Lynn Poole shows a chart of epochs and notes that man didn't appear until the Pleistocene period. Dr. George Carter, department chair and professor of geography at Johns Hopkins University, discusses the possibility of a land bridge from Siberia to Alaska across the Bering Strait, based on human tools found with mastodon remains. Likewise, zoologist R.G. Gilmore has discovered that animals crisscrossed the Bering Strait between ice periods. In describing the history of the study of pre-history, Dr. Carter names W.H. Holmes and Ales Hrdlicka as men who led the opposition to the previously generally accepted belief in the existence of a glacial age man in America. With Willard F. Libby's 1951 discovery that all living things contain radioactive carbon, remains could be dated, challenging previous beliefs. From evidence such as stone tools, Dr. Carter speculates that man entered America about 40,000 years ago. He creates a timeline based on the degree of skill in making tools, the degree of weathering on tools, and the date of the existence of the lake where the tools were found. Dr. Carter also discusses physical geography and carbon-14 dating of tools along the southern California coast. Using charts and photos, he shows how reading California river valley records also yields data about sea level, climate, and glaciers. In the controversial Texas Street site in San Diego, Dr. Carter claims he has discovered hearths, crude stone tools, and dart points corresponding to the last interglacial period.