This chapter explores the evolutionary emergence and significance of metazoans, emphasizing their pivotal role in the expansion of biological complexity. It examines how the transition from unicellular to multicellular life introduced radically new body organizations that enabled novel forms of motility and more dynamic interactions with the environment. These new organizational structures also made possible the emergence and evolution of nervous systems, which in turn supported coordinated movement, sensory integration, and increased behavioral complexity. The chapter traces the origins of neural networks and analyzes the interplay between body plans and neural development. It highlights how enhanced motility and sensorimotor integration led to the emergence of the most sophisticated forms of agency in the biological world. Ultimately, the discussion underscores the reciprocal relationship between brain development, bodily organization, and ecological adaptation—factors that have collectively shaped the evolutionary trajectory of animals.