Ahh yes, but what makes us human? You or I. Is it even physical or is it our personality, or our consciousness, or that which animates us, what theologians would call a soul? All these and more constitute what it is to be human. Yet can anyone say with certainty when any or all of these begin?
This is why conception is the crucial determining factor. If a heartbeat can be detected almost immediately and it has its own blood and circulatory system in less than a month, what then of those unmeasurable qualities such as consciousness, personality, reasoning and even awareness? Sandra Lee Smith could certainly shed more light on the rapidity of human embryonic growth.
Beyond that; if you plant a particular type of seed, will it grow to be anything but what is encoded in the seed itself. Therefore a human egg inseminated with human sperm, cannot be anything other than human.
We over complicate the matter when we ask when does it become human or viable or even ask what constitutes a human. This is simply so, because its DNA and RNA will never change unless attacked by by outside influence or by bad genetic reproduction. Even the later is however the result of cell division. Human cells. From conception till decomposition. We are human from the moment of conception. All the necessary ingredients are already present in the egg to be modified and encoded by the sperm. Hair color, eye color and pretty much everything else is predestined, only to await the entrance of the soul animation that accompanies the lifeforce. Just because it may appear to look more like primordial ooze than a recognizable human being doesn’t make it any less.
If you or anyone else can give be a reason why it is that although life begins at conception, humanity does not, I’m all ears.