Evolution is any change across successive generations in the  heritable characteristics of biological populations. The process of evolution involves mutations in the heritable characteristics which lead to a variability within a population. Characteristics which improve an organisms (animal or vegetable) chance of survival are then passed on to succesive generations which leads to gradual changes in the appearance or attributes of an organism. The evolution of the horse began 55 million. years ago in the eocene era, with the first known ancestor the eohippus.