Various forms of life exist, such as plants, animals, fungi, protists, archaea, and bacteria.
The criteria can at times be ambiguous and may or may not define viruses, viroids, or potential artificial life as "living".
Since its primordial beginnings, life on Earth has changed its environment on a geologic time scale.
To survive in most ecosystems, life must often adapt to a wide range of conditions.
Some microorganisms, called extremophiles, thrive in physically or geochemically extreme environments that are detrimental to most other life on Earth.
Aristotle was the first person to classify organisms.
However, many other biological definitions have been proposed, and there are some borderline cases, such as viruses.
Cells are sometimes considered the smallest units and "building blocks" of life.
There are two kinds of cells, prokaryotic and eukaryotic, both of which consist of cytoplasm enclosed within a membrane and contain many biomolecules such as proteins and nucleic acids.
Biophysicists have proposed many definitions based on chemical systems; there are also some living systems theories, such as the Gaia hypothesis, the idea that the Earth itself is alive.
Another theory is that life is the property of ecological systems, and yet another is elaborated in complex systems biology, a branch or subfield of mathematical biology.
"Figures of speech," Gleaves Whitney has observed, "are all of the ways in which human beings bend and stretch words to heighten meaning or create a desired effect" (.