*sneaking back in to comment on this*:

"of course life begins at fertilisation in every species on this planet" (yes...in ovulating species)

That may or may not be the case, depending on certain subjective definitions of biological status. Obviously, something relevant & important occurs. When an egg is fertilized and cell division begins, a zygote is produced and begins mitosis.... and if not interrupted, can eventually become a new living individual.
   However, agreement on these terms and the sequence is not really what the controversy is about. It is not even mostly about the stage at which a fetus (my spell checker's spelling)could survive outside the mother's womb, although that is important for certain decisions.
   What is really being question is when...and whether something called a 'soul' enters the zygote/fetus. IF there are such things as souls which are 'automatically' inserted by some divine process into each zygote, the argument goes, then interfering with the process amounts to 'killing' a human being.
   What is usually not discussed is the specific premises that are implied by this viewpoint. To shorten all this, the very concepts of 'divine being' & 'soul' and what it means to interfere with them is a major item for debate and discussion. We see zygotes and measure them and photograph them - we have no way to document the addition of 'souls.

   Because the very basic concepts are a matter of differing opinions, each individual must come to a decision about how to decide the issue, and it makes no sense for someone who believes one thing to have any right to interfere in the decisions of anyone on the other side!
   There are LONG philosophical arguments about explicating this situation, but a practical view would be that "mind your own business" is a good idea.