Although I agree that conditions had to be absolutely perfect for life to exist and evolve in the way human, animal and plant life has on Earth I am afraid that I feel you are being very naïve in thinking that it is impossible for life to exist elsewhere in the Universe.
When we consider the billions of galaxies with billions of stars and trillions of planets orbiting those stars in my opinion it is a mathematical certainty that life exists elsewhere. In fact the Universe may well be teeming with life.
That life will probably range from the amoeba just crawling out of the soup to highly technically advanced and intelligent forms of life far superior to us. When we consider that Earth is a relatively young planet compared with others in the Universe the probability of life elsewhere must be immense.
I am not saying that any life which does exist are genetically the same as ours because their 'building blocks' may have been far different from ours.
It appears to me that more and more scientists are embracing the idea that life may well exist on Europa and Enceladus albeit it extremely primeval life.
Also as far as Earth being visited by aliens is concerned I am afraid I take this suggestion with a pinch of salt. Although it is entirely possible it would take a super intelligent species to find us let alone having the technology to travel possibly millions of light years to reach us.
Even any life that may have evolved in our Galaxy, the Milk Way the chances of finding Earth would be like trying to find a proverbial needle in millions of haystacks. Although we have found many Exoplanets no one yet knows if any of those are habitable by humans.
Poseidon
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Adam D say: While that is mostly true (other conditions that allow life to evolve are also possible), you have to look at the odds. Because we exist, the probability of the existence of life is nonzero. The universe is almost unimaginably large - we can talk about numbers of stars and how many of those host planets, but the numbers are so large that they are pretty meaningless when you try to understand just how big they are. Given that many opportunities, even an extremely rare event should occur multiple times.
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