Published April 27, 2015-Updated March 15, 2018

The study of life is the domain of biology as a natural discipline. Biology examines all living organisms, and focuses on structure, function, growth, evolution, distribution, as well as the taxonomy of life forms. Life itself is hard to define, because there is such an extraordinary diversity in its manifestations. It is more like a process, rather than a substance or state. Is it possible to develop a definition for life? Is there something that is common to all life? A good definition must be broad enough to encompass all known life forms, as well as leave room for the forms of life that we probably find elsewhere in the universe. What about precursor or hybrid forms of life, like viruses? And what about artificial life? Furthermore, many properties of living things (for example, movement, orderliness, or energy use) can also occur in non-living things. Wind moves, crystals grow, and fire uses energy, but we do not consider these processes to be alive.


Life developed early in the history of the planet; it is worthwhile to review the timeframes.

  • The universe is approximately 13.7 billion years old; we currently believe that it had a starting point, the “Big Bang.”

  • Our sun is about 5 billion years old. The earth, which was formed from an accumulation of cosmic dust, is around 4.6 billion years old.

  • The early earth, shortly after its formation 4.6 billion years ago, was a strange planet that could not have supported life by any definition we might use today.

  • However, 3.5 billion years ago (approximately 1 billion years after its formation), the earth was teeming with life in the form of organisms that resemble modern-day bacteria. This development is extremely fast, especially considering that the molten Earth needed half a billion years to cool enough to form a solid rock surface.

  • Some biologists now think that there is evidence for life even earlier, at about 4 billion years ago.

Most scientists today believe that life arose spontaneously from non-living matter. How exactly this happens is still a mystery, and even today many religious people believe that life could only have come into existence through an act of creation, based on a supreme being of some kind. Other scientists defend the “panspermia hypothesis,” which claims that life on earth arrived from other planets. Both of these alternatives circumvent the question of how living matter can emerge from non-living matter.

Building Blocks

The most fundamental property of life is a dependence on organic chemistry, a chemistry based on organic compounds (molecules built around carbon). There are four general classes of organic compounds:

  1. Amino acids: the building blocks of proteins.

  2. Nucleic acids: they include DNA and RNA.

  3. Carbohydrates: this class includes sugar, starch, and cellulose (wood fiber).

  4. Lipids: fats and hormones.

Organic compounds have complex properties. The first step toward the origin of life must involve the formation of organic compounds from inorganic ones. However, organic compounds are generally produced only by living things, and as far as we know, the early earth was entirely inorganic. In order to solve this problem, experimental science had to step in. A now-classic experiment by Stanley Miller in 1953 demonstrated that it is possible and relatively easy to synthesize many organic compounds under conditions simulating those thought to have existed on the early earth. Once these organic compounds are present, they can interact and form the building blocks of life. Living systems have the characteristic that they organize in a hierarchical fashion, from molecules to cells to organisms to communities of species to ecosystems.

Functions and Principles

Something is considered to be alive when it exhibits all or most of the following traits:

  1. Homeostasis: Regulation of the internal environment to maintain a constant state; for example, electrolyte concentration or sweating to reduce temperature.

  2. Organization: Being structurally composed of one or more cells — the basic units of life.

  3. Metabolism: Transformation of energy by converting chemicals and energy into cellular components (anabolism) and decomposing organic matter (catabolism). Living things require energy to maintain internal organization (homeostasis) and to produce the other phenomena associated with life.

  4. Growth: Maintenance of a higher rate of anabolism than catabolism. A growing organism increases in size in all of its parts, rather than simply accumulating matter.

  5. Adaptation: The ability to change over time in response to the environment. This ability is fundamental to the process of evolution and is determined by the organism’s heredity, diet, and other external factors.

  6. Response to stimuli: A response can take many forms, from the contraction of a unicellular organism to external chemicals, to complex reactions involving all the senses of multicellular organisms. A response is often expressed by motion; for example, the leaves of a plant turning toward the sun (phototropism), and chemotaxis.

  7. Reproduction: The ability to produce new individual organisms, either asexually from a single parent organism, or sexually from two parent organisms.

These complex processes are called physiological functions. They have underlying physical and chemical bases, and they consist of signaling and control mechanisms that are essential to maintaining life. These seven traits can be unified into three principles that operate across all levels of organization and form a useful way to look at systems as a whole.

  1. Information and Evolution: This principle explores how the structure and organization of living things is encoded in the DNA molecule, how this information is transmitted and modified, and the implications of these processes for understanding life at all scales of organization.

  2. Development and Homeostasis: The second principle considers two related issues for understanding the workings of complex organisms: how single cells (i.e., fertilized eggs) proliferate and transform into complex, multicellular organisms and how the various parts of complex organisms remain coordinated and maintain their integrity in the face of various challenges.

  3. Energy and Resources: This principle explains how living systems obtain the energy and other materials needed to maintain their highly ordered state and the implications of these processes for understanding the organization of biology at all levels of scale. This principle is especially interesting because it dictates the structure of all levels of organization.


Viruses have genes, but no cell structure. They are genetic replicators rather than forms of life. They are mechanisms at the edge of life: they possess genes, evolve by natural selection, and replicate by creating multiple copies of themselves through self-assembly. They have no metabolism and they require a host cell to make new versions of themselves. The existence of viruses indicates that life could have started as a process by which organic molecules begin to assemble themselves.

Philosophical Approaches to the Definition of Life

There have been three main philosophical approaches to the problem of defining life:

  1. Life as a property of nature: Aristotle, for instance, views life as animation. As such, it is an irreducible property of nature and does not need any further definition.

  2. Life as a mechanism, a biochemical process: This kind of thinking starts with Descartes and his distinction between res extensa and res cogitans: body and mind are separate dimensions; therefore we can study life as if it were merely a material process.

  3. Life as organization. This approach looks at larger systems that encompass individual life forms and how they emerge. It reflects Darwin’s concept of variation and evolution through natural selection, and it also considers the idea of life as an emergent property of some complex systems.

As we enter a new era of space travel and artificial intelligence, the search for a better understanding of life will most likely transform our existing approaches. Future research will integrate a view of organisms and their action with evolutionary theory and complex systems theory. We are still far from understanding how life can emerge from inorganic matter, and in how many forms this emergence can occur. In his book, “The Way of the Cell,” Franklin Harold suggests a definition of life that is useful to capture many aspects of life, and yet broad enough for future discoveries.

Living organisms are autopoietic systems: self-constructing, self-maintaining, energy-transducing autocatalytic entities in which information needed to construct the next generation of organisms is stabilized in nucleic acids that replicate within the context of whole cells and work with other developmental resources during the life-cycles of organisms, but they are also systems capable of evolving by variation and natural selection: self-reproducing entities, whose forms and functions are adapted to their environment and reflect the composition and history of an ecosystem.”

Harold, F.M., 2001. The Way of the Cell: Molecules, Organisms and the Order of Life, New York: Oxford University Press. Page 232.

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