I submit that life generates 'pollution', deadly to itself if the environment doesn't dillute and carry it away. While some life creates 'poisons' to beat down the competition or avoid, in general, there's always byproducts or removed toxins to a life form. Case in point, plant life would have poisoned itself with oxygen without oxygen consuming life forms (and fire). The same thing goes for animal life creating co2 but in this case, fire doesn't help. It's a symbiotic relationship between different forms of life that have different requirements for sustenance and have different sorts of waste.

While I don't really believe the life on earth formed on earth. In that 100 or 200 million yrs or so between the time life is known to exist and the great 'splat' that formed the moon and IMHO turned the earth into one big blob of molton mess surrounded by a blanket of co2 plus our atmosphere and probably with quite a bit of h2o vapor that there really isn't that much time for anything to really get formed. After all, things gotta cool down quite a bit for the surface to solidify again and the h2o to become liquid. And, there's a heck of a lotta co2 that needs permanent sequestering by inorganic means. Anyway - panspermia seems to make a bit better sense. Call it the not invented here syndrome.

However, it would seem Drake's eqn was made in a time where the concepts of astronomy were gradual slow events in an almost static ballet of the heavens. Inhospitable conditions in the realm of space beyond earth were qualified as vacuum, heat and cold (oh and maybe that pesky van allan belt up there just beyond the atmosphere). There was virtually no concept of transients - other than super novae which tended to be considered quite local in effect. Modern times has brought about an understanding of significant amounts of transients, including super massive ones, and radiation factors that one would possibly expect to virtually offer the universe a built in sterilization feature. Even the earth has virtually gone through some minor cycles that did severe damage to the existing life and we're likely out in the boondocks compared to the majority of stars and their potential planets.

It's interesting where those to conflicting facets leave us. Perhaps life formed in this solar system from venus before it turned into the easy bake oven we're familar with or perhaps from mars. The fact venus still has all that excess co2 or currently has that excess co2 makes one wonder although I think there are theories that think it was once an earth style planet before the sun's radiant energy output rose beyond what venus could handle. We have found recently meteorites evidently from mars containing what might actually be fossilized life forms - as announced a few years ago by the previous president and NASA. It would seem a panspermia mechanism exists.

Personally though, I'm still awaiting word on the discovery of intellegent life on earth. Even here, it seems to be a rather rare commodity and perhaps a rather transient one.