Vegetation depends mainly on climate, and animals depend on the amount of food they eat. The shrinking of habitats and the loss of their carrying capacity have intensified competition between plant and animal species. One way of survival was to migrate to better habitats and food. Migration also brought some species to areas where they had never been found before. Once they found favourable conditions, they settled permanently, enriching the biota of the area. Migration did not only take place within continents. At certain times, land bridges evolved between continents and islands separated by seas: this is how also humans were able to reach almost all parts of the world during the Ice Age.
During the glaciations, continents were connected by temporary land bridges, opening up new pathways for life. Animals migrated and plants spread across the Bering Strait between North America and Eurasia, and between Europe and England, through the land connections that had been established. Between North and South America, the Isthmus of Panama became much wider than it is today.
The ancestors of modern horses migrated from North America to Eurasia across the Bering Strait about 1.7 million years ago. In a short time, they moved across Eurasia and made it even to Africa. By the end of the Ice Age, however, they had become extinct in the Americas, partly due to the arrival of migrating people.
However, in Eurasia, wild horses did not die out, and were domesticated around 6500 years ago. Much later, in the historical past, the colonizers from Europe introduced horses to the Americas, where they were unknown to the indigenous Indians. The mustangs of the Native Americans are also descendants of feral domesticated horses that were brought to the continent by European immigrants.