What difference does it make when the human *spirit* enters a body? If it's alive and it's human, then killing it is murder, regardless of when someone thinks a spirit or sould takes up residence.
--If it's alive and it's human,--
One cell is NOT a human.
The problem with the way this debate proceeds currently is the tendency to argue past one another ... one poster claims a single cell is definitely not a human being, another argues that if the cell is alive then to kill it means it was a human life, and yet another poster claims a continuum of life since the onset of the human species ties all levels of life together as if all levels and nuances are equal.
All the arguments applied could be sorted out with two basic facts established:
1) if one believes there is a human spirit, when does that human spirit take up residence with the human body?
2) when is there an organism present not just parts of an organism?
If we start with the common belief that a human spirit is what makes humans unique to the range of life on planet Earth, the state of not currently knowing definitvely when the spirit takes up residence with the human body argues that we ought not be slaughtering alive unborn humans since we don't know for sure that we are not murdering fellow humans. As to the second question, well science has already answered that and 'organism' is present even as a single cell at zygote age, the meandering naybob minds of some obfuscation-minded freepers notwithstanding. [And before some pompous ass tries to ditch this reasoning with the twinning argument: just because two or more organisms may be present tomorrow, doesn't mean there isn't at least one there today.]