In 1985 Freeman
John Dyson wrote a little book Origins of Life, in which he
argued that metabolic reproduction and replication are logically separable
propositions, and that natural selection
does not require replication, at least for simple creatures. In
higher-level life as seen today, reproduction of cells and replication of
molecules occur together. But there is no reason to presume that this was
always the case. According to Dyson, it is more likely that life originated
twice, with two separate kinds of organisms, one capable of metabolism without
exact replication, and the other capable of replication without metabolism. At
some stage the two features came together. When replication and metabolism
occurred in the same creature, natural selection as an agent for novelty became
more vigorous.
Eigen and
Orgel had demonstrated (cf. Part 53), by two
different experiments (one involving templates and the other involving
enzymes), that a solution of nucleotide monomers can, under suitable conditions
in the laboratory, give rise to a nucleic-acid polymer molecule (RNA) which
replicates and mutates and competes with its progeny for survival. Living cells
use both templates and enzymes for making RNA. This work pointed to a possible
parasitic development of RNA-based life in an environment created by a
pre-existing protein-based life.
During
millions of years of chemical (and later also biological) evolution, the
initial primitive but living cells diversified and refined their metabolic
reaction pathways. In particular, they evolved the synthesis of ATP (adenosine
triphosphate) through some autocatalytic
reaction mechanisms (cf. Part 46). ATP is the
main energy-carrying molecule in all present-day cells. ATP-carrying primitive
cells had an evolutionary advantage over other, less efficient, cells. In time,
other molecules like AMP (adenosine monophosphate) emerged; or perhaps AMP came
first, and then ATP.
Although ATP
and AMP have similar chemical structures; they play totally different roles in
present-day cells. ATP is the universal biological currency for energy. AMP, on
the other hand, is one of the nucleotides
in the structure of the RNA molecule.
If ATP loses
two of its three phosphate groups, it becomes AMP. Dyson argued that, although
the primitive cells had no genetic apparatus to begin with, they were loaded
with ATP molecules which could easily convert to AMP molecules. Accidentally,
in one such cell which happened to be carrying AMP and other nucleotides (the
‘chemical cousins’ of AMP), the Eigen
experiment for synthesizing RNA happened spontaneously. With some help from
pre-existing enzymes, an RNA molecule got produced. Once created, it went on
replicating itself because of the proclivity of base A to hydrogen-bond with
base U, and of G to hydrogen-bond with C.
Thus, RNA
first appeared as a parasitic disease in the cell. Although most such cells
died of disease, some evolved to survive the infection, à la Lynn Margulis (cf.
Part 51). In such
cells, the parasite gradually became a symbiont. Further evolution resulted in
a situation in which the protein-based life learnt to make use of the ability
for exact replication provided by the chemical structure of RNA.
Is it really
true that proteins emerged before RNA?
The early evidence came from laboratory experiments done during the 1950s. The
well-known experiments by Miller and others (cf. Part 53) demonstrated
that amino acids form easily in a reducing atmosphere from the still simpler
molecules, in the presence of ultraviolet radiation. What about nucleotides?
They are more
difficult to synthesize. A nucleotide has three parts: an organic base, a
sugar, and a phosphate ion. The phosphate ion occurs naturally as a constituent
of rocks and sea water. The sugar
(ribose) part can be synthesized with substantial efficiency from formaldehyde.
And the synthesis of an organic base was demonstrated by Oró in 1960. He
prepared a concentrated solution of
ammonium cyanide in water, and just let it stand. Adenine was self-created,
with a 0.5% yield. Guanine also got synthesized in a similar way. But the catch here is that it is difficult
to imagine how such high degrees of concentration of ammonium cyanide could
occur in Nature, although some possible scenarios have been suggested.
Dyson has
given an updated version of his earlier ideas, in the book Life: What a Concept!(2008). In his updated
model there are six stages in the evolution of chemical complexity, leading to
the emergence of life.
Stage 1. The early cells were just little bags of some kind
of cell membrane (as I have described in the lipid-first model in Part 54). This is the ‘garbage bag model’ for Stage 1. And inside the bag there
was a more or less random collection of organic molecules, with the
characteristic that small molecules could diffuse in through the vesicle membrane,
but big molecules, once synthesized, could not diffuse out. Thus the ‘garbage
bag’ situation was conducive to the conversion and retention of small molecules
into large molecules. The higher concentration of organic material in the bag
led to a higher efficiency of the chemical processes involved.
This evolution did not involve any replication processes. ‘When a cell
became so big that it got cut in half, or shaken in half, by some rainstorm or environmental
disturbance, it would then produce two cells which would be its daughters,
which would inherit, more or less, but only statistically, the chemical
machinery inside.’
Stage 2. Parasitic RNA appeared in some of the cells in
Stage 2. ATP had appeared in one of the garbage bags by a random process in
Stage 1, and the cell hosting it had a metabolic advantage over other cells.
Therefore many cells with large amounts of ATP got created. Then, again by
chance, ATP changed to AMP in one of the cells. In due course, AMP and its
chemical cousins polymerized into a primitive form of RNA. Thus there was
parasitic RNA inside these cells, forming a separate form of life, which was
pure replication without metabolism. To quote Dyson: ‘Then the RNA invented viruses.
RNA found a way to package itself in a little piece of cell membrane, and
travel around freely and independently. Stage two of life has the garbage bags
still unorganized and chemically random, but with RNA zooming around in little
packages we call viruses carrying genetic information from one cell to another.
That is my version of the RNA world’.
Stage 3. This stage started when the protein and the RNA
systems started to collaborate. This happened after the emergence of the
ribosome. Although this arrangement had the rudiments of the modern cell, the
genetic information was shared mostly via viruses travelling from cell to cell.
This was some kind of open-source
heredity. The chemical inventions made by one cell could be shared with
others. Evolution went on in parallel in many different cells. The best
chemical devices could be shared between different cells and combined, so the
chemical evolution was very rapid, as it occurred in parallel by many pathways.
Stage 4. Speciation and sex appeared in Stage 4, and that
marked the beginning of the Darwinian era, when species appeared. ‘Some cells
decided it was advantageous to keep their intellectual property private, to
have sex only with themselves or with the members of their own species, thereby
defining species. That was then the state of life for the next two billion
years, the Archeozoic and Proterozoic eras. It was a rather stagnant phase of
life, continued for two billion years without evolving fast.’
Stage 5. Multicellular organisms appeared in Stage 5,
which also involved death.
Stage 6. This is the stage when we humans appeared.
No comments:
Post a Comment