The theory of evolution is supported by biochemical evidence; many of the same molecules and biochemical processes occur within all living organisms, from single-cell bacteria to humans. Originally, scientists couldn't understand how the process of evolution began, but they later discovered that RNA possesses catalytic properties.
Catalytic RNA can make copies of itself, thus allowing the process of evolution to begin. As this process itself evolved, DNA became the nucleic acid primarily responsible for inheritance. The common molecules and chemical reactions shared by all life forms are believed to have arisen early in the process of evolution.
Organisms' DNA sequences also show evidence for common ancestry. Organisms that are biologically similar posses similar DNA sequences, whereas organisms that are biologically different possess different DNA sequences. Regardless, the DNA sequences of organisms that are less biologically similar still share common characteristics, indicating that a "tree of life" exists.
Vital proteins also show evidence for common descent. Their presence in all life forms indicates that, much like other common molecules and chemical reactions, the proteins were some of the original building blocks of life. Hereditary pseudogenes, DNA that doesn't function and serves no observable purpose, are also evidence supporting the theory of evolution.