The only realistic view is that a human life arises gradually, which is not much help in making personal decisions or devising public policy.
Darwin based his theory on generalizations that were strictly empirical. You can go out and see that organisms do vary, that variations are inherited, and that every organism is capable of increasing its numbers in sufficiently favorable circumstances.
The moment-of-conception fallacy implies that fertilization is a simple process with never a doubt as to whether it has or has not happened.
Ever since then, all descendant vertebrates have had the forward end of the digestive system and the forward end of the respiratory system very much involved with each other. This manifests itself in the human body with a crossing of the two systems in the throat.
