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.
Neither can do it on their own. A sperm or egg can't do it without the other. A zygote can't do it without a uterus, a placenta, an amniotic sac, an umbilical cord, a constant source of oxygen and nutrients, efficient removal of carbon dioxide, urea, electrolytes, and other waste products, warmth, protection, etc. The only difference between an unfertilized egg cell and a fertilized egg cell is the number of chromosomes. If that is what you are going to base "personhood" on, then exfoliating your skin becomes mass genocide.
That is what the science says - human life begins when a sperm cell attaches to an egg.
But it is not a human being. A fertilized egg cell is no more a person than an acorn is a tree.
If you don't think abortion is okay at 8.5 months, then viability is arbitrary too because consciousness is fairly subjective.
Not really. The structures in the brain required for any degree of consciousness or higher level thinking don't even exist prior to around 26 weeks.
And again, viability isn't anything objective, because you cannot define human consciousness at any specific point during development to any degree of accuracy.
See above. A fetus without a functioning cerebellum cannot be conscious.