Most animals have a dorso-ventral (back-to-belly) body axis which determines the position of the central nervous system - dorsal in humans, ventral in insects.
Though there are obviously morphological differences, the same signaling molecules of bone morphogenetic protein (BMP) molecules establishes the dorso-ventral axis including the central nervous system in both insects and vertebrates, which led to the conclusion that this molecular mechanism was already present in the common ancestor.
Tracing the origin of the dorso-ventral axis has not been easy but sea anemones have provided some answers.