"To suggest that the variety of life forms that are possible is some kind of evidence that we can't have been visited by life forms that are similar to us is more an act of faith than logical reasoning."

It isn't actually, because the scientific basis for this reasoning has been found right here, on earth, and gives an insight to the incredible improbability of our own existence as a species able to comprehend tour own origin.

The Burgess Shale was discovered in the mountains of British Columbia in 1909 by a chap called Charles Walcott. This (and others like it discovered since) assemblage of animals 520 years old had with it a number of phyla of animals which no longer exist. In fact, most of the phyla present in the shales represent animals whose evolutionary paths stopped some time after the shales were laid down. Of all the weird and wonderful animals of the Burgess Shale only spiders, insects, crabs, worms and one small swimming animal with a concentrated bundle of nerve fibres running the length of it's body - a chordate, the earliest one ever found and possibly our ancestor.

Why this one fragile animal should survive the extinction event that befell the Burgess Shale fauna is unknown, but it could hinge on the tiniest mutation that animal had - remember natural selection works not because a species is better or more advanced than any other or is particularly well adapted to it's current environment, but because it has some physical advantage for sudden change in it's living environment that enables it to survive. Generalists are in it for the long term, specialists are mere evolutionary blips (bad new for hummingbirds in the long run then . . .).

The significance of all this is that to reach the stage we have, carbon-based bipedal breathers of oxygen is dependent on a massive number of variables - way too many to comprehend. Rewind the tape of life on earth back to the Burgess Shale, press play again and the outcome could be entirely different - there would be no way of predicting which animal would eventually give rise to Homo sapiens. Rewind the tape a million times, you get a million different outcomes.

So the chances of another animal turning up with a physical configuration that is essentially a variant of our own is almost nothing. The universe is of course a very big place, so it could happen but in reality the chances of aliens looking like cute doe-eyed elves with no external genitalia is so small as to be infinitesimal. As is our own existence as a species.

I highly recommend Stephen J Gould's excellent Wonderful Life for anyone interested in this subject - it really is an incredible book.