The needless search for extraterrestrial fossils on Earth

It is argued that possible traces of life that differ fundamentally from life on Earth will not be recognized with the knowledge we have nowadays; traces of life that do not differ fundamentally from those on earth will not be recognized as extraterrestrial, either because such life may have originated on Earth (and have made a space trip afterwards), or because life on Earth may have come from the same source from where the life forms on the meteorite were derived.