>> >> Genes use cell machinery for self-replication in the same way as
>> >> viruses do. Or may be you don't consider viruses replicators?
>> >
>> >Viruses can only exist because there are cells. They are replicated by
>> >cells.
>>
>> But viruses have their own goals and meanings; in this sense they are
>> autonomous and alive. If you think that viruses are not replicators,
>> then all parasitic organisms are also not replicators because they
>> require a living host to live in. They can not replicate themselves
>> without a host. Do you accept this? May be you consider replicators
>> only autotrophic organisms?
>
>The difference is that viruses are informational parasites. The replication of
>their genome has to be done by other organisms. Other parasites have their own
>replication machinery, and depend on hosts for metabolic reasons only. Just
>like any organism (except a few bacterial species) relies on others for its
>metabolism. After all, predation (eating other organisms) is an extreme form of
>parasitism.
Mario, you have not answered my question. I don't mind if you can call viruses
informational parasites. The questions are: are they replicators and
are they alive?
There are many other cases when host-parasite relationships
involve interpretation of signs. For example, some insect parasitoids
release host hormones which are interpreted by the host. Without
this interpretation parasitism will not be successful and there
will be no self-reproduction of the parasite.
>> Stevan Rose has a much broader concept than yours. He considers
>> replication and selection at several hierarchical levels (both
>> structural and temporal).
>
>Maybe he needs all these levels because his understanding of the basic
>explanations of what life really is about is more limited. Also much of his
>criticisms on biology are simply dishonest. Although I disagree with Dawkins on
>several points, I even sometimes felt the need to defend Dawkins when reading
>Rose's account. Several excesses of genetic determinism are rightly pointed out
>by Rose, but others are flawed. Take the example of the failure of genetics to
>explain schizophrenia. Is this a problem of genetics? Well, it is generally
>accepted that it rather is a problem of clinical diagnosis, whereby different
>disorders are lumped together under a single name. Etc., etc. Also, he puts
>things too black and white, while modern biologists are well aware of things he
>phrases as being problematic. Finally, most of his remarks after all have been
>provided by biologists, which shows that certainly not all (and actually few)
>biologists are genetic determinists.
It is hard to find a book with no flaws! I can assume that something
was not well interpreted in the book of Rose although I have not
checked that. But my point was that Rose have a multi-level hierarchical
view on evolution which is absent in your paper. You also don't
distinguish between adaptation and adaptability; between adaptation of
organisms, genes, and populations.
>> >> You present a very interesting idea that protocells may develop
>> >> information molecules (e.g. RNA) in order to communicate with each
>> >> other. Some aspects of this hypothesis are not clear to me.
>> >> First, do you assume that protocells without information molecules
>> >> were replicating (i.e. producing multiple protocells that had a
>> >> similar components and functions to the parent protocell?
>> >
>> >They could divide very simply, which is not the case for a genetically
>> >encoded cell. By growing (which is metabolism which can be accepted to
>> >have been able without genetic encoding), fission occurred almost
>> >naturally. This can be considered as replication, but it is so simple and
>> >does not in any way resemble present cells.
>>
>> Why not? There may be several species of non-encoded protocells that
>> differ in their chemical composition. Then there may be evolution of
>> these protocells, some of them may grow and reproduce faster than others.
>
>There can be not much evolution, since the content of these protocells are
>cycling chemical molecules. All that will happen is possible increase of the
>number of such molecules but not the kind of evolution as we see it after
>informational molecules (like polynucleotides) were developed. One of the
>reasons is because this cycling between the molecules is a process, and much
>deviation of molecules is not allowed, since otherwise the process stops (when
>an intermediate disappears, the whole cycling is halted).
I don't argue that it is much EASIER to evolve for cells that have
polynucleotides than for cells that that don't have polynucleotides.
My point was that open-ended evolution is POSSIBLE even without
polynucleotides. If there is a self-reproducing attractor, occasional
mutations (transfer to other attractors) to not detroy the process
because systems are continually multiplying. If one system leaves the
attractor the rest of the systems continue cycling. And there may be
many closely located attractors so that the evolution may go indefinitely.
If this process continues sufficiently long, it will eventually reach
an attractor with information molecules like RNA, and further evolution
will become simpler.
>> >> Second, do you think that information molecules were communicated
>> >> to offspring or to any other protocells?
>> >
>> >That is what I claim. Not to offspring initially. Initially nucleotides
>> >were carriers of information that was exchanged between protocells, just
>> >like we use printed matter to exchange information.
>>
>> My impression is that it is more natural to assume that initial
>> communication was to offspring protocells. Otherwise it is not
>> clear what benefit (in terms of fitness) a protocell may get from
>> releasing an information molecule? And what benefit another cell
>> which may differ from the producer of the message may get from
>> interpretation of this molecule?
>
>What benefit has science to a third world farmer? What benefit had science in
>the time of Newton? Not much, it just was an extra possibility on top of a
>complex community. That is how I see the origin of life, not like Alexei (whose
>ideas are quite similar to the RNA-world hypothesis). Life was the inadvertent
>outcome of the existence and acitivities of a complex community, not of a
>competition between individual cells. Most people always start thinking from
>the point of view of the individual (its benefits etc). The really rewarding
>way to look at evolution is from 'the point of view' of information.
>Individuals are not really important when trying to understand what evolution
>is about.
When you consider evolution from 'the point of view' of information, you
forget that information has sense only for some class of interpreters.
That is why it is important for me to consider a hierarchy of interpreters.
There is no information with a universal meaning.
>> >> You may be interested in my hypothesis on symbiotic origin of life:
>> >> http://www.gypsymoth.ento.vt.edu/~sharov/biosem/txt/isas98.html
>> >> I suggest that protocells resulted from a symbiosis of linear
>> >> self-reproducing polymers with self-reproducing membranes.
>
>I read it and found several problems. E.g., you go from nonheritability
>(polymers which shed off molecules at the end of the chain, whereby you state
>that indeed this supplies no heritability) to heritability ("all that has to
>occur is that each daughter cell inherits at least one of two identical
>polymers") without explaining how this happened. Indeed, before it can occur
>that the daughter cells each inherit an identical string of each polymer, you
>should have some means to make sure that can you can make exact copies. But how
>this (most important) transition is achieved is not explained, just assumed.
Polymerization is already an inheritance! Read "Genetic takeover" by
Cairns-Smith. It is inheritance because monomers are attached
selectively, and the attached monomer has the same selectivity as
the previous monomer. Imagine the polymer growing and then breaking
into parts that continue growing. The whole system is one self-reproducing
attractor. Offspring molicules belong to the same attrctor, and this is
inheritance. If offspring molecules will move to another attractor, it
would be a mutation.
>> A polymer may grow by attaching monomers; then it breaks into parts
>> and these parts continue growing. It is already self-reproduction.
>
>It is not because it lacks heredity. Have polymer AGTC, which grows by adding C
>A A and T and then splits half way. Did polymer AGTC reproduce? No, now we have
>AGTC en CAAT.
>There is no reproduction since there is no heredity.
You think that heredity may occur only by duplicating linear sequences
of symbols! This is wrong. Heredity may be by duplicating of a single sign,
or a non-ordered set of signs. For example, one species may have a 'genome'
{A,B,C} which is a non-ordered set, and another species may have a
'genome' {C,D,E,F}. Both will be well inherited. The assumption that
only lenear sequences bear information has no grounds. I agree that
linear sequences are EASIER to handle, but there may be many other
methods of inheritance.
>> The same may happen with membranes that may incorporate monomers
>> from environment. By the way, have you read books of A.G. Cairns-Smith?
>> Although he definitely oversells his "clay hypothesis", but his
>> description of molecular self-replication is quite good.
>
>I did not, but he is quoted in the paper (since Maynard Smith & Szamary quote
>him) for the obcell theory.
>I do not say that membranes cannot replicate. But the result after 4 billion
>years of evolution will be membranes.
No, membranes can evolve too. I am sure that membranes of protocells
at the beginning of life were quite different. They may have very
different glicolipids: there may be different sugars, and different
length of lipids. Lipids could be shorter or longer, and so on.
Of course, the function of membranes pretty much remained the
same with small variations. It is hard to expect that membranes
alone could evolve into more complicated structures. But all
biological structures have internal limitations. For example,
cells could never develop silicon chips.
Best regards!
-Alexei
-------------------------------------------------
Alexei Sharov Research Scientist
Dept. of Entomology, Virginia Tech, Blacksburg, VA 24061
Tel. (540) 231-7316; FAX (540) 231-9131; e-mail sharov@vt.edu
Home page: http://www.gypsymoth.ento.vt.edu/~sharov/alexei.html