Lifeforms

Lifeforms, or living organisms, are entities that exhibit a set of characteristics used to distinguish living systems from nonliving matter: organized structure, metabolism, growth, reproduction, response to stimuli, homeostasis, and the capacity to evolve. Most recognized life on Earth consists of cells, and life processes run on energy captured from the environment and converted into usable chemical forms.

Biochemically, life is largely carbon-based and water-dependent, utilizing nucleic acids to store genetic information and proteins as catalysts. The cell is the basic unit of life, and organisms range from single-celled microbes to complex multicellular forms. Diversity is vast, spanning bacteria, archaea, and eukaryotes, with extensive variation in metabolism, habitat, and body plans.

Taxonomy and classification commonly follow the three-domain framework: Bacteria, Archaea, and Eukarya. Within Eukarya are kingdoms such as plants, animals, fungi, and various protists. Viruses challenge the definition of life because they replicate and evolve but lack independent metabolism and cellular structure, placing them at the boundary between living and nonliving matter in many discussions.

Origins and evolution are topics of ongoing study. Life on Earth is thought to have arisen more than 3.5 to 4 billion years ago through abiogenesis, with early metabolic networks and genetic information networks likely evolving toward DNA-based life. The emergence of oxygenic photosynthesis reshaped Earth's atmosphere and supported greater biological complexity.

Astrobiology explores life beyond Earth, studying extremophiles to broaden the known conditions that can sustain life and seeking biosignatures such as atmospheric gases, isotopic patterns, or organic molecules on other worlds. The study of life informs ecology, medicine, and environmental science, and raises questions about biodiversity and humanity’s responsibilities to other organisms.