Life

Life is a characteristic that distinguishes physical entities with biological processes from inanimate objects. In biology, life is defined by a set of properties that include organized structure, metabolism, growth, reproduction, response to stimuli, homeostasis, and the capacity to adapt and evolve through natural selection. No single criterion universally defines life, and definitions emphasize different aspects such as cellular organization, genetic information, or evolutionary potential.

Most known life is cellular, consisting of cells that carry out metabolism, maintain order, and reproduce. Genetic material, typically DNA, stores information necessary for growth and heredity. Life ranges from unicellular organisms to complex multicellular forms. Viruses occupy a controversial position: they reproduce and evolve but rely on host cells for metabolism, so many scientists do not classify them as living.

Life on Earth arose through chemical evolution, a gradual transition from simple molecules to self-replicating systems. The earliest evidence dates to about 3.5 to 4 billion years ago, documented in fossil records and ancient biomolecules. Evolution by mutation and natural selection has generated vast diversity and adaptation across ecosystems.

Life requires liquid water and a source of energy and nutrients; organisms inhabit oceans, soil, air, and extreme environments. Biosignatures such as atmospheric gases, isotopic patterns, and cellular remains guide the search for life beyond Earth in astrobiology.

The concept of life intersects science, philosophy, and culture, and debates continue about possible nonstandard life forms, artificial life, and the definition of life itself.