The arrow of time, from the origin of the universe to the present and beyond, spans several major epochs of history. Cosmic evolution is the study of the many varied changes in the assembly and composition of energy, matter, and life in the thinning and cooling universe. Inserted along the arrow of time are seven "windows" outlining some of the key events in the history of the universe:

• evolution of primal energy into elementary particles,
• evolution of atoms into galaxies and stars,
• evolution of stars into heavy elements,
• evolution of elements into solid, rocky planets,
• evolution of those elements into the molecular building blocks of life,
• evolution of those molecules into life itself
• evolution of advanced life forms into intelligence, culture, and technological civilization.

LEARNING GOALS

Studying this chapter will enable you to:

Summarize the process of cosmic evolution as it is currently understood.

Evaluate the chances of finding life in the solar system.

Summarize the various probabilities used to estimate the number of advanced civilizations that might exist in the Galaxy.

Discuss some of the techniques we might use to search for extraterrestrials and to communicate with them.

Are we unique? Is life on our planet the only example of life in the universe? These are difficult questions, for the subject of extraterrestrial life is one on which we have no data, but they are important questions, with profound implications for the human species. Earth is the only place in the universe where we know for certain that life exists. In this chapter we take a look at how humans evolved on Earth and then consider whether those evolutionary steps might have happened elsewhere. Having done that, we will assess the likelihood of our having Galactic neighbors and consider how we might learn about them if they exist.