selves. Others go in for a complete remodeling before adulthood, breaking down their juvenile bodies and using the parts to make something very different. Some live for more than a century. The water flea Daphnia completes its life cycle inside a week.
It’s easy to imagine what an all-conquering evolutionary super-beast—a Darwinian demon—would be like. It would grow to large size and sexual maturity in a flash and produce vast numbers of hearty offspring, forever. Such a paragon would monopolize the energy available to life. But, of course, this isn’t possible. Life’s resources are finite. Every feature of every plant and animal represents a choice made by evolution about how to invest energy. And every choice precludes other choices. Organisms must choose whether to put their resources into muscular legs or wings, gaudy tails or flowers, fearsome horns or juicy fruit. They must also choose what to invest in and when: whether to become sexually mature at a young age, and get on with mating and breeding before any mishaps befall them, or delay reproduction until they are bigger and better equipped to compete for resources or mates. They must choose whether to produce a few large offspring, each of which stands a good chance of surviving, or lots of little ones, most of which will die young. When they reproduce, they must decide whether to hold something back for future breeding seasons or fling everything on the one shot and die in the attempt. The result of all these decisions is called an organism’s life history.
Choice—trade-offs, in other words—is fundamental to life’s design. And there are still deeper unities to be pursued. Even in a finite world, the diversity of life makes it look as if there are an astronomical number of biological options. In fact, metabolism controls what choices can be made, and understanding metabolic rate peels back the multiplicity of life histories to reveal one sleekly efficient mechanism for turning energy into offspring.
D’Arcy Thompson noticed that organisms grow quickly in their youth and then ever more slowly thereafter. Some, such as birds, mammals, and insects, stop growing when they reach adulthood and start