Why aren't microspheres and protocells living beings?

[AnkurGarg]

Why aren't microspheres and protocells considered as living beings? What I know is that they do reproduce and exhibit metabolism.. so they fulfill the basic criteria of living systems.

I am really confused about it!

 

[atyy]

There isn't a sharp division between life and non-life, so it's mostly a matter of convention.

 

[stedwards]

I define life as a self similar replicating system. Neither protocells nor microspheres fit the bill, but crystals and viruses do.

 

[AnkurGarg]

But the Protocells do replicate,adapt,mutate,organize etc..They have all the features of a living cell?

 

[stedwards]

But the Protocells do replicate,adapt,mutate,organize etc..They have all the features of a living cell?

Facinating! I needed to read a bit more closely. They're apparently still theoretical but provide what seems a plausible path to the origin of organic life.

If you had the RNA without the membrane, the RNA would become dispersed. However if there were a single self-replicating strand this wouldn't be a problem; no membrane required. Is there a reason a single strand wouldn't work?

Also, I've been trying to tweak my own definition of life to include mules and such--in fact, any of the many branches of the evolutionary tree that have terminated. Any idea?

 

[AnkurGarg]

Facinating! I needed to read a bit more closely. They're apparently still theoretical but provide what seems a plausible path to the origin of organic life.

If you had the RNA without the membrane, the RNA would become dispersed. However if there were a single self-replicating strand this wouldn't be a problem; no membrane required. Is there a reason a single strand wouldn't work?

Also, I've been trying to tweak my own definition of life to include mules and such--in fact, any of the many branches of the evolutionary tree that have terminated. Any idea?

OMG ! Either you are talking about some high standard biology beyond my scope..or there's something wrong with your English!:P Please explain in simple words what you are saying...

Regards.

 

[stedwards]

Probably some of both. ;) Looking over what I've written, by "strand" I meant a strand of RNA. By "cellular membrane" I meant the envelope which contains the RNA. Concerning protocells, I read this article. It looks like ideas concerning the origins or organic life have improved in the last couple decades by one idea. One very good one!

RNA is really what makes things happen in cells. The DNA is the hardcopy. Everything else is up to the RNA. The RNA builds DNA, transcribes DNA and makes proteins. (I once had to take a test in this stuff.)

By the way, I see I didn't address your repeated query:-


Most all people want to see a minimum level of complexity before they define something as life. As such, some will, but some won't include viral particles as life as they are so simple. (The assumed level of simplicity of viruses as a replicating system is myopic, but that's another issue.)

For instance, how big does a plant have to be before you wouldn't call it a bush, but a tree?

It's a fuzzy distinction as Atyy pointed out. Everyone has a tendency to draw the distinction differently. To set a lower bar, to exclude simple systems such as crystals, they invent requirements of "metabolism" and "respiration", and other stuff, as defining qualities of life. They're just confused.

 

[Torbjorn_L]

There are several different issues here:

Q1 Whether or not simple self-replicating systems can be considered life.
Q2 Whether or not it helps making a testable "definition" of life.
Q3 Whether or not life emerged as protocells.

All these Qs are non-trivial, so there isn't any consensus on this.

To take Q2 first, many astrobiologists find it unnecessary to define "life". McKay find 3 senses of definitions, none of them really helpful:
"What is life? This is a question that is often asked and typically confused.

The confusion starts from the several uses of the word “life” in English. There are at least three usages as exemplified by the following questions:

1) Is there life on Mars?

2) Is there life in this organism?

3) Is life worth living?

The definition of “life” in these three usages is quite different. In the first case, life refers to a collective phenomenon, in the second case it refers to the ability of an individual organism to metabolize and grow, and in the third case life refers to the history of activities that an organism undertakes. The first two usages are of direct relevance to astrobiology."

"Many commentators hold the view that an effective search for life on other worlds requires that we first have a concise, agreed on, definition of life. This is not the case. Along this line, it has been suggested that once we understand life we will be able to produce a completely mechanistic and predictive theory of life. The example of water is sometimes used. Water is simply defined as two hydrogens joined with one oxygen. However, life is not a simple substance like water, rather it is a process, more like fire than water. There is no simple definition of fire. If life is like fire then even with a complete mechanistic and predictive theory of life we may still not be able to define it in any simple closed form. The search for life on other worlds can be based on what life does rather that its definition. One of the things that life does is build up large specialized molecules, such as DNA and proteins."

[ http://www.astrobio.net/news-exclusive/life-tricky-often-confusing-question/#sthash.8RvkQ7QK.dpuf ]

My own opinion here is that when people drag out the NASA definition (metabolizing self-replicator) they often, if not always, do so to try to identify an individual organism as a motivation to define life. (Say, how to quickly identify any extant cells on Mars.) But it isn't a good motivation, evolution as the process of life acts on whole living populations, so populations (of sets of alleles, in modern cells) are the vehicles that does that life do. As McKay says: "It is often pointed out that the definition of life as a system capable of evolution implies that single, isolated individuals not of child-bearing age are not “life.” This is nonsense and confuses the first and second cases of “life.”"

Now Q1 becomes slightly unhelpful even, except for fast and easy categorization. As atyy notes, it is a matter of [unhelpful] convention.

Q3 is the interesting part. It goes back to this:

If you had the RNA without the membrane, the RNA would become dispersed. However if there were a single self-replicating strand this wouldn't be a problem; no membrane required. Is there a reason a single strand wouldn't work?

Well, it would be very unlikely that a variation of strands would immediately hit on a self-replicator. Indeed now that people have managed to make non-chiral, geophysically produced strands self-replicating, they find that so far only cross-replicators can do this and that they are shorter than self-replicators. I.e. a left-handed RNA replicates all the right-handed strands due to better specificity, and that includes the right-handed replicator that reproduces the left-handed pool of RNA strands.

Such a pool should dilute the cross-replicators eventually even if they grab local strands. So a membrane is likely necessary. Dunno if it has been shown, but I have seen the claim somewhere.

But I hear it is worse, such a pool would tend to shrink the strand lengths until they can't reproduce. To chemically select for longer strands, and start the "selection" part of life's processes, you need a hydrothermal vent that do metal atom PCR in cracks or pores of ~ 0.1 mm size (or larger). Thus far they have managed to do DNA with the usual PCR enzymes as proof-of-principle. So a whole alkaline hydrothermal vent is likely necessary to start chemical self-replication.

Picking that apart a Hadean alkaline hydrothermal vent had metabolism* (which produces the nucleotides), inorganic cells with membranes (that support metabolism and chemical replication), and self-replicating chemistry.

Last week I read Martin's and Lane's latest (?) paper where they, quite convincingly I think, show why the last universal ancestor needed to be dependent on an alkaline hydrothermal vent in order to evolve chemiosmosis and then become independent of the inherited pH differential. It seems simply impossible for a free-living protocell to have evolved chemiosmosis, while there is a mechanistic, even selective, route to it in a vent. Possibly protocells evolved chemically elsewhere and then infected vents to make the leap to robustly metabolizing cells. Else it seems easier that life evolved around vents in toto, and spread between isolated such as inert 'spores' at times.

This ties in with bacteria and archaea having evolved the different flagellum respectively archellum afterwards, as well as different root metabolisms, membranes and pumps for chemiosmosis.

[Personally I think, if M&L et al are correct, that the LUCA had pili for movement inside and outside the vents, and was not as tied to the inorganic compartments as they propose. The archaea archellum evolved from a, shared, pili ancestor mechanism.]


* Shown by a neat proof-of-principle paper a few weeks ago, how such vents had to produce simple organics and possibly why some seem to do so a little bit even today.

 

[newjerseyrunner]

I define life as a self similar replicating system.

A [future generation] 3D printer could theoretically replicate itself.

 

[Torbjorn_L]

A [future generation] 3D printer could theoretically replicate itself.

That is true too, such a definition needs a more stringent context if you want to apply it robustly. Else at its simplest a crystal or a fire would fulfill the definition. So we need "biochemical self-replicating system" at the minimum. (Unless we plan to let capable software and hardware in as forms of life.)

That still leaves out evolution, the process of observed life, which is why the (most general) NASA definition includes it. [Most often the NASA definition goes something like "metabolizing evolutionary system" if you unpack it.]

 

[Guco]

You could conceive that in the future humanity may posses such advanced technology, that it will be possible to create individual carbon-lifeforms that do not mutate or procreate, but are capable of indefinitely maintaining themselves by intake of energy. At some point defining biological concepts is more about language and logic than biology itself.

 

[newjerseyrunner]

That is true too, such a definition needs a more stringent context if you want to apply it robustly. Else at its simplest a crystal or a fire would fulfill the definition. So we need "biochemical self-replicating system" at the minimum. (Unless we plan to let capable software and hardware in as forms of life.)

I see no reason to restrict life to biochemical or self-replicating. Would Star Trek's Data be considered alive? Is a Donkey considered alive?

Professor Spier had a nice definition: “A regime that contains a hereditary program for defining and directing molecular mechanisms that actively extract matter and energy from the environment, with the aid of which matter and energy is converted into building blocks for its own maintenance and, if possible, reproduction.”

 

[rbelli1]

So we need "biochemical self-replicating system" at the minimum.

When we try to define life we also need to avoid circular reasoning. Torbjorn is essentially saying we need a living self-replicating system to make life.

BoB

 

[Torbjorn_L]

I see no reason to restrict life to biochemical or self-replicating.

The given reason was the need for a a more stringent context if you want to apply the definition robustly. (I.e. in the context of biological life.)

Of course you can make it more wide, as long as you are clear with the context. Data is a jolly fellow.

When we try to define life we also need to avoid circular reasoning. Torbjorn is essentially saying we need a living self-replicating system to make life.

We don't need to avoid "circular reasoning" here, since we are looking at empirical science. Such science is based on circular dependencies (if that is what you mean by "reasoning"?).

For example the use of observations and their constraint under hypothesis testing in order to verify the very observation itself (under some mutually agreed on quality of low uncertainty). The same happens for theories when they undergo hypothesis testing, they predict the observations they are based on.

It can also be seen in the classical definitions of processes. Evolution is the process that takes living populations to living populations through generational, survivable changes. Gravity is the process that makes a test mass accelerate due to another mass. In both cases the axiomatic descriptions are constrained by the existence of the objects they describe the dynamics of.

"Torbjorn is essentially saying we need a living self-replicating system to make life."

I think my longish comment explicitly rejects such sophistry (if that is what it is supposed to be), since it described a pathway from geochemistry of a planet to the biochemistry of life.

Evolution used to be confined to living populations for the reason given above, no one knew how to extend it away from a definable 'axiomatic' scope of pre-existing biological populations, but astrobiology blur the boundaries as atyy notes. E.g. the first instance of selection may have been thermodiffusive selection for longer strands among a strand population rather than selection among populations of more extensive cellular metabolic replicating systems. Alas, the same blurring as for 'life' is unavoidable.

 

[rbelli1]

Your statement that I referenced:

So we need "biochemical self-replicating system" at the minimum.

Simply states that in order to have life we need a living system. Life as you stated in #10 being more than the self replicating 3D printer newjerseyrunner refers to.

While I agree that if we are looking at life we have an example of a living thing it doesn't really advance the conversation. Why a grasshopper alive and a self replicating 3D printer not is the big question.

To be science an experiment attempts to DISPROVE the hypothesis. When all conceivable negative tests fail it is said to be proven (usually with caveats, conditions, qualifications, etc).

As to your statement that all of science is circular your examples contradict your assertion. Evolution is the term we use for the process of generational change. Saying that in order to have evolution we need an evolving system simply states the same thing twice. We have change and survival as part of the definition. Gravity is the term we use for the attraction between masses. We have mass and an acceleration. In both cases the term is on one side of the equation and the other side has no need for the original term. Both gravity and evolution can be completely (to the best of your knowledge) described without using gravity or evolution or any synonymous terms.

To make a more useful argument you will need to state that biochemistry is [insert geochemical and other non bio/life terms here]. I don't have the answers either but I don't say life is life and that means I have made a great discovery.

Evolution used to be confined to living populations for the reason given above, no one knew how to extend it away from a definable 'axiomatic' scope of pre-existing biological populations, but astrobiology blur the boundaries as atyy notes.

How is there a great leap of using evolution to describe pre-existing living populations to [pre-existing] living populations that are not on earth? I know we use evolution to describe many nonliving systems but we don't have to consider that when we are talking about living systems. The discussion started out trying to find a bright line between living and nonliving so saying that we just don't know where it is simply reiterates the OP's inquiry.

If we all come to a conclusion that we have found the GUT* of life and then somewhere outside of Earth we point to something that we can all agree is alive but falls outside that theory we will have to modify it (the theory not the organism). Just the same there is no crisis if we find that everything we find off Earth follows the current models.

BoB

*Grand Unified Theory (my apologies to particle physics)

 

[Torbjorn_L]

Your statement that I referenced:
Simply states that in order to have life we need a living system.

Ah, so it is sophistry! You took my discussion of _newjerseyrunners's_ different description and put that as my own description. Note that I said "such a definition", meaning it wasn't mine.

I described in my earlier comment when such a description is sufficient (most biology), and when it isn't (astrobiology).

To be science an experiment attempts to DISPROVE the hypothesis. When all conceivable negative tests fail it is said to be proven (usually with caveats, conditions, qualifications, etc).

That is what I described, testing to verify the very observation itself (under some mutually agreed on quality of low uncertainty). I am sorry if it was unclear.

Saying that in order to have evolution we need an evolving system simply states the same thing twice.

You are saying that, not I. More sophistry. Sorry, but I find that boring - why should I, at the minimum be interested in discussing what is unfactual pertaining to the thread and/or the subject, or at the maximum is some form of defamation?

In both cases the term is on one side of the equation and the other side has no need for the original term.

That is the equation that results after observing the process and, in so doing, applying the classical definitions. You can't observe the process without the existence of the objects the equations describe the dynamics of. And that is the inherent cross-dependency I was describing, between theory and observation.

Again I find little interest in responding to my analysis as it is.

Look, you are obviously more interested in (sophistic) argumentation than in astrobiology. I am not, so I hope this comment will suffice.

 

[rbelli1]

So we have gone a bit off-topic here.

My original point was that using the term bio-mechanical in a definition of life doesn't get us anywhere because the goal of the definition is in the term itself. We need to work out a definition that doesn't need to point at living things to be complete.

BoB

 

[Torbjorn_L]

My original point was that using the term bio-mechanical in a definition of life doesn't get us anywhere because the goal of the definition is in the term itself. We need to work out a definition that doesn't need to point at living things to be complete.

A scientific definition needs to be testable. Hence it has to refer to the subject.

I am sure you mean something else. But I have already described how what, say philosophers, describe as 'circular' definitions are testable. (Newtonian definition of mass; darwinian definition of evolution.)

These types of definitions are what they are, no worse or no better than others. But, as all other testable definitions, they are useful.