Space is big but time is big too
About 3.8 billion years ago the Earth suffered the Late Heavy Bombardment: a storm of asteroids so fierce that the entire surface was re-transformed into molten lava again much as it was when it was formed. Somehow, immediately when the Earth was cool enough again about 200 million years later, the first life we can find evidence of arose. If there was life on Earth before then all evidence is lost as far as we can tell. If there was life before then it's reasonable to expect that the reason life started up again so soon is because some of it was smacked right off, made several laps around the sun, and landed right back here at a suitable time when it could take root again. Else it subsisted in subsea volcanic vents until the worst was over, but as I said - the evidence is lost.
In the time since then Earth has been smitten innumerable times well enough to take Life from the surface of the Earth and spread it not just to the nearest planets but even the nearest stars. Recent modelling proves this. It does not take 200 million years for life to travel to the nearest star. There is no chance whatever that Life from Earth has not fallen on the planets in the habitable zone of these nearby stars. The only question is if enough of it survived the long journey to start again there. We know that the Earth contains natural nuclear reactors that generate heat for billions of years, that life subsists on these without air miles beneath the surface - that the biosphere that exists more than 1km below the surface is actually far more massive than the entire rest of life on Earth. We also know that many asteroids are rich in heavy metals rare on the surface of the Earth because during the molten phase those heavy elements sank to the Earth's core and what heavy metals we have on the surface now are relics of asteroid impacts generally.
And now I come to the "time is long" portion of this tale: Since the Late Heavy Bombardment began 4.2 billion years ago our solar system has orbited the galactic central black hole Saggitarius A* seventeen times. With an imperfect orbit and many stars and other masses crossing our path in that period considerable mixing has occurred throughout the Milky Way. In fact high-speed interstellar matter (mostly dust but some interstellar dark objects as large as a stadium or larger) speed unnoticed through our solar system every year on hyperbolic paths that mean they'll never orbit our sun - picking up our interplanetary dust and life along the way. Sometimes they hit a planet, and that's a bigger deal than an equivalent mass of asteroid or comet because of its high inertial energy. Sometimes they'll miss all the planets in our system, but land on planets in another - maybe after impacting an asteroid or comet and shattering into a million pieces. If Life was first born on Earth by now the entire Milky Way galaxy is swimming in our effluent. It is a plausible theory that the Late Heavy Bombardment was itself caused by a rogue stellar system that flew so close by that the orbit of some it its comets impacted the sun, and some of our comets impacted it and its planets. We can't know. But it gets better.
You see the Milky Way galaxy is 13.2 billion years old. More than three times as old as Earth. It's possible in that time that a minor galaxy has passed entirely through it. If life is sturdy enough to bear the transit between the stars then it is exceedingly unlikely that it was born here but that it fell here and took root. In fact the likelihood that Earth is the origin of genesis is in that case so remote as to be unworthy of consideration. In that case not only are these worlds out there in the habitable zones likely to be capable of sustaining life of some sort after a hundred million years of biogenic terraforming - they are almost certain to have life already for billions of years in much the same pattern as Earth and to have already enjoyed the benefits of that terraforming - to degrees more or less than Earth has.
Asteroid impacts and wandering bodies are not the only things to eject masses out of a solar system. Nova and supernova events do this too: sterilizing the sun-side of the body of course, but blasting inner bodies into a shotgun shell cone and stripping outer planets of their moons for an interstellar planet-sized journey leaving night side life intact.
Earth has enjoyed a number of miraculous transformations of its ecosystem over the past few billion years due to the evolution of life. Photosynthesis was a key one. It's not possible to know if that has occurred elsewhere without we go look. As for "where are they?" well, our modest intellectual capability was improved by a large number of other unlikely incidents including the death of the dinosaurs, the frequent ice ages, various random evolutionary successes and so on - and now we have radio astronomy for less than 100 years and can see not very far. Only in this past YEAR we have discovered DFT (direct fission thermal) as a means of propulsion for interstellar travel thanks to datamining of Voyager craft launched over 35 years ago. We're just now discovering how to make the miracle material - graphene - that solar sails must be made of to be effective and haven't mastered its perfect production on vast scales yet. We're ephemeral and likely to go the way of the dodo shortly if for no other reason than that the end of the current interglacial will result in resource contention that ends in global thermonuclear war and a great deal of uncertainty about the survival of mankind but certainly the loss of its knowledge and culture. So this condition that we find ourselves in is both unlikely to occur and likely so brief that we're unlikely to observe a nearby world that is in it at the brief moment that it happened. If we knew where the next steps in physics would take us we might know better what to look for - but even then it would likely be too far to see as increased science leads us to increased efficiency with leads to diminished evidence of activity you can see from lightyears away.
I think it would be nice if we would send some robot probes to these nearest stars to check the lay of the land. It will be a few years yet before we have the resources to do so - we have to exploit the asteroids first, particularly Ceres, in order to have the raw materials and science to get even a robotic probe that far in a human lifetime. Manned interstellar travel will take so much more time with current science that I'm unlikely to see the launch of that voyage but I would like to see us start to try. Generation ships would of course be the first to go and likely be picked up partially along the way as knowledge of physics improved to where their propulsion was surpassed.
Biting the hand that feeds IT
