The investigation of molecular traits for evolutionary studies is a valuable alternative to the classic morphological approach. Since the mid-1980s molecular evolution has extended its field of investigation into the past. This has become possible with development of the polymerase chain reaction, which allows amplification of, and hence the study of, DNA from individuals long since dead. Many ancient, mostly extinct, animals and plants have been studied in order to determine their phylogenetic relationships with extant species. The biochemical properties of ancient DNA are also under investigation. The modifications that this molecule undergoes over long periods of time are of great interest as attempts are made to retrieve DNA from increasingly older remains.