I'm going to go off in a bit of a different direction here than most other commenters talking about the 'hard' science. I can't tell you how much life on earth would tell us about life elsewhere in the universe.

But to me there's actually a problem with that very line of thinking. This ultra-reductionistic view of abiogenesis and consequently E.T.

There are some pretty weak individual cases for E.T but put together the case for E.T becomes rather strong imo.

1. Panspermia. Life may have entered earth from comets. It has been shown that bacteria can survive just below the surface of comets, even bacteria that we're exposed to radiation after being put on the surface of satellites for months had some survival rates. The oldest hibernating bacteria ever discovered was revived after 150 million years.

Earth may just be a drop-off point for life in the universe rather than that fabled place where the big co-incidence and the 'soup of life' originated.

1. There are some UFO cases that aren't urban myths about cattle having their organs removed. There's the Wow! signal picked up by SETI. There are several reports from trained military pilots describing encounters with extremely fast and maneuverable aircraft.

   Of course this is where we edge into the territory of people throwing the terms 'conspiracy theorist' and 'sheep' at each other. I will concede that this isn't hard evidence but it certainly doesn't make the existence of alien life 'less' plausible.

It's kind of silly to see people spam links to the 'Fermi paradox' not because it's totally invalid but because it's premise is at least a little bit questionable.

1. How aware are fish of our (mankind's) existence. Humans have only stepped outside earth for the last half-century.

Fish can't conceive of mountains. Now just because it's dark in the woods, you're scared and you don't have a flashlight doesn't mean there's a predator stalking you.

Sorry if i'm not being clear enough but i'm pointing towards epistemology(the philosophy of how knowledge is gained). My view points to a bit more holistic approach than to go for complete agnosticism and say that we only know about life on earth thus we know nothing of the odds of there being life elsewhere. Like the forest i was just talking about.

Even if you don't know anything about that neck of the woods you do know that dangerous predators live in some forests. We know Earth is a planet in the universe. Far from being the only planet orbiting a sun in a galaxy that's far from being the only one.

So 'leaning' towards believing in E.T rather than being extremely doubtful or neutral towards the question seems more rational to me.