The lowest part of the human brain, in a structural sense, is the brainstem. It connects the brain to the spinal cord and is responsible for many essential functions, including breathing, heart rate, and blood pressure.

However, if you're asking about the most primitive part of the brain, it would be the medulla oblongata, which is the lowest part of the brainstem. This region is responsible for vital, involuntary functions like breathing, heart rate, and blood pressure. It's considered the most primitive part of the brain because it's very similar to the brainstem of other animals.

It's important to note that "lowest" can have multiple interpretations in this context. You might also consider the cerebellum, which is located at the back of the brain, to be the lowest part, but it's not the most primitive.