[YeGoblynQueenne]

>> Unless the individuals in the species driving the settlement have very long lifetimes (> 100 y) it is dicult to see how a galactic scale culture can arise (i.e. commerce etc. Krugman (2010)).

Why is a mere "more than 100 years" considered a "very long lifetime"? Is there any reason to assume that the majority of intelligent species, capable of technological civilisation, that may inhabit the galaxy, will not have a lifetime measured in thousands, or even hundreds of thousands of years?

The lifetime of the averarge star is a few billion years. We can assume that mos species will not have naturally evolved lifetimes lasting bilions of years, but a) species may evolve lifetimes lasting millions of years, or, b) a technologically advanced species may extend its lifetime indefinitely.

A species whose individuals lived for a few hundred thousand years would have plenty of time to visit the Earth by travelling in sub-relativistic speeds, from a significant portion of the galaxy. A species whose individuals lived for a few million years would have plenty of time to visit the Earth and wait for our own civilisation to die out. A species whose individuals lived for billions of years could pay us an intergalactic visit and still have time for tea.

Extremey long lifetimes are not impossible and they are not even particularly improbably. Here we are wondering whether there are other technological civilisations among the stars. Why should we assume that they are anything like we are?

[ryandvm]

Long _natural_ lifetimes are a fundamentally at odds with evolutionary progress. The lifespan of a species is directly related to how quickly that species can iterate generationally. Fewer generations mean less natural selection, which means a less evolved life form. Also, if the organisms don't die off fast enough, they end up competing with their own offspring for resources.

That said, I would imagine that once most life forms get to the point that they are able to extend their lifespans with technology, they probably always do. They just don't evolve anymore...

[YeGoblynQueenne]

>> Long _natural_ lifetimes are a fundamentally at odds with evolutionary progress.

I'm not sure I see that. For example, humans live many orders of magnitude longer than bacteria and yet we have both evolved just fine.

Anyway, I don't see why a species is pressed to evolve quickly in absolute terms. If all species on a given world evolve lifetimes many orders of magnitude longer than on the Earth, then there's no reason for any particular species to hurry.

I think how long or short a species' lifetime is can only be understood in the context of the environment in which it evolves. In any case, we don't know anything about life on other worlds, and my point is that we can't really make such big assumptions based on the single example we're aware of.