-u godan verbs historically ended in ふ /fu/ (and earlier /pu/), and were written that way until the 20th century kana reform. So the historical conjugation pattern of a verb like tamau was tamafa-/tamafi-/tamafu-/tamafe- + additional inflectional endings. The /f/ became a /w/ before /a/ but weakened and was lost entirely in other positions, leading to the modern pattern of tamau, tamaimasu, etc. but tamawanai.