1. The History of Life on Earth
Chapter 25
The History of Life on Earth

2. Past organisms were very different from those now alive
Overview: Lost Worlds
Past organisms were very different from those now alive
The fossil record shows macroevolutionary changes over large time scales including
The emergence of terrestrial vertebrates
The origin of photosynthesis
Long-term impacts of mass extinctions

3. Fig. 25-1
Figure 25.1 What does fossil evidence say about where these dinosaurs lived?

4. Fig 25-UN1
Cryolophosaurus

5. Concept 25.1: Conditions on early Earth made the origin of life possible
Chemical and physical processes on early Earth may have produced very simple cells through a sequence of stages:
1. Abiotic (nonliving) synthesis of small organic molecules (e.g. aa & nitrogenous bases)
2. Joining of these small molecules into macromolecules (e.g. protein & nucleic acids)
3. Packaging of molecules into “protocells” (membranous droplets w distinct internal envir.)
4. Origin of self-replicating molecules (inheritance)

6. Synthesis of Organic Compounds on Early Earth
Earth, along with the rest of the solar system, formed about 4.6 billion years ago.
Earth’s early atmosphere likely contained:
Water vapor, and
Chemicals released by volcanic eruptions (nitrogen, nitrogen oxides, carbon dioxide, methane, ammonia, hydrogen, hydrogen sulfide)

7. Early Hypothesis (Reducing Atmosphere):
A. I. Oparin & J. B. S. Haldane hypothesized:
The early atmosphere was a reducing environment
Stanley Miller & Harold Urey conducted lab experiments that showed:
The abiotic synthesis of organic molecules in a reducing atmosphere is possible

8. However, the evidence is not yet convincing that the early atmosphere was in fact reducing
Instead, the first organic compounds may have been synthesized:
Near submerged volcanoes & deep-sea vents
Amino acids have also been found in meteorites
Small organic molecules polymerize when they are concentrated on hot sand, clay, or rock

9. Fig. 25-2
Figure 25.2 A window to early life?

10. Protocells Key properties of life: Replication, & Metabolism
Aggregates of abiotically produced molecules
Surrounded by a membrane or membrane- like structure
Exhibit simple reproduction & metabolism
Maintain an internal chemical environment

11. Experiments demonstrate that:
Protocells could’ve formed spontaneously from abiotically produced organic compounds
For example:
Small membrane-bounded droplets called liposomes can form when lipids or other organic molecules are added to water

12. (a) Simple reproduction by liposomes Maltose (b) Simple metabolism
Fig. 25-3
20 µm
Glucose-phosphate
Glucose-phosphate
Phosphatase
Starch
Amylase
Phosphate
Maltose
Figure 25.3 Laboratory versions of protobionts
(a) Simple reproduction by
liposomes
Maltose
(b) Simple metabolism

13. Self-Replicating RNA and the Dawn of Natural Selection
The first genetic material:
Was probably RNA, not DNA
Ribozymes:
RNA molecules
Found to catalyze many different reactions
For example, ribozymes can make complementary copies of:
Short stretches of their own sequence, or
Other short pieces of RNA

14. 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
These strata are the richest source of fossils

15. Figure 25.4 Documenting the history of life
Present
Rhomaleosaurus victor,
a plesiosaur
Dimetrodon
100 million years ago
Casts of
ammonites
175
200
270
300
4.5 cm
Hallucigenia
375
Coccosteus cuspidatus
400
1 cm
Figure 25.4 Documenting the history of life
Dickinsonia
costata
500
525
2.5 cm
565
Stromatolites
600
Tappania, a
unicellular
eukaryote
Fossilized
stromatolite
3,500 1,500

16. Rhomaleosaurus victor, a plesiosaur
Fig. 25-4b
Figure 25.4 Documenting the history of life
Rhomaleosaurus victor, a plesiosaur

17. Fig. 25-4c
Figure 25.4 Documenting the history of life
Dimetrodon

18. Casts of ammonites Fig. 25-4d
Figure 25.4 Documenting the history of life
Casts of ammonites

19. Coccosteus cuspidatus
Fig. 25-4e
Figure 25.4 Documenting the history of life
4.5 cm
Coccosteus cuspidatus

20. Fig. 25-4f
Figure 25.4 Documenting the history of life
1 cm
Hallucigenia

21. 2.5 cm Dickinsonia costata Fig. 25-4g
Figure 25.4 Documenting the history of life
Dickinsonia costata
2.5 cm

22. Tappania, a unicellular eukaryote
Fig. 25-4h
Figure 25.4 Documenting the history of life
Tappania, a unicellular eukaryote

23. Fig. 25-4i
Figure 25.4 Documenting the history of life
Stromatolites

24. Fossilized stromatolite
Fig. 25-4j
Figure 25.4 Documenting the history of life
Fossilized stromatolite

25. Animation: The Geologic Record
Few individuals have fossilized, and even fewer have been discovered
The fossil record is biased in favor of species that
Existed for a long time
Were abundant and widespread
Had hard parts
Animation: The Geologic Record

26. How Rocks and Fossils Are Dated
Sedimentary strata reveal the relative ages of fossils
Radiometric dating determines the absolute ages of fossils
A “parent” isotope decays to:
A “daughter” isotope at a constant rate
The half-life of an isotope:
Is the time required for half the parent isotope to decay
Each isotope has a known half-life

27. 1/2 1/4 1/8 1/16 Accumulating “daughter” isotope
Fig. 25-5
Accumulating
“daughter”
isotope
Fraction of parent isotope remaining
1/2
Remaining
“parent”
isotope
1/4
Figure 25.5 Radiometric dating
1/8
1/16 1 2 3 4
Time (half-lives)

28. Radiocarbon dating:
Can be used to date fossils up to 75,000 years old
For older fossils:
Some isotopes can be used
They date sedimentary rock layers above & below the fossil

29. Magnetism of rocks:
Can provide dating information
Reversals of the magnetic poles leave their record on rocks throughout the world

30. The Origin of New Groups of Organisms
Tetrapods:
The group of animals to which mammals belong
Synapsids:
Ancestor of mammals
Evolution of unique mammalian features:
Occurred through gradual modifications
Can be traced from ancestral synapsids through the present

31. Synapsid (300 mya) Temporal fenestra Key Articular Quadrate
Fig
Synapsid (300 mya)
Temporal
fenestra
Key
Articular
Quadrate
Therapsid (280 mya)
Dentary
Squamosal
Figure 25.6 The origin of mammals
Temporal
fenestra

32. Very late cynodont (195 mya)
Fig
Early cynodont (260 mya)
Key
Articular
Temporal
fenestra
Quadrate
Dentary
Squamosal
Later cynodont (220 mya)
Figure 25.6 The origin of mammals
Very late cynodont (195 mya)

33. Key events in life’s history (Geologic Record)
The geologic record is divided into:
The Archaean eon
The Proterozoic eon:
The archean & the proterozoic eons together lasted approximately 4 billion years

34. Key events in life’s history (Geologic Record), cont.
The Phanerozoic eon:
The last half billion years
Encompasses most of the time that animals have existed on earth
Encompasses multicellular eukaryotic life

35. Boundaries Between Geological Divisions
Major boundaries between geological divisions:
Correspond to extinction events in the fossil record

36. Ceno- zoic Meso- zoic Humans Paleozoic Colonization of land Animals
Fig. 25-7
Ceno-
zoic
Meso-
zoic
Humans
Paleozoic
Colonization
of land
Animals
Origin of solar
system and
Earth 1 4
Proterozoic
Archaean
Prokaryotes
Figure 25.7 Clock analogy for some key events in Earth’s history
Billions of
years ago 2 3
Multicellular
eukaryotes
Single-celled
eukaryotes
Atmospheric
oxygen

37. The First Single-Celled Organisms
Stromatolites:
The oldest known fossils
Are, multilayered rock-like structures
Composed of layers of bacteria & sediment
Date back 3.5 billion years ago
Prokaryotes were Earth’s sole inhabitants from 3.5 to about 2.1 billion years ago

38. Photosynthesis and the Oxygen Revolution
Most atmospheric oxygen (O2):
Is of biological origin
Produced during photosynthesis (H2O-split step )
Photosynthesis O2:
Is reacted with dissolved iron, and
Precipitated to form banded iron formations
The source of O2:
Was likely bacteria similar to modern cyanobacteria

39. Fig. 25-8
Figure 25.8 Banded iron formations: evidence of oxygenic photosynthesis
For the Discovery Video Early Life, go to Animation and Video Files.

40. By about 2.7 billion years ago, O2 began accumulating in:
The atmosphere, and
The rusting iron-rich terrestrial rocks
This “oxygen revolution” from 2.7 to 2.2 billion years ago
Posed a challenge for life
Provided opportunity to gain energy from light
Allowed organisms to exploit new ecosystems

41. The First Eukaryotes The oldest eukaryotic cell fossils:
Date back 2.1 billion years
An endosymbiont:
Is a cell that lives within a host cell
The endosymbiosis hypothesis:
Proposes that mitochondria and plastids (chloroplasts and related organelles) were formerly small prokaryotes living within larger host cells

42. In the process of becoming interdependent:
Prokaryotic ancestors of mitochondria & plastids probably gained entry to the host cell as:
Undigested prey, or
Internal parasites
In the process of becoming interdependent:
The host & endosymbionts became a single organism
Serial endosymbiosis:
Supposes that mitochondria evolved before plastids thru a sequence of endosymbiotic events

43. Break Slide
Thur, Jan 23, ‘14

44. Endoplasmic reticulum Nucleus
Fig
Cytoplasm
Plasma membrane
DNA
Ancestral
prokaryote
Endoplasmic reticulum
Nucleus
Nuclear envelope
Aerobic
heterotrophic
prokaryote
Photosynthetic
prokaryote
Mitochondrion
Figure 25.9 A model of the origin of eukaryotes through serial endosymbiosis
Mitochondrion
Ancestral
heterotrophic
eukaryote
Plastid
Ancestral photosynthetic
eukaryote

45. Key evidence supporting an endosymbiotic origin of mitochondria and plastids:
Similarities of inner membrane structures & functions of these organelles to those of living prokaryotes
Similar division “splitting” process of these organelles to that in some prokaryotes
These organelles transcribe and translate their own DNA
Their ribosomes are more similar to prokaryotic than eukaryotic ribosomes

46. The Origin of Multicellularity
Evolution of eukaryotic cells allowed for:
A greater range of unicellular forms
A second wave of diversification resulted in:
Evolvement of multicellularity giving rise to:
Algae
Plants
Fungi
Animals

47. The Earliest Multicellular Eukaryotes
DNA sequence comparisons date the common ancestor of multi-cellular eukaryotes to:
1.5 billion years ago
The oldest known fossils of multicellular eukaryotes are of small algae that lived about:
1.2 billion years ago

48. The “snowball Earth” hypothesis:
Suggests that:
Periods of extreme glaciation confined life to:
the equatorial region, or
deep-sea vents
from 750 to 580 million years ago