That’s the question pro-life and pro-abortion folks have been arguing for years. And there are so many different answers for that question that I highly doubt it will ever be answered.
But here’s another question that ought to be asked more often: ‘When does the fertilized egg become a human being?’ And here’s the answer: at fertilization/conception.
All organisms on the planet contain DNA (save for a few bacteria). DNA is the blueprint for construction and maintenance of the organism it’s in. And DNA is very specific to the organism it’s found in: scientists can determine the source of DNA found in samples simply because, for instance, chicken DNA is different than dog DNA. This is sensible, since DNA is the blueprint for making the organism: you wouldn’t expect frog DNA to make a parakeet, would you? Of course not; it’s going to make a frog.
In asexually-reproducing organisms, like some bacteria and single-celled protozoans), the DNA is simply replicated, with each half receiving one complete DNA ‘package’. In sexually-reproducing organisms, though, it’s a bit more complicated. Each gamete (sex-cell, or egg and sperm) in a sexually-reproducing organism carries a ‘half-blueprint’, if you will; half of the DNA necessary to construct and maintain a new organism (this happens through meiosis). In order to form a complete DNA molecule, those half-molecules must bind together; this happens when the female egg and the male sperm ‘connect’ at the point of fertilization (in humans, we call this ‘conception’). Once the blueprint is complete, construction begins on a replica of whatever provided the DNA.
I’ll say that again, in a slightly different way: at conception/fertilization, the DNA molecule is complete, and begins construction on a new, replica organism.
And that’s all it will construct: a replica of the parents which provided the egg and sperm. It will not construct any other type of organism: human DNA will not construct a frog or a bat or a prairie chicken or a monitor lizard or a carp or a gibbon or an orangutan – it will only construct another human being.
So when does a human become a human? The moment it has a full and complete strand of DNA blueprint.
At the moment the human egg and the human sperm come together at fertilization.