Complex sentence: McCollum got his Ph.D. from Yale in two years, but stayed at Yale for another year as a postdoctoral researcher working with Thomas Osborne and Lafayette Mendel on problems of plant protein composition and diet.
Split into simpler sentences: McCollum got his Ph.D. from Yale in two years. He remained for another year as a postdoctoral researcher working with Thomas Osborne and Lafayette Mendel on plant protein and diet.