We have an extraordinary relationship with dogs - closer than with any other animal on the planet. But what makes the bond between us so special? From the latest fossil evidence, to the sequencing of the canine genome, to cognitive experiments, dogs are fast turning into the new chimps as a window into understanding ourselves. Why are dogs so good at reading our emotions? Horizon meets Betsy, reputedly the world's most intelligent dog, and compares her incredible abilities to those of children. Man's best friend has recently gone one step further - helping us identify genes responsible for causing human diseases.