The Galactic bulge is of importance to understand the formation of the Milky Way, as a massive and old stellar component. Studying the bulge is, however, hampered by the crowd of stars from various components, as well as extremely high and variable foreground extinction. The presence of two red clumps (RCs) observed in the color-magnitude diagram of the Milky Way bulge provides a great opportunity to study the structure and formation of the bulge. This double RC is widely accepted as evidence for a giant X-shaped structure originated from the bar instability. We suggested, however, a drastically different interpretation based on the multiple stellar population phenomenon as is observed in globular clusters (GCs), where the bright RC is from He and Na enhanced second-generation stars, while the faint RC is representing first-generation stars with normal abundances. Because these two scenarios imply very different pictures of the formation of the bulge, understanding the origin of the double RC is of crucial importance. As supporting evidence for multiple population scenario, here we report our discovery that the stars in the two RCs show a significant difference in CN-band strength. We also found that the red giant branch stars in the outer bulge are divided into two groups according to Na abundance in the [Na/Fe] - [Fe/H] plane. Since these chemical patterns and characteristics are only explained by stars originated in GCs, this is evidence that the double RC is due to the multiple population phenomenon. Furthermore, this result indicates that the outer bulge was mostly assembled from disrupted proto-GCs in the early history of the Milky Way.