WILLIAM R. LEONARD is Associate Professor of Anthropology at Northwestern University, Illinois. He is a biological anthropologist whose research focusses heavily on aspects of physiology, nutrition and health. He has extensive fieldwork experience in Siberia and Latin America examining how human populations adapt to extreme environments. MICHAEL H. CRAWFORD is Professor of Anthropology at the University of Kansas. His research focuses on anthropological genetics particularly in populations in the Americas. He has also written The Origins of Native Americans (1998/ 2001), ISBNS 0521 592801 & 0521 004101, and edited Different Seasons: Biological Aging in Mennonites of Midwestern United States (2000).