D.Hajiyev was born in 1929 in Ilisu village of Gakh region. In 1952 he graduated from the pediatric faculty of the Azerbaijan State Medical Institute (now Azerbaijan Medical University).
In 1961, D.Hajiyev defended PhD and gained name of doctor of sciences. In 1963 he was elected professor of the Department of Zoology of Vertical Zoology at the Azerbaijan State University (now Baku State University), and in 1964 he became the professor. In 1972 he was elected a corresponding member of the Azerbaijan Academy of Sciences (AAS).
D. Hajiyev started his career in 1952 as a doctor at Morfovka Regional Hospital in Crimea. In 1954, he was invited to the H. Zardabi Museum of Natural History, where he worked as a junior researcher, scientific secretary and senior research fellow. In 1964 he was appointed as the head of the Department of General Biology of the Azerbaijan State Medical Institute, and since that time has been the Vice-Rector of the Institute, Chairman of the Scientific-Medical Council of the Ministry of Health. In 1968-1970 he headed the Paleobiology sector of the Institute of Geology (now the Institute of Geology and Geophysics), and since 1971 - Laboratory of Evolutionary Paleontology. In 1996, he served as Chairman of the Special Biology Council at the Higher Attestation Commission.
The area of scientific researches of D. Hajiyev covers many areas of paleontology, paleobiology, paleothiology and paleopathology.
In the 1960s, D.Hajiyev played a great role in the study and promotion of the ancient human settlement discovered by M. Huseynov in the south of Nagorno-Karabakh. He confirmed that the stone tools found in the 5th layer of Azikh cave belong to the Middle Achilles culture. According to the scientist, the lower jaw of the primordium found in this layer belongs to a woman aged 20-22. The findings are kept in the Gold Fund of the Azerbaijan State Museum of History.
The Honored Scientist of Azerbaijan D. Hajiyev is the author of more than 120 scientific works, including 12 monographs and textbooks.
Under the guidance of D.Hajiyev 3 doctors of sciences and 12 doctors of philosophy were trained. In addition to his scientific research, he was engaged in pedagogical activities at the Azerbaijan Medical University and Azerbaijan State University.
D. Hajiyev was a member of several foreign academies and scientific societies. In 1970 he was awarded the Medal of Honor, in 1971 he was awarded the Red Banner of Labor, in 1972 he was awarded the honorary title of Honorary Worker. He was awarded with the diploma and prize named after Academician H.A.Aliyev.
Corresponding member of ANAS Demir Hajiyev died in 1999 in Baku.