Asked by: Rich Webb, Northampton


It reflects the evolutionary history of locomotion. Our wormy ancestors slithered on the sea floor, so undulated side-to-side. Fish inherited that movement, for which a vertical tail is best.

Their distant land mammal descendants evolved to run with limbs underneath: an unstable gait allowing rapid direction changes. To extend the stride, their spine flexes up-and-down. Marine mammals kept this movement, for which a horizontal tail is best.

Read more:


Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.