Why is the brain stem sometimes called the reptilian brain and?

The brain stem is often referred to as the "reptilian brain" because it is the most primitive part of the brain, responsible for basic life functions such as heart rate, breathing, and reflexes. This term highlights its similarities to the brains of reptiles, which possess similar structures that govern survival instincts and automatic behaviors. The concept is rooted in the triune brain theory proposed by Paul MacLean, which categorizes the brain into three evolutionary layers, with the brain stem representing the oldest evolutionary component.