Many changes took place between those little animals and today's horse. These changes are best explained as adaptations to its changing ecological niche. From a small forest-dweller eating nuts and fruit to a larger forest browser eating leaves and small branches. Finally, the modern horse is a grazer on open grassland, with different teeth, legs for running and much larger size. Major changes happened in the mid-Miocene when the climate became cooler, and grassland began to replace forests. This change continued, and several groups of mammals changed from browsers to grazers.