Origins of Life on Earth
Where did life come from?
Early Ideas
Spontaneous Generation
 Believed life “arose” from nonliving objects.
 Mice came from piles of grain
 Bees were produced from carcasses of cattle
 Maggots came from rotting meat
 Lice from sweat
Francesco Redi
1668
 Hypothesized maggots arose from eggs that
were to small to see with the naked eye.
 Tested his hypothesis by placing meat in
several jars.
 ½ open to air directly
 ½ covered with gauze
 Gauze covered were exposed to air
 Maggots formed in open jars
 No maggots formed in gauze covered jars
 DISPROVED SPONTANEOUS GENERATION!!!
Francesco Redi
Francesco Redi
A
B
C
John Needham
o
Attacked Redi’s conclusions
o
Sealed gravy in a bottle
o
Heated the bottle to kill the living organisms in it.
o
After several days, examined the bottle under a microscope
o
Found it swarming with microscopic organisms
o
Concluded the microorganisms “could only have come from
the juice of the gravy.”
Lazzaro Spallanzani
» Considered “Spontaneous Generation”
» Knew of Redi’s & Needham’s experiments
» Believed Needham was wrong.
» Thought Needham didn’t kill bacteria by heating.
Lazzaro Spallanzani
» Prepared identical gravy to Needham
» Put ½ the gravy into one jar, the other ½ into another jar
» Boiled both jars
» Sealed one, left the other open to the air
» After a few days, the open one was teeming with life
» The sealed jar had no living things in it.
Lazzaro Spallanzani
» Spallanzani concluded that the microorganisms did not come from the
gravy, but entered the jar through the air (Air was a Vital Force).
» People still believed that Spallanzani’s work was flawed.
» They said that air was necessary for spontaneous generation.
» By sealing the jar, the air was kept out.
Louis Pasteur
 Finally disproved “Spontaneous Generation”
 Placed nutrient broth similar to Needham and
Spallanzani into some flasks with long curving necks
 Boiled the flasks thoroughly
 Waited for one year
1864
Louis Pasteur
Louis Pasteur
 No microorganisms formed – even though open to the air
 After a year, he broke the neck of some of the flasks
 All developed microorganisms
 Conclusion: Life from Life
Where did life come from?
That’s the million dollar question
Early Ideas
First atmosphere contained
 Water vapor (H2O)
 Carbon Monoxide (CO)
 Nitrogen (N2)
 Hydrogen Sulfide (H2S)
 Hydrogen Cyanide (HCN)
 No free oxygen
 Could not support life as we know it
 Geological evidence supports this because rocks from this
period contain no iron oxide (Rust) or other compounds
that require free oxygen to form
The Molecules of Life
 Stanley Miller and Harold Urey
 Miller recreated what he thought might have been earth’s
earliest atmospheres by mixing
-ammonia
-water
-hydrogen
 in a flask & exposed the flask to electric sparks (Lightning)
Miller-Urey
Organic Soup?
In a few days, “Organic Soup” – complex molecules
- urea
- acetic acid
- lactic acid
- several amino acids
- ATP
- Nitrogenous base adenine
Complex Molecules of Life
Formation of Complex Molecules
 Russian Alexander Oparin and American Sidney Fox showed
that the organic soup that formed in the oceans could form
other compounds
 In the absence of Oxygen, amino acids tend to link together
to form chains.
What do chains of amino
acids form?
Proteins
Protolife
Others compounds form carbohydrates,
alcohols, and lipids
Collections of molecules like this tend to
gather into round droplets
Some of these droplets grow and even
reproduce
Some break down glucose
These droplets are called “proto-life” because
they are not “alive”, but have begun to
perform tasks necessary for life
Protolife to Cells
DNA & RNA must have somehow been
created molecules can replicate
First Cells
Several hypotheses
Some believe first cells came from a
shallow pool containing organic soup
when the organic soup is dried, lipids
form spheres around small DNA
molecules
Given time, a form of DNA that is
capable of replicating could have arisen
First Cells
G. Cairns-Smith and J. Bernal:
noted attractive forces concentrate
amino acids, DNA and RNA onto clay
crystals
Held together, these may have formed
lengths of proteins and DNA
First Cells
Several scientists believe the first cells formed
near volcanic vents deep in the ocean
Very high temperatures
Lots of sulfur compounds
Assortment of chemicals
Strong currents to mix them
Deposits of clay
First Cells
scientists who duplicated these conditions
observed the spontaneous synthesis of amino
acids and RNA
First True Cells
First cells were prokaryotes
Resembled some bacteria
Heterotrophs
Anaerobes (can live without oxygen)
Photosynthesis
Heterotrophic cells could have survived for a
long time without difficulty finding “Food”
Sooner or later, the food would run out.
Complex molecules in the organic soup would
be depleted.
Some organisms would have to develop
complex molecules from simple ones
Natural selection would favor ability to
harness outside energy source
Stromatolites
First cells probably used Hydrogen
Sulfide instead of Water
Successful, spread rapidly and common
3.4 million years ago
Grew in mats called Stromatolites
Still can find stromatolites in special
habitats, but fossils have been found all
over the world
Stromatolites
Life from Non-life
We said that life did not come from
non-life, how did life start?
Earth then … and now
Earth of today is very different from earth of
billions of years ago.
No oxygen to break down organic compounds
Today, such compounds cannot remain intact
in the natural world long enough to give
another start to life
Road to Modern Organisms
2.2 billion years ago, a more modern form of
photosynthesis evolved.
The use of H2O instead of H2S
Deadly gas?
This put out deadly gas …
OXYGEN!
Effects of photosynthesis
Over the next 500 Million years, the
waste product given off by some
organisms changed the earth from an
anaerobic planet to an aerobic planet.
Nearly 1/5 Oxygen
Oxygen
Because oxygen was toxic to anaerobes, they
had to survive in areas with no oxygen such
as deep in mud, or other places oxygen does
not reach.
Oxygen was beneficial to those that survived
Oxygen that reached the upper atmosphere
was changed from O2 to O3 (Ozone) that
shielded earth from much of the UV light
from the sun.
Aerobic Metabolism
Evolution of Aerobic Metabolism
organisms evolved that used Oxygen in
their metabolic pathways (remember
chapter 6)
18X more efficient than anaerobic
respiration
Eukaryotes
Eukaryotic cells
between 1.4 and 1.6 billion years ago,
the first eukaryotic cells evolved.
Endosymbiont Hypothesis
Endosymbiont Hypothesis
Lynn Margulis
Noticed Chloroplasts and Mitochondria both
had their own DNA
Circular piece of DNA similar to bacterial
plasmid
Hypothesized that organelles were originally
prokaryotes that somehow ended up inside
another cell
Reproduction
Sexual reproduction
Sexual reproduction was an extremely large
step towards evolution
Asexual reproduction makes exact copies of
cells. Only chance of change is by mutation.
Sexual reproduction shuffles genetic material
every time.
offspring never are exactly like parents
Multicellular
A few hundred million years after
evolution of sexual reproduction –
multicellular life
Great adaptive radiation
Life was on its way.