I want to know what evolutionary pressures would lead to merfolk. Here are characteristics required:
must be around human height, strength, and intelligence
must swim at least 4 mph
it having gills is preferred, however lungs are fine too
must be capable of complex tool use
would be better if they had an improved sense of sight
must be able to detect water vibrations
Note: By merfolk I don's specifically mean the half fish variety just aquatic humanoids with the listed characteristics. Also the world is in a medieval level of technology.