The Adamic Cycle inaugurated 6000 years ago by the Manifestation of God called Adam is only one of the many bygone cycles. Bahá’u’lláh, as you say, is the culmination of the Adamic Cycle. He is also the Inaugurator of the Bahá’í Cycle. Obviously there must have been Prophets and Manifestations in the ages preceding the Adamic Cycle. This is supported by the following statement revealed by Bahá’u’lláh.
(Universal House of Justice, Lights of Guidance, p. 500)