Final answer:
Primitive vertebrates have smaller and less complex brains compared to lower mammals and humans. Mammalian brains, especially humans', have a neocortex that enhances mental abilities and is responsible for complex brain functions.
Step-by-step explanation:
The difference in brain structure between primitive vertebrates, lower mammals, and humans can be seen in the size and complexity of the brain. While all vertebrates have a brain, mammals, including humans, have larger and more complex brains compared to primitive vertebrates. Mammalian brains have a unique layer of nerve cells called the neocortex, which is particularly developed in humans. The neocortex plays an important role in complex brain functions and increases mental abilities.