1. CHAPTER 25 ORIGIN OF LIFE ON EARTH
Past organisms were very different from those now alive
The fossil record shows macroevolutionary changes over large time scales, for example:
The emergence of terrestrial vertebrates
The impact of mass extinctions
The origin of flight in birds
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2. Figure 25.1
Figure 25.1 What does fossil evidence say about where these dinosaurs lived?

3. So where did the first organic molecules come from?
Miller-Urey and others demonstrated that organic materials can be produced from inorganic molecules under certain circumstances.

4. MILLER-UREY EXPERIMENT
Conditions of Early Atmosphere of Earth Before Life
Products of Experiment
Flask with H, methane, ammonia, H20 (no free oxygen! O2)
Spark (to represent lightning)
Some amino acids, and cytosine and uracil

6. Figure 25.2a
Under Volcanic Conditions (green) the results were even more notable than the original experiment (orange) 20
200 10
100
Number of amino acids
Mass of amino acids (mg)
Figure 25.2 Amino acid synthesis in a simulated volcanic eruption.
1953
2008
1953
2008

7. “Bubble Theory”
RNA monomers have been produced spontaneously from simple molecules
Small organic molecules polymerize when they are concentrated on hot sand, clay, or rock
This can form a type of “bubble”
AKA : Protocell, vesicle, micelle
Provides an enclosed protected area for reactions to take place
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8. Protocells
Replication and metabolism are key properties of life and may have appeared together
Protocells may have been fluid-filled vesicles with a membrane-like structure
In water, lipids and other organic molecules can spontaneously form vesicles with a lipid bilayer
First cells may have formed on sand or clay in warm-shallow water.
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9. Which came first? DNA, RNA, or Protein
DNA first- Probably not. What does DNA require in order to replicate?
RNA first
Not only can RNA replicate, but it can sometimes act like an enzyme (ribozyme).
Protein first
Not only can proteins perform countless functions, but they can also replicate (prions).

10. Self-Replicating RNA and the Dawn of Natural Selection
The first genetic material was probably RNA, not DNA
RNA molecules called ribozymes have been found to catalyze many different reactions
For example, ribozymes can make complementary copies of short stretches of RNA
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11. The First Eukaryotes
The oldest fossils of eukaryotic cells date back 2.1 billion years
Eukaryotic cells have a nuclear envelope, mitochondria, endoplasmic reticulum, and a cytoskeleton
The endosymbiont theory proposes that mitochondria and plastids (chloroplasts and related organelles) were formerly small prokaryotes living within larger host cells
An endosymbiont is a cell that lives within a host cell
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12. heterotrophic eukaryote Ancestral photosynthetic
Figure
Plasma membrane
Cytoplasm
DNA
Ancestral
prokaryote
Nucleus
Endoplasmic
reticulum
Photosynthetic
prokaryote
Mitochondrion
Nuclear envelope
Aerobic heterotrophic
prokaryote
Figure 25.9 A hypothesis for the origin of eukaryotes through serial endosymbiosis.
Mitochondrion
Plastid
Ancestral
heterotrophic eukaryote
Ancestral photosynthetic
eukaryote

13. Concept 25.2: The fossil record documents the history of life
The fossil record reveals changes in the history of life on Earth
Sedimentary rocks are deposited into layers called strata and are the richest source of fossils
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14. Stromatolite fossils are the remains of prokaryotes
Figure 25.4
Present
Dimetrodon
Rhomaleosaurus
victor
100 mya
1 m
175
Tiktaalik
0.5 m
200
270
300
4.5 cm
Hallucigenia
Coccosteus
cuspidatus
375
400
1 cm
Figure 25.4 Documenting the history of life.
Dickinsonia
costata
500
525
2.5 cm
Stromatolites
565
600
Fossilized
stromatolite
1,500
3,500
Tappania
Stromatolite fossils are the remains of prokaryotes

15. How Rocks and Fossils Are Dated
Sedimentary strata reveal the relative ages of fossils
The absolute ages of fossils can be determined by radiometric dating
A “parent” isotope decays to a “daughter” isotope at a constant rate
Each isotope has a known half-life, the time required for half the parent isotope to decay
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16. Remaining “parent” isotope Time (half-lives)
Figure 25.5
Accumulating
“daughter”
isotope
Fraction of parent
isotope remaining 1 2
Remaining
“parent”
isotope
Figure 25.5 Radiometric dating. 1 4 1 8 1 16
Time (half-lives)

17. Concept 25.3: Key events in life’s history include the origins of single-celled and multicelled organisms and the colonization of land
The geologic record is divided into the Archaean, the Proterozoic, and the Phanerozoic eons
The Phanerozoic encompasses multicellular eukaryotic life
The Phanerozoic is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic
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18. Geologic Time Scale Page 515
Table 25.1
Geologic Time Scale Page 515
Table 25.1 The Geologic Record

19. Cambrian Explosion
Concurrent events that may have lead to an overwhelming increase in the number of species
Hox Genes
Predator / Prey Relationships (defined by cephalization- having a head)
Sequestration of carbon dioxide into algae and fossilization (what is happening to that carbon today?)
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20. Time (millions of years ago)
Figure 25.10
Sponges
Cnidarians
Echinoderms
Chordates
Brachiopods
Annelids
Molluscs
Figure Appearance of selected animal groups.
Arthropods
PROTEROZOIC
PALEOZOIC
Ediacaran
Cambrian
635
605
575
545
515
485
Time (millions of years ago)

21. EXTINCTION
Major boundaries between geological divisions correspond to extinction events in the fossil record
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22. Mass Extinctions
The fossil record shows that most species that have ever lived are now extinct
Extinction can be caused by changes to a species’ environment
At times, the rate of extinction has increased dramatically and caused a mass extinction
Mass extinction is the result of disruptive global environmental changes
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23. (families per million years):
Figure 25.15
1,100 25
1,000
900 20
800
700 15
600
(families per million years):
Total extinction rate
Number of families:
500 10
400
300 5
200
Figure Mass extinction and the diversity of life.
100
Era
Paleozoic
Mesozoic
Cenozoic Q
Period E O S D C P Tr J C P N
542
488
444
416
359
299
251
200
145
65.5

24. A number of factors might have contributed to these extinctions
Intense volcanism in what is now Siberia
Global warming resulting from the emission of large amounts of CO2 from the volcanoes
Reduced temperature gradient from equator to poles
Oceanic anoxia (low oxygen) from reduced mixing of ocean waters
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25. Is a Sixth Mass Extinction Under Way?
Scientists estimate that the current rate of extinction is 100 to 1,000 times the typical background rate
Extinction rates tend to increase when global temperatures increase
Data suggest that a sixth, human-caused mass extinction is likely to occur unless dramatic action is taken (by 2050)
Did you have any plans for the second half of this century?
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26. Adaptive Radiations
Adaptive radiation is the evolution of diversely adapted species from a common ancestor
Adaptive radiations may follow
Mass extinctions
The evolution of novel characteristics
The colonization of new regions
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27. Evolutionary Novelties
Most novel biological structures evolve in many stages from previously existing structures
Complex eyes have evolved from simple photosensitive cells independently many times
Exaptations are structures that evolve in one context but become co-opted for a different function
Natural selection can only improve a structure in the context of its current utility
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28. E: Squid (Similar to Mammal)
Figure 25.26
A; Limpit (Patella)
B: Some mollusks
C: Nautilus
D: Marine snail
E: Squid (Similar to Mammal)
(a) Patch of pigmented cells
(b) Eyecup
Pigmented cells
(photoreceptors)
Pigmented
cells
Epithelium
Nerve fibers
Nerve fibers
(c) Pinhole camera-type eye
(d) Eye with primitive lens
(e) Complex camera lens-type eye
Epithelium
Cornea
Cellular
mass
(lens)
Fluid-filled
cavity
Cornea
Lens
Figure A range of eye complexity among molluscs.
Retina
Optic nerve
Optic
nerve
Optic nerve
Pigmented
layer
(retina)

29. Page 531 Figure 25.27 Figure 25.27 The evolution of horses. Holocene
Equus
Pleistocene
Pliocene
Hippidion and
close relatives 5
Nannippus
Pliohippus
Sinohippus
Neohipparion 10
Callippus
Miocene
Hipparion
Megahippus
Hypohippus 15
Archaeohippus
Anchitherium
Parahippus
Merychippus 20 25
Millions of years ago
Miohippus
Oligocene 30
Haplohippus 35
Figure The evolution of horses.
Palaeotherium
Mesohippus
Key
Pachynolophus
Grazers 40
Epihippus
Browsers
Propalaeotherium
Eocene 45
Orohippus 50
Hyracotherium
relatives 55
Hyracotherium