Prospective candidates generally must be a member of their diocese's Fellowship of Vocation for at least one year before their Bishop will consider putting them forward for the Selection Conference, who does so on the recommendation of their Diocesan Director of Ordinands (DDO) and after satisfactory performance at psychological and academic testing at a Pre-Selection Conference, usually held in January.
There are no reasons to believe that humans are exceptional, they have all required enzymes (DDO, DAO).