Transcript Origin of Life

Origin of Life
Biogenesis
Earth History
Earliest Life Forms
The Theory of Biogenesis
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Before the seventeenth century,
most people believed that living
things could arise from nonliving matter.
Flies could come from rotting
meat, mice could come from old
clothes and dirty straw, etc. This
belief was called the Theory of
Spontaneous Generation.
The combined work of several
important scientists disproved
this theory.
Redi’s Experiment
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In 1668, Redi designed a controlled experiment that
showed that flies did not arise from rotting meat.
Control Group
Experimental Group
Maggots and flies appeared on the uncovered meat, but
the net-covered meat produced no flies. The flies came
from eggs laid by other flies, not the meat.
Spallanzani’s Experiment
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Spallanzani tested the hypothesis that bacteria arose by spontaneous
generation. He boiled broth and allowed it to sit in containers for
several days. One group of containers was open to the air, and one
group was sealed shut. Only the open containers grew bacteria.
Control Group
Experimental Group
Pasteur’s Experiment
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Pasteur disproved spontaneous
generation conclusively in the mid
1800’s. He used a curved-neck flask
containing nutrient broth and boiled
it.
It was left open to the air for an entire
year. Nothing grew.
After a year, the curved neck was
broken off and one day later, bacteria
appeared in the broth.
Pasteur deduced that bacteria from
the air had fallen into the flask, and
that they had NOT arisen
spontaneously, but from pre-existing
life, in other words, by BIOGENESIS.
Broth in open
flask is boiled
No growth
after one year
Neck of flask
is broken and
growth
occurs after
one day!
Earth History
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About 4.6 b.y.a, the solar
system was formed. Out of
swirling clouds of gases, the
sun and the planets and their
moons condensed. Eventually
they cooled and attained their
present forms.
Sediments from the earth’s
crust have been dated using
radioactive clocks. This
technique has been used to
accurately estimate the age of
the earth.
The First Organic Molecules
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How did the 92 naturally-occurring elements found on
earth assemble into organic compounds? An early
explanation was proposed by Alexander Oparin in 1923.
Oparin said that the atmosphere of the early earth
contained no free oxygen. It was a reducing atmosphere
made primarily of NH3, H2, H2O, and hydrocarbons such
as methane (CH4).
The surface of the early earth was above 100oC. At
those temperatures, the atoms found in the atmosphere
would have had enough energy to form simple organic
compounds such as amino acids spontaneously.
As the earth cooled, these compounds would have fallen
with the rain and collected in the early oceans.
Miller-Urey Experiment
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In 1953, Stanley Miller and Harold Urey tested Oparin’s theory.
They attempted to duplicate conditions on the early earth and see
if organic compounds would form. They produced many organic
compounds, including amino acids.
electrode
Water
vapor forms
Water vapor,
CH4, H2, NH3
condenser
H2O
Organic compounds
After Miller-Urey
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More refined experiments since the 1950’s have continued
to produce a variety of organic compounds, including
amino acids, ATP, and the nucleotides that make up DNA.
Other planetary scientists have suggested that the early
atmosphere contained large amounts of CO2, which
interferes with the formation of organic compounds.
Current theories now include the
possibility that these organic
compounds arose in sheltered areas
such as parts of the ocean near
undersea volcanic vents or the
entrances to hot springs.
From Molecules to Cells
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Under certain conditions, cell-like structures can be formed
spontaneously in the lab from solutions of organic molecules.
These include microspheres, which are small spheres surrounded by
protein molecules organized like a membrane. Coacervates are
another type of structure, which are a group of droplets containing
such compounds as amino acids, proteins, and sugars.
Coacervates and microspheres exhibit some of the properties of
living things, such as growing, or budding, or concentrating
“nutrients” within their membranes. Thus, some properties of life can
occur without the presence of genes.
They cannot respond to the pressures
of natural selection, however, because
they cannot pass on their characteristics.
Earliest Life Forms
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In Australia, fossils of early life forms have
been found that are 3.5 billion years ago.
 Studies comparing DNA and RNA have
shown RNA to be more versatile. It can
even act as an enzyme in certain cases,
catalyzing its own reactions. RNA
molecules have “heredity” and can
respond to changing environmental
pressures (natural selection).
 RNA plays a critical role in the replication
of DNA, protein synthesis, and other basic
chemical processes in the cell.
 Therefore, scientists now believe that life
probably started with self-replicating forms
of RNA, not DNA.
The First Prokaryotes
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Because the atmosphere of the early earth contained no
free oxygen, the earliest organisms must have been
anaerobic.
Because the early oceans were rich with organic
molecules like amino acids and carbohydrates, the early
living things were probably heterotrophs, taking those
molecules into their cells as nutrients.
As the populations of heterotrophs grew, they depleted
their environment of nutrients, thus causing an
environmental selection for the survival
of autotrophs, when they arose.
The first autotrophs were chemosynthetic, not photosynthetic.
Photosynthesis and Aerobic Respiration
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When photosynthetic autotrophs appeared about 3.8
b.y.a. and reproduced in large numbers, they produced
oxygen in large amounts, which accumulated in the
atmosphere. This was toxic to many early organisms.
Some organisms, however, used oxygen to bond to
other substances in their cells, thus preventing damage.
This was the first step in the development of aerobic
respiration.
It took about a billion years for oxygen to accumulate in
the atmosphere to its present concentration of about
21%. The oxygen eventually rose to the upper layers of
the atmosphere where it formed ozone (O3).
Ozone acts as a shield from UV radiation that allowed
life to migrate onto the land.
The First Eukaryotes
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The first eukaryotes probably arose by the process of
endosymbiosis, in which a larger cell engulfs but does
not digest a smaller cell. The two cells then exist in a
relationship that is beneficial to both.
It is thought that between 1.5 and 2.0 b.y.a. a small
aerobic prokaryote began to live and reproduce inside of a
larger anaerobic prokaryote. These small aerobic
prokaryotes evolved into mitochondria.
Small aerobic
prokaryote
Large prokaryote
with engulfed
smaller prokaryotes
Eukaryotes
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Later there was another successful influx of photosynthetic
prokaryotes similar to cyanobacteria into larger cells. These small
photosynthetic prokaryotes evolved into chloroplasts.
Evidence supporting the theory of endosymbiosis includes the
following: 1) Mitochondria and chloroplasts each have their own
genes on a circular piece of DNA like that of prokaryotes. 2) They
also make their own enzymes, and they are also surrounded by two
membranes instead of one. 3) They reproduce by binary fission, like
prokaryotes do.
mitochondria
Small photosynthetic
prokaryote
Photosynthetic
Pre-eukaryote