With a discipline which translates, literally, to ‘study of life’, you’d think that biologists would have agreed upon a technical definition of ‘life’ way back when they first got started. It certainly helps to know what you’re studying. Unfortunately, like some other fundamental bits of biological terminology (‘species’, for example), the definition of ‘living’ has been subject to discussion and debate for a very long time.
Life, according to Princeton’s WordNet, is “the organic phenomenon that distinguishes living organisms from nonliving ones.” This is commonsense enough—but precisely what that ‘phenomenon” is requires some elaboration.
Some organisms are obviously very much alive, like this cheetah …
Before the advent of microscopes, defining life was a generally intuitive task. Animals are definitely alive, plants require a bit more inspection but it’s clear they’re living too—albeit generally in a less mobile way. When microscopes came onto the scene, they revealed a world of microscopic life that looks every much unlike our own, macroscopic world. Bacteria and single-celled organisms came to be understood as living things easily enough, though, thanks mostly to their reproduction and mobility. It was (and is) viruses that have become a sticking point of a modern biological definition of “life”.
… Some are not so obvious, like sponges.
Viruses have been, quite aptly, called “organisms at the edge of life”. Indeed, most modern scientists don’t consider them alive at all, instead preferring to refer to them as “replicators”. Viruses are little more than packets of DNA protected by proteins. They have genes, and they evolve via natural selection like most organisms, but they have no metabolism of their own. Even compared to single-celled bacteria, they are absurdly simple. So much so that scientists today hope that studying them may help reveal something off the earliest origins of life.
Bacteria are not so obviously ‘alive’ as macroscopic plants and animals.
Even today, biologists don’t have a concrete definition of life. Usually, definitions of life are broken down into a set of descriptive criteria, most of which need to be met for something to be considered alive. Some of the most established criteria, as enumerated by Wikipedia, are as follows:
Homeostasis: self-regulation of the internal environment. Humans regulate our internal body temperature, and single-celled organisms can alter things such as chemical concentrations inside the cell.
Organization: being structured, generally, of one or more cells. This is a criterion bacteria would fail.
Metabolism: being able to transform chemicals or energy into cellular materials and being able to decompose organic matter. You eat food, organic matter, which you break down into energy for your body. Plants, via photosynthesis, convert the energy from light into food for themselves.
Growth: this one is fairly self-explanatory, though it’s helpful to note that a living thing increases in size in all of its parts, not by simply accumulating matter. A termite is alive, but the mound which it ‘grows’ by adding more and more material to, does not.
Adaptation: the ability to change over time in response to the environment. This criterion is fundamental to the process of evolution.
Response to stimuli: this one is broad, with a wide range of examples. Plants, for instance, grow toward sunlight (phototropism); you pull your hand away from a flame if it burns you. But less obviously, a unicellular organism could respond to chemicals in its environment by contracting.
However, several alternate definitions have been proposed. They range from the absurdly technical to the elegantly simplistic: life as a network for negative feedbacks (i.e. regulatory mechanisms) which are subordinated to positive feedbacks (growth, reproduction) versus life as “a self-sustained chemical system capable of undergoing Darwinian evolution.”
Furthermore, Dr. Daniel E. Koshland Jr., of the Department of Molecular and Cell Biology at UC Berkeley in California, wrote an essay for Science in 2002 titled “The Seven Pillars of Life”. His ‘pillars’ are a Program, Improvisation, Compartmentalization, Energy, Regeneration, Adaptability, and Seclusion. You can read his full essay here, which does a better job of explaining his pillars than I could.
As we continue to learn more about the rest of the universe, and we explore alien planets in greater and greater detail, our definition of life will be very important. Should we ever discover life elsewhere, it may be even more challenging to our conception of living/nonliving than viruses.
FURTHER READING AND CITATIONS:
Ask a Scientist: Definition of Life
“The Seven Pillars of Life” (Science)
The Ecosystemic Life Hypothesis
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