History of Life on Earth
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Chemical Evolution (prebiotic evolution) – most
biologists believe that life developed from nonliving
matter
Alexander Oparin (Russian) and John B. S. Haldane
(England) were the first scientists (independently) to
advance the idea that simple organic molecules could
form spontaneously from more simple raw materials
(1920’s)
they noted that the oxygen-rich atmosphere of today
would not have permitted the spontaneous formation of
organic molecules
they speculated that the Earth’s early atmosphere was
very low in oxygen and rich in hydrogen in the form of
hydrogen gas (H2), methane (CH4), and ammonia (NH3)
– also contained carbon dioxide (CO2), water vapor
(H2O), carbon monoxide (CO), and nitrogen (N2)
Conditions on primordial Earth
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Earth is about 4.6 billion years old
Earth was very hot when first formed
four requirements must have existed for chemical
evolution:
1. little or no O2 – Earth’s early atmosphere was
probably strongly reducing which would cause
any free oxygen to react and form oxides and be
removed from the atmosphere
2. a source of energy – early Earth was a place of
high energy
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violent thunderstorms with torrential rainfall
widespread volcanic activity
bombardment from meteorites (caused cataclysmic
changes in crust, ocean, and atmosphere)
intense radiation (including UV radiation, since there
was no ozone layer and younger suns emit more UV
light)
3. presence of chemical building blocks –
water, dissolved inorganic minerals
(present as ions), and the gases present
in the early atmosphere
4. time for molecules to accumulate and
react with one another – Earth is
approximately 4.6 billion years old, the
earliest traces of life are approx 3.8
billion years old
Oparin and Haldane’s hypothesis is tested by
Stanley Miller and Harold Urey in the
1950’s
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they designed a closed apparatus that
simulated conditions that presumably
existed on early Earth
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they exposed an atmosphere rich in H2,
CH4, H2O, and NH3 to an electrical
discharge to simulated lightening
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analysis of the chemicals produced in a
week revealed that amino acids and other
organic molecules had formed
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recent evidence indicates that organic polymers
may have formed and accumulated on rock or
clay surfaces (rather than in a “primordial soup” in
the sea)
clay consists of microscopic particles of
weathered rock and may have acted as a site for
early polymerizations because it binds organic
monomers and contains zinc and iron ions that
might have served as catalysts
lab experiments using clay have confirmed that
organic polymers form spontaneously from
monomers on hot rock or clay surfaces
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Protobionts – scientists have been able to
synthesize several different protobionts
(assemblages of abiotically produced organic
polymers)
exhibit many characteristics of living cells –
division after growth, maintaining an internal
environment different from the external fluids
Microspheres
• protobionts formed by adding water to polypeptides
• microspheres show an electrical potential, may
absorb materials from the surrounding environment
• microspheres may give clues as to the evolution of
the cell membrane
• membranes are made of phospholipid bilayers with
proteins
• scientists have heated amino acids without water
and produced long protein chains – when water is
added, stable microspheres (coacervates) are
formed
• microspheres can accumulate compounds inside
them and become more concentrated than outside,
they also attracted lipids and formed a lipid-protein
bilayer around them
Protobionts
Microsphere
Liposome
The first cells probably assembled from organic
molecules
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Cells were evident in microfossils 3.5
billion years old, perhaps even 3.8 billion
years ago
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The first cells were prokaryotic
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Stromatolites offer more fossil evidence –
rocklike columns composed of many
minute layers of prokaryotic cells (usually
cyanobacteria)
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living stromatolite reefs are still found in
hot springs and in warm, shallow pools of
fresh and salt water
Fossilized Stromatolites
– 3.5 billion years old
Modern day
stromatolites
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A crucial step in the origin of cells was
molecular reproduction
both DNA and RNA can form
spontaneously on clay, so… which came
first?
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RNA is self-catalytic and is believed to have
appeared first (according to the proposed model
of the “RNA World”)
chemistry of prebiotic Earth gave rise to selfreplicating RNA that functioned both as enzyme
and substrates for their own replication
RNA has catalytic properties – enzymatic RNAs
are called ribozymes (in modern cells, ribozymes
help catalyze the synthesis of RNA and process
precursors into rRNA, tRNA, and mRNA)
ribozymes may have catalyzed the synthesis of
RNA, and processed RNA molecules
RNA could also catalyze protein formation
(catalyzes peptide bonds formation) – protein
catalysis of RNA formation happen later
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DNA probably evolved after RNA – it’s a
more stable molecule
may have evolved from RNA making
double stranded copies of itself
stability of DNA provides advantages as
the information storage molecule
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The first cells were probably heterotrophs
fermented organic molecules from the
aqueous environment – appeared 3.1 –
3.4 billion years ago
first cells were anaerobes, free O2 not
available
as concentration of free organic
molecules in environment declined,
photosynthetic organisms had a selective
advantage
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first photosynthetic organism were autotrophs which split
H2S as a hydrogen donor (purple and green sulfur
bacteria)
the first photosynthetic organisms to use H2O as a
hydrogen donor were the cyanobacteria (released O2 as
by-product)
source of the first free oxygen in aquatic environment and
atmosphere – O2 existed in significant quantities by 2
billion years ago
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Aerobes appeared after oxygen increased in
atmosphere
aerobic respiration was “added” to glycolysis
after free O2 became available
aerobic organisms are much more efficient in
converting glucose to ATP
carbon dioxide produced helped to stabilize
concentration of CO2 and O2 in atmosphere (byproduct of each process – photosynthesis and
aerobic respiration – are raw materials for other
process)
O3 begins to accumulate in upper atmosphere to
form ozone (protection from UV radiation) –
allows organisms to live in more shallow water
and ultimately on land
Evolution of Eukaryotic cells
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evolved from prokaryotes about 2 billion years
ago
Endosymbiont Theory – first proposed by Lynn
Margulis – suggests that mitochondria were
originally independent prokaryotic aerobic
organisms which developed a symbiotic
relationship with another prokaryote
aerobic prokaryote was engulfed by endocytosis
but not digested
aerobic prokaryote continued to function and
formed a symbiotic relationship with host
similar process occurred later with the host cell
and photosynthetic prokaryotes (which became
chloroplasts)
other evidence:
• mitochondria and chloroplasts grow and divide
like cells
• they have a naked loop of DNA like prokaryotes
• they synthesize some of their own proteins using
70s ribosomes, like prokaryotes
• they have double membrane as expected since
cells were taken into a vesicle by endocytosis
• cristae are similar to mesosomes of prokaryotes
• thylakoids are similar to structures containing
chlorophyll in photosynthetic prokaryotes
Theories for the Origin of Species
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Lamarck
Darwin-Wallace
Panspermia – life may have originated
elsewhere and come to Earth from space
Comets contain a variety of organic
compounds and may have delivered both
organic compounds and water to early
Earth
Special Creation – followers of many
religions believe that God created life
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Evidence for Evolution
Evidence from geographical distributions (biogeography)
if evolution did not occur, we would expect to find a given
species everywhere that it could survive – in reality this
does not occur
example – Australia (a separate land mass for millions of
years) has distinctive animals (marsupials and
monotremes) not found anywhere else
200 million years ago, Australia and other continents
were joined together (Pangea)
Australian continent gradually separated from others
monotremes and marsupials continued to thrive and
diversify in Australia
isolation of Australia prevented placental mammals
(which arose elsewhere at a leter time) from competing
with monotremes and marsupials
in other areas of the world, placental mammals
outcompeted the marsupials and monotremes (which
were eliminated)
Evidence from biochemistry
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all living organisms use DNA (or RNA) as their genetic
material
all use the same universal genetic code
all use the same 20 amino acids in their proteins
Variations in specific biochemical molecules can indicate
phylogeny (line of evolutionary evidence)
– phylogeny can be studied by comparing the structure
of a protein or other biochemical
– hemoglobin is often used – amino acid sequences of
vertebrates are compared for the number of
differences
– the more difference there is, the longer the timespan
since the two species had a common ancestor
– evidence seems to show that differences tend to
accumulate at a constant rate – differences can be
used as an evolutionary clock
Evidence from homologous anatomical
structures
• at an early age, vertebrate embryos are
very similar
• limbs of vertebrates show striking similarities in
their bones, despite being used in many different
ways
• ex. pentadactyl limb – this limb has a basic
pattern of bones including 5 digits (fingers/toes)
– pentadactyl limb is used differently by different
mammals – appearance varies but same basic
structure
Evidence from Paleontology – based on fossil record
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fossils are difficult to explain without evolution
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examples: evolution of horse, Archaeopteryx
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fossil record provides direct evidence for evolution
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fossils range from bone, teeth, shells, to actual body parts
preserved in bogs, tar, amber or ice
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Fossils most commonly form in aquatic environments and
typically form in sedimentary rock
Various methods are used to determine the age of fossils
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Radioactive dating – radioisotopes decay in a
characteristic and known rate
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Half-life is the amount of time required for 50% of the
radioisotope to decay – at the end of this period of time
(any where from fractions of seconds to thousands of
years, depending on isotope) radioactivity will be half of
what it was before
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Carbon dating – excellent method of determining
the age of organic material (relatively young, tens
of thousands of years old)
uses radioactive isotope of carbon, Carbon-14
14C is unstable and spontaneously changes to 14N
all living organisms contain 14C in the same
proportion as found in the atmosphere
when organisms dies, process of incorporating
new carbon into body stops (ie, feeding)
half-life of carbon is 5730 years
Potassium-40 is used to date very old fossils
(hundreds of millions of years old)
40K decays to form 40Ar
half-life of Potassium-40 is 1300 million years
Modern examples of observed evolution
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Darwin’s finches – finches of the Galapagos islands
all finches range in size, 10-20 cm long and are of
brownish or black coloration
shapes of beaks varied greatly from island to island
based on different feeding habits resulting from different
selective pressures
Rosemary and Peter Grant studied two species of
finches on the Galapagos – Geospiza fortis and
Geospiza scandens
G. fortis had a short, wide beak for feeding on large, hard
seeds - during 1982-83 there was a severe El Nino
bringing heavy rain and changing the vegetation – by
1991, only 37% of the original population remained and
those on average had longer more narrow beaks
(compared to the original population) to eat the new
smaller softer seeds being produced
the population of G. scandens on the same island did not
change over that time since its food and diet remained
the same
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other examples include the Peppered
Moth in England, insect resistance to
pesticides, antibiotic resistance in
bacteria (through mutations or by
transfer of plasmids with resistance
genes btw bacteria)