The earliest fossils we have of primordial life forms on Earth date back nearly 4.3 billion years, almost immediately after the Earth formed. As far as the chances go of finding life on other planets, that's pretty encouraging. It may even have happened more than once in our solar system.

But we're not just looking for life. We're looking for civilizations, and here's the flip-side: If there was ever life on Mars or Venus, it's not there now.

We live on a perfect planet in a perfect orbit around a perfect sun. If we were slightly closer to the sun than we are, we probably wouldn't be here. If we were any further away than we are, we probably wouldn't be here. If we didn't have plate tectonics, there would be no water cycle and we would have a runaway greenhouse effect that would have long since turned the Earth into Venus. If the Earth was much smaller than it is, then our mantle would be narrower and would already have cooled and solidified, which would mean no magnetic field and solar radiation would have blasted away most of the atmosphere and turned the Earth into a frozen desert, like Mars.

If our sun were a red dwarf, like 80-85% of the stars that exist, then it would be so much smaller that we and every other planet in orbit would be tidally locked, meaning one hemisphere would always be facing the sun, being perpetually baked and fried, and the other hemisphere would be a permanent night side, eternally dark and frozen. Most red dwarves are also flare stars, prone to wild and unpredictable variations in light and heat output. If our sun suddenly got twice as bright and twice as hot and stayed that way for several hours, we'd be fucked. If it did that every few days or weeks at random and unpredictable intervals, there would be no stable climate on any planet in the solar system. Almost no red dwarf produces enough consistent UV radiation to kick start organic chemistry at all, and these are the vast majority of stars that exist. If we were around a much brighter and more massive star than our sun, then it would have burned through all of its hydrogen and gone nova eons ago, and taken us out with it.

If we were in a binary or a trinary star system, like most star systems are, then the orbits of all of the planets would be erratic and wobbly, also creating hugely unstable climates, likely incapable of sustaining liquid water on the surface. We also live in a relative void in the interstellar medium. It's extremely rare to have a star whose nearest stellar neighbour is more than four light years away. It we were in a denser stellar neightborhood, closer to the center of the galaxy than we are, we probably wouldn't be here. If we were further out than we are, then there's another whole set of factors counting against us and we probably wouldn't be here. If we were too close to a black hole, or a neutron star or a starbirth region with supergiants going nova every couple million years, or a gamma ray buster, then we wouldn't be here.

If we didn't have Jupiter in the outer solar system, with its huge gravity eating up all the rocks careening around all over the place that want to come crashing into us, then we wouldn't be here. Even as it is, one of them occasionally gets through. The likelihood of a planet the size of Jupiter forming around a star the size of our sun is actually quite low.

We have all of these factors working in our favour, and yet even here, on this perfect planet in its perfect orbit around a perfect star, in a relatively quiet corner of a relatively stable galaxy; even here, it has taken 4.3 billion years, a third of the age of the universe, for those first organisms to reach stage that we're at now, and no evidence exists that it happened earlier.

Of course it's possible for it to have happened somewhere else, but if it has then it's almost certainly so rare and so distant that we will never meet them. For all practical purposes of the word, we are alone, and we almost certainly always will be.