Modern definitions are more complex, with input from a diversity of scientific disciplines.
However, many other biological definitions have been proposed, and there are some borderline cases of life, such as viruses.
Throughout history, there have been many attempts to define what is meant by "life" and many theories on the properties and emergence of living things, such as materialism, the belief that everything is made out of matter and that life is merely a complex form of it; hylomorphism, the belief that all things are a combination of matter and form, and the form of a living thing is its soul; spontaneous generation, the belief that life repeatedly emerges from non-life; and vitalism, a now largely discredited hypothesis that living organisms possess a "life force" or "vital spark".
The earliest known life forms are microfossils of bacteria.
3.45 billion year old Australian rocks are reported to have contained microorganisms.
Life is a characteristic that distinguishes physical entities that do have biological processes, such as signaling and self-sustaining processes, from those that do not, either because such functions have ceased, or because they never had such functions and are classified as inanimate.
Various forms of life exist, such as plants, animals, fungi, protists, archaea, and bacteria.
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.
Abiogenesis describes the natural process of life arising from non-living matter, such as simple organic compounds.
Death is the permanent termination of all biological functions which sustain an organism, and as such, is the end of its life.
Extinction is the process by which an entire group or taxon, normally a species, dies out.
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.