Answer
This adaptation could offset iron loss during menstruation in a population that would have had to adapt to lower levels of vitamin D.
Work Step by Step
This could be an adaptation that prevents iron-deficiency anemia and vitamin D deficiency in human females around menstruation, during which iron loss can cause dangerous anemia in some circumstances. Humans evolving nearer to the poles must also compensate for lower levels of vitamin D, which requires iron for activation.
