All living organisms share several key characteristics or functions: order, sensitivity or response to the environment, reproduction, growth and development, regulation, homeostasis, and energy processing. When viewed together, these characteristics serve to define life.
NASA’s current working definition of life
is “a self-sustaining chemical system capable of Darwinian evolution.”
Szent-Györgyi, who was awarded the 1937 Nobel Prize in Physiology or Medicine for his discovery of the action of ascorbic acid:
“In my hunt for the secret of life, I started research in histology. Unsatisfied by the information that cellular morphology could give me about life, I turned to physiology. Finding physiology too complex I took up pharmacology. Still finding the situation too complicated I turned to bacteriology. But bacteria were even too complex, so I descended to the molecular level, studying chemistry and physical chemistry. After twenty years' work, I was led to conclude that to understand life we have to descend to the electronic level, and to the world of wave mechanics. But electrons are just electrons, and have no life at all. Evidently, on the way I lost life; it had run out between my fingers.”
Growth and development.
Permanence and change.
Daniel E. Koshland Jr. (University of California at Berkeley) formulated the “Seven Pillars of Life”. They are as follows:
1. A program
Autonomy is one important characteristic of life. But there are many others, including the following:
Reproduction. A living organism should be able to reproduce. However, some nonliving things, like crystals and bush fires, can reproduce, whereas viruses, which many people would regard as living, are unable to multiply on their own. Mules are certainly living, even though, being sterile, they cannot reproduce. A successful offspring is more than a mere facsimile of the original; it also includes a copy of the replication apparatus. To propagate their genes beyond the next generation, organisms must replicate the means of replication, as well as replicating the genes themselves.
Metabolism. To be considered as properly alive, an organism has to do something. Every organism processes chemicals through complicated sequences of reactions, and as a result garners energy to enable it to carry out tasks, such as movement and reproduction. This chemical processing and energy liberation is called metabolism. However, metabolism cannot be equated with life. Some micro-organisms can become completely dormant for long periods of time, with their vital functions shut down. We would be reluctant to pronounce them dead if it is possible for them to be revived.
Nutrition. This is closely related to metabolism. Seal up a living organism in a box for long enough and in due course it will cease to function and eventually die. Crucial to life is a continual throughput of matter and energy. For example, animals eat, plants photosynthesize. But a flow of matter and energy alone fails to capture the real business of life. The Great Red Spot of Jupiter is a fluid vortex sustained by a flow of matter and energy. Nobody suggests it is alive. In addition, it is not energy as such that life needs, but something like useful, or free, energy. More on this later.
Complexity. All known forms of life are amazingly complex. Even single-celled organisms such as bacteria are veritable beehives of activity involving millions of components. In part, it is this complexity that guarantees the unpredictability of organisms. On the other hand, a hurricane and a galaxy are also very complex. Hurricanes are notoriously unpredictable. Many nonliving physical systems are what scientists call chaotic -- their behavior is too complicated to predict, and may even be random.
Organization. Maybe it is not complexity per se that is significant, but organized complexity. The components of an organism must cooperate with each other or the organism will cease to function as a coherent unity. For example, a set of arteries and veins are not much use without a heart to pump blood through them. A pair of legs will offer little locomotive advantage if each leg moves on its own, without reference to the other. Even within individual cells the degree of cooperation is astonishing. Molecules don't simply career about haphazardly, but show all the hallmarks of a factory assembly line, with a high degree of specialization, a division of labor, and a command-and-control structure.
Growth and development. Individual organisms grow and ecosystems tend to spread (if conditions are right). But many nonliving things grow too (crystals, rust, clouds). A subtler yet altogether more significant property of living things, treated as a class, is development. The remarkable story of life on Earth is one of gradual evolutionary adaptation, as a result of variety and novelty. Variation is the key. It is replication combined with variation that leads to Darwinian evolution. We might consider turning the problem upside down and say: if it evolves in the way Darwin described, it lives.
Information content. In recent years scientists have stressed the analogy between living organisms and computers. Crucially, the information needed to replicate an organism is passed on in the genes from parent to offspring. So life is information technology writ small. But, again, information as such is not enough. Though there is information aplenty in the positions of the fallen leaves in a forest, it doesn't mean anything. To qualify for the description of living, information must be meaningful to the system that receives it: there must be a "context." In other words, the information must be specified. But where does this context itself come from, and how does a meaningful specification arise spontaneously in nature?
Hardware/software entanglement. As we shall see, all life of the sort found on Earth stems from a deal struck between two very different classes of molecules: nucleic acids and proteins. These groups complement each other in terms of their chemical properties, but the contract goes much deeper than that, to the very heart of what is meant by life. Nucleic acids store life's software; the proteins are the real workers and constitute the hardware. The two chemical realms can support each other only because there is a highly specific and refined communication channel between them mediated by a code, the so-called genetic code. This code, and the communication channel -- both advanced products of evolution -- have the effect of entangling the hardware and software aspects of life in a baffling and almost paradoxical manner.
Permanence and change. A further paradox of life concerns the strange conjunction of permanence and change. This ancient puzzle is sometimes referred to by philosophers as the problem of being versus becoming. The job of genes is to replicate, to conserve the genetic message. But without variation, adaptation is impossible and the genes will eventually get snuffed out: adapt or die is the Darwinian imperative. How do conservation and change coexist in one system? This contradiction lies at the heart of biology. Life flourishes on Earth because of the creative tension that exists between these conflicting demands; we still do not fully understand how the game is played out.
The cell's self-generated “electrome”: The biophysical essence of the immaterial dimension of Life?
Over 100 different definitions of “Life” have been published in the past.5 To my knowledge, none takes all presently known dimensions and types of organization of living systems into account in an unambiguous and holistic way as in the definition that I logically deduced.5,6 It meets all criteria a good definition of Life should meet according the philosophers of science Schetjer and Agassi.10 In this deduction opposing the situations “still alive” vs. “just not alive any longer,” instead of by following the classical procedure of comparing the properties of living- versus non-living matter, yielded the view that communication activity is what “Life” is all about. What we call “Life” is nothing other than the total sum of all acts of communication exerted by a given sender-receiver compartment at moment t, at all levels of its compartmental organization (cell organelle, cell, tissue,…, whole organism,…, population, community, Gaia level).
Any biological compartment, whatever its degree of complexity dies at the very moment that it irreversibly (to exclude regeneration) loses its ability to communicate at its highest level of compartmental organization. It can be shown by electrophysiological methods that a cell is dead from the moment that its voltage gradient over its plasma membrane is irreversibly lost. “Death” refers to a particular level of compartmental organization, namely the highest one. In the case of e.g. a vertebrate, the brain is the highest (coordinating) level of communication. A brain-dead person is no longer a person, but a corpse which is an aggregate of cells and tissues. The transition from “still alive” to “no longer alive” involves a drastic change, namely a total and irreversible collapse of the communication activity (= handling of information) at the highest level of compartmental/communicational organization of the dying entity under consideration.
Last edited by Otangelo on Sun Feb 07, 2021 8:04 am; edited 13 times in total