Life

Life is the characteristic that distinguishes living organisms from non-living matter. Living things are recognized by their use of energy, their capacity for growth, their response to environmental changes, the maintenance of internal stability in their structure, and their ability to reproduce and pass on their hereditary information to offspring.

Biologists lack a single agreed-upon definition. A list of characteristics is commonly used instead, arising from difficult cases such as viruses, which cannot reproduce independently and require a host cell. For this reason, life is sometimes understood as a process rather than as a thing.

General characteristics

Typical living organisms possess a cell, the smallest living unit. They carry out metabolism, the chemical transformations that convert energy and materials from food or light. They maintain homeostasis, the preservation of internal conditions such as temperature and pH. They also transmit hereditary information encoded in DNA or, in some viruses, RNA.

Divisions of life

Modern classification systems divide life into three major groups: bacteria, archaea, and eukaryotes. The third group includes animals, plants, fungi, and single-celled organisms with a nucleus. The classification is based on comparison of hereditary sequences, which reveal deeper relationships than outward appearance.

Origins of life

Life on Earth is estimated to have begun millions of years ago, with evidence found in ancient rocks. Theories of abiogenesis attempt to explain how simple chemical substances became self-reproducing systems. Laboratory experiments have shown that basic materials such as amino acids can form under natural conditions, but the subsequent steps remain a matter of scientific debate.