Buddhists believe Gautama Buddha was the first to achieve enlightenment in this Buddha era and is therefore credited with the establishment of Buddhism. A Buddha era is the stretch of history during which people remember and practice the teachings of the earliest known Buddha. This Buddha era will end when all the knowledge, evidence and teachings of Gautama Buddha have vanished. This belief therefore maintains that many Buddha eras have started and ended throughout the course of human existence.[web 15][web 16] The Gautama Buddha, therefore, is the Buddha of this era, who taught directly or indirectly to all other Buddhas in it (see types of Buddhas).

Answer this question, if possible (if impossible, reply "unanswerable"): Who taught directly or indirectly to all other Buddhas?
Gautama Buddha