It is extremely likely that lives before earth exists and the number might be astronomical. Billions of years of billions of habitable planets must have been quite good for life.
Anyway, considering the fact, we can be a transplant of life of another planet. Maybe a life in another planet was advanced enough to send a life into the universe, in order to preserve their kind of life. But there may have been a dangerous period of possibly billions of years, so they might have determined that they would send out very basic form of life, with DNA containing information about the senders (it can be true because over 90% of our genes don't seem to do much to influence any features of us).
Are we the aliens? Are we waiting for the senders to come and contact us?