Nick, thanks.

Yes, I was aware of the examples you've cited, which I alluded to in my closing paragraph (albeit vaguely). But there it is. The parallel I was drawing between the technological intelligence adaptation and the flight adaptation may not have been drawn as well as it could have been. But I think you and I are agreeing on this point, that there is more than one way to get there.

I don't think the road taken to technological intelligence on our planet is the only road that exists in the universe. There are no doubt 'roads not taken' on this world that have been taken on other worlds.

Our understanding of evolution is fraught with so much political and emotional baggage, and there's so much we don't know. For instance, do all Earth-like worlds (near twins of Earth) produce roughly similar life following generally similar evolutionary pathways? Or not? Will every 'Earth' produce creatures like ourselves? Or will a very high percentage of them not produce beings like us at all? If not, does that mean there will be no technological civilizations on such worlds if there are no beings like us? Likewise, are there worlds notably different from Earth that can also produce intelligent tool-users that build civilizations?

Noboby knows. I sure don't. Neither does anyone else.

What I've found intriguing about the metallicity findings are the implications for the development of complex life. It appears there are some general constraints that are in place for the entire galaxy. Civilization-builders like us seem unlikely to have come along much earlier than we have. We may even be the most advanced civilization in the galaxy. If not, then there's a good chance we're near the front of the pack.

This whole area of speculation is fascinating. It's probably all the more fascinating because it is so speculative. We're free to invent theories and hypotheses all day long because there's no real way of knowing which are going to be right and which are going to be wrong.

Now what would you say - in your opinion - is the earliest date that civilization-builders could have arisen on our planet?