A Great question but an unconvincing answer!
In this book Ajit Varki addresses a very critical and important question that would strike any one looking at human evolution - how come our species is the only one that attained self consciousness? Why didn't any other species cross the barrier? The dinosaurs did get plenty of time - even by cosmological standards, 200 million years is a decent time period.
Most probably I would not have read this book, except that I happened to meet Ajit Varki at a conference recently. (Btw, the conference was 'From Bones to Genomes' at Sitges close to Barcelona - a fantastic conference with speakers like Matt Ridley, Svante Paabo, Chris Stringer etc - making it a veritable heaven for those interested in Evolution, Genetics and Population history). His book was also mentioned in some side discussions and that prompted me to download the kindle version (talk about the benefits of technology!) and take a stab at it.
However the approach of the book was rather surprising. Ajit had decided on the answer upfront - that humans were able to cross the 'intelligence barrier' by being able to deny the inevitability of death. To make it clearer, according to Ajit the barrier that prevents any species from attaining self consciousness is that it will not have the neuronal connections or capabilities to handle the fact that death is inevitable. Only the human brain developed in such a way that both self consciousness and death denial ability developed together at some time (only once) in our past and that was the turning point. The book is then one long argument (in the author's words) to try to convince himself and the reader that the argument is correct. (lot of cherry picking!) Ajit acknowledges that there is no clear evidence to prove his thesis and also acknowledges that it is not falsifiable but does try to be as fair as possible in outlining that. He still gets carried away with his theory and even attempts to compare the situation with that of Darwin's Evolution theory in the late 19th century. But he ignores that Darwin did not start with a conclusion - he had discovered a large number of facts that led him to a conclusion which is not the case here.
The book is an interesting read and the author is a very knowledgable person that makes the narrative fun. But I have to say that his answer to an all-important question is possible but not probable.