Ch. 17
The History of Life
17.1 How Did Life Begin?
17.2 What Were the Earliest
Organisms Like?
17-1 How Did Life Begin?
A. Pre-Darwinian thought held that all species were
simultaneously created by God a few thousand
years ago
B. Until 19th century, most people believed in
spontaneous generation (abiogenesis) where
new organisms sprang up from both non-living
matter.
17-1 How Did Life Begin?
• Spontaneous generation advocates believed
– Maggots were thought to arise from decaying
meat
– Microbes were thought to arise from broth
– Mice were thought to arise from mixtures of
sweaty shirts and wheat
• Francesco Redi (1668) disproved maggots-frommeat idea
– No maggots developed when he kept flies away
from uncontaminated meat
17-1 How Did Life Begin?
17.1 How Did Life Begin
• Louis Pasteur and John Tyndall (mid 1800s)
disproved broth-to-microorganisms idea
– Microorganisms didn’t appear in the sterile broth
unless the broth was 1st exposed to existing
microorganisms
– Experiment demolished the notion of spontaneous
generation but didn’t address how life on Earth
originated in the first place
17.1 How Did Life Begin
The broth in a flask is
boiled to kill preexisting
microorganisms
The long, S-shaped neck
If the neck is later broken off,
allows air, but not microorganisms
outside air can carry
to enter the flask
microorganisms into the broth
17.1 How Did Life Begin
http://education-portal.com/academy/lesson/the-origin-of-life-on-earth-theories-andexplanations.html#lesson (good intro to Primordial soup and Oparin and Mill-Urey Exp)
• Alexander Oparin and John Haldane showed that
spontaneous generation formation of complex
organic molecules necessary for life would NOT
be permitted in today’s oxygen-rich atmosphere
• Oxygen reacts with other molecules breaking
chemical bonds
• An oxygen-rich environment tends to keep
molecules simple
17.1 How Did Life Begin
ammonia (NH3)
water vapor (H2O)
EARLY
hydrogen gas (H2)
methane gas (CH4)
EARTH
high temperatures
electrical storms
CONDITIONS
frequent volcanoes
comets
17.1 How Did Life Begin
• The first living things arose from non-living ones
– Organic molecules can form spontaneously under
prebiotic (before life) conditions
– Stanley Miller and Harold Urey (1953) designed
an experiment that simulated prebiotic evolution
– http://www.ucsd.tv/miller-urey (online Miller-Urey Activity)
• Simulated early Earth’s atmosphere by mixing
the gases in a flask and adding an electrical
charge (simulate lightening)
17.1 How Did Life Begin
Urey-Miller Experimental Apparatus
• Simple organic molecules
appeared in just a few days
• Small molecules likely
present in early atmosphere
can combine to form large
organic molecules (nucleic
acids, amino acids,
proteins, lipids) if electrical
energy is present
17.1 How Did Life Begin
Miller-Urey
Experiment
An electric spark simulates
a lightning storm
electric spark
chamber
CH4
NH3
H2
H2O
Energy from the spark
powers reactions among
molecules thought to be
present in Earth’s early
atmosphere
Boiling water adds
water vapor to the
artificial atmosphere
boiling chamber
condenser
cool water When the hot gases in
the spark chamber are
flow
cooled, water vapor
condenses and any
soluble molecules
present are dissolved
water
Organic molecules
appear after a few
days
17.1 How Did Life Begin
• Modern geochemists believe early atmosphere
was slightly different from that modeled in
Miller and Urey’s experiments
• Additional experiments with more realistic (but
still oxygen-free) simulated atmospheres have
also yielded organic molecules
• Electricity not the only suitable energy source
(heat and UV light)
17.1 How Did Life Begin
• Prebiotic synthesis neither efficient nor fast
• prebiotic molecules threatened by sun’s UV
radiation since Earth lacked an ozone layer
• Ozone layer is a region high in today’s
atmosphere made up of ozone molecules
(O3 – formed when solar energy splits O2 molecules
into individual O atoms which then react with other
O2 molecules)
Ozone
Formation
Ozone layer
• UV radiation can
provide energy for
formation of organic
molecules but can also
break them apart
• organic molecules may
have accumulated under
rock ledges or bottoms of shallow seas away
from UV radiation
17.1 How Did Life Begin
• RNA may have been the first self-reproducing
molecule
– DNA was probably not the earliest informational
molecule
• DNA requires large
complex protein enzymes
–Instructions for
building these enzymes
are coded in DNA
http://www.sciencechannel.com/tv-shows/throughthe-wormhole/videos/through-the-wormhole-fromrna-to-dna.htm
17.1 How Did Life Begin
• RNA can act as a catalyst
(speeds rxns)
– Thomas Cech and Sidney Altman (1980s)
discovered a cellular reaction that was catalyzed by
a protein, a small RNA molecule and coined the
term ribozyme
– Dozens of natural occurring
ribozymes have been discovered
since; found to catalyze reactions
including attaching amino acids to
growing proteins
17.1 How Did Life Begin
• Ribozyme discovery led to hypothesis that RNA
preceded origin of DNA
– first ribozyme continued to evolve and gradually
developed into is present role as intermediary
between
DNA and
protein
synthesis
17.1 How Did Life Begin
• Self-replicating molecules on their own
DON’T constitute life
– membrane-like vesicles
may have enclosed
ribozymes
– Protocells – spherical
collection of proteins &
lipids containing ribozymes
(stepping stone to 1st cells)
17.2 What Were the Earliest Organisms Like?
• Earth formed 4.5 b.y.a and was HOT
– Meteorites smashed into forming planet; kinetic energy
converted into heat on impact
• Geologic evidence suggests Earth cooled
enough for water to exist in liquid form 4.3
b.y.a.
• Oldest fossil organisms found (so far) are
approximately 3.4 b.y.o
• Life arose 3.9 b.y.a. in Precambrian era
How Do We Know How Old a Fossil Is? 17-1
http://education-portal.com/academy/lesson/the-history-of-life-on-earth-timeline-and-characteristicsof-major-eras.html#lesson (cuts off after a while)
1. Relative Dating (before 20th century)
a. Law of Superposition - Fossils in deeper rock
layers were older than fossils found in shallower
rock layers
2. Absolute Dating (1896)
a. Radiometric dating – when the nuclei of
radioactive elements spontaneously break down,
or decay, into other elements
b. Each radioactive element decays at a different
rates
How Do We Know How Old a Fossil Is? 17-1
2. Radiometric dating (cont.)
a. Time it takes for half of a radioactive element’s
(isotope) nuclei to decay at a specific rate is called
half-life.
b. Half-life of Carbon-14 = 5,730 years
Half-life of Potassium-40 = 1.25 billion years
c. Can estimate how much time has passed by
measuring the proportion of decayed nuclei to
undecayed nulcei
Half-lives
256 14C atoms
at time 0
128 14C and
128 14N atoms
after 5,730
years or
1 half-life
64 14C and
192 14N atoms
after 11,460
years or
2 half-lives
32 14C and
224 14N atoms
after 17,190
years or
3 half-lives
Half-lives
16 14C and
240 14N atoms
after 22,920
years or
4 half-lives
8 14C and
248 14N atoms
after 28,650
years or
5 half-lives
Half-lives
4
14C
and
252 14N atoms
after 34,380
years or
6 half-lives
2 14C and
254 14N atoms
after 40,110
years or
7 half-lives
Half-lives
Proportion of Isotope Left vs. Half Lives
Proportion of isotope left
1
1/2
1/4
1/8
1/16
0
1
2
3
Half-lives
4
5
How Do We Know How Old a Fossil Is? 17-1
3. Hyphen notation of radioisotopes (element
symbol and mass number)
Examples: C-12, C-14, O-16, O-18
4. Carbon-14 dating – compare ratio of C-14 and C12 and use the ratio to determine age
ISOTOPE
Carbon-14
Uranium-235
Potassium-40
Uranium-238
HALF LIFE (years)
5,730
704,000,000
1,250,000,000
4,500,000,000
How Do We Know How Old a Fossil Is? 17-1
Sample Problem 1:
The half-life of Thorium-230 is 75,000 years.
If a scientist has 40.0g of Thorium, how
much will remain after 225,000 years?
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
How many half-lives have past?
225-75 = 150
150-75 = 75
75-75 = 0
3 half-lives have past
How much Thorium is left?
40/2 = 20g
20/2 = 10g
10/2 = 5g
5g of Thorium remaining after
225,000 years
2. The half life of carbon-14 is 5,730 years. How long
will it take for ½ of the sample to decay?
1 half-life = 5,370years = ½ the sample decays
3. If a biologist has 64.0g of C-14, how long will it take
until 8.0g remain un-decayed?
64/2 = 32g
1 half-life
5,730 x 3 half-lives =
32/2 = 16g
1 half-life
17, 190 years
16/2 = 8g
1 half-life
3 half-lives
17.2 What Were the Earliest Organisms Like?
The first organisms were anaerobic (don’t require
oxygen to metabolize nutrients) prokaryotes (lack
membrane bound nucleus)
- Obtained nutrients & energy by absorbing organic
molecules from their environment (heterotroph
hypothesis)
17.2 What Were the Earliest Organisms Like?
• Some organisms evolved ability to capture sun’s
energy when organic molecules were used up
(photosynthetic bacteria)
– Photosynthetic organisms
release oxygen
– Oxygen reacted with iron
within Earth’s crust to form iron oxide (rust)
which is why rocks formed during that time contain
lots of iron
17.2 What Were the Earliest Organisms Like?
• Excess oxygen began accumulating in
atmosphere (approx. 2.3 b.y.a) probably
produced by bacteria like cyanobacteria
– Very likely we are breathing some of the recycled
oxygen expelled more than 2 b.y.a!!!!
17.2 What Were the Earliest Organisms Like?
• Aerobic metabolism arose in response to
dangers posed by oxygen
• Oxygen can react with organic molecules
breaking them down
• Oxygen may have exterminated many anaerobic
organisms (natural selection)
• Evolution of aerobic metabolism was significant
because aerobic organisms can harvest more
energy per food molecule than anaerobic
organisms
17.2 What Were the Earliest Organisms Like?
• Some organisms acquired membrane-enclosed
organelles
• Ability to compartmentalize functions inside the
cell improved efficiency of early cells
• First
eukaryotes
(membranebound
organelles)
appeared 1.7
b.y.a.
17.2 What Were the Earliest Organisms Like?
• Mitochondria and chloroplasts may have arise
from engulfed bacteria
• Endosymbiosis hypothesis proposes that early
eukaryotic cells acquired precursors of
mitochondria and chloroplasts by engulfing
certain types of bacteria
aerobic
bacterium
Endosymbiosis
Hypothesis
•
Evidence? Many
distinctive biochemical
features shared by
eukaryotic organisms
and living bacteria
An anaerobic,
predatory prokaryotic
cell engulfs an aerobic
bacterium
Descendants of the
engulfed bacterium
evolve into mitochondria
photosynthetic
bacterium
The mitochondriacontaining cell engulfs a
photosynthetic bacterium
Descendants of the
photosynthetic bacterium
evolve into chloroplasts
https://www.youtube.com/watch?v=bBjD4A7R2xU
17.2 What Were the Earliest Organisms Like?
D-Anaerobic, prokaryotic, heterotrophs evolve from protocells
(heterotroph hypothesis)
B- Evolution of autrophic pathways
chemosynthesis/photosynthesis
H-Anaerobic, prokaryotic, autotrophs evolve (cyanobacteria)
E-Oxygen released by photosynthesizers
I- Aerobic prokaryotes evolve
C- Endosymbiosis of aerobic prokaryotes (mitochondria) into
anaerobic autotrophic prokaryotes
G-Endosymbiosis of autotrophic prokaryotes (chloroplasts) into
aerobic prokaryotes
A-Evolution of eukaryotes
F-Evolution of multicellular organisms
Review the following:
Ch. 1: Introduction to Life on Earth
1. Read Ch. 1 summary of key concepts on pg. 16
2. Chapter 1 Vocab
--------------------Ch. 17-1 and 17-2: History of Life-------------------1. Read Ch. 17-1 and 17-2 summary of key concepts on pg. 334
2. Chapter 17-1 and 17-2 Vocab