The Theory of Natural Selection proposed by Charles Darwin is the mostly accepted theory which proposes that the higher animals descended from the lower animals. All the living beings share certain characters because the traits are inherited from the parents to offspring and new species evolve from the existing species.
Modern humans share most of the traits with the Chimpanzees, which indicates that both of them share a common ancestor.
Thus, the blanks can be filled with common ancestor; chimpanzee.