What is life?

There are several definitions of life, all of them with problems, because it is like defining "green" on a spectrum: What you end up with are threshold definitions, which end up being subjective. When is it considered dead? The fetus forms from living cells, and again, what is considered an important stage in the development from zygote to independent organism is a matter of subjective definition.In humans children aren't fully capable of independent support until 10 years old or so - about the time they become able to reproduce.In bacteria, they reproduce by budding\division and as soon as the cell divides into two daughter cells the new individual(s?) are "born" -- and this process is similar to how the cells in multicellular life form from the original zygote, but when does that division and multiplication form something we identify as - in the above example - human? Such cell division and multiplication goes on throughout your life, so when are you complete?wikipedia: life

quote:

---

Life is a characteristic of organisms that exhibit certain biological processes such as chemical reactions or other events that results in a transformation. Living organisms are capable of growth and reproduction, some can communicate and many can adapt to their environment through changes originating internally.[1] A physical characteristic of life is that it feeds on negative entropy.[2][3] In more detail, according to physicists such as John Bernal, Erwin Schrdinger, Eugene Wigner, and John Avery, life is a member of the class of phenomena which are open or continuous systems able to decrease their internal entropy at the expense of substances or free energy taken in from the environment and subsequently rejected in a degraded form (see: entropy and life).[4][5]

​

An entity with the above properties is considered to be a living organism, hence, a 'life form'. However, not every definition of life considers all of these properties to be essential. For example, the capacity for evolution is sometimes taken as the only essential property of life; this definition notably includes viruses, which do not qualify under narrower definitions as they are acellular and do not metabolize.

---

It's a spectrum from naturally occurring chemical reactions to organisms we recognize as life.You could say life uses self-replicating molecules to reproduce and evolve, and it feeds on negative entropy. It may not have been a "cell" as we know it. Perhaps it was just self-replicating molecules in a lipid bubble.http://scienceweek.com/2005/sw050325-1.htm

quote:

---

The following points are made by Ers Szathmary (Nature 2005 433:469):

​

1) In investigating the origin of life and the simplest possible life forms, one needs to enquire about the composition and working of a minimal cell that has some form of metabolism, genetic replication from a template, and boundary (membrane) production.
...
3) Basically, there are two approaches to the "minimal cell": the top-down and the bottom-up. The top-down approach aims at simplifying existing small organisms, possibly arriving at a minimal genome. ...
...
6) The bottom-up approach aims at constructing artificial chemical supersystems that could be considered alive....

---

Another concept to throw in the pot is LUCA - the last universal common ancestor (or last universal ancestor)wikipedia: LUCA

quote:

---

The last universal ancestor (LUA, also called the last universal common ancestor, LUCA, the cenancestor or "number one" in slang) is the most recent organism from which all organisms now living on Earth descend. Thus it is the most recent common ancestor (MRCA) of all current life on Earth. The LUA is estimated to have lived some 3.5 to 3.8 billion years ago (sometime in the Paleoarchean era).[1][2]

​

It is possible that all of LUA's contemporaries became extinct and only LUA's genetic heritage lived to this day. Or, as proposed by Carl Woese, perhaps no individual organism can be considered a LUA, but the genetic heritage of all modern organisms derived through horizontal gene transfer among an ancient community of organisms.[7] Another hypothesis to explain the paucity of alternative life forms is panspermia, the inoculation of Earth by life carried on meteorites.

---

Of course panspermia doesn't answer the question of when life began, it just moves it to another location.The theory I personally prefer is that some large pre-biotic molecules (see PAH's) form in space - part of the export of materials from stars\novas - and that these combine to form life where conditions are favorable, such as on early earth. A partial panspermia theory.See also Carl Woese and the "RNA World" theory.Enjoy.

---

we are limited in our ability to understand
by our ability to understand
Rebel American Zen Deist
... to learn ... to think ... to live ... to laugh ...
to share.

---

• • • Join the effort to solve medical problems, AIDS/HIV, Cancer and more with Team EvC! (click) • • •

Let's not confuse the issue. Actually, doctors and lawyers have already addressed this issue - see Legal Death, Legal Life - we have laws that deal with this issue, driven by the issue of organ transplants:

quote:

---

The real question is when does this continuum of life begin to be a distinct living breathing heart thumping thinking human being. On common moral grounds, it is important to be consistent at both ends of the spectrum of life. Thus the concept of beginning needs to be consistent with current medical practice in determining when a human life has ended. This criteria has been developed over a significant period of time with a lot of ethical input from all sides into the specific ethical considerations involved.

​

Legal Death

​

The legal standard of death is very clear - from
the Legal Definition of Death (click):

UNIFORM DETERMINATION OF DEATH ACT
1. [Determination of Death.] An individual who has sustained either
(1) irreversible cessation of circulatory and respiratory functions, or
(2) irreversible cessation of all functions of the entire brain, including the brain stem, are dead.
A determination of death must be made in accordance with accepted medical standards.

​

That's the legal nuts and bolts of it: either failure of {heart\lung} system or total brain failure. Any person with either of these failures is universally and legally considered to be dead.
The word "irreversible" is used to refer to common medical practical limits to resuscitation.

---

If you want to discuss the issue of when a human life begins, I suggest addressing it on the Legal Death, Legal Life thread. That way we can stick to just the issue of the original beginning life forms and what is necessary to form life from non-life. Exactly, so the question is what you need to do to kill a bacterium while leaving all the parts intact and undamaged ...?Would you not agree that bringing a bacterium back to life would be similar to assembling the necessary parts to form life? Perhaps if we take a bacterium and remove parts without killing it, reducing it down to an irreducible minimum to remain alive, then we can get a top down impression of the threshold for the beginning of life. A bud is still part of the - living - parent organism, they become - living - individuals when they become separated, but they still form a continuum of life from the original population. The question is how that original population formed.I'd say the minimal requirements are:

• Self-replicating molecules
• Consumes energy and materials to form new molecules
• Held together into a consistent unit by some membrane
• Divides into multiple membranes containing self-replicating molecules
• Replication and division are imperfect, thus allowing selection and evolution to operate

See (you can fast forward to ~minute 3.45 to get past the politics)This makes and argument from the bottom up -- so we have narrowed the focus on the spectrum to a (hypothetical) set of chemicals to a (hypothetical) minimal bacterium.The definition offered by Dr Adequate (Message 8) would be a "bottom up" definition, achieving the threshold of self-replication of molecules. Surface catalysis has been demonstrated using clay substrates, and this also seems to have a bias to using one-handed molecules (one of the curiosities of the issue)There is also a discussion of the various definitions from Joseph Morales.Enjoy.

---

we are limited in our ability to understand
by our ability to understand
Rebel American Zen Deist
... to learn ... to think ... to live ... to laugh ...
to share.

---

• • • Join the effort to solve medical problems, AIDS/HIV, Cancer and more with Team EvC! (click) • • •

Thanks. I thought Morales covered all the problems with every known definition, showing that there is no definition that does not run into problems with distinguishing "life" from "non-life". What we have are subjective "operating" definitions that depend on our world views and consensus with other people, we just can't refine this down into words that apply in all cases.What we have is a pick list of traits that we observe in living organisms, such as reproduction, evolution and net consumption of energy, and that once sufficient degrees of enough of these traits are observed we can judge whether our personal subjective threshold has been crossed, but it is done on a case by case basis.Enjoy.

---

we are limited in our ability to understand
by our ability to understand
Rebel American Zen Deist
... to learn ... to think ... to live ... to laugh ...
to share.

---

• • • Join the effort to solve medical problems, AIDS/HIV, Cancer and more with Team EvC! (click) • • •

Thread title: What is Life? Well we experience life and non-life, and there does seem to be an animating energy, but we have not yet identified it. Certainly there is energy transfer between cells via ATP iirc that powers cell reactions and actions (muscles etc).Is life just a synergy between active cells that breaks down causing a cascade effect at death? Certainly the Uniform Code of Death means death can be declared while certain organs are still functional and can be transplanted to living humans.Edited by RAZD, : added