He identifies five stages in human evolution based on the fossil record, marked by the successive appearance of (1) the earliest hominins; (2) the australopithecines; (3) Homo and Paranthropus; (4) Homo erectus; (5) the journey towards Homo sapiens. This scheme can be simplified into three phases. First there was the evolution of bipedalism, which led to the spread of Australopithecus species across Africa. Next came the evolution of Homo erectus, and finally we get the evolution of Homo sapiens.
Maslin takes the decisive event to have been the appearance of H. erectus, whose brain was considerably larger than its predecessors' and whose body size and morphology were close to those of modern humans. H. erectus made more sophisticated tools than earlier species and eventually went on to colonise much of the Old World outside Africa. This three-part scheme is the framework Maslin uses throughout the book, as he considers how plate tectonics, global and local climate change, and celestial mechanics influenced human evolution.
Tectonics is important in the story of human evolution because it caused the development of the East African Rift Valley system. Rifting happens when a plume of hot magma rises beneath a continent, causing a bulge that forms a plateau. As the pressure rises further the plateau ultimately fractures, allowing the central part to slip down and create a huge valley, with large mountain ranges on each side.
In East Africa this led to increased aridity, fragmentation of the vegetation, and the development of many lake basins. The region then became highly responsive to climate change, which affected how our ancestors evolved. Bipedalism evolved against this background, although there are still uncertainties about how exactly it came about.
Superimposed on these tectonic events there were also major climatic changes caused by celestial mechanics. In the long term it is mainly tectonics that is responsible for such changes, but in the shorter term celestial mechanics are also involved. This is a complex topic; Maslin says that it is the hardest subject that he teaches.
The Earth's orbit around the Sun and the angle of its axis of rotation vary or 'wobble'. We now know that this 'orbital forcing' drove the glacial–interglacial cycles that are fundamental characteristics of the Quaternary Period, the past 2.5 million years.There were periods when the climate in East Africa was highly variable, cycling between extremely high rainfall, with widespread lakes, and hyper-arid conditions when the lakes dried up and much dust was blown into the surrounding oceans. This variability, Maslin suggests, may have played a part in the evolution of H. erectus, favouring brain enlargement that helped in coping with the adverse conditions and probably enhanced sociability.
Further brain development led to increasingly complex behaviour in the modern humans who succeeded H. erectus, although there was an unexplained delay between the first appearance of anatomically modern humans 200,000 years ago and the development of sophisticated art and technology 50,000 years ago; Maslin has a discussion of this problem. His final chapter looks at the challenging prospects for humanity in the context of the far-reaching and exponentially accelerating changes that we are making in our world.
It's not entirely clear whom this book is intended for. Parts of it, particularly the discussion of celestial mechanics, are quite demanding, although Maslin writes clearly and avoids jargon. But from scattered remarks I get the impression that he was really writing an introductory text for his students—which doesn't mean that the rest of us won't enjoy listening in.