4Our solar system is more than six billion years old. Scientists believe there was life on Mars four million years ago. Many people on Earth believe they are the only life forms in the universe but, with millions of undiscovered planets out there, can that really be true? Find out how life began on Earth, how far humans have come in a short period of time, and how we might measure up to other worlds. Includes 21st Century Skills and Content, sidebars, activity, table of contents, index, and glossary.