In the book Chatwin develops his thesis about the primordial nature of Aboriginal song. The writing does not shy away from the actual condition of life for present day indigenous Australians, it does not present the songlines as a new-age fad but from an appreciation of the art and culture of the people for whom they are the keystone of the Real. While the book's first half chronicles the main character's travels through Outback Australia and his various encounters, the second half is dedicated to his musings on the nature of man as nomad and city builder.
The basic idea that Chatwin posits is that language started as song, and the aboriginal Dreamtime sings the land into existence. A key concept of aboriginal culture is that the aboriginals and the land are one. By singing the land, the land itself exists; you see the tree, the rock, the path, the land. What are we if not defined by our environment? And in one of the harshest environments on Earth one of our oldest civilizations became literally as one with the country.
This central concept then branches out from Aboriginal culture, as Chatwin combines evidence gained there with preconceived ideas on the early evolution of man, and argues that on the African Savannah we were a migratory species, moving solely on foot, hunted by a dominant brute predator in the form of a big cat: hence the spreading of "songlines" across the globe, eventually reaching Australia (Chatwin notes their trajectory generally moves from north-east to south-west) where they are now preserved in the world's oldest living culture.