Twins have been of interest to scholars since early civilization, including the early physician Hippocrates (5th century BCE), who attributed different diseases in twins to different material circumstances, and the stoic philosopher Posidonius (1st century BCE), who attributed such similarities to shared astrological circumstances. More recent study is from Sir Francis Galton's pioneering use of twins to study the role of genes and environment on human development and behavior. Galton, however, was unaware of the difference between identical and DZ twins.