exam 1 (the history of life on earth Flashcards
Some hypotheses (how and where) about the origin of life on earth
- Spontaneous (not all at once!), abiotic
generation, “primordial soup” hypothesis
(most hypotheses) - Occurred in a “warm little pond” (Darwin)
- Chemical evolution hypothesis (A.I. Oparin
and J.B.S. Haldane, 1920s) - Occurred around hydrothermal vents
(Corliss, 1977; others since) - Volcanic activity helped to jump start life
- Life had an extraterrestrial origin
Fossil Evidence in Ancient Rocks
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)
Where did life originate?
• One hypothesis is that life came about at deep hydrothermal vents • Chemistry • Gradients in temperature, pH, chemicals • Energy source • Protected, stable environment
What are the minimum
requirements of life?
• 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)
Chemical evolution hypotheses
envision a series of significant steps in
the evolution of life. For example:
- Small organic molecules (e.g. fatty acids, amino
acids, simple sugars, nucleotides) form from
simple inorganic molecules and accumulate - Simple organics join together to form larger more
complex macromolecules (polymers such as
proteins, polysaccharides, nucleic acids) - Membrane bound “protocells” form
- “Genetic” informational molecules that can selfreplicate form
- Metabolic pathways form
Approaches to answering questions,
testing hypotheses about the origin of
life
• 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
Experimental evidence and tests of the
chemical evolution hypothesis
• 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
Protocells
• 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”
What was the First Genetic Molecule?
• 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?
The evidence for RNA (ribozymes) as
the first hereditary molecule
• 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
The Geological Record and Major
Events in the History of Life
• 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
Photosynthesis and the generation of
oxygen
• Solar energy is transformed into chemical
energy which is used to synthesize organic
molecules (for food and materials) from
inorganic molecules
Photosynthesis on early earth
• 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)
Oxidative Metabolism (cellular respiration, aerobic metabolism)
• 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)
How and when did first eukaryotes
evolve?
• 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
