Humananity is not defined by the ability to think. Even before the first thought the human embryo is not just a collection of cells. It's a distinct organism. It is dependent on the mother but those cells are independent in the sense that it directs itself on how to form. If you could remove an embryo from the womb and give it a good environment and nutrients it would form just the same. The unborn baby isn't slowly assembled out of parts to make a whole, the embryo is already functioning, it's a distinct human life. It is already human. If you define humanity by thought would you say a 3 year old is less human than a 10 year because their thoughts are less complex? What about when you're sleeping, are you less human when you're unconscious? Humanity is also not defined by it's viability. Is a wounded soldier who will die without life support all of a sudden less human or less valuable because his life depends on an outside source?
The embryo is a distinct human being that can direct it's own formation. It is separate from the mother's cells. And that is why it is a human life.
I explained my poor wording somewhere else in this thread.
Why do you feel an unviable collection of cells is a human life? It has no thought, that is what makes us human. At that point its a collection of cells leaching off the mother