N. Bishop
Part II: Replication First (Gene First) or RNA
World Hypothesis
CLAIM: The first molecule of life was RNA which
can both store information and has catalytic
ability to copy itself.
N. Bishop
TIMELINE
Building blocks of
RNA form and
assemble into
nucleotides
RNA nucleotides
form polymers
Some RNAs are
catalytic – can
make copies of
themselves
RNAs attract
amino acids –
serve as template
for polypeptides
Protein favors
RNA survival –
later surrounded
by membrane
Later DNA stores
info, proteins
take over as
catalysts
N. Bishop
Part II: Metabolism Hypothesis
CLAIM: Life began as interlocked networks of
chemical reactions (metabolism) and became
more complex over time.
N. Bishop
TIMELINE (Iron-Sulfur World Hypothesis)
H2S from hydrothermal vents
transfers energy
by donating e-
Iron compounds
become catalyst
when they
receive e-
Self-sustaining
network of
chemical
reactions forms
Chemical network becomes
enclosed in membranes or
iron-sulfur “bubble”
N. Bishop
Iron compounds
hold & donate eto carbon
compounds
Part III: SUPPORT for Replication First
Building blocks of RNA form and assemble into
nucleotides
1. Such chemical reactions could have occured
on early Earth
2. These chemical reactions occur in space
3. Nucleotides can form without creation of
building blocks (sugar, base, phosphate)
N. Bishop
Part III: SUPPORT for Replication First
RNA nucleotides form polymers
1. Clay can attract nucleotides and act as
catalyst for forming polymers
2. Polymers can form on frozen sea ice
3. Polymers can form near hot springs
N. Bishop
Part III: SUPPORT for Replication First
Some RNAs are catalytic – can make copies of
themselves
1. Catalytic RNA exists – ribozymes
2. We are just beginning to discover all the
reactions that RNAs are involved in!
N. Bishop
Part III: SUPPORT for Replication First
RNAs attract amino acids – serve as template for
polypeptides
1. Current cells can do this – tRNA and
ribosomal RNA
2. RNAs created in lab can do this
N. Bishop
Part III: EVIDENCE AGAINST Replication First
1. How could RNAs form in the presence of so
many building blocks?
2. RNA nucleotides can link together more than
one way.
3. Replication by ribozymes is slow and has a high
error rate.
4. RNA is unstable. Could it persist long enough for
life to begin?
N. Bishop
Part III: SUPPORT for Metabolism First
1. Experiments have produced a variety of organic
molecules using metal as a catalyst.
2. Some proteins in our cells do require the presence of
metal atoms to carry out catalysis.
3. Pores in hydrothermal vents provide containers for
concentrating chemicals and for maintaining
temperatures suitable for chemical reactions
4. Cells escaping iron-sulfide bubbles at different times
could have chemically different membranes like
current bacteria and archaea
N. Bishop
Part III: SUPPORT for Metabolism First
5. Thermophilic bacteria
that thrive on sulfur,
methane and hydrogen
are at the root of the
evolutionary tree.
N. Bishop
Part III: EVIDENCE AGAINST Metabolism First
1. Iron-sulfur catalyst cannot catalyze CO2
(expected in early ocean)
2. Simulations indicate iron-sulfur catalyst very
slow
3. Electron transfer near hydrothermal vents
too slow
N. Bishop
WHY ARE THESE HYPOTHESES NOT THEORIES?
AS USED IN SCIENCE:
Theory - an explanation of some aspect of
the natural world that has been repeatedly tested
and confirmed through observation and experiment.
Hypothesis – a testable explanation made on the
basis of limited evidence as a starting point for
further investigation
N. Bishop
MISCONCEPTIONS!
“Evolution is a theory about the origin of life.”
Evolutionary theory deals mainly with how life
changed after its origin NOT how life started. Most
evolutionary studies are focuses on how life branched
and diversified after life started.
However, science does try to investigate how life
started.
N. Bishop