Just as other animals have evolved, so have humans.
We share many of the same physiological characteristics of other animals; as mammals we bear live young and are warm blooded, as vertebrate animals we have skeletons and muscles, and as self-propelled beings our brains have similar structures and functions to many other animals. As our bodies and brains have evolved, the workings of the brain can be assumed to be physical properties that occur due to evolutionary selection. There is no room for a mind or spirit that is separate to the physical workings of the brain. The psychological processes that operate appear as marvellous systems and maybe difficult to quantify, but they are still physical processes.