Transcript Black with Gold - Cloudfront.net

Chap 26: Early Earth and The Origin of Life
This is basically an introductory chapter without too much stressful material. I
hope this outline helps.
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An Intro To the History of Life
Six Kingdoms
•
Fig 26.1 shows you 6 kingdoms that represents one way of classifying
organisms.
• Bacteria: such as E. coli
• Archaea: such as T. aquaticus. These prokaryotes love special
environments such as hot springs, or high salt environments or they
love to live in methane-rich environments
• Protists: such as Paramecium, Euglena
• Plants:
• Fungi: like those growing between your toes or on Teva sandals
• Animals
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Figure 26.1 Some major episodes in the history of life
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Life on Earth originated between 3.5 and 4.0 billion years ago.
•
Most organisms were unicellular at that time.
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No water
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Oldest known rocks are in Greenland at 3.8 billion years old.
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Oldest organisms are fossilized in rocks in Australia and are 3.5 billion
years old.
• These resemble present day bacteria
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Prokaryotes dominated evolutionary history from 3.5 to 2.0 billion years ago.
•
What are prokaryotes?
• Prokaryotes are described, in detail in Chap. 27. Basically:
• they are either bacteria or archeabacteria
• most have cell walls made of peptidoglycans (sugar polymers)
• many or motile by either a flagellum, helical filaments or by
gliding on self-secreted slime.
• they have a nucleoid
• the have plasmids
• they transfer genes by transformation, conjugation or transduction.
• they have diverse nutritional requirements
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Figure 26.3 Early (left) and modern (right) prokaryotes
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Figure 26.4x Stromatolites in Northern Canada
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Figure 26.4 Bacterial mats and stromatolites
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Oldest fossils of prokaryotes are in fossilized mats called
stromatolites.
Oxygen began accumulating in the atmosphere about 2.7 billion years ago
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Initially the environment was anaerobic and oxygen was a “poison.”
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PS evolved fairly early in prokaryotes but water was not the source
of electrons and oxygen was not the product.
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It wasn’t until cyanobacteria evolved that PS split water and released
oxygen.
• Initially this oxygen dissolved in the water, eventually saturated the
water and then escaped into the atmosphere.
• Oxygen in the water also formed precipitates with iron forming the
marine sediments.
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Figure 26.5 Banded iron formations are evidence of the vintage of oxygenic
photosynthesis
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Eukaryotic life began by 2.1 billion years ago
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These cells are larger and more complex.
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They evolved from symbiotic relationships between relationships
between prokaryotes (endosymbiont theory)
Multicellular Eukaryotes evolved by 1.2 billion years ago
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Unicellular protists as well as multicellular forms evolved such as
algae, fungi, plants and animals.
Animal diversity exploded during early Cambrian period
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Cambrian period was 543 million years ago
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huge “radiation” of animal types.
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Plants, fungi, and animals colonized the land about 500 million years ago
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How did the land ever become inhabitable?
• terrestrial surfaces were probably first dampened by cyanobacteria
and other prokaryotes.
• adaptations to prevent dehydration had to occur in the cellular
structure of the plants(waxy cuticle on plant leaves)
• reproduction could no longer rely on the dissemination of eggs and
sperm into water.
• Plants colonized land in the presence of fungi since fungi could aid
in the decomposition of matter and absorb water. Both then were
available to plants. Many plants today are associated with fungi at
their roots.
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The Origin of Life
First cells may have originated by chemical evolution
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Abiotic synthesis of organic monomers
• amino acids and nucleotides formed. So covalent bonds were
made.
• polymers were made from these monomers
• molecules needed to replicate themselves. RNA? DNA?
Proteins? What catalyzed the reactions?
• membranes formed around these polymers forming protobionts.
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RNA may have been the first genetic material
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RNA was probably the first genetic material and first catalyst.
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So, the enzyme was around before more and more molecules could be
made.
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Ribozymes are catalytic RNA molecules. They remove introns from
the RNA.
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RNA sequences made of certain bases were more stable, replicated
faster and created fewer errors in this replication.
• Those RNA molecules that could tolerate the environment would
“survive.”
• Those replicated RNA molecules that produced copying errors
produced diversity that allowed for selection based on the stability of
these new molecules and their new shape and folding.
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Figure 26.11 Abiotic replication of RNA
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Protobionts can form by self-assembly
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Lab experiments have shown that circular droplets will form
spontaneously-liposomes
• These liposomes allow for water to move in and other and
therefore react to different environments.
• Selectively permeable
• Some split into smaller liposomes
• If enzymes are added to the environment, the will be taken in by
the protobiont
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Natural Selection could refine protobionts containing hereditary information
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Protobionts could have packaged RNA within their membranes
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If the RNA was a catalyst, it could then begin reactions, form
polymers, structures, some stable to remain within a cell.
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If some metabolic process was started, energy could be harnessed
within the cell.
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Figure 26.13 Hypothesis for the beginnings of molecular cooperation
RNA acts as a
template to form
polypeptides,
forming a
diversity that
could undergo
selection.
The
polypeptides
then could aid
in replicating
certain RNA
molecules.
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Figure 26.16 Our changing view of biological diversity
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