Why do humans have 23 pairs of chromosomes?

Human body cells usually contain 46 chromosomes arranged as 23 pairs. Twenty-two of those pairs are autosomes, and one pair is the sex chromosomes, typically XX or XY. During the production of eggs and sperm, the chromosome number is cut in half by meiosis so that fertilization restores the full set. Closely related primates have different chromosome counts, showing that chromosome number can change during evolution; humans’ chromosome 2 is known to have formed by fusion of two ancestral chromosomes.