ORIGIN AND EVOLUTION OF LIFE ON OTHER WORLDS: LESSONS FROM THE HISTORY OF LIFE ON EARTH

摘要

Life on Earth is the only form of life we know at the present time. However, that life originated and has evolved under the influence of physical and chemical laws presumed to pertain throughout the universe. Therefore, the course that life has taken on Earth, while undoubtedly unique in some respects, may hold lessons applicable to the origin and evolution of life on other worlds. The major facts of life on Earth include (1) its rapid onset early in the window of opportunity for its survival, (2) conservation of form and function by stabilizing natural selection in static and homogeneous environments, (3) diversification of form and function by directional natural selection in heterogeneous or changing environments, (4) the prevalence of relatively simple, microscopic forms over complex, macroscopic organisms, and (5) the rare but inevitable tendency toward a high degree of complexity in a small fraction of life. A reasonable inference from these facts is that life is relatively pervasive but mostly simple, microscopic, and sequestered in protected environments on other worlds, with the evolution of complex forms rare but occasional, and the emergence of technology rarer still but ultimately inevitable.