May be Due to a comet which was fall on earth through which the life chain was started on earth, at the very first instance. Say for any type of life on earth like insects, human, animals etc. How they appear on earth ! Do you think that there is a duplicate of earth in this universe and that duplicate earth is very old than our earth!.
Or the first "life" was brought here only due to a comet struck on earth, eighter in the form of small and single cell bacterias !, and later that evolved.
what would be the short cut in finding life on another planet!
Or from the unknown destination life process was started. !