According to the most widely accepted view, Native Americans were originally inhabitants of northeastern Asia who gradually migrated to North America between 12,000 and 15,000 years ago. At that time, the planet was gripped in the last of the major ice ages. Because greater amounts of water were trapped in the polar ice caps, sea levels dropped by several hundred feet. Lower sea levels created a number of land bridges, including one between modern-day Siberia and Alaska. Once on North American soil, these ancestors of modern-day Native Americans made their way southward and, within a couple of thousand years, inhabited all of the Americas from the subarctic region to the tip of South America.