This video segment from The Human Spark explores some of the branches of our family tree as host Alan Alda visits the Museum of Natural History in New York City. Ian Tattersall, the curator of anthropology at the museum and one of the world’s leading experts on human evolution, presents several skulls from the museum’s collection, pointing out how the size and shape of the skulls evolved over time. John Shea, one of the leading experts on stone-age technology, demonstrates how to make a stone axe, a tool that was used unchanged by our ancestors for millions of years.