This study explores the diachronic evolution of doctoral dissertation writing in terms of interactive and interactional metadiscourse at three time intervals of 1966, 1986, and 2016 through examining the salient textual features and the pattern of change involved in the metadiscourse in question. One hundred and eighty authentic doctoral dissertations were retrieved from humanities and social sciences (HSS) and sciences and engineering (SE) which generated a 5.16 million words corpus. Findings show that metadiscourse has considerably increased for SE and drastically decreased for HSS. Therefore, it is claimed that academic writing tends to be moving toward more objective and audience responsible texts in HSS and less objective and more author responsible texts in SE. This suggests that the former is inclined to get less persuasive and reader-oriented while the latter appears to be more persuasive and reader-friendly. Based on the findings, a theoretical model of academic writing evolution is proposed which focuses on three writing aspects, namely, informality and subjectivity, reader-orientedness and persuasiveness in the hope of offering implications for better understanding and constructing academic writing across disciplines.