This link: http://www.channel4.com/programmes/inside-natures-giants/4od/player/3418172 takes you to Channel 4's 4OD page. You will need to register.
Between 6:00-7:05 minutes Professor Richard Dawkins explains the evolution of the leg bones of the horse, as is discussed in the student book.
(The rest of the programme is fascinating, and covers other aspects like adaptation and evolution).
Source: 4OD
