Can a sperm or egg differentiate into a human being?
Absolutely, given the right conditions. A human skin cell could be made to grow in to an entire human given the right conditions. A fertilized egg cell will only grow in to an entire human given the right conditions. Your cut off of fertilization is complete arbitrary and not based on any scientific data. The only difference is the number of chromosomes.
That's not true, a sperm or egg cell can't on it's own differentiate into an entire human, only a zygote can do that. And so human life begins at conception, so there isn't any arbitrary cut off. That is what the science says - human life begins when a sperm cell attaches to an egg. This isn't about data, the biology is clear on when human life begins.
If you don't think abortion is okay at 8.5 months, then viability is arbitrary too because consciousness is fairly subjective. The brain at viability in the womb is no where near advance enough to experience any level of lucidity or awareness.
Please point out where I have ever argued for abortions at 8.5 months. We are talking only about abortions of non-viable fetuses. Creating strawmen like this only weakens your argument, as does using clearly hyperbolic terms like pro-abortion and celebrating abortion. Thinking that women should have a little bit of bodily autonomy is not the same as celebrating abortion.
I didn't say you were arguing this - only pointing out that human life does not magically become something else only because the brain isn't fully developed yet. Consciousness isn't what defines a human.
Not before they are viable they do not.
And again, viability isn't anything objective, because you cannot define human consciousness at any specific point during development to any degree of accuracy. Even post birth, the brain is so under developed that "consciousness" is hardly achieved, if achieved at all. Science isn't even sure how consciousness works, so to pinpoint it through development in the womb is just pseudo science.