Most multicellular animals possess a nervous system, a network of many interconnected nerve cells. This network allows an animal to sense and respond to the environment, coordinate the action of muscles, and control the internal function of its body. Nerve cells, or neurons, are the basic functional units of nervous systems.

Nervous systems first evolved in simple animals. These early nervous systems could sense basic features of the environment, such as light, temperature, chemical odors, and physical forces such as touch or pressure. Guided by these cues, early nervous systems could trigger movements useful for obtaining food, finding a mate, or choosing a suitable habitat. As animals evolved more complex bodies with increasingly specialized organ systems for respiration, circulation, digestion, and reproduction, their nervous systems also became more complex. These more complex nervous systems could regulate internal body functions in response to sensory cues received from the environment. They also made possible more sophisticated behaviors that relied on improved decision making, learning, and memory. These abilities, possessed by many invertebrate and vertebrate animals, increased their fitness by improving their ability to survive, find and select a mate, and reproduce.