Everywhere I look describes how the 5 sacral vertebrae start fusing together during adolescence and are totally fused by the time you're 30 or so. But nobody seems to explain why this happens.
It seems like it would be disadvantageous for women, since presumably having a fused sacrum makes giving birth more difficult- apparently post-partum fractures are not uncommon (http://rheumatology.oxfordjournals.org/content/42/2/384.long). So why does it happen?
EDIT: Matthew McTernan gave a great answer, but I am greedy and would like to know more: Why does the sacrum only start fusing together in adolescence, and then finish in your late 20s - early 30s? Most major development ends in your early 20s. Does this have something to do with reproductive concerns? If so, why does it happen on the same timescale in men and women?