In Michael Crichton's works, he discusses the average lifespan of a species, noting that it typically spans around four million years. This timeframe reflects the natural cycles of evolution and extinction over geological periods. However, this average significantly shortens for mammals, which tend to last only about one million years before becoming extinct.

This observation underscores the fragility of mammalian species in the context of Earth's biodiversity. The rapid pace of extinction among mammals highlights the impact of environmental changes and evolutionary pressures. Crichton's exploration of these concepts serves as a reminder of the challenges species face in adapting to their surroundings.