exam 1 (the history of life on earth

Question 1

Some hypotheses (how and where) about
the origin of life on earth

Answer 1

1. Spontaneous (not all at once!), abiotic
   generation, “primordial soup” hypothesis
   (most hypotheses)
2. Occurred in a “warm little pond” (Darwin)
3. Chemical evolution hypothesis (A.I. Oparin
   and J.B.S. Haldane, 1920s)
4. Occurred around hydrothermal vents
   (Corliss, 1977; others since)
5. Volcanic activity helped to jump start life
6. Life had an extraterrestrial origin

Question 2

Fossil Evidence in Ancient Rocks

Answer 2

It has been thought that earth was not
habitable for life until about 4.2-3.9 billion
years ago. But recent evidence is
extending that earlier
• Fossils of early prokaryotes 3.5 by; more
recent discoveries 3.77 by old
• Carbon isotopic evidence of life has been
aged at 3.95 billion years (controversial)

Question 3

Where did life originate?

Answer 3

• One hypothesis is that life came
about at deep hydrothermal vents
• Chemistry
• Gradients in temperature, pH, chemicals
• Energy source
• Protected, stable environment

Question 4

What are the minimum

requirements of life?

Answer 4

• Water
• Raw materials: Inorganic precursors from
which organic molecules can be built
• C, O, N, H, various minerals needed to
form organic molecules (building blocks
for proteins, lipids, carbohydrates,
nucleic acids)
• Energy
• Appropriate temperature range
• A reducing or neutral environment (lack of
oxygen and other oxidizing molecules)

Question 5

Chemical evolution hypotheses
envision a series of significant steps in
the evolution of life. For example:

Answer 5

1. Small organic molecules (e.g. fatty acids, amino
   acids, simple sugars, nucleotides) form from
   simple inorganic molecules and accumulate
2. Simple organics join together to form larger more
   complex macromolecules (polymers such as
   proteins, polysaccharides, nucleic acids)
3. Membrane bound “protocells” form
4. “Genetic” informational molecules that can selfreplicate form
5. Metabolic pathways form

Question 6

Approaches to answering questions,
testing hypotheses about the origin of
life

Answer 6

• Study the geological, fossil record
• Attempt to simulate conditions on early
earth in the laboratory (most common)
• Biochemical “modeling”
• Verify, test possible mechanisms for each
step in the chemical evolution of life
• Attempt to create, under laboratory
conditions, simple forms of “life”
(protocells) that possess some of life’s
main characteristics (e.g. reproduction,
metabolism, growth, evolution)
**Important to appreciate that the actual
process had many millions of years to

Question 7

Experimental evidence and tests of the

chemical evolution hypothesis

Answer 7

• Stanley Miller (Harold Urey)—1953; tested
Oparin and Haldane hypothesis
• Simulated what they thought to be the
chemical composition of earth’s ancient
atmosphere
• Provided energy input in the form a spark
• Collected an analyzed the chemicals
formed
• After one week, numerous types of
amino acids were identified as well
as some fatty acids
• Many more recent experiments,
simulating various environments and
conditions since
• These experiments produced:
– Amino acids, fatty acids, sugars,
components of nucleotides,
phospholipids

Question 8

Protocells

Answer 8

• Abiotic vesicles with membrane-like bilayers
– Can maintain an internal environment separate
from external environment
– spontaneously form when organic molecules (like
lipids) are mixed together in water; may also have
involved micro air bubbles
• These vesicles
– “grow”
– “reproduce”
– Have selective permeability, can absorb substances
– Can carry on metabolic reactions
• The field of “synthetic biology”

Question 9

What was the First Genetic Molecule?

Answer 9

• A paradox surrounds the hypothesis that the
first genetic molecule was one of those three
molecules. What is this paradox?
• In order for DNA or RNA to make proteins,
need pre-existing proteins (enzymes)
• Is there another molecule, other than DNA,
that could fulfill the requirements of a
genetic molecule?

Question 10

The evidence for RNA (ribozymes) as

the first hereditary molecule

Answer 10

• Can carry information
• Has variable 3D (tertiary) structure
• Catalytic, self replicating (can catalyze their
own synthesis)—acts as an enzyme as well
as a carrier of information
• Experiments have demonstrated that it can
mutate and undergo natural selection
• The “RNA World” hypothesis
Experiments are underway to
determine how RNA may have formed
spontaneously
• The molecule is made up of sugar,
phosphate, and nucleobase
• Each of these had to form spontaneously,
then bind together, then polymerize
• If RNA was the first genetic molecule, life
later “adopted” DNA. Why?
• Newer experimental evidence has
generated building blocks of RNA, amino
acids (building blocks of proteins), and a
precursor to lipids that form cell
membranes
• Generated simultaneously from the same
simple reactants thought to be present on
early earth (hydrogen cyanide, hydrogen
sulfide, carbon containing molecules,
phosphates)
• Still must demonstrate how they could

Question 11

The Geological Record and Major

Events in the History of Life

Answer 11

• Earth’s origin: 4.6 billion years ago
• From 4.5 to about 3.8 bya, earth was hellish.
Debris impacts, ocean boiling away, covered
with molten rock.
• Earliest evidence of life: 3.5-3.95 bya
• Thus, life may have originated about . . . . .?
• First oxygen started to accumulate about
2.7 bya, probably mostly from cyanobacteria
• The world was populated solely by
prokaryotes until about 1.8 bya
• About 2.35 bya oxygen levels spiked due to
the activities of photosynthetic
cyanobacteria. This is known as the Great
Oxidation Event
• The first evidence of eukaryotic cells appears
1.8 billion years ago

Question 12

Photosynthesis and the generation of

oxygen

Answer 12

• Solar energy is transformed into chemical
energy which is used to synthesize organic
molecules (for food and materials) from
inorganic molecules

Question 13

Photosynthesis on early earth

Answer 13

• The origin of photosynthesis, and associated
accumulation of O2
in the atmosphere, were
incredibly important events in earth’s history.
Why?
– Facilitated the evolution of oxidative
metabolism (cellular respiration)
– Led to the origin of chloroplasts
– Led to the origin of eukaryotes, more
complex life (ancestors of all present day
organisms) including animals
– Led to the accumulation of ozone (screens
damaging UV radiation)

Question 14

Oxidative Metabolism (cellular
respiration, aerobic metabolism)

Answer 14

• A very efficient metabolic pathway that
releases energy from complex organic
compounds
• Adopted by almost all organisms on earth
• Complex organic molecules are oxidized to
release energy which is transformed into
chemical energy (ATP)

Question 15

How and when did first eukaryotes

evolve?

Answer 15

• Eukaryotic cells are very different, much
more complex than prokaryotes (internal
membrane systems, mitochondria,
chloroplasts, etc.)
• Appeared 1.8 bya, after the GOE
• How did this significant leap in complexity
arise?
• The endosymbiont theory (Lynn Margulis):
– Chloroplasts and mitochondria were
previously free-living prokaryotes
– Became associated as internal symbionts
within larger cells

Question 16

The Cambrian Explosion

Answer 16

• First fossilized animals—about 650 mya
• 535-525 mya during the Cambrian Period
• Significance?
• A “big bang” of animal evolution
• Perhaps most phyla of animals living today
had their origins around this period of time
and just before it
• The span of time is considered very short
• Evolutionary rates estimated to be 5 times
faster than average rates

Question 17

Colonization of Land

Answer 17

• Occurred in stages
• The terrestrial environment required new
adaptations relating to:
– Resisting desiccation
– Support to contend with gravity
– Reproduction
• Prokaryotes
• Fungi, plants
• Arthropods and other invertebrates
• Tetrapods