Complex sentence: Gilbert returned to Harvard in 1956 and was appointed assistant professor of physics in 1959; in 1964 he was promoted to associate professor of biophysics and promoted again in 1968 to professor of biochemistry.
Split into simpler sentences: Gilbert returned to Harvard in 1956 and was appointed assistant professor of physics in 1959. In 1964 he was promoted to associate professor of biophysics and promoted again in 1968 to professor of biochemistry.