PowerPoint for BI 520-01 Spring 2013

1. The Origin and Evolution of Life on Earth
―There is no fundamental difference
between a living organism and lifeless
matter. The complex combination of
manifestations and properties so
characteristic of life must have arisen in
the process of the evolution of matter.‖
Oparin, A.I. The Origin of Life, Foreign
Languages Publishing House (1924)
Dover publication (1938).

2. Spontaneous Generation?
A 17th century recipe for the
spontaneous generation of mice:
• place sweaty underwear and
husks of wheat in an open-
mouthed jar
• Wait 21 days while sweat from
the underwear penetrates the
husks of wheat and changes
them into mice!
Although such a concept may seem laughable today, it was consistent
with other widely held cultural and religious beliefs of the time and
people believed it up through the 1900‘s!

3. Pasteur Refutes Spontaneous Generation
Spontaneous generation was laid to rest in 1859 by Louis Pasteur.
Pasteur boiled meat broth in a flask, heated the neck of the flask in a flame until it
became pliable, and bent it into the shape of an S.
Air could enter the flask, but airborne microorganisms could not - they would settle by
gravity in the neck—no microorganisms grew.
When Pasteur tilted the flask so that the broth reached the lowest point in the
neck, where any airborne particles would have settled, the broth rapidly became cloudy
with life. Pasteur had both refuted the theory of spontaneous generation and
convincingly demonstrated that microorganisms are everywhere - even in the air.

4. Spontaneous Origin if Life
Probable or Improbable?
Suppose the spontaneous generation of life is a reasonably improbable
event—say it has a chance of one in a million (10-6 / yr) of happening in any
particular year.
During a human lifetime, roughly 100 years, the chances of it happening
would be quite small—about 10-6 per year x 102 years = 10-4.
But during the Earth‘s lifetime, about 4.5 billion years, the chance that it will
happen at least once is very close to one. How close? The chance per year
that it doesn‘t happen is 999,999/1,000,000. The chance that it doesn‘t happen
in 4.5 billion years is 0.999,999 4,500,000,000 = 0 to nine decimal places.
In other words, this incredibly long time has transformed the event from one
which is highly improbable to an event which is now almost inevitable!
Suppose, instead, that the probability of the spontaneous origin of life in any
given year on Earth were one in a trillion, or 10-12 / yr.
The probability that life would arise on Earth at any time during its existence
would now be about 10-12 per year x 4.5 x 109 years = 4.5 x 10-3, a very small
number. If this were the case, we are here only through an extraordinary
stroke of luck!

5. Tornedo in the Junkyard
Many scientists have argued that the latter scenario is more likely. They
base this supposition on a specious argument, such as calculating the
probability of assembling a protein out of amino acids in a purely random
fashion.
Consider a protein consisting of 100 amino acids. Any ‗slot‘ in the chain
can be occupied by one of twenty possibilities. The random assembly of
a particular amino acid sequence to form the protein has a probability of
(1/20)100, or about 10 -130. The probability of randomly assembling all the
other organic molecules that make up life boggles the mind!
The distinguished astrophysicist, Fred Hoyle, estimated that the odds of
cellular life arising were about one in 1040000! He commented…
“The chance that higher life forms might have emerged in this way is
comparable to the chance that a tornado sweeping through a junkyard
might assemble a Boeing 747 from the materials therein.
…Life as we know it is, among other things, dependent on at least 2000
different enzymes. How could the blind forces of the primal sea manage
to put together the correct chemical elements to build enzymes?”

6. What‘s Wrong?
Spontaneous generation of life was not a purely random process!
Proteins, RNA, DNA and the other molecules of life did not suddenly form
The evolutionarymolecular building Dawkins saysformation is a function
randomly out of biologist, Richard blocks. Their in ―The Selfish Gene,‖
— the laws of chemistry and biochemistry. The process is decidedly not
of
―At someThe formation wasremarkable molecule was formed by accident.
random. point a particular a multi-step process in which ever more
We will call itassemblages emergednot necessarily have been the biggest
complicated the Replicator. It may from slightly simpler ones that
or most complicated molecule a much less it had the‗first molecule‘ which
preceded them, but there was around, but complex extraordinary
property of being able to make copiesthough its formation might be very
might be characterized as alive, even of itself. This may seem like a
unlikely‗an accident.‘
termed sort of accident to happen. So it was. It was exceedingly
improbable. In the lifetime of a man, things that are improbable may be
treated for practical purposes as impossible. That is why you will never
win the big prize on the football pools. But in our human estimates of
what is improbable and what is not, we are not used to dealing in
hundreds of millions of years. If you filled in pools coupons every week
for a hundred million years, you would very likely win several jackpots.‖
Once a Replicator formed, mutations would occur and natural selection
would begin—having a strong say in the way subsequent organic
molecules used by living organisms would emerge and proliferate.
Modern proteins, DNA and RNA were still a long way away.

7. More of What‘s Wrong
The spontaneous generation of life did not depend on the random
assembly of a specific replicator, a specific molecule or any specific
process. It depended on the emergence of some replicator—or an
assembly of some set of simple molecules that eventually led to
replication via some process that worked. In other words, calculations of
probability must take into account that there are many possible pathways
to life that might work—not just one that is unique. A simple example
His friend, perhaps possessing a deeper understanding of statistics
Herein help— fallacy behind the simplistic probability calculation carried
might lays the
says,by Hoylegolfer many others). If wegolf. to turn back his ball to 4.5 billion
out “Yes, a (and playing a round of had He tees up the clock play the
Consider John, but the damned ball could end up on some blade of
grass!hole. starttakes out hisbillionwould exactly the same yardsof life based
years AfterHe things out again, of and hits his ball 220 kind straight
first and all—there are a “driver” them out here so unless you smacked
the ballthe fairway. He then walks down the molecule capable ofhole and
upon exactly the same information-storing fairway towards the one of
down into a sand trap, it’s 100% probable that it would land on
them!”his ball lying in the grass. He exclaims something different? Bill, will
replication emerge on Earth—or would it be
finds to his friend, “Wow! Put
another way, if we find life elsewhere in the universe—or even our own
solarlook at that? What are the chances that my storage basis and this
you system—will it have the same information ball would land on
replicate in the sameprobability must be a billion to one— that’sare
blade of grass? The way as ours? Not likely—many pathways almost
available just like there are many ‗blades of grass‘ available to our golfer.
impossible.
What we are hinting at here is that the probability of emergence of some
kind of life—given the right conditions such as exist on Earth—could
indeed be 100%.

8. Spontaneous Emergence of Life—Yes!
A current school of thought suggests that life is an
inevitable consequence of ―cosmic evolution,‖ shaped by
the fundamental laws of physics. All around us we see
evidence of self-organization that has been taking place on a
cosmic scale since the Universe began.
Superclusters, clusters and galaxies have formed out of
stars and giant clouds of gas and dust—pulled together by
the action of gravity. Stars and their solar systems formed in
the same way. This large scale organization of matter was
driven by the energy expended during gravitational
contraction. the Sun and other stars, gravitational
In the case of
contraction heated them enough for nuclear fusion to start in
their cores—unleashing another source of energy that now
bathes all the planets in any solar systems that accompany
them. This energy source ultimately drove the emergence of
complexity that we now see on Earth—including life, itself.

10. Life—An Inevitable Consequence of
Cosmic Evolution
Life is an emergent property of cosmic evolution that
“… Laws of complexity spontaneously generate much of the order of
inevitably arises in those locations where (i) suitable
the natural world…
materials are exposed to (ii) a suitable energy source in (iii)
Life is a natural property of complex systems. over a the number of of
a suitable environment that is stable When long period
different molecules in a chemical soup passes a certain characterized
time. The theoretician, Stuart Kauffman, has threshold, a self-
sustaining network of reactions—an autocatalytic metabolism—will
life thusly—
suddenly appear. Autocatalytic metabolisms arose in the primal waters
spontaneously, built from a random conglomeration of whatever
happened to be around. …The collective system does possess a
stunning property not possessed by any of its parts. It is able to
reproduce and evolve. The collective system is alive. Its parts are just
chemicals.
…If all this is true, life is vastly more probable than we have
supposed. Not only are we at home in the Universe, but we
are far more likely to share it with unknown companions.‖

11. Stepping Towards Complexity
1. Synthesis of simple organic
molecular building blocks
2. Polymerization—assembly of
simple building blocks into long
chains.
3. Origin of the first replicator—the
RNA world
4. The emergence of genetically
encoded protein synthesis
5. The emergence of DNA as the
vehicle of information storage
6. Last Universal Common
Ancestor (LUCA)—the first cell
7. Origin of the eukaryotic cell
8. Origin of multicellular life—
specialization
9. The Cambrian Explosion
10.Origin of humans

12. Cosmic Dust
Cosmic dust grains, like this
one < 0.1 mm across, are
Some researchers have arguedfound in Gianthave taken a
that it would Molecular
billion years for complex organic molecules to form onof
Clouds. Most are made Earth
graphite and silicate
―others say they could form in less than 100 million years.
compounds.
The evidence for life‘s early appearance on Earth is
overwhelming so either those arguing for slow formation are
wrong…
Ices made of
or the organics formed slowly in GMC‘s and then fell to
H2O, CO2, CO, CH4, formaldeh
Earth, like manna from heaven…and the time required for
The icy mantles facilitate yde (H2CO) (which might play
formation would be irrelevant!
the formation of other more an role in the formation of the
complex molecules. Over simple sugar, ribose), and
130 have been identified methanol (CH3OH) form
and 65 of them are organic! mantles around dust grains.

13. Comets
Organic molecules have been found in
comets
Spacecraft visits to comets 1P/Halley (in
1986) and 81P/Wild 2 (in 2004) found even
more complex organic compounds than
have been found in interstellar space by
remote sensing. They remain in frozen form
Giotto images
Halley‘s nucleus
in dust grains ejected from cometary nuclei
in 1986 as the comet approaches the inner Solar
system.
Prebiotic molecules contained on these dust grains—if
captured by Earth‘s gravity—would float in the atmosphere
for many years―ultimately falling to the surface—and the
molecules would remain intact! Thus the delivery of cometary
material containing prebiotic molecules that could ‗jumpstart‘
the origin of life on Earth would be a virtual certainty!

14. Meteorites
Over 70 different amino acids have been
found in the Murchison meteorite!
Nucleotide bases, sugars and other
organic compounds such as
alcohols, carboxylic acids, amines and
amides have also been found—in other
meteorites as well!
Murchison
meteorite, which Complex organic molecules formed in
fell near the town space are able to survive passage
of through Earth‘s atmosphere as well as
Murchison, Austral ground impact.
ia in 1969
Slight excess of L over D-amino acids → processing on dust
grains by circularly polarized light → explains chirality of amino
acids and other biotic compounds!

15. This 4.5 billion-year-old rock, labeled meteorite ALH84001, is believed to have
once been a part of Mars and to contain fossil evidence that primitive life may have
existed on Mars more than 3.6 billion years ago. The rock is a portion of a
meteorite that was dislodged from Mars by a huge impact about 16 million years
ago and that fell to Earth in Antarctica 13,000 years ago. The meteorite was found
in Allan Hills ice field, Antarctica, by an annual expedition of the National Science
Foundation's Antarctic Meteorite Program in 1984. It is preserved at the Johnson
Space Center's Meteorite Processing Laboratory in Houston.

16. Miller—Urey Experiment
Simulate primitive
atmosphere H2O, CH4,
NH3 and H2
Subject it to lightning
Organic molecules
produced—including
the sugar, ribose, and
all 20 amino acids!
Early atmosphere was
a secondary—
CO2, N2, H2O, some CO
and H2
Still obtain organics

17. Black Smokers
Where oceanic plates separate, hot
mantle oozes up to build new crust—sea
water is heated up to temperatures of
350 oC and it dissolves and exchanges
minerals with the rock
Near black smokers, this heated water
enriched in gasses, minerals (such as H2,
H2S, CO, CO2, HCN and NH3) and ions is
ejected back into surrounding cooler sea
water where it interacts with catalytic
clays
Sudden drop in temperature of this
enriched water from 350 oC down to 2 oC
facilitates chemical reactions that
produce simple organic molecules and
polymers—including amino acids

18. Chicken and Egg Paradox
Which came first—the chicken or the egg?
Proteins can‘t make more proteins without
DNA and RNA, but DNA and RNA can‘t be
made (and its information store accessed)
without proteins
…or can it?
There would be no ‗chicken and egg‘ paradox if the first living
organisms did not require proteins at all. Could some simple single-
stranded RNA-like polymer have spontaneously formed that could
catalyze its own replication without the aid of proteins? Eventually,
such a structure would have evolved the ability to make proteins that
would greatly accelerate the replication process. Later on DNA
appeared as a more robust form of information storage, thanks to its
superior chemical stability.
This idea is known as the ―RNA world hypothesis.‖

19. Evidence for RNA First
It received support with the discovery of ribozymes, which are types of
RNA that act as catalysts. RNA enzymes are ‗living fossils‘, still found in
today's DNA-based life. In 2001, the 3-d structure of the ribosome was
deciphered—they consist of RNA and proteins—but the key catalytic
sites of ribosomes were revealed to be composed of RNA. The proteins
are of peripheral functional importance.
The formation of the peptide bond that binds amino acids together into
proteins, is now known to be catalyzed by an adenine residue in the
rRNA of the ribosome: thus, the ribosome is a ribozyme. This finding
suggests that RNA molecules were most likely capable of generating
the first proteins, i.e., they came first.
The existence of the ribosome supports the hypothesis that a simple
RNA replicator appeared before DNA because the RNA in the ribosome
contains in its structure ‗fossil‘ evidence of the existence of an earlier
‗RNA world.‘
The hypothesis received further support from experiments in which
ribozymes were produced in the laboratory that catalyze their own
synthesis—such as the RNA polymerase ribozyme.

20. Polymerization of RNA
A dilemma—the sugar of one nucleotide must form a bond
spontaneously with the phosphate of the next…and so on.
However, this process of polymerization in water is not
thermodynamically favored since it involves the release of
water into surrounding water and thus the process will not
happen spontaneously—in fact, the reaction goes the
other way—polymers in water will undergo hydrolysis and
break down eventually into individual monomers!
However, if there is some external energy source to drive
the binding of monomers—along with the presence of
some catalyst to facilitate the process—then
polymerization in a water solution can occur.

21. Clay as a Catalyst
Polymerization in a water
solution can occur when
clay minerals are present in
the mix. Lab experiments
have shown that they
catalyze the polymerization
of RNA chains up to 50
nucleotides long!
Montmorillonite consists of 3 "layers"—a layer
containing aluminum sandwiched between two
silicate layers. They concentrate nucleotides and
provide metal ions to catalyze their polymerization.

22. Thermodynamic Argument for the
Emergence of RNA and DNA
Life is ‗an irreversible thermodynamic process‘ which arises and persists
to produce entropy. The production of entropy is not merely incidental to
the process of life, but in fact it is the very reason for its existence. The
absorption and transformation of sunlight into heat is the most important
irreversible process generating entropy in the biosphere. Thus, life most
probably began as a catalyst for the absorption and dissipation of
sunlight on the surface of Archaean seas. The resulting heat could then
be used to drive other irreversible processes such as the water cycle,
hurricanes, and ocean and wind currents.
RNA and DNA are the most efficient of all known molecules for
absorbing the intense UV that penetrated the early atmosphere and are
remarkably rapid in transforming this light into heat in the presence of
liquid water. Thus, the origin and evolution of life were inseparable
from water and the water cycle and would have resulted from the
natural thermodynamic imperative of increasing the entropy production
of the Earth in its interaction with its solar environment.

23. Entropy—an Aside
The Second Law of Thermodynamics tells us which
configuration came first—Nature tends from order to
disorder in isolated systems—a statement that paraphrases
the Second Law of Thermodynamics—or more technically—
the entropy of isolated systems always increases.

24. Entropy and Life
An infusion of energy is needed to generate and support any system that
is more complex than its surroundings—order has to be maintained.
Earth a system morelong wavelength photons back into space
Life is
emits more complex than its surroundings. Its order needs to
than short wavelengthenergy—fromreceives from the Sun.sink‘.
be maintained by a flow of photons it a ‗hot source‘ to a ‗cooler
The Sun, the Earth and surrounding of Thermodynamics,an isolated
According to the Second Law space—or universe— is this
process increasesthe entropy of the Universe.
system—more entropy must be returned into space from the Earth than it
receives from the Sun—in other words, the organization of complexity in
Earth‘s biosphere requires the removal of entropy.

25. The First Replicator
Simple nucleotides could have ‗polymerized‘ forming short strands of
RNA in a variety of possible ways. Many combinations that formed
would break apart because the strength of their bonds wasn‘t very great
and the energy required to break the nucleotide chain was
correspondingly small. However, certain base pair sequences have
catalytic properties that strengthen the bonds of a forming chain,
allowing them to stay together for longer periods of time. Such chains
The idea of emergence of RNA ribozymes has recently been given
would grow longer Lincoln andmore matching nucleotides faster until
credence by Tracy and attract Gerald Joyce of the Scripps Research
they eventually formed two RNA ribozymes in the lab from individual RNA
Institute who ‗evolved‘ at a faster rate than they broke down. Thus, they
would begin catalyzed the on early Earth. each other. The evolution of
strands that to proliferate reproduction of
these two RNA ribozymes capable of self-replication took about an hour.
Theiremergence of an example of natural selection in action—it the
The emergence was such chains would mark the origin of
occurred as a result of of life—the beginning of ancandidate enzyme
first primitive form molecular competition between ‗RNA world‘ in
mixtures
which different forms of RNA compete with each other for
free nucleotides and are subject to natural selection. The
most ‗fit‘ RNA molecules—ribozymes—the ones able to
efficiently catalyze their own replication—would survive and
evolve, ultimately forming modern RNA.

26. RNA Replication Without Enzymes
Thermodynamic arguments hypothesize that RNA became self-
replicating when the temperature of the primitive seas had cooled to
somewhat below the denaturing temperature of RNA (around 70±15 °C).
During the night, the surface water temperature would be below the
denaturing temperature and single-stranded RNA could act as a
template for the formation of double-stranded RNA. During the
daylight, double-stranded RNA and DNA would absorb UV light and
convert this directly to heating of the ocean surface, raising the local
temperature enough to allow for denaturing of RNA (breakup into 2
single strands). The copying process would be repeated during the cool
period overnight.
Such a temperature assisted mechanism of replication is similar to
polymerase chain reaction (PCR), a routine laboratory procedure to
multiply DNA segments.
Thus, RNA/DNA at the beginning of life did not require enzymes for self-
replication—reproduction was instead promoted by the day/night
fluctuation of the sea-surface skin temperature about the denaturing
temperature of RNA/DNA!

27. Proteins Enter
Eventually, however the first replicator appeared in Earth‘s
primordial soup, RNA chains developed catalytic properties
that help amino acids bind together. These amino acid
chains were primitive proteins that assisted the synthesis of
RNA, giving those RNA chains that had this catalytic
property a highly selective evolutionary advantage.
The ability to catalyze one step in protein synthesis
(aminoacylation) of RNA has been demonstrated in the lab in
a short (five-nucleotide) segment of RNA.
Furthermore, competition between various RNA chains may
have favored the emergence of chains that acted
cooperatively—which could have opened a pathway to the
formation of the first proto-cell!

28. The Emergence of the Cell
No other ‗mechanism of life‘ has proved to be ‗more fit‘ than the cell. It is
virtually impossible to imagine any other mechanism that offers more
advantages than those derived from the cell. For example—
Molecules synthesized within a cell‘s membrane do not escape into the
surrounding environment and their concentrations are maintained at
desired levels by regulating their formation and break-up.
Transport of molecules thru the cell is regulated by other cellular
molecules that act together in a coordinated way.
Since the molecules that are produced by the cellular machinery are
based upon genetic information stored in the cell‘s DNA and since these
molecules interact with the cellular membrane, those cells with the most
favorable interaction are the ones that tend to proliferate—in other words
genes evolve based upon their products.
This physical interaction with cell membranes allows for joint
maintenance of different genes within the cell‘s DNA and makes possible
co-evolution towards enhancing synergistic function. In essence, the cell
membrane makes it possible for all cellular components to function and
evolve as one well-coordinated unit. So—how did cellular life emerge?

29. The First Cell
The current cell is the result of
some three and one-half billion
years of evolution. The first cell was
much more primitive in its structure
and make-up.
Proteins, if the RNA-first hypothesis is
correct, did not exist, so the
phospholipid bilayer that make up
current cell membranes could not
have been bio-synthesized and the
protein channels that are responsible
for molecular transport through cell
membranes could not have existed.
The first primitive membranes were most likely fatty lipids, which
spontaneously formed vesicles in water. Recent experiments have
demonstrated that membranes formed out of amphiphilic-lipid chains
spontaneously assemble into bi-layers and then spontaneously form
vesicles. Such vesicles could have enclosed double-stranded RNA
segments which then served as compartments for their replication.

30. Recent Experiments
Certain membranes made of simple fatty acids, such as oleic acid, were
shown to be semipermeable—small molecules like nucleotides and
amino acids pass through them, but large polymers do not.
Thus, vesicles that form from such fatty acids would spontaneously take
in these simple monomers but would keep the larger polymers that
formed inside from passing back out.
The formation of vesicles that grow—and divide—has been
demonstrated in the lab.
Experimenters have incorporated short segments of single-stranded DNA
into such vesicles immersed in an aqueous solution containing DNA
nucleotides. The nucleotides passed through the vesicle membrane
spontaneously and once inside the ‗model protocell‘ lined up its
matching nucleotides on the DNA template and then reacted with each
other to form a complementary DNA strand. The experiment
demonstrated that the first protocells that spontaneously formed
contained some RNA-like polymer carrying genetic information and
replicated, grew and divided without the aid of any enzymes. Proteins
were not necessary.

31. Protocell Growth & Division
(1) Membrane
forms around RNA
and nucleotides
If protocell formed
pass through—
near volcanic
complementary
vent—heat provided
strand forms.
by heated water and
(2) RNA double
cooling occurs when
strand completed.
convection carries
(3) Heat breaks
protocell into cooler
double strand
water. Continued
circulation would lead
(4)cell growth until it
to Cell incorporates
more fatty acids
breaks in two.
and membrane
grows.
(5) Protocell divides
and daughters
repeat cycle.

32. The First Cell
(1) Evolution starts with a protocell
(3) Metabolism begins: Other emerge as a mutated RNA sequence. The
(2) RNA catalysts (ribozymes)ribozymes catalyze metabolism—chains of
chemical reactions that enable protocells to tap cell membrane.
ribozymes accelerate replication and strengtheninto nutrients from the
environment and produce energy.
Replication no longer needs an external stimulus!

33. The First Cell—Continued
(5) Proteins appear: Eventually, natural selection would produce the cell
(6) DNA appears: over: Some proteins could a more robust molecule like
(4) take the enzymes would produce form bridges across
membrane that made proteinsentry could carry outand nutrients needed
DNA to store genetic information that thenucleotides RNA would be to act as
ribozymes allowing selective and of main task of a variety of tasks, such
tobridge between DNA and proteins. The RNA world would be taken over by a
a support chemical reactions within the cell. In the cell membrane even
as assisting with replication and strengthening addition some proteins
would act as enzymes taking over the task of ribozymes.
DNA world
more.

34. LUCA Emerges
Organisms resembling modern
bacteria adapt to living virtually
everywhere on earth and rule
unopposed for billions of
years, until some of them begin to
evolve into more complex
organisms.
Life as we know it has begun with
LUCA —the Last Universal Common
Ancestor.

35. Origin of the Eukaryotic Cell
Protists are unicellular and are the simplest
of eukaryotes.
Some carry out photosynthesis, such as
diatoms—a major group of algae.
Others move around and act like animals—
such as amoebae.
… and finally some act like fungi—they act
as decomposers by releasing enzymes into
dead organisms that break it down —
releasing materials useful to other
organisms into the surrounding
environment—such as water molds
... which during the period of 1845–1860, due to wet growing
Some simple protist was most likely the first eukaryote to
seasons, infested all of Ireland‘s potato crops and led to the death of 1/3
emerge on Earth, approximately 2 Gya, which eventually gave
of Ireland‘s population!
rise to the entire line of eukaryotes

36. Endosymbiosis

37. Early Evolution and Rise of O2
First organisms had simple metabolism
Atmosphere was O 2 free, must have been anaerobic
Probably chemoheterotrophs
• Obtained nutrients from organic material
• Obtained nutrients from inorganic material
• Modern archaea appear to be close to the root of the tree of life
• Obtaining energy from chemical reactions involving hydrogen, sulfur and iron
compounds (all abundant on early Earth)

38. Early Evolution
Natural selection probably resulted in rapid diversification
Modern DNA has enzymes that reduce the rate of mutations
RNA is not so lucky, more likely to have copying errors
Higher mutation rate in early evolution than now

39. Photosynthesis
Most important new metabolic process evolved
gradually
Organisms that lived close to ocean surface
probably developed means of absorbing sunlight
(UV in particular)
Once absorbed, developed method of turning it
into energy
• Modern organisms of purple sulfur bacteria and green
sulfur bacteria much like early photosynthetic
microbes, use H2S instead of H2O for photosynthesis

40. Photosynthesis
Using water for photosynthesis developed later, perhaps 3.5
billion years ago
First appearing in cyanobacteria (blue-green algae)
By product of O2, released into atmosphere
Changed the world!

41. Rise of O2
O2 is highly reactive
All initial O2 would react with rock and minerals in
water
O2 could not accumulate in atmosphere until
surface rock was saturated
Rocks 2-3 bill. Yr old called banded iron
formations, show atmosphere had <1% of current
amount of O2
Rock evidence suggests that O2 amounts in
atmosphere began to rise about 2.0 bill. Yr ago
Clear evidence of O2 near current levels appears
only 200 million yr ago
• Find charcoal (fossil fuel)

42. Rise of O2
Rise of O 2 would have created a crisis for life
O2 reacts with bonds of organic materials
Surviving species avoided effects of O 2 because they lived or migrated to
underground locations
• Many anaerobic microbes found in such locales today

43. Early Eukaryotes
Fossil evidence dates to 2.1 bill. Yr ago
Dates to when O2 rising in atmosphere
DNA evidence suggests that prokaryotes and
eukaryotes separated from common ancestor
much earlier
O2 played a key role in eukaryote evolution
• Cells can produce energy more efficiently using aerobic
metabolism than anaerobic metabolism
• Adaptations of aerobic organisms could develop
adaptations that required more energy that would be

44. The Cambrian Explosion
Animal branch of the tree of life
Different classifications based on body plan
All known body plans made appearance in fossil record in a time span of 40
million years
• <1% of Earth’s age
• Animal diversity began 545 mill. Yr ago

45. Colonization of Land
Life flourished where liquid water exist
Life on land was more complicated
• Had to develop means of collecting solar energy above ground and nutrients below
Life in shallow ponds or edges of lakes
• Water evaporates
• Natural selection favored that which could withstand periods of drought

46. Colonization of Land
DNA evidence suggests that plants evolved from
an algae
It took only 75 mill. Yrs for animals to follow
plants out of water

47. Mass Extinctions

48. Mass Extinctions
Possible Causes
• Impacts
• Impact sites found for K-T boundary
• Suspected for Permian extinction 245 mill yr ago
• Active volcanism
• Climate change
• External influence for copying errors
• Increase in solar particles or radiation hitting surface
• Local supernova

49. Primate Evolution
Monkeys, apes, lemurs and humans have common ancestor that lived in
trees
Tree life
• Limber arms for swinging between branches
• Eyes in front of head for depth perception
• Offspring would be born more helpless than other animals

50. Emergence of Humans
Did NOT evolve from gorillas or monkeys
Share a common ancestor that lived just a few million years ago
98% of human genome is identical to genome of the chimpanzee
2% difference in genome separates the success of humans verses chimps
• Also indicates evolution of intelligence is complex

51. Emergence of Humans

52. Emergence of Humans
After hominids diverged from chimps and
gorillas, evolution has followed a complex path
Numerous hominids species existed, some during
the same time period
• All humans are the same species
First skull fossils that are identical to modern
human skulls dates to 100,000 yr old
Our ancestors shared the Earth with
Neanderthals
• Went extinct 35,000 years ago

53. Emergence of Humans

54. Cultural and Technological Evolution
Have not undergone biological evolution in
40,000 years
• Mutation rates are slow
Dramatic cultural changes
• Transmission of knowledge between generations
• Spoken to written word, thousands of years
• agriculture
Technological evolution
• Result of coupling between science and technology
• About 100 years between industrial revolution to landing
on the Moon and generating weapons of mass destruction