Matter characterized by the ability to metabolize nutrients (process materials for energy and tissue building), grow, reproduce, and respond and adapt to environmental stimuli. Fossil evidence suggests that Earth's first living organisms, bacteria and cyanobacteria, arose about 3.5 billion years ago. All known life-forms possess either DNA or RNA. Viruses, which possess DNA and RNA, cannot reproduce without a host cell and do not metabolize nutrients, and it is uncertain whether they should be classified as living or nonliving. Scientists disagree on the likelihood of extraterrestrial life. See alsoDrake equation.