Today, Asia and North America are separated at their closest point by 85 kilometers (58 miles) of chilly ocean waters. But ocean levels were much lower 15,000 to 20,000 years ago, and a large land mass connected the continents, allowing the first of many waves of immigrants to cross into North America. Climate researchers say that massive sheets of ice covered the inland areas of present-day Canada, so the migrants must have stuck close to the Pacific coast in order to reach the ice-free regions of North and South America.
