Ch. 17 The History of Life

Ch. 17 Ch. 17 The History of The History of LifeLife

17.1 How Did Life Begin?17.1 How Did Life Begin?

17.2 What Were the Earliest 17.2 What Were the Earliest

Organisms Like?Organisms Like?

17-1 How Did Life Begin?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?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-from-meat idea– No maggots developed when he kept flies away

from uncontaminated meat

17-1 How Did Life Begin17-1 How Did Life Begin??

17.1 How Did Life Begin17.1 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


The broth in a flask is boiled to kill preexisting

microorganisms

The long, S-shaped neckallows air, but not microorganisms

to enter the flask

If the neck is later broken off, outside air can carry

microorganisms into the broth

• 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 Begin17.1 How Did Life Beginhttp://education-portal.com/academy/lesson/the-origin-of-life-on-earth-theories-and-explanations.html#lesson (good intro to Primordial soup and Oparin and Mill-Urey Exp)

ammonia (NH3) hydrogen gas (H2) water vapor (H2O) methane gas (CH4)

EARLY EARTH CONDITIONS

high temperatures frequent volcanoes electrical storms comets


• 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)


Urey-Miller Experimental ApparatusUrey-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

CH4 NH3 H2 H2O

electric sparkchamber

condenser

An electric spark simulatesa lightning storm

Organic moleculesappear after a few

days

Energy from the sparkpowers reactions amongmolecules thought to bepresent in Earth’s early

atmosphere

When the hot gases inthe spark chamber are

cooled, water vaporcondenses and anysoluble molecules

present are dissolved

Boiling water addswater vapor to the

artificial atmosphere

boiling chamber

water

cool waterflow

17.1 How Did Life Begin17.1 How Did Life BeginMiller-Urey Experiment

• 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)



• 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

• 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/through-the-wormhole/videos/through-the-wormhole-from-rna-to-dna.htm


• 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

• 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


• 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)


• 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

17.2 What Were the Earliest Organisms Like?17.2 What Were the Earliest Organisms Like?

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

http://education-portal.com/academy/lesson/the-history-of-life-on-earth-timeline-and-characteristics-of-major-eras.html#lesson (cuts off after a while)

How Do We Know How Old a Fossil Is? 17-1How 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-livesHalf-lives

256 14C atoms at time 0

128 14C and

128 14N atoms

after 5,730 years or

1 half-life

Half-livesHalf-lives64 14C and

192 14N atoms


2 half-lives

32 14C and

224 14N atoms


3 half-lives


240 14N atoms


4 half-lives

8 14C and

248 14N atoms


5 half-lives


252 14N atoms


6 half-lives2 14C and

254 14N atoms


7 half-lives

Pro

port

ion

of is

otop

e le

ft

1/4

1/8

1/16

1

1/2

30 4 52Half-lives

1

Proportion of Isotope Left vs. Half LivesProportion of Isotope Left vs. Half Lives

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 C-12 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

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? How much Thorium is left?

225-75 = 150

150-75 = 75 40/2 = 20g

75-75 = 0 20/2 = 10g

3 half-lives have past 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

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)



• 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?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!!!!

• 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


• Some organisms acquired membrane-enclosed organelles• Ability to compartmentalize functions inside the

cell improved efficiency of early cells• First eukaryotes (membrane- bound organelles) appeared 1.7 b.y.a.


• 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


https://www.youtube.com/watch?v=bBjD4A7R2xU

photosyntheticbacterium

Descendants of thephotosynthetic bacteriumevolve into chloroplasts

The mitochondria-containing cell engulfs aphotosynthetic bacterium

Descendants of theengulfed bacteriumevolve into mitochondria

aerobicbacterium

An anaerobic,predatory prokaryotic cell engulfs an aerobicbacterium

Endosymbiosis Hypothesis

•Evidence? Many distinctive biochemical features shared by eukaryotic organisms and living bacteria


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 evolveC- Endosymbiosis of aerobic prokaryotes (mitochondria) into

anaerobic autotrophic prokaryotesG-Endosymbiosis of autotrophic prokaryotes (chloroplasts) into

aerobic prokaryotesA-Evolution of eukaryotes

F-Evolution of multicellular organisms

Review the followingReview the following::

Ch. 1: Introduction to Life on Earth1.Read Ch. 1 summary of key concepts on pg. 162.Read the Ch. 1 Learning Outcomes on pg. 173.Complete the Chapter 1 Review

- Pg 17; 1-7 and 1-64. Ch. 1 online resources at http://www.masteringbiology.com4.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. 3342. Read the Learning Outcomes (LO1 and LO2) on pg. 3353. Complete the Chapter 17 Review

- Pg 335; any question pertaining to LO1 and LO24. Ch.17-1 &17-2 online resources at http://www.masteringbiology.com4.Chapter 17-1 and 17-2 Vocab

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Ch. 17 The History of Life