Claim: Earth is full of life of all different sorts, which is a unique feature of the planet.

Life is a characteristic distinguishing physical entities having 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 . The criteria can at times be ambiguous and may or may not define viruses , viroids , or potential artificial life as `` living '' . Biology is the primary science concerned with the study of life , although many other sciences are involved .   The definition of life is controversial . The current definition is that organisms maintain homeostasis , are composed of cells , undergo metabolism , can grow , adapt to their environment , respond to stimuli , and reproduce . 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 '' . Modern definitions are more complex , with input from a diversity of scientific disciplines . 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 . Abiogenesis describes the natural process of life arising from non-living matter , such as simple organic compounds . Properties common to all organisms include the need for certain core chemical elements to sustain biochemical functions .   Life on Earth first appeared as early as 4.28 billion years ago , soon after ocean formation 4.41 billion years ago , and not long after the formation of the Earth 4.54 billion years ago . Earth 's current life may have descended from an RNA world , although RNA-based life may not have been the first . The mechanism by which life began on Earth is unknown , though many hypotheses have been formulated and are often based on the Miller -- Urey experiment . The earliest known life forms are microfossils of bacteria . In July 2016 , scientists reported identifying a set of 355 genes believed to be present in the last universal common ancestor -LRB- LUCA -RRB- of all living organisms .   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 . Later , Carl Linnaeus introduced his system of binomial nomenclature for the classification of species . Eventually new groups and categories of life were discovered , such as cells and microorganisms , forcing dramatic revisions of the structure of relationships between living organisms . 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 . Cells reproduce through a process of cell division , in which the parent cell divides into two or more daughter cells .   Though currently only known on Earth , life need not be restricted to it , and many scientists believe in the existence of extraterrestrial life . Artificial life is a computer simulation or man-made reconstruction of any aspect of life , which is often used to examine systems related to natural life . 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 . Fossils are the preserved remains or traces of organisms .