The life prolongation project was an ambitious attempt at bioengineering that ultimately produced a tragedy.
On a world identical to Earth, circa Earth year 1960, scientists worked to develop a means of extending the life of a cell far beyond normal. The result of this work was the life prolongation complex, a series of viruses that quickly became an epidemic, killing the adult population. Children were granted the extended lifespan, but the onset of puberty caused the complex to manifest its fatal symptoms.
The notes left behind from this project provided Doctor McCoy of the USS Enterprise vital clues necessary to develop an effective vaccine. (TOS: "Miri")