Your topic 'Extraterrestrial Life' [ETL] is an intriguing one for the simple fact that it is a personal discourse at best since there is not one shred of evidence uncovered or not that an ET has EVER visited this planet-regardless the hype of the 'Ancient Aliens' TV show nor does ETL exist around another rocky planet. [Yes, I am well aware of the various planet hunting ideas and techniques employed today and in the past-I even look for planets on Zooniverse]
Now, life as we know it on this rocky planet is a mishap, accident, calamity. In order for me to say that one has to read the right books, listen to the many speeches, conversations, discussions, etc... on the subject and not to rely on those hideous yet simple-minded TV shows.
There is only one fact we can rely on, and it is the exact fact that this planet established a place or position in space 3.5 billion years ago that was capable, through all the ramifications of early solar system confusion and make-up originality to form this planet within a 'habitable zone' of a star. We have to realize this as a given condition of our existence from ALL that came before.
This means minimally that one, for a lack of a better word, 'intelligent life- form' [human being] incubated on Earth and saw other various life-forms including flora and fauna we see today and that which have also been documented from the past planet's history that we study at Colleges and University's around the world.
Therefore, personally speaking and presuming in and on a very large historical data format, that there would seem to be one, intelligent or not, life-form per galaxy.
However, when employing the famous Drake equation, and there are other additions to this formula as well as other ideas of exclusions and inclusions to this formula and others, years ago I calculated, and this was well before any extra-solar planets were discovered in the early 1990's, that 2 to 3 other civilizations could be in this Milky Way galaxy! Civilizations, yes, because that is what the equation asks to solve.
So far we have made discoveries [not very many] of other planets around other stars and have documented the various solar systems containing gas and rocky planets, with perhaps a few total water worlds thrown in but yet to be confirmed. Rocky planets [perhaps with an atmosphere-of sorts] are the main thing here for thought, since we are on one. Mentally consider all the conditions that have taken place in the past that we know about to produce a life-form of any kind! You will soon see, and I would consider you to be an intelligent person, that our plant has had a very nasty formation and structural development in itself-and this is more than likely a very typical situation. With that thought in mind, place it in another solar system and include a moon with other planets circling that star. Trust that this world is in that star's habitable-zone too and must undergo all the exact same conditions of early planet formation we know about today to make a world like ours.
Do you see any difficulty? Can you imagine 'different' but NOT 'same'? Can you imagine a moon pounding this planet and gathering through gravity its smaller parts to orbit it? Perhaps to some degree not exactly that way, but a moon none-the-less acting gravitation-ally on the planet's infrastructure to distort and replace repeatedly the top soil and whatever is on it with its subsequent atmosphere for billions and millions of years. This kind of action accounts for the planet to form into a smooth hot rocky ball of water and soil. Are there other worlds that have endured this torture of nativity the exact same way as the Earth? I said earlier...one per galaxy!
Then there is a question of life itself and what I am talking about here are the same conditions the Earth tolerated during the subsequent 'quiet' years of a constant swing around the Sun establishing a near perfect orbit and with its rotation and tilt. Its like picking the exact order the balls fall in the Power Ball lottery holding 1.3 billion dollars for the next draw. Do you know those 5 numbers with the bonus?
No one does.
Photosynthesis is the most important by-product of this planet-I consider. Its description to 'come about' in nature is endless. But it happened. Would this process happen on the ETL world too? It would have to, to make sense of other alien world activity. Would there be the same but similar flora and fauna on that ETL world...there would have to be something similar I suspect.
Then would those dinosaurs die out? Or are they still there? Were there any at all? Does a planet require dinosaurs to have future intelligent life and do these dinosaurs have to die out for that to happen. This idea encompasses all the same requirements and conditions of the early Earth and what it tolerated to become a nice blue [polluted] dot in the future! [tic]
Trust you are a thinking man, Paul, and I will stop here for your comments before I continue the discourse with man as an accident on this planet.