Earlier quoted context omitted.
As an atheist it would seem dualism would be out of bounds so given that your brain is you in a very literal sense what other factor do you believe defines humanness? I think therefore I am seems like the strongest possible argument here.
No. Brain alone does not define humanness. You can do a few thought experiments, such as: lab-grown full function brain, would it be treated as a human? lab-grown baby with brain development stopped at day x, would it be treated simply as a pile of meat? adult after accident left with x% brain function, would it be treated simply as a pile of meat? Just use your brain, you will see it is not that easy.
If the baby or the human had a sufficiently damaged or deficient brain it WOULD be a pile of meat. If they weren't treated as such it would be because of an emotional connection as opposed to a logical reason. This is all you've posited in this comment. Emotional situations designed to confuse the issue. I am using my brain, specifically to cut through the fog.