spontaneously
spontaneous + -ly
https://gyazo.com/41922dd29944b31c3602fae91c569043
adverb
as a result of a sudden impulse and without premeditation:
自発的に; 自然に; 無意識に.
e.g. the crowd spontaneously burst into song.
without apparent external cause or stimulus:
e.g. a surprising number of early pregnancies spontaneously abort
e.g. mutations can occur spontaneously.