The world around us is truly amazing. Ever since a yo ung child I always contemplated to myself how us and other living organisms came to be. What shocks me to this day is
how there are so many animals that share common features with us. Two eyes, limbs, a nervous system, a brain, etc. Just recently I came across a science article about turtles that have bioluminescence? I was in awe because I only thought fireflies and other deep sea animals had this feature. But my question is whats our root? Not just our roots but other animals as well. Nature is so perfectly balanced but yet complex to our understanding. Why cant dolphins create their own underwater cities? Like we do on land? Humans are the most intelligent species, we are just a species to nature and is there a species superior to us? We may think we dominate this world but I truly believe there is a far more advanced breed of life upon us, watching us, reading us and we dont have the slighest clue. Scientists devote their lives trying to.understand our world without a hint that another species may potentially have the knowledge we fervently crave. Be it this advanced species be extraterrestrial or share a mutual home.

your questions mostly relate to the common ancestors from an evolutionary perspective. take a read of:
http://evolution.berkeley.edu/evolibrary/article/evo_02
https://en.wikipedia.org/wiki/Common_descent

could there be other forms of life we are unaware of - yes anything is possible but there is no good evidence for that at present.