Hi Nigel,
Thanks again for your answer, but again I cannot find the data point I
am after in all the interesting information you have provided! So I
will try again.
Purely as an illustration or analogy, consider the growth of the human
body. It starts at conception having many embryonic _stem_ cells.
These all have the potential to differentiate into the many varieties of
cell types that are required to form all the organs in the body. Once
they have fully _differentiated_, they seem to lose the plasticity that
they once had when stem cells and can no longer go back and reproduce
into different types of organ cells.
Elements in the phylogenetic tree (made up of life forms instead of
cells) seems to possess similar traits. There are elements such as the
coelacanth which has reproduced itself for ~350 million years with
"scarcely a whisker out of place", while the offspring of some very near
neighbour has crawled out of the water onto dry land and formed into all
the reptiles, dinosaurs, birds, mammals, and finally as human, looked
back at its own evolutionary history. I think all must agree that this
enormous difference in the evolutionary potential of such near
neighbours is truly remarkable! If this has an explanation I would love
to hear it. But I am getting sidetracked.
My point is that there are some life forms which seem to be like stem
cells and have the plasticity to evolve into a countless variety of
other life forms. And there are others which seem to be like fully
differentiated cells which have spent their evolutionary potential and
can no longer produce other forms. So if one draws the phylogenetic
tree, there must be a trunk (or root) of simple life forms at the centre
from which all large branches proceed, leading finally to leaves at the
outer extremities where the fully complex but often evolutionarily spent
life forms exist.
One could put the first creature that crawled from the sea (which you
failed to name but which I might call a proto-frog) as one of the major
branches, and things like the bat (which seems have remained unchanged
for 50 million years) as one of the leaves.
What I would mostly like to find out is whether the DNA information
content seems to increase or decrease as we go from trunk to leaves? I
appreciate it may be difficult to know the information content when you
can't pick the difference between random numbers and vital information.
But if one was to "zip" or compress the DNA letters so as to at least
get rid of duplicates and repetitive strings, that would provide an
upper limit.
So do "simple" organisms like stromatolites and cyanobacteria seem to
have significantly more, or significantly less DNA information than
"complex" organisms like vertebrates?
John
On 27/08/2014 7:35 PM, Nigel Dyer wrote:
Hi John
Evolutionary principles can help understand how the first self
replicating cell originated. For example all the evidence suggests
that it came from an RNA based predecessor, where RNA is replicated
and splits into chunks to form enzymes etc. We are currently finding
RNA has far more roles than we had previously realised, plenty of
scope for RNA based lifeforms.
As for whether there is an inevitability of humanoid based life forms,
no there is nothing that suggests that we were inevitable. In 10
million years time it might be that the descendants of todays mice
(see Douglas Adams) or dolphins who are in many ways as advanced as we
are might be asking themselves the same question, and they are not
sexually compatible with us.
There is an emergent phenomena that gives rise to more complex life
forms that are better fitted than their predecessors to survive.
Darwin describes well the process that makes this happen, and all that
we have found in genetics supports and can be understood based this idea.
OK this is something that I would like to find out. I can see no
ratchet mechanism to help complexity develop from simplicity rather than
vice-versa. Every molecular level mutation that you can imagine must be
effectively reversible. Thus every micro-evolutionary step must also be
reversible. Thus every macro-evolutionary change must also be
reversible _if the selection pressure is removed or reversed_ - No?
We have a similar situation in physics: Every microscopic interaction
is time reversible, therefore you would expect that every macroscopic
event can happen just as well in reverse. And indeed it could, but in
practice it doesn't. You can very quickly pick whether a film is being
run in reverse or in the forward direction. So what determines the
arrow of time? The way that this is usually explained is with
probability - the second law of thermodynamics - the inevitable increase
of entropy. This is what ensures for instance that objects that are
hotter than their environment will cool towards their environment rather
than warm up even more.
If you were to apply this law that is so overriding in physics to
evolution, it would say that things must go in the opposite direction to
the one that we always seem to see them go. If you like, the evolution
of man into microbes is almost infinitely more probable than microbes
evolving to man. But since that is not what we see in the historical
record, and since it flies in the face of all orthodoxy, it is best
quietly forgotten and never mentioned!
Do you know of a convincing ratchet mechanism that ensures that "mount
improbable" will always be climbed and never rolled back down? Or do
you think there is plenty of rolling back down that happens but that
just never gets mentioned?
However, the more find out about biological processes, the more we
seem to rule out alternative none DNA/RNA ways that life could
occur. Basically evolution (and our chemists) seems to have explored
just about every available option, and there is nothing else that
comes close to doing what DNA/RNA can do. If life exists elsewhere,
I find it increasingly difficult to see how it can be anything other
than DNA/RNA based. If it is DNA/RNA based then that would therefore
just be an example of parallel evolution, which we already have lots
of examples of within nature. What will then be interesting will be to
see what the similarities and differences are in the way that the
DNA/RNA encodes information (e.g. coding for proteins, which is more
abitary), which would be the only way that we could determine whether
there was a common ancester.
My particular heresy/unproven hypothesis is that I beleive that some
of the information in DNA is stored in a 'non-local' form (similar to
Sheldrake's morphogenetic fields), so in principle could be shared
with an alien DNA based life form, which could mean that the aliens
might indeed turn out to be hairy humanoids. I await the arrival of
aliens with interest so that these various hypothesies can be tested.
Nigel
