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Chapter 25
Great Idea:
All life on Earth evolved from single-celled
organisms by the process of natural selection.
Chapter Outline
• The Fact of Evolution
• Chemical Evolution
• Natural Selection and the Development of
Complex Life
• The Evolution of Human Beings
• How did life start- origin of life.
• How did the astonishing variety of life that
we observe come about.
• Fossil record.
– Fossils- minerals replace the natural materials
of the “hard parts” of organisms.
• Mass extinction.
• Genetics
The Origin of Life
What are the properties of living systems?
1. Self duplication with information transfer.
2. Discrete change.
Living systems are systems that reproduce themselves
closely, but that mutate as well and can reproduce their
The word evolution simply means a change in
genetic content.
Populations evolve but individuals do not.
The Fact of Evolution
The Fact of Evolution
• Evolution
– Ongoing process of change
• Scientists accept evolution as fact
– Debate various theories
There are two approaches to tracing species:
1. Examine the fossil record - Paleontology
2. Examine the genes of living organisms Molecular genetics
The Fossil Record
• Fossils
– Organism’s hard parts preserved
• Turned to rock
• Replaced by minerals
– l
• Fossil record
– Fossils found, catalogued & analyzed
– Shows transitions
– Incomplete
• Three key ideas
– Older fossils more different
– Increasing complexity with time
– Most species have gone extinct
The Biochemical Evidence
– Evidence for evolution
– Changes slowly
– Also compare amino acid sequences
• Cytochrome C
– Humans and chimps identical
– Rattlesnake 86% overlap
Evidence from Anatomy:
Vestigial Organs
• Vestigial organs
– Internal features
– No useful function
• Example
– Appendix: humans
– Wings: penguins
– Hind legs: whales
Chemical Evolution
Life requires: Carbon, Nitrogen, Oxygen, Hydrogen
Reducing Atmosphere:
Carbon - CH4
Nitrogen - NH3
Hydrogen, Oxygen - H2O
Non-reducing Atmosphere:
Carbon - CO2
Nitrogen - N2
Hydrogen, Oxygen - H2O
There are considerable questions
regarding the atmosphere of the
primitive Earth.
It is clear that there was no free oxygen.
Free oxygen did not occur until living organisms
evolved photosynthesis.
The Miller-Urey
experiment. Several
of the chemical
compounds thought
to have been
present on the early
Earth were mixed
and subjected to
discharges. Within
a few weeks, amino
acids had formed.
The spark is like lightning!
Stanley Miller's experiment, (1953)
The spark is like lightning!
bases, and
amino acids formed
Some Origin of Life Scenarios
1. Life began in hydrothermal vents
2. Inorganic catalysis was important
a. Clay minerals were the first living things
b. Nitrogen fixation by oxides
3. Extraterrestrial origin of life
RNA Enzymes
• Early life was simpler
– RNA catalyzed reactions
– Created proteins
– Proteins led to DNA
The Window of Opportunity
• Chemical processes
– Occurred rapidly
• Recent discoveries
– Schopf
• Evidence of life 3.5 billion years ago
• First Cell
– 4.0-3.5 billion years ago
The First Cell
• First cell
– No competition
– Multiplied rapidly
• Special characteristics
– 20 amino acids
Black Smokers
• Black Smokers
– Deep-ocean floor
– Vents of mineral rich hot
– Diverse ecosystem
• Most primitive bacteria
• Hydrothermal zones
– Site of life’s origin?
• Protected
• Ideal chemical environment
Life three miles deep at a midAtlantic hydrothermal vent.
Natural Selection and the
Development of Complex Life
Natural Selection
• Darwin
• Natural Selection
– Variation in populations
• Some variation heritable
– More individuals born than
will survive
• Adaptation
• Artificial selection
The Story of Life
• First cell
– Natural selection
• mutations
• Mutations
– Most not beneficial
• Environment
– Impacts evolution
• Eukaryotes
• Colonies
• Hard Shell
– Cambrian explosion
Geological Time
Punctuated Equilibria
Mass Extinctions and the
Rate of Evolution
• Rate of extinction
– 10%-20% extinct in 5-6 million
• Mass extinctions
– 30%-90% extinct
• Mechanisms
– asteroid
• Evolution
– Gradualism
– Punctuated equilibrium
Mass extinction
The geological record show a number of instances in
which the number of different species dropped
dramatically These are mass extinctions brought about
by some change in the environment:
Meteor impact
Global climate change
Glaciation (Ice ages)
Continental Drift
Global warming (rise in sea levels)
The diversity of life on Earth has not increased steadily, but
has gone through a series of sharp changes.
This graph of marine animals in the fossil record is
indicative of the overall growth in families and of
The extinction of the dinosaurs is labeled 5.
The location of the giant crater near the Yucatán
Peninsula in Mexico. The effects of the asteroid’s
impact were recorded in rocks as far away as the red
dashed circle.
The Evolution of
Human Beings