Here is a proposition: Life is the phenomena of the emergence in a specific environment providing free energy and material of a self-sustained chemical system that reproduce and which over long period of time will evolve a diversity of forms which will gradually spread over larger and larger ecological niches and which will gradually channel a large proportion of the available free energy of the planet.

In the context of astrobiology, NASA's Working Definition of Life may be a very general but appropriate formulation: “Life is a self-sustaining system capable of Darwinian evolution.”

Found in "An Introduction to Astrobiology" by Iain Gilmour and Mark A. Sephton (Cambridge University Press) but also at this website --> http://www.astrobio.net/interview/5580/forming-a-definition-for-life-interview-with-gerald-joyce

While at first look that seems a question rather easy to answer, it happens that we lack a complete and self-consistent answer to it. We are, perhaps, lacking some fundamental knowledge that would allow us to give a sound definition (much as the lack of the concept of atoms impeded a rigorous definition of what water is). In any case, if reproduction or Darwinism is involved, the mule would be a non-living entity. I highly recommend the book by Pier Luigi Luisi "The Emergence of Life", for me, it was truly enlightening.

Although we may not be able to define what is life, it does not mean that we do not tacitly (Polanyi's tacit knowledge) know what is life. If it is the case then we would be able to recognize it under totally different material clothing. Maybe all intelligent forms of life in the Universe have this capacity in common.

I agree that we can tacitly recognize life even if extremely different to terrestrial. Some trends, as complexity, self-organization, metabolism... must be present and would a tell-tale about its nature

Dear Louis, Jordi, Joachim,

when it comes to defining life, simple litmus tests can be used to test any definition. For example to test the suggestion that life requires reproduction, one can simply imagine a sterile organism (which complies with the property of life). As a sterile organism cannot reproduce, and as a sterile organism is a living being, reproduction cannot be an obligatory aspect of a definition of an entity which complies with the concept of life.

And to test the properties of metabolism, mobility, energy degradation, reactivity to stimuli, etc. one can use as a test a frozen or dried bacterium. Any frozen or dried bacterium will not show these properties, which nevertheless are suggested as relevant for a definition of life. Yet, while being a frozen/dried system, the bacterium still possesses the property of life (it will 'live' again when thawed/wetted). Again such a simple mental play can be used as a litmus test, and will show that these criteria are not obligatory for a definition of life.

In fact, also the subject of evolution (definition of NASA) seems to be vulnerable to a simple litmus test. For example a sterile cat cannot produce offspring, and will therefore never involve in any evolution process. Yet the cat is a living being. The confusion about the role of evolution may arise from the fact that while most organisms result from reproduction, they may not necessarly have to be the result of evolution.

In relation to the above and other litmus tests, I have recently, and after 20 years of study and passion for the subject, suggested a new approach for a systems-definition of the concepts of 1. a living being, and 2. the abstract concept of life. Please be invited to find more information about this (as well as my papers on this subject) at: http://the-operator-theory.wikispaces.com/Definition+of+life%2C+the+organism%2C+and+death

My most recent points of view on the definition of life have been discussed in three chapters of my recent book: The Pursuit of complexity.

I additionally discuss the topic on the talk page of Wikipedia, because the definition of Wikipedia can be considered as to shows circularity, and has problems with the limus test game. It is quite interestingto see that such a fundamental concept in biology is treated on wikipedia in the way it is now. A circular reasoning is something that in philosophy would lead to serious discussions.

Kind regards, Gerard Jagers op Akkerhuis

Dear Gerard,

I do not find that the existence of a sterile individual within a species is really a problem for including reproduction into a requirement for life. There is no species that is completely sterile. If it happens, this species will soon get extincted. To render some individual sterile or unwilling to reproduce may even be an advantage for a population in period of starvation. So reproduction is not a individual requirement for an organism to be alife but it is collective requirement for a population of living organisms.

The frozen or dried bacteria is not into a active living state. When it is frozen, its living process is suspended and it is not living during that period. Living is not a thing but a process, if the process is suspended or temporarilty stopped, then the organism is not living during that period. This is not an argument against using metabolism as a requirement for a living process.

Regarding the issue of reproduction, or any other absolute atribute of life, I remember an anecdote of Daniel Koshland; he once told that in a meeting of very eminent scientists that gathered in an attempt to build a definition of life, one of them pointed reproduction as a universal property of life; initially everyone accepted that claim, until another guy said: "then, one rabbit is dead, but two of opposite gender are alive". The idea that Koshland tried to transmit with that story is that though life is undoubtedly something very intuitive to us, define it is very difficult. Another issue is the mistaking between an adjective (alive) and a noun (life), the latter encompassing a much more abstract idea of the phenomenon, closer to a definition, and the former just being a case of the phenomenon.

And regarding NASA's definition, it is tricky, because evolution is a process, not a tangible entity susceptible to a punctual measurement; in earth we have enough geological, fossil and current elements to infer evolution, but in another world, it is unlikely, so perhaps we should seek for properties or indicators that could not be occurring without an underlying evolutive process, which is also extremely hard to determine. Remember Manfred Eigen's hypercycles, chemical systems capable of self-sustainment, and capable of evolution through time (cuasi-species), but whom of us would categorically claim that such system is alive?

Perhaps there is no discrete property or atribute to distinguish between living and non-living matter, and it should be that way because our distinction between living and non-living things is based merely on intuition and empathy for what we are and what we know. Anyway, scientific community is going to have to build at least a working definition, accurate enough at least to permit us to explore possible living systems without the problems of detection and result interpretation from previous cases, case in point: viking mission expermients on mars.

I also recommend you to read this essay on this topic, very enlightening

http://blogs.scientificamerican.com/brainwaves/2013/12/02/why-life-does-not-really-exist/

Daniel,

It is a good article. I agree as most people would agree that we are not able to define life. I even agree with a minority that think that even when we will have a good abiogenesis theory, we will not even be able to define life. But I disagree that what cannot be defined does not exist. I love my children, I am sure we will never define what love is and I am sure it exists.

I agree with you, the header of the article is way too radical. The material phenomenon of what we call life does exist, independent of if it does have a differential and universal property that differentiate it from non living matter, or if we are able to distinguish it. At the end, it does not matter if the way we see and name the world has formal failures or is based in an intuitive logic, Chesterton once said that we cannot find sense in something with logic if we have not already found it without logic, and so, as a matter of fact, the formal study of the definition of life came after the intrinsic belief of its existsnce, the same with love. There are some things that are inherent to our perception of the world, our fundamental biases, and that's ok. Nonetheless, I am stil intrigued about a working definition for the search of extraterrestrial life, or perhaps the solution is to send people and they will tell us if something is alive or not hehehe (joke...or not?)

Daniel,

In a few years, we will have the technology to observe directly the surface of earth-like planet in the hospitable zone of other star in this galaxy. I am confident that we will observe on some of these planet : forests and multiple other direct evidences such as as chemistry of the atmosphere. I do not think we will need a definition of life to draw the obvious conclusions.

What kind of technology are you referring to, Louis?

There are a lot of research on future astronomical instrument for the direct observation of exoplanets. Just one paper on the subject:

http://oro.open.ac.uk/24945/1/

which conclude with:

''Within a period of approximately 200 years, it can reasonably

be expected that high-resolution spectroscopy and

then high-angular direct imaging will improve considerably

our knowledge of nearby exoplanets and possible global

biomarkers. For the latter approach, large interferometers

will be inevitable. The highly desirable next step would be

direct visual observation of the morphology of life-forms on

these planets, should such life-forms exist.''

SIM ~ 2015?-? ~ SIM, the "Space Interferometry Mission," was originally supposed to be launched in 2005. Due to severe budget cuts, it has been delayed to "no earlier" than 2015 or 2016. Assuming it is actually built and launched, the craft is supposed to use optical interferometry to accomplish the goal of direct imaging of exoplanets. It has the additional goal of constructing a map of our Galaxy.

The Gemini Planet Imager (GPI), is perhaps the most ambitious of the systems

currently being developed, employing, like Project 1640, an

apodized pupil Lyot coronagraph for starlight suppression.

The goals of the project are to provide images and spectra

of roughly 100–200 young planets around nearby stars,

with a similar field of view as Project 1640. First light is

expected in 2011.

The TPFI/Darwin concept is currently expected to be the last to be

flown, after a TPF-C or similar type is flown, owing to the

perceived complexity and cost of a system that requires five

free-flying spacecraft, all at cryogenic temperatures.

These large-aperture telescopes would be designed to image

planets as small as an Earth-twin, unless the majority of

stars have zodiacal disks that are 10–100 times thicker than

our own solar system’s zodiacal dust. These devices, which

in principle could be constructed now and launched within

a few years, could begin to tackle the question of life in

other planetary systems. The key to this is the detection of

chemical abundances in a planet’s atmosphere that are not

consistent with thermochemical equilibrium and could only

be generated by biological activity.

Thank you very much, Louis, for that great information. I hope we get to see at least some of the discoveries of these technologies. And, undoubtedly, direct observation will have the upperhand in life forms detection.

Daniel,

Since the ancient atomists, the idea that the earth and the solar system had been formed from an original nebula was known. Descartes and Kant, Laplace have created modern versions of this ancient nebula hypothesis. What is implicit in this hypothesis is that life had to emerge from the inanimate matter on this planet and so that all forms of life had evolved. Even the idea of natural selection had been put forward by ancient atomists and re-invented by Maupertuis. But most of these early philosopher thought that as soon as the earth and water had cool down life not only emerge immediately but emerge constantly from inanimated matter. Maybe they are right. Maybe we will find experimental evidence that some very very primitive forms of life are evolving right now in certain niche deep down of the oceant around thermal vents or deep down in the rock out of nuclear decay processes. Finding such evidences would also be a tremendous discovery. Finding life elsewhere in the solar system is also a possibility that we are presently exploring and that will take several decade to explore. Communication with other intelligent form of life is probably the most exciting search that would be the biggest scientific discovery of all time. My personal hope is based on the speculation that it may be possible to communicate instantly accross the universe though the vacuum at very high energy level. Do not ask me to produce the physics of this speculation. The reasoning is the following. Quantum mechanics have established that some physical phenomena are not constainted by the speed of light. In particle accelerator we experience the world at higher energy level, a world that is similar to the whole universe in the far past when the universe was dense and less differentiated and in a sense similar to the vaccuum. Maybe, a big maybe, the vacuum is a global reality of the whole universe and it is possible at very very high energy level to send and receive global messages and that such global communication medium already very busy carrying communication among million of advance civilisation accross the universe right now. It is only up to us to tune in when we will know how and then we will do our first broadcast: ''Hello everybody my name is Gaia''.

The terrestrial genetic code existed even before the earth scenario. The code existed even before of what we can consider life, or living matter. Here I share with you a recent paper of my own authorship, where it demonstrates that the probability of life tends to zero than to one and it includes more details about similar topics.

Article Evolution through the stochastic dyadic Cantor Set: the uniq...

Dear Daniel, 

I address you to our (with Elena Kadyshevich) Life Origination Hydrate Theory (LOH-Theory). The definition of the phenomenon of life and the history of this definition is detailed by us in  "Generalized hypothesis of the origin of the living-matter

simplest elements, transformation of the Archean atmosphere, and the formation of methane-hydrate deposits", Physics-Uspekhi 50 (2) 175-196 (2007). The problems of living matter origination are considered by us in a number of papers, the last of wich are published in "Life (Basel)", 2012; "J. Molecular Evolution", 2014; "Mini-Rev. in Oraganic Chemistry", 2015; "Chirality", 2016. I also recommend you the paper in "Global J. Science Frontier Res.", 2012, where new ideas which require the subsequent development are specified. All these papers are available in my and Elena Kadyshevich pages at the ResearchGate site:  https://www.researchgate.net/profile/Victor_Ostrovskii  https://www.researchgate.net/profile/Elena_Kadyshevich 

Defining th phenomena of life and death  

As of today it has not been possible to define and explain the phenomena of life and death in biology – the science of life. For nearly six decades now, biologists have been promoting molecular gene (DNA) as the blueprint of life. The world now believes that it is a chemical molecule (DNA) that encodes genetic program responsible for the development, heritable traits and behaviour of an organism. According to the current molecular (material) concept, a gene is a stretch of DNA that encodes a functional product, a single polypeptide chain or RNA molecule. The entire collection of genes is the genome that is supposed to constitute the genetic program of the organism.

The molecular gene concept is chemically untenable for three simple reasons.

·         The chemical structure of a substance encodes chemical information that is responsible for its physical and chemical characteristics, and behavior. By proposing that genetic program is encoded by the DNA structure, scientists are superimposing biological information over chemical information. How can a structure encode two different kinds of information?

·         Static chemical structure cannot encode dynamic biological (genetic) program.

·         A body soon after death is materially (chemically) identical to its living counterpart. The DNA in the dead body is intact. If DNA encodes the genetic program how can the body die when the biological information (DNA) is intact?

It is high time biologists realized that the molecular gene concept is wrong.   

In 1909, Danish botanist Wilhelm Johannsen introduced the notions of “genotype” and “phenotype”. In addition, he proposed the term “gene” for the elements of the genotype. Johannsen had reservations with respect to gene’s particulate nature and had also warned against assuming genes for a particular character [Johannsen, W. 1911. The genotype conception of heredity. The American Naturalist 45, 129-159]. Thus the gene originally proposed was a nonphysical (nonmaterial) entity. The nonphysical gene agrees well with the Scriptural revelation of the invisible or intangible soul – rooh or nafs mentioned in the Quran (Q. 15:26-29, 6:93) and ‘Spirit’ mentioned in the Bible (Genesis 2:20-23) – as the cause of life. The nonmaterial genetic information can be explained in the light of computer model of the organism.

A computer or a robot has basically two components namely, the visible hardware and the invisible (nonphysical) software. The nonphysical software of computer or robot stored on its hard disk may be considered as its soul that determines its functioning. Similarly, an organism is natural biocomputer or biorobot. It also has visible hardware and invisible software. The cell, the basic unit of a living system, is a biochip. The chemical structures in the cell (organelles and nuclear structures including DNA), tissues and organs at the level of the organism constitute the hardware (phenotype). The development and functioning of the hardware are determined by the biosoftware (nonphysical genetic program). The biosoftware is not coded in the form of a molecule. It has no chemical structure or visible features and is comparable with a computer program stored on a physical medium (e.g., hard disk). The storage device for biological information (nonphysical genetic program or the soul) in the cell is chromosome. Chromosome is the hard disk of organism. It is the removal (or in computer parlance ‘deletion’) of biosoftware (nafs) from the biosystem that results in death (Q. 6:93). That is, the biosoftware is irreversibly lost from the body at death. A dead body is therefore like a computer without software. This will explain why life cannot be created from chemicals (without involving a living organism), why a dead cell cannot be cultured and why it cannot be  restored to life. Accordingly, the phenomenon of life can be defined as the manifestations of the execution of the biosoftware in the phenotype, and death as the loss of biosoftware from the body cells. Computer, robot, etc. that run on man-made programs can be considered as forms of ‘artificial life’. (For more details, visit www.islamicscience.in). 

It is impossible to create life from nonlife (chemical molecules). If that is possible we should be able to restore life to a dead cell through chemical means. We need not go for producing prosthetic cell through chemical means to conduct that experiment. A dead cell at the instant death occurs is prosthetic cell. It has all the material constituents including genome (DNA) except life. The proposed theory of nonphysical biological program is falsifiable through restoration of life to a dead cell chemically without involving a living cell.  

P. A. Wahid,

''It is impossible to create life from nonlife (chemical molecules).''

Life on this planet and most likely in billion of billion of other planets has to emerge from chemical processes.  We do not understand how this happen but it is obvious that this has happen on this planet which at some point had no life. 

"Life on this planet and most likely in billion of billion of other planets has to emerge from chemical processes.  We do not understand how this happen but it is obvious that this has happen on this planet which at some point had no life."  

The general notion that life emerged on this planet from nonlife is wrong. Life emerged on Earth with the appearance of divine biological information (please see the attached paper). A living cell or body is not just a chemical substance; it also contains nonphysical biosoftware stored on the chromosome. One can compare a living organism with its artificial counterpart, say a robot. It has the material hardware and nonmaterial software stored on its disk. Likewise, the living organism has material hardware or the so-called phenotype and nonmaterial divine biosoftware stored on its chromosomes, the hard disk. The chemical structure of a substance encodes divine chemical information, which is responsible for its physical and chemical properties and behaviour. 

We will never be able to create life by chemical means without involving a living organism at any stage because DNA or any other chemical structure does not encode biological information. It is the deletion of the nonphysical biosoftware from the chromosomes that causes death of the cell. If life can be  created from nonlife, the simplest method to prove it is by restoring life of a dead cell chemically.

The failure of all human efforts to create life from nonlife will be the ultimate scientific proof for the existence of God. Biology is poised to prove God sooner or later.