What makes you human to me is your brain. That's why I feel no compunction about killing a fly, regret about killing a dog, and think it is wrong to commit murder.

So if I suffer an accident and suffer brain death, pull the plug. Appearances notwithstanding, I would no longer be meaningfully human.

Conversely a zygote with no brain is less to me than a fly. A fetus with pain reflexes but no higher thought is less to me than a dog. And even a newborn baby is not fully human. Lots of potential is there, but not yet reality.