The authors, good reductionists both, naturally start at the bottom and work their way up. A brief discussion of general concepts such as progress, complexity, and transition leads naturally into the question of what life is and a glance at non-living systems that approach it in some ways (the Oklo reactor and the chemoton). They then offer a potpourri of prebiotic chemistry (diversity without replication, the Miller-Urey experiment, surface metabolism, extraterrestrial organic chemistry) and mathematical models (replication accuracy and error thresholds, hypercycles and stochastic correctors). This leads to debates about the primacy of proteins and nucleic acids, and the origins of translation and the genetic code. (Smith and Szathmáry are obviously supporters of RNA-world approaches, but they give I think a fair appraisal of theories of Kauffman and Dyson based on autocatalytic protein networks.)
Then there are chapters on the origins of protocells (membranes, the negibacterial double membrane, the origin of chromosomes) and of eukaryotes (mitosis, intracellular membrane systems, mitochondria and other organelles, centrioles and undilipodia). Next come chapters on sex (with a little about the nature of species), intragenomic conflict, and symbiosis. And four chapters cover development: one on simple organisms (self-assembly and its limits, cell cycles and temporal differentiation, yeast budding, the alga Volvox, and slime moulds); one on gene regulation, cell differentiation, and the preconditions for the metazoa; one on spatial patterns and positional information; and one on the connections of development with evolution (levels of selection, gene homologies, zootypes and archetypes). This is all, again, at a very high level: there is, for example, barely a page on homeobox domains.
There was always going to be something disconcerting about an attempt to say something meaningful about the origins of societies and the origin of language in so little space, but the chapters on these topics were not as simplistic as I had feared they might be. The first moves from the Prisoner's Dilemma and insect sociality to "the Social Contract game" — and Rousseau, Plato, Durkheim, and Adam Smith. The second provides an introduction to basic syntax and then touches on language disorders, possible protolanguages, and connections of language evolution with tool use.
Major Transitions is clearly laid out, with a good selection of diagrams and helpful sectioning, but the amount of whitespace is actually excessive: not only does every chapter begin on a right-hand page, forcing many blank pages, but each opens with a table of contents, simply repeating material from the main table of contents, followed by yet another blank page. Some of this space could have been used for an annotated further reading list for each chapter, which would have made the book much more useful as a starting point for more specialised study.