Life's Origins - Biofundamentals
8/25/08 11:41 AM
Life's Origins
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Where did all of the different types of organisms living on earth come from?
This is one of the fundamental questions biology attempts to answer. Did
organisms arise a one specific time point, or have different types of organisms
appeared at different times during the history of the earth?
It used to be widely believed that various types of organisms, such as flies, frogs,
and even mice, could arise spontaneously, from non-living matter.
Flies, for example, were thought to appear from rotting
flesh and mice from wheat.
If true, spontaneous generation would have
profound implications for our understanding of
biological systems.
A key event in the conceptual development of modern
biology occurred in 1668 with the publication of
Francesco Redi's "Experiments on the generation
of insects".
Redi's hypothesized that spontaneous generation did
not occur. He assumed that the organisms that
appeared had developed from "seeds" deposited by
adults.
"He who experiments increases
knowledge. He who merely speculates
piles error upon error."
- Arabic epigraph quoted by Redi.
This hypothesis predicts that if adult flies are kept
away from rotting meat, for example, maggots will not
appear - no matter how long you wait.
To test this hypothesis, Redi set up two sets of flasks both contained meat.
One set was exposed directly to the air (and so to flies), the other was sealed with
paper or cloth. Maggots appeared only in the flasks open to the air. Redi concluded
that organisms as complex as insects could arise only from other insects.
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The invention of the microscope, and its application to biological
materials by Antony van Leeuwenhoek (1632-1723) and
Robert Hooke (1635-1703), led to the discovery of a completely
new (and totally unexpected) world of microscopic organisms the protozoa, microscopic fungi, and bacteria.
Although it was relatively easy to
So, naturalists observe, a
generate compelling evidence that
flea has smaller fleas that
macroscopic organisms, such as
on him prey; and these have
insects, could not arise
spontaneously, it seemed possible smaller still to bite ’em; and
that microscopic (and presumably
so proceed ad infinitum.
much simpler) organisms might be
- Jonathan Swift
able to form spontaneously.
A number of scientists began to explore the reproduction of microbes. In
particular, Lazzaro Spallazani (1729-1799), showed that after a broth was boiled, it
would remain sterile (that is, without life) as long as it was isolated from contact with
fresh air. He concluded that microbes, like larger organisms, could not arise
spontaneously, but were descended from other microbes.
One possible criticism of this experiment was that perhaps the process of boiling the
broth destroyed some vital component necessary for the formation of microbes, or
perhaps fresh air was the "vital" ingredient.
In 1862, Louis Pasteur carried out a particularly convincing experiment to address
these concerns. He sterilized broths by boiling them in special "swan-necked"
flasks.
The flask was open to the air but because of the
shape of its neck, airborne organisms could not
reach the broth.
The liquid remained sterile for months. Once the
neck of the flask was broken, however, the broth was
quickly overrun with microbial growth.
Based on such experiments, a consensus was reached that neither microscopic nor
macroscopic organisms could arise spontaneously, at least in the modern world.
Because of this consensus, studying spontaneous generation was no longer
considered a smart way to advance one's career in biology.
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What types of controls could you add to Redi's experiments?
What is a control experiment?
How would biology change if spontaneous generation were
occurring today (say between your toes)?
Are scientists close minded, not to continue to look for
spontaneous generation in the modern world?
How much time do we have (to generate life)?
In the absence of spontaneous generation, how did life originate? Where do
organisms come from? If organisms can arise only from pre-existing organisms,
where and when did the first organisms appear?
To begin to answer these questions, it is best to start at the beginning.
The current scientific model for the origin of the
universe is known as the "Big Bang".
Edwin Hubble (1889-1953) was the first to realize
that the fuzzy nebula that astronomers saw were
in fact galaxies, like our own Milky Way, composed
of hundreds of thousands to millions of stars.
These galaxies were moving away from one
another. He concluded that at one point in the
past, all of the matter and energy in the universe
had been concentrated in a single point.
Based on this hypothesis, it is possible to estimate the age of the
universe at 13.7 +/- 0.2 billion (10 9 ) years (a billion years is a gigayear
and Gya stands for gigayears ago).
This is a length of time well beyond human comprehension; it is
sometimes referred to as deep time.
The earth and the other planets formed ~ 4.5 x 109 years ago: we use
the symbol "~" to mean "approximately".
The earliest period of earth history is known as the Hadean, after
Hades, the Greek god of the dead.
The Hadean period lasted from the formation of the earth, ~4.5 x 109
years ago, until the formation of the oldest preserved rocks, which are
~3.8 x 109 years old.
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~3.8 x 109 years old.
The Hadean is also defined as the period before the appearance of life. The first
evidence of biologic processes appear in rocks that are ~3.8 to 3.5 x 109 years old.
Evidence of life: Fossils provide the most dramatic evidence of the history of life
on earth. Fossils are formed only in sedimentary rock. There are a number of
different types of fossils.
Chemical fossils are molecules that, as far as we know, can be synthesized
naturally only through biological processes. Their presence in ancient rock implies
that living organisms were present at the time the rock was formed. What makes
them problematic is that there may be abiological mechanism that we have not yet
discovered that could produce them.
Trace fossils can be subtle or obvious. Burrowing
animals can leave tunnels and disrupt layers of
sediment. Animals that walk can leave footprints.
Organisms without hard parts, such as jelly fish, can
leave impressions, much like footprints.
Dinotracks at Dinosaur Ridge, in Morrison, Colorado
Structural fossils are the mineralized remains of
organisms.
They can be as simple as a single tooth, scale or shell, or
as complex as a complete skeleton. Generally, as
organisms developed hard parts (bones, shells) their
fossilization became more likely; nevertheless,
fossilization is a rare occurrence.
More often the dead organism is digested, and nothing
recognizable remains. The study of what happens to the
bodies of organisms when they die is known as
taphonomy.
Nevertheless, over the billions of years that life has existed on earth, many fossils
have been created; as erosion removes surface layers of rock, these fossils come to
light.
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light.
Fossils of various types suggest that the first living microbes were present on earth
by ~3.5 x 109 years ago, and perhaps earlier.
Change over time: Early on in the history of geology, it
was discovered that fossils of specific types were found
associated with rocks of specific ages - this correlation was
so robust, that rocks can be accurately dated by the types
of fossils they contain.
The oldest rocks contain only very simple organisms.
Life's impact: Based on fossil
evidence, it would appear that for a
period of ~2 x 109 years, microscopic
organisms were the only form of life on
earth.
During this period, photosynthetic
bacteria captured light and used that
energy to transform CO2 (carbon
dioxide) and H2 O (water) into sugars
(carbohydrates).
During this reaction, molecular oxygen
(O2 ) is produced as a waste product.
Roll over
to discover that in the cretaceous period, O 2
levels are thought to have reached ~35%, that is almost
twice the current level!!!
Over time, O 2 began to accumulate in the atmosphere. By about 300 x 106 years
ago, atmospheric O 2 levels reached ~35%, almost twice the current level.
Some hypothesize that such high levels of atmospheric oxygen made possible the
evolution of giant flying insects.
Because O 2 is a highly reactive compound, its appearance posed challenges and
provided opportunities to many organisms. O 2 can be toxic and can also be used to
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extract the maximum amount of energy from food.
Around 109 years ago, trace fossil burrows appeared;
these were likely to have been produced by simple
worm-like, macroscopic (meaning larger) metazoans
moving along and through the mud on the ocean
floor.
About 600 x 106 years ago, new, more complex
structural fossils begin to appear in the fossil record.
The first of these were the Ediacaran organisms. It
remains unclear how they are related to later
organisms.
By the beginning of the Cambrian age (545 x 106
years ago), a wide variety of organisms had appeared,
many clearly related to modern organisms.
These cambrian organisms show a range of body
types. Most significantly, many were armored,
suggesting adaptations against predators.
Since the fossil record does not contain all types of
organisms, we are left to speculate on what the
earliest metazoans looked like.
How do scientists estimate the age of the universe?
Why are fossils only found in (originally) sedimentary rock?
Why are only a few organisms fossilized on death?
What factors would influence the probability that a particular
organism, or type of organism, would be fossilized?
If life first arose ~3.5 to 3.0 x 109 years ago, why did it take so
long for complex multicellular organisms to form?
What kinds of adaptations would reflect the appearance of
predators?
What would evidence for pathogens look like in the fossil record?
When do you think pathogens first evolved?
The scientific study of life's origins: There are at least three possible approaches
to the study of life's origins.
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A religious (i.e. non-scientific) approach
postulates that life was created by a
supernatural being or process.
Different religious tradition differ as to the details
of this event(s), but since the process is
supernatural it cannot, by definition, be studied
scientifically.
"Intelligent design" creationists claim that we can identify those aspects of life that
could not have been created by natural processes, by which they mean the
processes of evolution. This approach abandons science, and displays a failure of
faith in the creative power of natural processes, as well as in our ability to discover
and understand them.
More significantly, it implies that the origins of life are, by definition, beyond the
ability of science to study. The lesson of history, however, is different. Predictions
as to what is "beyond the ability of science to explain" have consistently proven
wrong (often a few years after they are made!)
Another type of explanation would be to assume that advanced aliens
brought life to earth. This hypothesis is termed panspermia. Perhaps
we owe our origins to casually discarded litter from an alien vacation
visit!
Unfortunately, this does not really answer the question of how life
began, since those aliens also had to come from somewhere.
If there is life on other planets, and we could analyze it, it would be extremely
informative to know how similar it is to life on earth: are the same types of molecules
used? Is DNA used to store genetic information?
If it is very different? would we even recognize it as being alive?
Unfortunately, it is currently not yet possible to answer these questions - we really
have no idea whether life is common or rare (in the universe).
The death of vitalism: It was originally thought that life itself was basically a
supernatural process, too complex to obey the laws of chemistry and physics.
In this "vitalistic" view, organisms were thought to obey different laws from the "nonliving" world. For example, it was widely held that the molecules found in living
organisms, organic molecules, could not be made except by living organisms.
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organisms, organic molecules, could not be made except by living organisms.
This view was overturned when Friedrich Wöhler synthesized
urea in the lab in 1828.
Urea is a simple "organic" molecule. O=C(NH2 ) 2 . It is
naturally found only in living organisms and is a major waste
product. Urine contains lots of urea.
Wöhler's in vitro or "in glass" synthesis of urea was simple.
He took the inorganic compound ammonium cyanate
(NH 4 + CNO- ) and heated it; this led to the production of urea.
This was a simple demonstration, but it had a profound
impact – it revealed that there was nothing supernatural
about the synthesis of urea; it obeyed the laws of
chemistry.
Based on this and other observations, we can predict that
all molecules in cells can be synthesized outside of cells,
using the appropriate procedures.
This is very much the same approach that scientists use
when they consider the question of how life first arose.
They assume that life is a natural process, and that with
time, the steps involved can be recapitulated in the lab.
"It is often said that all the
conditions for the first
production of living organisms
are now present. But if (and oh!
what a big if!) we could conceive
in some warm little pond, with
all sorts of ammonia and
phosphoric salts, light, heat,
electricity, etc. present, that a
proteine compound was formed,
ready to undergo still more
complex changes, at the present
day such matter would be
instantly devoured or absorbed,
which would not have been the
case before living creatures were
formed." - Charles Darwin
(1887).
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A first approach to understanding what is involved in the
origin of life is to attempt to create living systems or their
precursors in the laboratory.
A early and influential example of this approach was the
Miller-Urey experiment.
These two scientists made a guess as to the composition
of earth's early atmosphere. They assumed the presence
of oceans and lightning.
They set up an apparatus to mimic these conditions.
They then passed electricity through their experimental
atmosphere.
After awhile (hours/days), they found that a complex mix of compounds had been
synthesized. Included in this mix were many different types of amino acids, the
building blocks of proteins.
Certain types of meteorites also contain complex organic molecules. It therefore
appears likely that the early earth was rich in organic (that is, carbon-containing)
molecules, the building blocks of life
Given that the potential building blocks were present, the question is, what set of
steps were involved in generating living systems?
The earliest proto-biotic systems were likely to be molecular communities of
chemical reactions isolated in some way from the rest of the "outside" world.
One possible model is that these systems were originally associated with the
surface of specific minerals, from which they extracted energy and which could
serve as catalysts for important reactions.
Over time, these systems acquired membranes and were able to exist free of the
mineral surface.
Such a isolated system has important properties
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Such a isolated system has important properties
that are likely to have facilitated the further
development of life.
First, because of the boundary, changes that
occur within one such structure are not "shared"
with neighboring systems. Such systems can
also "divide" in a crude way by fragmentation.
If changes within one system improves its
stability, ability to accumulate resources, or
efficiency of growth, that system, and its
progeny, are likely to become more common.
As changes accumulate, and are passed from
parent to offspring, the "organisms" will evolve.
"Scientists now believe they may be
able to make very simple 'artificial
cells' which can metabolize, replicate
and evolve." - Jack Szostak
What are the properties of life? What would you consider alive?
Does Wöhler's synthesis of urea mean that all organic molecules
can be easily synthesized in vitro?
Would it be a good way to forward a scientific career to attempt
to prove that a particularly complex molecule cannot be
synthesized in the lab?
How is the idea of "vitalism" similar to "intelligent design
creationism"?
Use Wikipedia | revised 25-Aug-2008
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