Evolution of self-replicating RNA?
Incubate billions of randomly generated RNAs
with labeled ribonucleotides.
Select labeled molecules, replicate (with mutation).
Repeat.
After 18 rounds of selection:
Can catalyze RNA polymerization faithfully.
1
Johnston et al. 2001 Science
The origin of life on earth required
a)
the formation of larger macromolecules such as polypeptides
from smaller organic molecules.
b) the formation of cell-like structures that enclosed basic
metabolic processes and a hereditary mechanism.
c) DNA to transmit information from parent to offspring.
d) All of the above.
e) A and B.
2
Steps in the Origin of Life (Oparin-Haldane Theory)
•
organic molecules form
•
•
•
macromolecules polymerize
•
•
•
minerals may have acted as a template
ex: proteins, nucleic acids
membrane-enclosed protocells form
•
•
can occur spontaneously (Miller-Urey experiment)
ex: amino acids, nitrogenous bases (part of DNA/RNA)
can occur spontaneously under laboratory conditions
a hereditary mechanism develops
•
RNA as both enzyme and genetic material
While the original Oparin-Haldane Theory implied that
these events must have occurred in the listed order, more
recent work (see reading) provides alternatives to the
order of events in the origin of life.
3
06 Life on Earth
First evidence for life: stromatolites & microfossils
Oxygenic photosynthesis
banded iron formations
oxygen revolution
endosymbiosis theory of eukaryotic development
multicellular organisms in Ediacaran
Cambrian explosion
4
Stromatolites (varying evidence of existence
beginning 3.45 BYA or more recently at 2.7 BYA)
Campbell 8th, Figure 25.1, www2.fiu.edu/~longoria/gly1101/proterozoic.html
5
Microscopic fossils of bacteria or cyanobacteria
from several sites, dated to 2.7–2.5 BYA
2.5-2.7 Ga microfossils
(Schopf, 2006. Phil. Trans. R. Soc. B 361: 869-885)
6
Cyanobacteria were the first organisms to perform
oxygen-producing (oxygenic) photosynthesis
Freeman 4th, Figure 28.10
7
Atmospheric oxygen spiked after cyanobacteria
became prevalent: The Oxygen Revolution
% Present
Day O2 Levels
Canfield (2005) Ann. Rev. Earth Planet. Sci. 33: 1-36
8
Banded Iron Formations
(Hayes, 2002, Nature 417: 127-128)
(Image courtesy of Dr. Pamela Gore,
Georgia Perimeter College)
9
Early Life, Photosynthesis and O2
Relative
abundance of
Banded Iron
Formations
Canfield (2005) Ann. Rev. Earth Planet. Sci. 33: 1-36
10
What caused atmospheric O2 to be present?
O2 is formed by photosynthesis and consumed
by respiration:
CO2 + H2O  “CH2O” + O2
“CH2O” + O2  CO2 + H2O
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Atmospheric O2 and the Ocean
•
Most organic matter
(“CH2O”) produced is
respired quickly.
Fig: Canfield (2005) Ann. Rev. Earth Planet. Sci. 33: 1-36
12
Oxygen Revolution is the greatest atmospheric
pollution event on record. It fundamentally changed
the oceanic and atmospheric conditions on earth.
% Present
Day Levels
• Atmospheric O2 and major biological events
– Earliest photosynthetic bacteria: ~ 2.5 BYA
– Earliest eukaryotes: ~ 2 BYA
– Multicellular organisms: ~ 0.6 BYA
13
At the beginning of the oxygen revolution, you
would predict that cyanobacteria populations
were
a)
b)
c)
d)
Dying off
Remaining steady in size.
Increasing in size.
No predictions can be made.
14
The accumulation of substantial quantities of free
O2 in our atmosphere
A. occurred with an increase in organic carbon in the
atmosphere.
B. created ideal conditions for the spread of anaerobic
organisms.
C. allowed the previously dominant species of nonphotosynthetic bacteria to occupy new habitats and
increase their population sizes.
D. Was caused by the evolution of oxygenic
photosynthesis.
E. C and D.
15
All organisms we know of on Earth today are
descended from a “common ancestor” that lived about
4 billion years ago. Over time, three domains arose.
4 Symbiosis of chloroplast ancestor
with ancestor of green plants
3 Symbiosis of mitochondrial ancestor
with ancestor of eukaryotes
2 Possible fusion of bacterium and
archaean, yielding ancestor of
eukaryotic cells??
1 Last universal common ancestor
(LUCA) of all living things
Campbell & Reece 7th Edition, Fig. 25.18
16
Freeman 4th, Figure 29.8
17
Endosymbiosis theory for the origin of mitochondria
Freeman 4th, Figure 29.9, 29.10
18
All plants and animals have mitochondria, but only plants
have chloroplasts, which enable the formation of chemical
energy from light through the same mechanism as in
cyanobacteria.
The endosymbiosis theory would predict that
a) mitochondria were engulfed first; chloroplasts were
engulfed later.
b) chloroplasts were engulfed first; mitochondria were
engulfed later.
c) Mitochondria and chloroplasts were engulfed by two
separate and independent lineages and do not coexist.
d) Any of these is possible.
e) Either a or b but not c.
19
Cambrian explosion
Opabinia
Hallucigenia
Image credits: paleobiology.si.edu/burgess/burgessSpecimens.html
20
If it were possible to conduct sophisticated microscopic
and chemical analyses of microfossils found in
3.2-billion-year-old stromatolites, then within such
microfossils, one should be surprised to observe
evidence of:
I. double-stranded DNA
a) II and IV
II. a nuclear envelope
III. a nucleoid
b) III only
IV. a nucleolus
c) II, III, and IV
V. nucleic acids
d) II only
e) all five of these
21
Which of these observations would fail to support the
endosymbiotic theory for the origin of eukaryotic cells?
a)
b)
c)
d)
rRNA sequence differences between eukaryotes
and mitochondria
size differences between some prokaryotic cells
and mitochondria
No genes in common between prokaryotes and
chloroplasts
All of these support the endosymbiotic theory.
22