Ascidians, very similar to last common ancestor of all vertebrates, have very important sensory organs just where our dopamine systems are. They cannot swim if those organs are destroyed. This is likely, evolutionarily, why substantia nigra (at just about the same place as those organs) controls movement in us.

Synapse

Sato, S., & Yamamoto, H. (2001). Development of Pigment Cells in the Brain of Ascidian Tadpole Larvae: Insights into the Origins of Vertebrate Pigment Cells. Pigment Cell Research, 14(6), 428–436. https://doi.org/10.1034/j.1600-0749.2001.140602.x