Download Lecture Details - Earth and Atmospheric Sciences

EAS 2200
The Earth System
Spring 2011
Lecture 15
Evolution of the Atmosphere and
Biosphere II
The RNA World
One macromolecule, RNA (ribonucleic acid), is the prime suspect because it can
Store, transmit and duplicate genetic information (like DNA)
Catalyze chemical reactions (unlike DNA)
Furthermore, certain riboenzymes have been shown to catalyze their own synthesis under
specific conditions.
This, however, is one idea of many. It remains controversial and there is not yet a
consensus on this matter.
From Macromolecules to Protocells
Life as we know it requires a barrier between itself and its surroundings.
Present cell walls are composed of a bilayer of phospholipids -each of which has a
hydrophobic and hydrophilic end. In water, phospholipids spontaneously arrange such that
the tails are shielded from the water, resulting in the formation of structures such as
bilayers, vesicles, and micelles.
Fatty acids (just hydrocarbon chains with a COOH on the end) have the same properties
and could have played this role in the first protocells.
Fatty acids formation could be catalyzed by clays in hydrothermal systems.
A fatty acid vesicle would be permeable to nucleotides, so the material needed to
synthesize additional RNA could accumulate in the cell.
The protocell would grow as it accreted micells and accumulated nucleotides, eventually
1
EAS 2200
The Earth System
Spring 2011
Lecture 15
becoming distorted and splitting.
Learn more and get videos at http://exploringorigins.org/.
The ‘metabolism first’ hypothesis
Some scientists, while agreeing it preceded DNA, think RNA is still far too complex for
“first life”.
One line of thinking is “metabolism first”
A contained (perhaps in a lipid vesicle) chemical system or cycle that exploits an energy
source (perhaps redox) to sustain the cycle, grow, and reproduce. Catalysis of reactions is
key.
Example: the combination and separation of amino acids in the presence of metal sulfide
catalysts with energy supplied by the oxidation of carbon monoxide to carbon dioxide.
Transition out of the RNA World
RNA has some disadvantages:
Not as chemically stable as proteins or DNA
Not as good a catalyst as proteins
Not as good at storing information as DNA.
Single-stranded RNA (if was indeed the basis of first life) was eventually replaced by
double-stranded DNA.
Some think DNA first evolved in viruses, which then infected RNA-protocells.
Some Observations about the Origin of Life
Improbable or not, it did happen (in one way or the other).
It happened relatively quickly.
Possibly present 3.8 Ga ago - and arguably far more complex than mere macromolecules.
The fossils and trace fossils of 3.5 Ga are remarkably similar to modern bacteria - they are
significantly evolved beyond “first life”.
The Earth (and solar system) were likely quite hostile places for the first few hundred
million years.
The time scale for the origin of life is no more than a couple of hundred million years - and
quite possibly less than one hundred million years.
Simple life might not be all that improbable after all.
Life later became more complex, but apparently in a series of ‘giant steps’ rather than
steadily.
Key Events in the Proterozoic
Huronian Glaciation 2.3-2.5 Ga
Rise of Atmospheric Oxygen 2.0-2.3 Ga
First Eukaryotes 2.7-1.2 Ga
Evolution
Evolution is the process by which novel traits arrive in populations and are passed on from
generation to generation.
The fundamental mechanisms driving this are mutation and natural selection.
There is certainly no question as to whether mutation, natural selection, and evolution
occur (witness antibiotic-resistant bacteria, HIV, and H1N1 flu).
And it has been observed in higher organisms (fruit flies, fish, birds) in the laboratory and in
the field.
Whether evolution has led to the biological diversity we see today (and to us) has been
2
EAS 2200
The Earth System
Spring 2011
Lecture 15
controversial in broader society, but not within science.
Development of Evolutionary Theory
Modern evolutionary theory has its roots in the 18 th century, contributors include James
Hutton, Erasmus Darwin (Charles’ grandfather), and Pierre Maupertuis.
Ideas of Jean-Baptiste Lamarck (1744-1829) particularly influential.
He thought, however, than inherited traits could be passed on.
Modern theory, which identifies natural selection as the mechanism, is due to Alfred
Russel Wallace and Charles Darwin – whose joint paper was presented in 1858.
Evidence for Evolution and Common Ancestry
Darwin’s evidence
Taxonomy
Comparative anatomy & embryology
Observed variation in domesticated and non-domesticated organisms
Biogeography
Fossil Record
Subsequent Evidence
Similarities of cellular biochemistry
DNA sequences
Chirality
Taxonomy
That organisms can be classified (Linnean system) based on their similarities in a
hierarchical manner suggests evolutionary relationship
(Why not a Jackalope?)
More on taxonomy and the tree of life
http://www.sciencemag.org/feature/data/tol/
http://tolweb.org/tree/
Comparative Anatomy
Fundamental similarities in anatomy of widely different organisms
(e.g., human arm, cat’s leg, whale’s fin, bat’s wing)
Comparative Embryology
Early development of all vertebrates is similar
Small Scale Changes - Domestic Animals
In a few thousand years, selective breeding has produced widely varying characteristics of
domestic animals such as the dog.
Variation & Selection
Light and Dark Moths in Britain
Biogeography
Distribution implies a history:
Species are different in widely separate, but similar, environments.
Species are absent from environments they could inhabit.
Closely related species are often found in close proximity.
The Fossil Record
Overall pattern on the long time scale is one of increasing complexity.
Biological succession:
3
EAS 2200
The Earth System
Spring 2011
Lecture 15
individual species are (usually) restricted to limited periods of geologic time.
It is possible to trace the ancestry of present species to ancient ones through a succession
of forms.
Species becoming increasingly diverse through time (with notable and important
reversals).
Only a tiny fraction of organisms are fossilized, so the fossil record provides only a glimpse
of ancient life.
Post-Darwin Evidence
At a cellular level, all life is remarkably similar
All rely on same fundamental set of chemicals & reaction pathways: DNA, RNA, similar
proteins; all use ATP, all autotrophs rely on the Calvin cycle, etc.
Complex chemicals (e.g., hemoglobin) are similar (but not identical) as well.
DNA/RNA sequencing
DNA/RNA sequences generally match the phylogenic tree based on morphology (but there
have been surprises)
You share >98% of your genes with chimpanzees and 94% with baboons.
Amino Acids
Of the many amino acids possible, life uses only 20.
Universal Chirality
All biologically generate amino acids are left-handed (abiotic amino acids can be either).
All nucleotides are right-handed.
Post-Darwin Genetics
Darwin was unaware of the mechanism of inheritance.
The term genetics was not coined until the early 20 th century.
Gregor Mendel (1822-1884), who was Darwin’s contemporary, worked out
fundamentals of genetics, but Darwin was unaware of it.
Molecular basis of inheritance, DNA, not discovered until 1950’s.
The points, in this context, are:
Once life emerged, it increased in diversity and complexity.
All organisms today are related and are descendents of a common Archean (or possibly
Hadean) ancestor.
Archean Life
Based on morphology and size, all Archean fossils appear to have been prokaryotes.
Compared to even single-celled eukaryotes, prokaryotes are:
Morphologically simple
Small
Internally simple.
Nevertheless, they profoundly changed the Earth.
Evolution of photosynthesis
Speculation that Isua carbon was the product of photosynthesis.
Early Archean microfossils look like photosynthetic cyanobacteria.
Cyanobacteria-related hydrocarbons (methylhopanes) found in 2.7 Ga Fortescue Group of
W. Australia.
Rise of Atmospheric Oxygen
A good deal of evidence suggests the oxygen became a significant component of the
4
EAS 2200
The Earth System
Spring 2011
Lecture 15
atmosphere around 2.3-2.0 Ga.
Before that time:
Banded Iron Formations
Detrital pyrite and uranite in sediments & paleosols
Mass-independent sulfur isotope fractionation (implying UV penetration of the atmosphere)
Iron-poor paleosols
After that:
Red beds (basically, hematite-rich sandstones)
Iron-rich paleosols
Banded Iron Formations (BIFs)
Banded iron formations are thought to form when deep water containing soluble Fe2+
upwelled and mixed with oxygen-bearing shallow water. The iron was oxidized to insoluble
Fe3+ and precipitated.
They are most abundant in the late Archean/early Proterozoic.
Prokaryotes & Eukaryotes
Prokaryotes are small, morphologically simple and internally simple, with little internal
differentiation (most notably, no nucleus).
The Eubacteria and Archea are prokaryotes
Eukaryotes are larger, morphologically diverse and internally differentiated, i.e., nucleus,
mitochondrion.
Some of these internal structures have their own DNA.
Eukaryota comprise all other organisms (plants, animals, monera).
Eukaryotic Roots
Key observation:
“the archaeal DNA replication machinery has striking similarity to that in eukaryotes and is
evolutionarily distinct from that in bacteria.”
“Many archaeal DNA replication proteins are more similar to those found in eukarya than
bacteria.”
This would suggest we eukaryotes are most likely descended from the Archaea, rather
than Bacteria.
Eukaryotes may have arisen through “Endosymbiosis”. Evidence:
Both mitochondria and chloroplasts can arise only from preexisting mitochondria and
chloroplasts. They cannot be formed in a cell that lacks them. (Your mitochondrial DNA
comes exclusively from your mother.)
Mitochondria (and chloroplasts) have own set of genes (a single circular molecule of DNA)
that are more like bacterial genes than eukaryotic genes.
DNA replication in mitochondria is independent of nuclear DNA replication and cellular
reproduction.
Both mitochondria and chloroplasts have their own protein-synthesizing machinery, and it
more closely resembles that of bacteria than that found in the cytoplasm of eukaryotes.
³ntibiotics that act by blocking protein synthesis in bacteria also block protein synthesis
within mitochondria. Conversely, inhibitors of protein synthesis by eukaryotic ribosomes
that do not affect bacterial protein synthesis do not affect protein synthesis within
mitochondria.
When did Eukaryotes Appear?
First undisputed eukaryote fossils are Meso- (middle) Proterozoic red algae (bangiophytes)
from the Canadian Arctic – 1.2 Ga.
Abundant Acritarchs (resting stage cysts) are known from the Neo (Late) Proterozoic.
5
EAS 2200
The Earth System
Spring 2011
Lecture 15
Possible molecular fossils (steranes) found in the Fortescue Group of W. Australia - 2.7 Ga
Molecular clocks suggest eukaryotes originated between 2.7 and 1.8 Ga
Grypania
Oldest macrofossil is Grypania in 2.1 Ga rocks from Michigan (and China).
What it is?
Colonial prokaryote? (blue-green algae)
Colonial eukaryote? (colonial algae)
Multi-cellular organism?
How do we know the Earth was Glaciated?
Three evidences of glaciation:
Glacial Till (tillite or diamictite): poorly sorted, containing angular blocks
Striations
Glacial marine sediments: large clasts in otherwise fine-grained, well-bedded marine sediment.
Huronian Glaciation
First clear evidence of a glacial event occurs in 2.3 Ga Huronian rocks of Canada (near
Lake Huron).
Curiously, these are sandwiched between pyrite- and urananite-bearing sediments of the
Archean and Proterozoic red beds.
So what caused the glaciations?
What was the atmosphere like in the Archean?
Rich in CO 2, possibly rich in CH4.
What kind of gases are these?
Greenhouse gases.
So why was the Earth not a “hot house”?
“Faint Young Sun Paradox”
Stars grow brighter as they age.
The Sun would have been about 30% less luminous during the Hadean than it is now.
So the paradox is that the sedimentary record shows liquid water, not ice, has existed
throughout almost all of Earth’s history.
How do we resolve it?
CO 2 and CH4
Enter Photosynthetic Life
What does photosynthesis do?
Consumes CO 2
Produces O 2
What does O2 do to methane?
Oxidizes it to water and CO 2
What happens to the greenhouse effect and climate when the atmosphere becomes
oxidizing?
It crashes.
Late Proterozoic Glaciations: Snowball Earth?
Peculiar Rock Sequences in the Late Proterozoic
Massive beds of limestone immediately overly tillites (i.e., very poorly sorted sediments
deposited by glaciers).
Carbonates are unusual in that they appear to be abiologic.
6
EAS 2200
The Earth System
Spring 2011
Lecture 15
Snowball Earth Hypothesis
Continents cluster near the equator where rainfall and rock weathering scrubs carbon
dioxide out of the air. Global temperatures fall, and large ice packs form in the polar
oceans. Albedo increases, driving temperatures even lower, etc.
Average global temperatures plummet to -50˚ C. The oceans freeze to depth of more
than a km, marine organisms die. With no rainfall, CO2 is not removed from the
atmosphere. As CO2 from volcanic eruptions accumulates, the planet warms and sea ice
slowly thins.
Concentrations of atmospheric CO2 increase 1,000-fold as a result of volcanic activity. In a
matter of centuries, surface temperatures soar to more than 50˚ C.
An intense cycle of evaporation and carbonic acid-rich rain erodes the rock and washes
bicarbonate ions into the oceans, where they form carbonate sediment. New life-forms-engendered by prolonged genetic isolation and selective pressure--populate the world as
global climate returns to normal.
7