The line was, you can buy yourself a N on the block. the N is not human, but a smart animal that you can train to do your bidding. The N becomes your property and you can use the N for whatever tasks you see fit.
Notice how the past reduced black americans to a term that literally meant a bundle of wood (antiquated definition). The black american went from being human to less. This justified the chattel slavery of the 1700s and 1800's.
With the fetus, the designation of "a clump of cells" dehumanizes it. The fetus goes from being human to something less. Now, you can do whatever you want to this "clump of cells" including having it dismembered.
Yes, people are clumps of cells. More complex clumps of cells as we mature. Developmentally, we hit peak in our late 20's/early 30's. So, you could say that anyone under 30 isn't fully human.
The point at which a fetus is considered human is easy for pro-lifers. It's at the moment of conception.
The pro-abortion advocates have no good point to draw a line. By the time the woman realizes she's pregnant, the fetus already has hands, feet, and other very human features. By this point, the baby can feel pain. The pro-abortion advocates don't draw a distinct line because there is no solid place to make a stand. The pro-abortion line is a slippery slope. What makes 12wks different from 13 wks in development? Also, the best was can do is estimate the development stage of the baby. Some people develop faster than others.
So, with all these variables, a strong case can be made from science that this is not only a distinct human life from the mother, but also that after the moment of conception, any points for abortion are arbitrary.