Life history theory is a theory of biological evolution that seeks to explain aspects of organisms' anatomy and behavior by reference to the way that their life histories - including their reproductive development and behaviors, life span and post-reproductive behavior - have been shaped by natural selection. These events, notably juvenile development, age of sexual maturity, first reproduction, number of offspring and level of parental investment, senescence and death, depend on the physical and ecological environment of the organism. Organisms have evolved a great variety of life histories, from Pacific salmon, which produce thousands of eggs at one time and then die, to human beings, who produce a few offspring over the course of decades. The theory depends on principles of evolutionary