Life is a characteristic distinguishing physical entities having signaling and self-sustaining processes from those that do not, either because such functions have ceased (death), or because they lack such functions and are classified as inanimate. Biology is a science concerned with the study of life.
The smallest contiguous unit of life is called an organism. Organisms are composed of one, or more, cells, undergo metabolism, maintain homeostasis, can grow, respond to stimuli, reproduce and, through evolution, adapt to their environment in successive generations. A diverse array of living organisms can be found in the biosphere of Earth, and the properties common to these organisms—plants, animals, fungi, protists, archaea, and bacteria—are a carbon- and water-based cellular form with complex organization and heritable genetic information.
The Earth was formed about 4.54 billion years ago. The earliest life on Earth existed at least 3.5 billion years ago, during the Eoarchean Era when sufficient crust had solidified following the molten Hadean Eon. The earliest physical evidence for life on Earth is biogenic graphite in 3.7 billion-year-old metasedimentary rocks discovered in Western Greenland and microbial mat fossils found in 3.48 billion-year-old sandstone discovered in Western Australia. Nevertheless, several studies suggest that life on Earth may have started even earlier, as early as 4.25 billion years ago according to one study, and even earlier yet, 4.4 billion years ago, according to another study. The mechanism by which life began on Earth is unknown, although many hypotheses have been formulated. Since emerging, life has evolved into a variety of forms, which biologists have classified into a hierarchy of taxa. Life can survive and thrive in a wide range of conditions.
Biology Basics: Characteristics of Life. (n.d.). Retrieved August 30, 2014, from http://biology.about.com/od/apforstudents/a/aa082105a.htm