Prosimians are the most primitive extant primates; they have characteristics similar to forms that were ancestral to monkeys, apes and humans. Prosimians are the only primates native to Madagascar, and are also found in Africa and Southeast Asia. They include lemurs, the Aye-aye, bushbabies, and tarsiers. With the exception of the tarsiers, all of the prosimians are in the suborder Strepsirrhini. Due to this reason the classification is not phylogenetically 'valid', as they do not share a unique last common ancestor, and anatomical traits.
The adapids are an extinct grouping that were most certainly prosimians and closely related to the strepsirhines. The omomyids are another extinct group of prosimians but they are believed to be haplorrhines, closely related to the tarsiers, but an outgroup to the rest of the haplorrhines.
The prosimians were once considered a suborder of the Primate order (suborder Prosimii)(Gr. pro, before, + simia, ape). They have been shown, however, to be paraphyletic - that is, the smallest clade of related species that includes all of the prosimians also includes other species - in this case all of the primates.This relationship is shown by the ranks (prosimians in bold) in the list below of the current Primate classification between the order and family level. The classification is used on a more behavioural term nowadays, due to the lack of a unique last common ancestor.