It appears that you are tacitly presuming that life typically co-evolves with a stellar system and that implicitly, life of earth complexity took a maximum of ~4.5b years to evolve(?). If this is case I'm curious what your thoughts on the recent paper by Sharov and Gordon might be. The paper applies statistical arguments to genomic complexity of earth organisms and argues that life as we know it on earth may in fact have taken as much as 9+/-2.5bn years to evolve (predating earth and the local star, potentially up to approximately the entire history of our galaxy and a significant percentage of the history of the stellar epoch)?
If life of complexity similar to that currently on Earth does take as much as 11.5 billion years to evolve, that does not leave a very large window for forerunner biologies and would tend to increase the probability that Earth life is in fact some of the most complex life to ever even have the opportunity to arise in our current light cone.
Is the position of their paper (basically: that life takes longer to evolve than we think) something you are already planning to address further in your article series? I'd be interested to hear what your interpretation might be.
This is the first in a series of posts I am putting together on a personal blog I just started two days ago as a collection of my musings on astrobiology ("The Great A'Tuin" - sorry, I couldn't help it), and will be reposting here. Much has been written here about the Fermi paradox and the 'great filter'. It seems to me that going back to a somewhat more basic level of astronomy and astrobiology is extremely informative to these questions, and so this is what I will be doing. The bloggery is intended for a slightly more general audience than this site (hence much of the content of the introduction) but I think it will be of interest. Many of the points I will be making are ones I have touched on in previous comments here, but hope to explore in more detail.
This post references my first two posts - an introduction, and a discussion of our apparent position in space and time in the universe. The blog posts may be found at:
http://thegreatatuin.blogspot.com/2015/07/whats-all-this-about.html
http://thegreatatuin.blogspot.com/2015/07/space-and-time.htm