Jake Parker, an artist, has theorized about the cars in Cars. They are part animal and part machine, in sci fi terms they would be cyborgs, or daleks. Jake has suggested a possible version, however the nervous system is a little out sized. There was a NFB cartoon from 1966, What On Earth, that purported to be a report from Earth by Martians explorers. This film also saw cars as alive and we human as the parasites. For even more cartoon anatomy studies check here.