How did life begin, both here and perhaps elsewhere? Are we alone, a chemical fluke, or are we the result of a process common in the universe? Prior to this century the question of our origin has been relatively unconstrained. With knowledge gained through experimentation and space science - about Earth, the solar system, and the universe - the question of our origins has become a serious scientific quest.

Exobiology, the study of the origin, evolution, and distribution of life in the universe, embodies this quest. With the recognition that: chemical evolution proceeded rapidly to life on Earth, life may be a natural consequence of planetary evolution, and planetary systems may be common in the universe, life is likely to be common in the universe. How can we find it?

With modeling, observation, and sampling in our solar system, we can learn of the planets' evolution through time. What is the relative importance of distance from the sun, bulk composition, and mass that determine whether the planetary body could have ever sustained life. What planetary bodies are the better candidates for life around other stars?

What are the potential signatures for life? So far, we have only one example, rather evolved life at that. How would Earth's biosphere appear from space? Technology has an obvious signature but technology has been here for an extremely limited period of time. For the last 2 billion years, Earth's biosphere has been dominated by oxygenic photosynthesis - ozone would be an indicator. Chemical disequilibria are maintained by biology and could be indicative of life, for example methane or nitrous oxide in the presence of oxygen. But what of early Earth? What combination of reduced species would be indicative of life in a reducing atmosphere? Perhaps Earth's atmosphere was dominated by volcanic activity during its first 1.5 billion years, swamping any biological signature. An alternative maybe to search for a predominance of an optically active pigment, like chlorophyll or rhodopsin.

Exobiology can contribute to the search for life elsewhere by broadening our understanding of the origin and evolution of life. Specific questions that could be addressed are: What planetary characteristics would indicate habitability? What planetary characteristics would be most diagnostic of life? and at what stage? What planetary characteristics would be most detectable?