Important words and concepts from Chapter 26,
Campbell & Reece, 2002 (3/25/2005):
by Stephen T. Abedon (abedon.1@osu.edu)
for Biology 113 at the Ohio State University
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Course-external links are
in brackets Click [index] to access site index Click here to
access text’s website Vocabulary
words
are found below |
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(1) Chapter title: Early Earth and the Origin
of Life
(a)
"The fossil record of past life is generally less and less
complete the farther into the past we delve. Fortunately, each organism alive
today carries traces of its evolutionary history in its molecules, metabolism, and anatomy… The evolutionary episodes of greatest
antiquity are generally the most obscure. This chapter is the most speculative
of the unit, for its main subject is the origin of life on a young Earth, and
no fossil record of that seminal episode exists."
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Quicky Review of the Origin of Life (supplemental discussion) |
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Singularity
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pre Big Bang |
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Inflation |
energized Big Bang |
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Universal expansion |
energy for expansion provided by inflation |
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Sub-atomic particles |
result of energy-to-mass conversion as expansion cooled |
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Atoms |
mostly Hydrogen |
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Gas-cloud inhomogeneities |
provided gravity wells |
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Stars |
the results of local gravitational collapse of gas clouds |
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Super novae |
forgers and disseminators of metals |
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Metals |
atoms other than H and He |
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Molecules |
more complex than, e.g., H2 |
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Accretion |
how planets form—the slamming together of sub-planet
particles and chunks |
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Planets |
local, non-gaseous gravity wells—substrate upon or within
which life evolves |
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Chemical evolution |
formation of more-complex chemicals, either in space or on
planets—requires energy source, e.g., sun and internal planetary heat |
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complex chemicals capable of templating their own
duplication (see “Logic of Origin of Life”) |
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molecules other than those carrying genotype responsible
for some of phenotype |
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Individuality |
protocells then cells |
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earliest true cells |
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Endomembrane System |
increase in complexity of cellular morphology—Eukaryotes |
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Endosymbiosis |
further increase in Eukaryote complexity and expansion of
biochemical repertoire |
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Multicellularity |
cooperative grouping of differentiated cells |
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Etc. |
animals, plants, fungi… |
(b)
[early earth and the origin of
life, early Earth, origin of life, origins of life (Google Search)]
[chemical history (The Chemical Context of Life)]
[cells: origins (Online Biology Book)]
[index]
THE ORIGIN OF LIFE
(a)
The earliest life was most likely prokaryotic
(b)
Prokaryotic cells are simpler than eukaryotic cells and life presumably began simpler and only
evolved greater complexity with time
(c)
The oldest rocks are about 3.8 billion years old; the oldest fossil
prokaryotes are about 3.5 billion years old
(d)
The oldest eukaryotic fossils are about 1.5 billion years old
(e)
See Figure 26.1, Some major
episodes in the history of life (between 4500 and 3500 million years ago)
(f)
See figure 26.2, Clock
analogy for some key events in evolutionary history (between 4500 and 3500
million years ago)
(g)
[prokaryotic origin of life
(Google Search)]
[index]
(a)
Life appears to have begun about as early in the Earth's history as one
may reasonably expect it to have begun
(b)
Earth in its formative years was much hotter than it is today e.g., probably
with episodes of crustal melting as well as much greater levels of geothermal
energy (hotter interior, greater levels of volcanism, etc.)
(c)
Life may even have begun more than once, with entire episodes of origin
obliterated by asteroid-impact generated melting of the Earth's crust
(d)
Life may have had a very hot beginning, perhaps in environments better
resembling those found at deep-sea vents than those found in
(e)
The keys to understanding the origin of life are considered in the “logic of origin of life” table below
(f)
See Figure 26.1, Some major
episodes in the history of life (between 4500 and 3500 million years ago)
(g)
See figure 26.2, Clock
analogy for some key events in evolutionary history (between 4500 and 3500
million years ago)
(h)
[origin of life, origins of life (Google Search)]
[index]
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Logic of the Origin of Life (synopsis &
supplemental discussion) (please
emphasize in your studies the material found below this table) |
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Large volumes |
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Diverse environments |
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Lots of time |
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Organic molecules |
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Energy |
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SOME DETAILS CONCERNING THE
ORIGIN OF LIFE
(a)
The Earth's early atmosphere appears to have lacked any more than trace
concentrations of molecular oxygen (O2)
(b)
These low-concentrations-of-molecular-oxygen conditions appear to have
been present during the first billion or so years of life's existence (ending
about 2.5 billion years ago as molecular oxygen generated by cyanobacteria finally began to accumulate)
(c)
See figure 26.1, Some major
episodes in the history of life (about 2500 million years ago)
(d)
See figure 26.2, Clock
analogy for some key events in evolutionary history (about 2500 million years
ago)
(e)
This lack of molecular oxygen is expected given that molecular oxygen
is highly reactive, especially in an environment that evolved in the absence of
molecular oxygen
(f)
This is because many materials that are stable in environments lacking
in molecular oxygen are readily degraded by (unstable in the presence of)
molecular oxygen
(g)
In the absence of oxygen, oxygen-labile materials can accumulate, only
ultimately to be destroyed (oxidized) once oxygen became abundantly available
(resulting in a loss of both the material and the material-destroying molecular
oxygen)
(h)
Such materials that accumulate in the absence of oxygen are termed reduced
(i)
The early Earth's atmosphere is thus described as a reducing atmosphere
(certainly it was not oxidizing)
(j)
Biomolecules tend to be somewhat reduced (certainly they do not
represent carbon in its most oxidized form)
(k)
Biomolecules are somewhat unstable in the presence of oxygen
(l)
Thus, only in an environment that lacks molecular oxygen could life
have slowly evolved from reduced carbon-containing materials found more
or less stably present in such an environment
(m)
Were oxygen present in large concentrations then the instability of
organic molecules in oxygen’s presence would have placed a too-stringent time
limit on the simultaneous evolution of self-replication and resistance to
oxygen
(n)
Molecular oxygen is a poison that all organisms that live in oxygenated
environments have had to evolve to deal with; numerous organisms still exist
that are incapable of survival in the presence of molecular oxygen (e.g.,
strict anaerobes such as Clostridium
tetani, the bacterium that lives in anoxic, deep puncture wounds and causes
tetanus)
(o)
[reducing atmosphere
(Google Search)]
[image: Stanley Miller’s
experiment (Cells: Origins)] [index]
(a)
The second key thing to keep in mind when pondering on the origin of
life is that life evolved in an environment(s) lacking in super-sophisticated
modern organisms
(b)
Thus, life did not have to exist, out of the box, anywhere nearly as
capable at growing and replicating as are modern organisms
(c)
In a world lacking in modern organisms, it doesn't take much to be a
better replicator than, for example, a rock
(d)
Or, in other words, in the land of the blind, the one-eyed woman is
queen
(e)
["no competition"
"origin of life" (Google Search)]
[index]
(a)
There has been much speculation that at an early stage in the evolution
of life, RNA was a very important player
(b)
RNA today serves as the bridge between genotype and phenotype (i.e., between DNA and protein, i.e., between information storage and interaction
with the environment)
(c)
However, the intricate patterns RNA is able to fold into allows it to
display phenotype all by itself (e.g., there exist numerous examples of
catalytic RNA)
(d)
RNA also serves as the hereditary material in numerous viruses and thus
can serve as a replicable, information carrying molecule, just as DNA can
(e)
Thus, RNA is a single molecule which possesses both genotype and
phenotype
(f)
Furthermore, RNA can give rise to DNA (in theory at least) via base pairing; an RNA-based organism could, at least in
principle, give rise to a DNA-based organism possessing similar if not
identical genotype (retroviruses, in fact, do this all the time)
(g)
Finally, the replication of RNA seems to be possible even in the
absence of complex replicative machinery
(h)
For all of these reasons, people posit that prior to the existence of
DNA-based genotype, organisms (or their precursors) possessed RNA-based
genotype
(i)
Note that this statement is not quite the same as claiming that RNA
served as the first replicator, only, probably, an earlier one than DNA
(j)
[RNA world (Google Search)]
[index]
(8) Scenario for the origin of self replication
(a)
Keep in mind that any substance that possesses qualities that make it
likelier that it will arise and likelier that it will persist, once present,
will tend to be more prevalent in an environment than a similar substance that
for whatever reason is less likely to arise and less likely to persist, once
present
(b)
Physical and chemical principles control whether a substance will be
more likely or less likely to arise and then to persist
(c)
For simple substances, the likelihood of arising depends on a
relatively short and simple history, e.g., two precursors might find themselves
in the same place, at the same time, react, and thereby give rise to the
substance
(d)
More complex substances may require more elaborate paths to their
existence; modern life forms represent an extreme in this latter regard
(e)
Proto-life presumably was a substance that was able to catalyze its own
existence, and which was able to carry information relevant to the catalysis of
its own existence
(f)
In this way the mere presence of the substance would tend to bias the
environment toward producing more of that substance (recall positive feedback
as well as exponential growth); the deposition of minerals, layer upon layer
upon preexisting material along crystalline planes follows a similar tendency
(g)
However, once such a substance additionally has the potential to vary
in its information content as well as pass that information content on to
copies of itself, there exists the potential for Darwinian evolution
(h)
Particularly, those substances that are more capable of self
replication (e.g., templated duplication) along with stable persistence would
come to dominate the environment
(i)
["self replicator"
"origin of life" (Google Search)]
[index]
(a)
Two additional key steps in the origin of life were the separation of phenotype from genotype and the
subsequent evolution of individuality
(b)
An RNA molecule capable of creating phenotype separate from itself
(primitive peptides, for example) would immediately face a problem of
inadvertent sharing of that phenotype with parasitic RNA which is unable to
create the phenotype in question, but is capable of taking advantage of its
presence
(c)
A way around the problem of inadvertent sharing of phenotype with
parasites is to prevent parasite access to phenotype
(d)
One way of accomplishing this is to put a wall around genotype and its
phenotype
(e)
Today we would call such "walls" plasma membranes, though keep in mind that the
original “walls” were not necessarily lipid bilayers (indeed, likely were not
lipid bilayers but instead something cruder and more permeable)
(f)
A protocell thus is born
(g)
See Figure 26.13, Hypotheses
for the beginnings of molecular cooperation
(h)
[protocell, protocells (Google Search)]
[index]
CLASSIFYING CELLULAR LIFE
FORMS
(a)
We can describe an ancestor that was universal to all living (extant)
life as an organism that possessed all of the basic molecular characteristics
shared by all extant organisms
(i)
DNA-based genotype
(ii)
Three-nucleotide codons using the
code still employed today
(iii)
A lipid-based separator between cytoplasm and the
extracellular environment
