Re: OT? evolution explains what?

Eric R <epic@xxxxxxxxxx> wrote:

> Ok, I've got a lot here and I look forward to first having a look at
> tree ring info but I'm wondering if there is anything more you can add
> if I alter my original question,
>
> "As far as the fossil record
> goes I'm trying to identify the specific reasons, other than
> similarities, to believe that the relevant fossils are evolutionary
> steps rather than merely other extinct "species".
>
> I'll change this to, "...other than anatomical similarities, and
> sedimentary and decay/erotion dating..."
>
> These 3 subjects provide the bulk of the reasoning for the theory that
> 1 creature mutated into another, right?
>
> but how far back must we believe there is a common ancestor...
>
> ..., there is no reason to assume that all creatures came from 1
> original starting cell, right? I'm no expert but the theory doesn't
> preclude the possibility that 1 cell may have formed and existed as a
> living thing for a billion years or whatever while other cells formed
> elsewhere by a similar or different process and made up of similar or
> different materials, right?

The answer seems to depend on what you want to define as a 'cell'. Since
the simplest independently living things we know of are bacteria, the
answer is that there is good genetic evidence that they all evolved from
a common ancestor. The problem is that the genetic evidence does not at
base require what we regard as a cell.

Back when 'life' was beginning there must have been replicating things
before there were cells. Best evidence at the moment is that it was RNA,
not DNA because RNA can be both information store and enzyme. Proteins
came along much later. Google for 'rna world' for more info on this.

OK so what is the evidence for unitary origins? Chirality is a good one,
molecules often come in mirror image forms, particularly things with
carbon atoms arranged in rings, the ring can 'flip' rendering any side
groups mirror imaged. Now life on this planet uses only one chirality
for things like amino acids (for proteins) and nucleotides (for rna &
dna). Thalidomide is only dangerous in one chirality, the problem is at
the time they had no way of separating the two forms. There is no
chemical or energetic reason for this choice, so it must have been
arbitrary. It may be that if we find life elsewhere in the universe it
will utilise molecules with opposite chirality.

As well as chirality rna and dna use the same 5 bases all over (rna use
Uracil, dna Thymidine) which is more good evidence. But, there has been
described recently a subgroup of the archaea (a major division of the
bacteria) which can use another base (whose name escapes me), but iirc
only to help spell STOP at the end of genes. Archaea also use a slightly
different genetic code, the three letters that spell which amino acid
goes into the protein chain.

It is unclear whether these differences arose before or after there were
what we would call cells. But because they didn't spread they must have
happened after replcating things began to wall themselves off so they
could control their own resources (a proto cell).

> So, if we admit the preceding paragraph what criteria ought we use to
> try to determine a relation as opposed to a total separate creature?
> This is obviously a very involved kind of question so I'm not really
> looking for an answer right now but just mentioning it to show what
> I'm trying to get clear in my head.

At a fundamental level to prove the interconnectedness of all things you
sequence 18S rna genes, common to all living things (apart from viruses,
they use the host's). By looking at the sequence you can draw trees of
relatedness, this sequence must have come from this one, which came from
this one etc. We now have clever algorithims for drawing such trees
which model different scenarios and give you probabilities. By measuring
how fast sequences change (callibrated wrt the fossil record and dating)
we can estimate how long ago such ancestors must have lived.

For deciding if fossil A is related to fossil B or animal C, we employ
cladistics. But what do they measure? that is the critical thing. they
give greater import to shared derived characters. So we say all things
with backbones are related because they all share a character that was,
and can only have been, derived from a single common ancestor. We know
that cephalopods (octopus and squid) are molluscs, like clams and snails
because if you look internally their anatomy is related, you can draw
diagrams showing which bit of a snail has swelled or shrunk and changed
to make an octopus. It helped that we still have nautiloids, those
octopus like animals with shells.

But, you have to be careful with shells as shells can be examples of
convergent evolution, arriving at the same point from different routes.
Staying with cephalopods, their eyes as very similar to those of us
vertebrates. So an unwary classifier could group the cephalopods as
sisters to the vertebrates due to the eyes alone. Much of the argument
in cladistics revolves around such things, 'I think your tree is wrong
because I don't think those two features are commonly derived' kind of
thing. Sequence comparisons from living members of the groups can help
to solve such questions.

For a good example of the problem, lagomorphs (rabbits, hares etc) have
hopped around the mammalian tree something chronic. They were thought to
be sister to the rodents, but sequenceing said they weren't, problem was
the sequences said they were sister to us primates but their feature
list says otherwise. It is a problem, where do you put the rabbits?

Peter
--
Add my middle initial to email me. It has become attached to a country
.

Relevant Pages

  • Re: The complete infinite binary tree has only countably many infinite paths, says WM.
    ...   That is not what you must do. ... Denote the leading node of any sequence of nodes by its number followed ... In that infinite tree, ...
    (sci.logic)
  • Re: Compact subsets of {0,1}^N
    ... > the case where a is the empty sequence, ie a sequence of length 0: ... > T be the tree of all finite sequences a such that S_a intersect K ... > f(every finite initial segment of a) is an initial segment of f. ... each of which is a sibling. ...
    (sci.math)
  • Re: Is Multi-Sequence Search Selection Macro Possible?
    ... It's a range object which I chose to name 'cll' and refers ... of running down each cell in column AF. ... that portion of the sequence should not have passed ... test in the macro for it to be completed successfully. ...
    (microsoft.public.excel.programming)
  • Re: Compact subsets of {0,1}^N
    ... >the intersection of a descending sequence of nonnul closed subsets of K. ... >> a sibling if it is the empty string or an n-sibling for some n. ... t' starts with t if and only if t' is a descendant of t; ... and in the other we're thinking about the tree structure. ...
    (sci.math)
  • RE: Automatic formula entry, file save and cell blanking
    ... and put them in the next as per the sequence given. ... put the mouse in the center of B1 and the cursor will become a hollow plus ... contain few thousand entries one in each row ranging from columns 1 to 18. ... intact) it in a given folder in C: with a name already specified in a cell ...
    (microsoft.public.excel.programming)