Transcript Chapter-181

Life’s Origin and Early Evolution
Chapter 18
Biology Concepts and Applications, Eight Edition, by Starr, Evers, Starr. Brooks/Cole,
Cengage Learning 2011.
18.1 Looking for Life
 Biochemical, genetic, and metabolic similarities
among Earth’s species imply that all like evolved
from a common ancestor that lived billions of
years ago
 Astrobiology
• The scientific study of life’s origin and distribution
in the universe
• Study extreme habitats  determines the range
of conditions that living organisms can tolerate
• High pressure, high temperature, dry
Looking for Life
 All metabolic reactions on Earth involve
interactions that occur in water solutions
• Conclusion  liquid water is considered an
essential requirement for life
 Life on Mar’s, Is so…
• it supports the hypothesis that life on Earth arose
as a consequence of processes that occurred
throughout the universe
• Find extraterrestrial microbes… this could mean
intelligent life in the universe is possible
18.2 Beginnings: The Big Bang
 Earth formed approximately 4.6 billion years ago
Big Bang Theory
 Big Bang Theory
• Model describing the formation of the universe as
a nearly instant distribution of matter through
space
• The universe began 13-15 billion years ago
• The Event:
• all existing matter and energy appeared and
exploded outward from a single point
Big Bang Theory
 The Event:
• Single elements of hydrogen and helium formed
• Over millions of years, gravity drew the gases together
and formed giant stars and eventually galaxies
• Asteroids collided and merged together
• The heavier the asteroid (pre-planetary rock)  the
more gravitational pull they exerted and the more
material they gathered
• 5 billion years ago  the sun was formed
• 4.6 billion years ago  Earth and planets in our solar
system formed
Conditions on Early Earth
 Scientists study stars and space to discover clues
about how our universe originated
 Organic compounds spontaneously self-assemble
under conditions possible on the early Earth
• First Air Contained  water vapor, carbon dioxide,
and gaseous hydrogen and nitrogen
• LITTLE OR NO OXYGEN
• Later, life began in salty runoff pooled in early seas
18.3 Stanley Miller’s Experiment
 Proposed that reactions in Earth’s early
atmosphere could have produced building
blocks for the first life
 Experiment
• Procedure: Placed water and gases (methane,
ammonia, and hydrogen gas) into a reaction
chamber
• Observation: As the mix circulated, sparks from
electrodes simulated lightning
• Result: Within weeks, a variety of amino acids
and other small molecules formed
18.3 Stanley Miller’s Experiment
Reactions at Hydrothermal Vents
 Hydrothermal vent
• Rocky, underwater opening where mineral-rich
water heated by geothermal energy steams out
• Reactions at vents can produce organic building
blocks
• Experiment:
• Hot water + carbon monoxide + potassium cyanide
+ metal ions (like those near vents) 
• Result: amino acid formation
Key Concepts: ABIOTIC SYNTHESIS
OF ORGANIC COMPOUNDS
 When Earth first formed about 4 billion years
ago, conditions were too harsh to support life
 Over time, its crust cooled, seas formed, and
organic compounds of the sort now found in
living cells may have formed spontaneously or
arrived in meteorites
18.4 From Polymers to Cells
 All cells have a
• plasma membrane,
• genome of DNA that is transcribed into RNA and
protein
• All cells replicated and pass on copies of genetic
material to descendants
 Self-replicating genetic systems require proteins
(including enzymes) and nucleic acids
Origin of Metabolism
 Proteins and nucleic acids may self-assemble
when certain conditions are met
• Clay-template hypothesis
• Clay (- charge) attach + charge molecules
• Low tide, evaporation concentrated the subunits
• Concentration and energy from the sun caused the
molecules to bond together as polymers
• Hydrothermal vent hypothesis
• High pressure, high temperature environment
• Iron sulfide in rocks donated electrons to dissolved
carbon monoxide  start reactions that lead to the
formation of larger organic compounds
Protocells:
Origins of the Plasma Membrane?
Laboratory produced protocell
•Lipid bilayer encloses RNA
•Cell grows by adding fatty acids and nucleotides
•Mechanical force causes protocell division
Origin of the Plasma Membrane
 Protocell
• Membranous sac that contains lipid-enclosed
collections of interaction molecules
• It is able to take up materials and replicate itself
• Hypothesized to have formed prior to the earliest
life forms
membrane-bound proto-cells
living
cells
Self-replicating system enclosed in a
selectively permeable, protective lipid sphere
DNA
RNA
formation of
protein-RNA systems,
evolution of DNA
enzymes and
other proteins
formation of
lipid spheres
Proposed
sequence
for the
evolution
of cells
spontaneous formation of lipids,
carbohydrates, amino acids, proteins,
nucleotides under abiotic conditions
Fig. 18.7, p.295
Origins of Self-Replicating
Genetic Systems
 Hypothesis: RNA world
• RNA stores genetic information, but breaks apart
easily and mutates often
• Ribozymes: Catalytic RNAs
 Switch from RNA to DNA
• Makes the genome more stable
• Defense against viruses that attack RNA-based
cells
18.5 Life’s Early Evolution
 3.8 billion years ago, oxygen levels in
atmosphere and seas were low
• Early prokaryotic cells probably were anaerobic
• Stromatolites
• Dome-shaped structures composed of layers of
bacterial cells and sediment
• Cyanobacteria and photosynthetic bacteria grow
here
 Divergence separated bacteria from ancestors
of archaeans and eukaryotes
Stromatolites
The Oxygen Atmosphere
 Cyanobacteria evolved an oxygen-releasing,
noncyclic pathway of photosynthesis
• Noncyclic photosynthesis arose through
mutations and modified the cyclic pathway
• Changed Earth’s atmosphere
 Increased oxygen favored aerobic respiration
• ATP-forming metabolic pathway
• Key innovation in evolution of eukaryotic cells
The Oxygen Environment
 Consequences of Life
1. Oxygen interferes with self-assembly of complex
organic compounds. Life no longer could arise from
nonliving materials
2. Oxygen put organisms that thrived in aerobic
conditions at an advantage. Aerobic respiration
allowed the evolution of multicelled eukaryotes
3. Some oxygen molecules broke apart and then
recombined as ozone (O3). The ozone layer
reduced the amount of solar UV radiation.
•
Without the ozone life could not have moved onto
land
Rise of Eukaryotes
 Eukaryotic cells branched off from archaean lineage
 Lipids are biomarkers for eukaryotes
• Biomarker  molecule produced only by a specific
type of cell (molecular signature)
 Red alga  1.2 billion years ago
• Oldest species to reproduce sexually
 870 mya  sponge-like animals
 543 mya  Animal diversity increased greatly during
a great adaptive radiation.
• All major animal lineages, including vertebrates were
represented in the seas
Key Concepts: ORIGIN AND
EARLY EVOLUTION OF CELLS
 Laboratory experiments and advanced computer
simulations support the hypothesis that
forerunners of living cells arose through known
physical and chemical processes, such as
tendency of lipids to assemble into membranelike structures when mixed with water
Key Concepts: ORIGIN AND
EARLY EVOLUTION OF CELLS (cont.)
 First cells probably were anaerobic prokaryotes
• Some gave rise to bacteria, others to archaeans
and to ancestors of eukaryotic cells
 Photosynthetic bacteria started releasing free
oxygen into the atmosphere
• Oxygen accumulated over time and became a
global selection pressure
18.4 Where Did Organelles Come From?
 Eukaryotic internal membranes may have
evolved through infoldings of cell membrane
DNA
infolding of plasma membrane
Fig. 18.10a, p.298
Fig. 18.10b, p.298
Endosymbiosis
 Endosymbiosis  One cell lives and reproduces
inside another
 Host and guest cells come to depend upon one
another for essential metabolic processes
 Mitochondria and chloroplasts may have
evolved by endosymbiosis
Key Concepts: HOW THE FIRST
EUKARYOTIC CELLS EVOLVED
 A nucleus, ER, and other membrane-enclosed
organelles are among the defining features of
eukaryotic cells
 Some organelles may have evolved from
infoldings of the plasma membrane
 Mitochondria and chloroplasts probably are
descendants of bacterial cells that became
modified after taking up residence in host cells
18.7 Time Line: Life’s Origin and Evolution
Fig. 18.12b, p.300
Key Concepts: VISUAL PREVIEW OF
THE HISTORY OF LIFE
 Key events in life’s origin and early evolution can
be correlated with the geologic time scale
 A time line for milestones in the history of life
offers insight into shared connections among all
organisms
Animation: Milestones in the history of
life
Animation: Miller's reaction chamber
experiment
Animation: Origin of organelles