Final answer:
During anaphase I of meiosis, the critical event is the separation of homologous chromosomes, where each moves to opposite poles of the cell, unlike anaphase II where sister chromatids separate.
Step-by-step explanation:
During anaphase I of meiosis, the event that must occur is the separation of homologous chromosomes. This is the stage where spindle fibers shorten, pulling the homologous chromosomes apart. Each chromosome of the homologous pair moves towards opposite poles of the cell, while the sister chromatids remain connected at their centromeres. It is important to note that the separation of homologous chromosomes is a key distinction of anaphase I in meiosis, differentiating it from anaphase II, where sister chromatids separate and move to opposite poles.