The Land Bridge Theory

During an Ice Age, when the world is colder, more of the world's water freezes into ice. So less water is in the oceans than before. This makes the level of the oceans lower, like the water in your bath is lower if you let some out. Because the ocean level is lower, more of the land sticks out, like your knees in the bath. 
In some places, this makes islands where before the land had been underwater. In other places, this makes a long bridge of land that connects two different continents. During the last Ice Age, about 12000 BC, there was a land bridge between East Asia and North America, leading from Siberia to Alaska, and most archaeologists think that some people and dogs crossed over this land bridge and began to live in North America at that time. These probably weren't the first people to come from Central Asia to North and South America - the first people may have come in an earlier Ice Age, or they may have come by boat.