The Dragons of Eden: Speculations on the Evolution of Human Intelligence is a Pulitzer prize winning 1977 book by Carl Sagan. In it, he combines the fields of anthropology, evolutionary biology, psychology, and computer science to give a well balanced perspective of how human intelligence evolved. One of the main issues featured in the book is the search for a quantitative way of measuring intelligence. Sagan shows that the brain to body mass ratio is an extremely good indicator, with humans having the highest and dolphins second. It does break down, however, at the extremely small end of the scale. Because a certain minimum size is needed to sustain life, smaller creatures (ants in particular) place disproportionally high on the list.