[Taxacom] Cyanobacteria, thylakoid membranes, and the Origin of Life

Kenneth Kinman kinman at hotmail.com
Mon Nov 12 20:51:51 CST 2018


Hi again,
       Could the all-important thylakoid membranes (which support most of life on Earth) be the remnants of an extremely ancient endosymbiont?   I have long wondered if higher Cyanobacteria (those which containing thylakoids) were the result of an extremely ancient endosymbiosis.  It would have been a very primitive Gloeobacter-like cyanobacterium (lacking thylakoids) containing one photosystem engulfing another cyanobacterium with the other photosystem.  If so, it would be the most important event in the history of life.
                            --------------------- Ken
________________________________
From: Taxacom <taxacom-bounces at mailman.nhm.ku.edu> on behalf of Kenneth Kinman <kinman at hotmail.com>
Sent: Saturday, October 27, 2018 8:56 PM
To: taxacom
Subject: [Taxacom] Cyanobacteria and the Origin of Life

Hi All,
         I have long believed that Cyanobacteria are the most primitive forms of cellular life to have survived to the present day.  And that the earliest cyanobacteria might not have been capable of oxygenic photosynthesis (at least one that required both Photosystems I and II).   The question is whether a recent October 2018 research paper might help support such an hypothesis.  Opinions (pro or con) are welcome.  Here is a weblink to the paper:            http://www.pnas.org/content/early/2018/09/26/1808176115

                               ----------------Ken Kinman

________________________________
From: Taxacom <taxacom-bounces at mailman.nhm.ku.edu> on behalf of John Grehan <calabar.john at gmail.com>
Sent: Wednesday, October 24, 2018 5:55 AM
To: taxacom
Subject: [Taxacom] Orthogeneisis

For anyone interested in such esoterica, below is the text of an article
published in The International Encyclopedia of Biological Anthropology
(2018). If anyone wants a pdf please contact me.

John Grehan

Like many controversial concepts, orthogenesis
will mean different things to different people.
For most evolutionary biologists the term is
likely to embody an obsolete view of evolution as
teleological or predetermined, or that it involves
some kind of “straight line” or linear evolution
that is incompatible with the reality of evolutionary
divergence. As emphasized by Darwin (see
Darwin, Charles R), evolution was (and still
is) largely seen as the result of random variation
subject to natural selection. But even Darwin
referred to “laws of growth” and other early
evolutionists saw that evolution was not entirely
“random” as novel variations failed to obfuscate
the existence of preexisting ancestral architecture
(To put this issue in simplistic terms: A penguin
in the water remains a bird and does not become
a fish.) There was also widespread acceptance that
evolution was generally irreversible—that while
individual characters may sometimes revert,
overall structural organization did not. Darwin
later expressed regret over his initial emphasis on
natural selection (see evolution).

The idea of orthogenesis was proposed in 1893
by the German evolutionist William Haacke, who
argued that if any kind of variation was possible,
one would expect reversals to be as frequent as
novelties. The rarity of such reversals was seen as
evidence that the evolutionary origin of variation
was not only limited, but constrained or biased by
preexisting biological organization. Orthogenesis
was subsequently adopted by a minority of evolutionists,
including some who applied it to theories
of vital forces or goal-driven evolution.The fossil
record was sometimes regarded as demonstrating
a linear direction to evolution, such as the classic
reduction of horse digits from five to one.This linear
concept is widely seen as having been refuted
by evidence that evolution was not linear, but
involved a process of diversification or branching
(e.g., Ernst Mayr (see Mayr, Ernst), George
Gaylord Simpson (see Simpson, George Gaylord),
Stephen Jay Gould (see Gould, Stephen
J.)—see Grehan and Ainsworth 1985). This
critique mistakenly conflated linear evolution
with orthogenesis. Haacke cited the reduction of
horse toes from five to one as an illustration of a
biologically driven trend, not a linear or purposeful
evolution. Whether or not horse evolution
was “linear” or a radiating “bush,” there was a
reduction of toes that took place over space and
time resulting in the single-toed horse of today.
This reduction may be attributed to “random”
variations that imparted increased fitness (see
fitness), or it may be attributed to a biological
process where new variants resulted in reduction
of toes. Horses survived because these reductions
were compatible with the requirements of
ecological survival. The apparent contradiction
between an orthogenetic trend and evolutionary
divergence was addressed by Leon Croizat (1958,
1964) in the panbiogeographic evolutionary synthesis.
He regarded differentiation as the result
of allopatric divergence across a widespread
ancestral range. The heterogeneous distribution
of genetic characters across this range provides
spatially different starting points that biased
subsequent differentiation and speciation.

In natural selection theory, a random variation
is spread through a population by differential
reproductive success (fitness). In orthogenetic
models, a new variant does not require differential
reproduction to become established. While
observations of biological organization and biogeographic
distribution led some evolutionists
to adopt orthogenetic models of evolution, there
remained the widespread objection that the
concept was “mysterious” because there was
no mechanism to explain the spread of novel
variants without requiring natural selection.
But in recent decades evolutionary biologists
have widely recognized that aspects of genomic
architecture, gene structure, and developmental
pathways are difficult to explain without
involving nonadaptive processes of genetic drift
and mutation. Gene expression is controlled by
complex pathways and networks of interacting
regulatory genes, and there has been no formal
demonstration of an adaptive origin for any
genetic network.Many features of transcriptional
networks can readily arise through nonadaptive
drift, mutation, and recombination. This raises
the question as to whether natural selection is
sufficient or even necessary for the origin of
gene-network topologies. Recombination events
are accompanied by unidirectional exchanges
known as gene conversion. There is evidence,
for example, that many eukaryotes, including
humans, are GC-biased—that is, their total DNA
complement contains a higher-than-expected
proportion of guanine–cytosine base pairs. In
humans, there is a genome-wide positive correlation
between crossover rate and GC content.
Other biological processes capable of producing
evolutionary trends without environmental selection
include the emergence of error minimization
in the genetic code, expansion of genome size
by mobile element proliferation, and widespread
reductions in genome size in multiple lineages of
mammals. Complementary to genetic and developmental
studies is the widespread existence
of trends in morphology involving reduction,
suppression, and fusion in complex multipartite
structures such as the angiosperm flower and
vertebrate skull.

The recognition of orthogenesis does not
imply a rejection of natural selection. Orthogenesis
implies only that biological mechanisms can
result in mutations and the spread of mutations
without requiring increased fitness. As natural
selection can also result in the spread of mutations
(variants), there are two kinds of dynamic
processes involved with evolution: one internal
and biological and the other external and environmental.
These are not two separable entities.
Earlier formulations of orthogenesis emphasized
a contrast between the roles of “internal” and
“external” forces in evolution, but no biological
process exists without an environment; and
conversely, the effects produced by environmental
forces are contingent upon the organisms
involved. How these biological and environmental
processes interact to “co-construct” organisms
and their ecological relationships through development
represents a new and largely unexplored
aspect of evolutionary biology—one that involves
intrinsic biological mechanisms, regardless
of whether they are labeled orthogenesis or
something else (Heads 2016).
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Nurturing Nuance while Assaulting Ambiguity for 31 Some Years, 1987-2018.
_______________________________________________
Taxacom Mailing List
Send Taxacom mailing list submissions to: Taxacom at mailman.nhm.ku.edu

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The Taxacom Archive back to 1992 may be searched at: http://taxacom.markmail.org
To subscribe or unsubscribe via the Web, visit: http://mailman.nhm.ku.edu/cgi-bin/mailman/listinfo/taxacom
You can reach the person managing the list at: taxacom-owner at mailman.nhm.ku.edu

Nurturing Nuance while Assaulting Ambiguity for 31 Some Years, 1987-2018.


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