In the past, there have been many attempts to define what is meant by "life" through obsolete concepts such as Odic force, hylomorphism, spontaneous generation and vitalism, that have now been disproved by biological discoveries.
Cells reproduce through a process of cell division, in which the parent cell divides into two or more daughter cells. 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.
The cell is considered the structural and functional unit of life. Some microorganisms, called extremophiles, thrive in physically or geochemically extreme environments that are detrimental to most other life on Earth. Since its primordial beginnings, life on Earth has changed its environment on a geologic time scale, but it has also adapted to survive in most ecosystems and conditions. Complex organic molecules occur in the Solar System and in interstellar space, and these molecules may have provided starting material for the development of life on Earth. The classic 1952 Miller–Urey experiment and similar research demonstrated that most amino acids, the chemical constituents of the proteins used in all living organisms, can be synthesized from inorganic compounds under conditions intended to replicate those of the early Earth. Life on Earth is probably descended from an RNA world, although RNA-based life may not have been the first life to have existed. The earliest known life forms are microfossils of bacteria. 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 Earth 4.54 billion years ago. The prevailing scientific hypothesis is that the transition from non-living to living entities was not a single event, but a gradual process of increasing complexity. Other definitions sometimes include non-cellular life forms such as viruses and viroids.Ībiogenesis is the natural process of life arising from non-living matter, such as simple organic compounds. One popular definition is that organisms are open systems that maintain homeostasis, are composed of cells, have a life cycle, undergo metabolism, can grow, adapt to their environment, respond to stimuli, reproduce and evolve. There is currently no consensus regarding the definition of life. Biology is the science that studies life. Various forms of life exist, such as plants, animals, fungi, protists, archaea, and bacteria. Life is a characteristic that distinguishes physical entities that have biological processes, such as signaling and self-sustaining processes, from those that do not, either because such functions have ceased (they have died) or because they never had such functions and are classified as inanimate.