be at the microbial level today.

Applying this to the 100 million or so sun-like stars...
I agree about variable "fi".

For any one given system with liquid water:
Chance of life = good.
Chance of intelligence = very slim indeed.

Maybe 2 per large spiral galaxy. (We are 1).

[PatrickHenry]

Maybe 2 per large spiral galaxy. (We are 1).

Sagan estimated thousands, and that was before we knew how common planet formation seems to be. But he always was a bit enthusiastic.

[edwin hubble]

"Maybe 2 per large spiral galaxy. (We are 1)."

But I do agree that it would be kind of lonely (and a lot less fun) if that were the case.