Re: Trees of Life


"alwaysaskingquestions" <alwaysaskingquestions@xxxxxxxxx> wrote in message news:5emunuF39fhcuU1@xxxxxxxxxxxxxxxxxxxxx

"Ron O" <rokimoto@xxxxxxx> wrote in message
news:1183203066.953173.103560@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Jun 30, 6:02 am, "alwaysaskingquestions"
<alwaysaskingquesti...@xxxxxxxxx> wrote:
There are two ways we know in which life can develop purely from
non-living
matter - photosynthesis and chemosynthesis.

Photosynthesis and chemosynthesis are not means of life developing.
They are terms used to describe how organisms extract energy from
their environment.

I worded that badly. The point I was trying to make was that they don't need
other pre-existing forms of life for their energy source.

Ah! Then a better way of wording it might have been "two ways we
know in which life can grow and reproduce purely from non-living
matter".

Still not quite right though. "Chemosynthesis" is kind of a catch-all
term. There are quite a variety of ways of making a living without
using light, and they can be as unrelated to each other as any of them
is to photosynthesis.

I think you need to learn a couple technical terms to express your
questions properly. "Autotroph" and "Heterotroph". Look em up.
Your questions below mostly focus on the relatedness (or not) of
autotrophs and heterotrophs. But you confuse it in your first
sentence by talking about two kinds of autotrophs.

Why then, do we try to construct a tree of life originating back to a single
point? As organisms using photosynthesis and chemosynthesis are entirely
independent, would it not be more logical to have two separate trees of
life - basically one for plants and one for animals, or do we have real
evidence of a common ancestor? (I'm consciously leaving cyanobacteria out
of this as I don't think it affects my underlying point.)

Hmmm. You may be more confused than I thought. Animals don't do chemosynthesis,
and neither do plants. Animals are heterotrophs. Plants are photosynthetic
autotrophs. Various bacteria do 'chemosynthesis'. All share a common
ancestor because they have more fundamental things in common - things like
DNA, a genetic code for building proteins, the same general membrane structure,
and lots of other things.

Plants and animals are even more closely related than that. They are both
eukaryotes, which means that they both have a nucleus, and a cytoskeleton,
and mitochondria, and histones. We can tell that they are closely related
by looking at and comparing the sequences of their histone proteins and
other proteins. The histones remain almost identical. Some other proteins
are more different, but still recognizably related - at least by comparison
to the same proteins in bacteria.

Science claims that the evidence indicates life going back to one
point (the lifeform that developed the genetic code that all known
extant life uses), but it is acknowledged that we know nothing about
how many different types of lifeforms may have combined during the
evolution of that common ancestor.

You don't, know much about chemosynthesis and photosynthesis if you
claim that they have to be two separate trees.

That's why I'm asking here :)

Just look up the
bacteria that produce H2S (chemo) and the ones that produce H20
(photo). You will note that they both use the same genetic code and
have similar electron transport systems.

Hmmm. Actually I'm not aware of any bacteria producing H2O by anything
like photosynthesis. You probably mean consuming or breaking down H2O, Ron.

What I'm wondering is whether they inherited the genetic code and electron
transport system from a common ancestor or whether they could have evolved
them separately.

Common ancestor, definitely. There is just too much about the genetic
code which is arbitrary. In Crick's words, "a frozen accident". The exact
same code would not have been invented twice.

Science doesn't even claim
that it knows what the first lifeforms used as an energy source. My
bet would be on chemosynthesis. Lifeforms probably just took up
molecules from their environment and did things with them. Some of
those chemicals may have been photoactivated, probably not in the
organism, but maybe they were.

I strongly doubt that any lifeform ever took up photoactivated molecules
from the environment. It is possible that photoactivated molecules in
the environment could have produced some kind of high-energy (but not
photoactivated) molecules in the environment and that the life-forms
then took these up. I doubt that it happened, but a lot of abiogenesis
researchers would disagree with me on this.

A secondary area intrigues me. Outside of those basic organisms that use
photosynthesis or chemosynthesis, all other forms of life grow and
develop by consuming other organisms - cow eats grass, man eats cow, etc.

Right. Cows and men are heterotrophs. So are our intestinal bacteria -
like E coli. They don't eat other organisms directly, but they do eat
organic material produced by other organisms. Cows and men and E coli
could not survive without other organisms in the world. Plants and
cyanobacteria, as autotrophs, could survive.

What is so intriquing?

Because it seems to me that that organisms starting to consume other
organisms could have been a major stepping stone in evolution.

I agree. But the question is complicated. Plants, for example, are
descended from heterotrophs. They are heterotrophs who developed a
symbiotic relationship with autotrophic cyanobacteria. But if you
go even further back in time - back before there were eukaryotes, it
is possible that plants and animals can trace their common ancestry
back to an autotrophic methanogen who got her energy from hydrogen.

What is the best source of chemicals in terms
of highest concentrations of the ones needed for life? Why wouldn't
lifeforms evolve to exploit other lifeforms for their energy needs?


Do we have any knowledge of at what point in evolution this process of
organisms consuming organisms began?

Probably as soon as there were enough around to bump into each other.

My guess would be that it was quite a bit later than this - probably not
until after the LUCA, which I assume was an autotroph. My reasoning is
that heterotrophy is difficult biochemically, and doesn't really make
much sense energetically until there is quite a bit of oxygen in the
air. But this opinion of mine is definitely not the general consensus.

.

Relevant Pages

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