What may some similarities between alien life and earthen life be? Also, how would that change humans perceptions on alien life.
The term "alien" specifically means "something foreign". Any extraterrestrial life would evolve on a planet with its own completely unique biospheres and geological history meaning that there would be absolutely nothing in common between anything on Earth compared to whatever is out there. In fact, we ourselves could be considered extraterrestrials to other aliens as we did not originate on their planet. The only things that we could have "common ground" with an aliens we come across would be perhaps certain organs that have similar functions, but have completely different chemical and bio-electrical reactions that power them. Thus an alien coming down in its flying saucer would not be able to digest anything of nutrients on Earth and vise versa with us coming to them.
As for cultural differences, think of it this way. At a US checkpoint in Saudi Arabia, when a soldier holds his hand out to an approaching civilian, In America, that means "Halt!". In Arabia, holding your hand out means "Hello!". The situation goes downhill from there. If we have difficulty understanding our own cultures between different people, how difficult do you think communicating with aliens would be, something intelligent with its own way of seeing and sensing the universe, with its own language having nothing in common with anything here on Earth, possibly not even biological when it makes contact with us?