In this portrait of Planet Earth-at just about the mid point of its probable lifespan-a biologist discusses the evolution of the network of life and the crucial role played by humans in determining the future of our world.

Unlike most books on earth history, which present the story of life on our planet in terms of one chronological period after another, the author discusses Earth's teeming diversity in terms of pivotal evolutionary developments. Among these he stresses the importance of symbiosis, sex, and altruism as key determinants of the Earth's biodiversity.

Symbiosis-when single cells began working together-sparked the sudden appearance of complex animals. Much later symbiotic relationships led to flowering plants that depended on animals for pollination and seed dispersal.

With the advent of sexual selection, there developed an astonishing world of complex behavior and a dizzying array of life forms. In humans, sexual selection exerted a great influence on the development of our large brains.

Altruism-when species learned to work together-resulted in even greater variety and complexity. In early humans, altruism gave rise to ever-widening social circles and the spread of culture.

The author also discusses the role of photosynthesis in establishing and maintaining life on earth; the evidence for ancient natural catastrophes, which caused widespread extinctions; and the importance of religion and the recent use of scientific reasoning in the development and the future of the human species.

This eloquent, panoramic perspective is well designed to foster an appreciation for the scope of life on Earth and to encourage wise stewardship of the natural world on which our survival depends.