CHAPTER 26
LECTURE
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The Tree of Life
Chapter 26
Origins of Life
• Cell is the basic unit of life
• Today all cells come from pre-existing cells
• The Earth formed as a hot mass of molten rock
about 4.5 billion years ago (BYA)
– As it cooled, chemically-rich oceans were formed
from water condensation
• Life arose spontaneously
– Ocean’s edge, hydrothermal deep-sea vents, or
elsewhere
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Fundamental Properties of Life
•
•
•
•
•
•
•
•
Cellular organization
Sensitivity
Growth
Development
Reproduction
Regulation
Homeostasis
Heredity
4
• Panspermia
– Earth may have been “infected” with life from some
other planet
– Meteor or cosmic dust may have carried complex
organic molecules to earth
– Kicked off evolution of life
• Frozen water found on Mars
5
Conditions on Early Earth
• Seems likely that Earth’s first organisms
emerged and lived at very high temperatures
• First organisms emerged between 3.8 and 2.5
BYA
• Early atmosphere composition not agreed on
– May have been a reducing atmosphere
– Would have made it easier to form carbon-rich
molecules
6
• In 1953, Miller and Urey did an experiment
that reproduced early atmosphere
– Assembled reducing atmosphere rich in
hydrogen with no oxygen gas
– Atmosphere placed over liquid water
– Temperature below 100ºC
– Simulate lightning with sparks
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• Found within a week that methane gas
(CH4) converted into other simple carbon
compounds
– Compounds combined to form simple
molecules and then more complex molecules
• Later experiments produced more than 30
carbon compounds including amino acids
– Adenine also produced
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• RNA may have been first genetic material
– Ribozyme activity
• Amino acids polymerized into proteins
• Metabolic pathways emerged
– Primitive organisms may have been autotrophic –
built what they needed
• Lipid bubbles could increase the probability of
metabolic reactions
– Leads to cell membranes
• Other innovations contributed to diversity of life
10
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Eras
Periods
Cenozoic
Eons
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Quaternary
Present
Appearance of humans
First primate
Tertiary
50 MYA
Bird radiation
North and South
America joined
by land bridge.
Uplift of the
Sierra Nevada.
Worldwide glaciation.
Mammal radiation
Pollinating insects
Mesozoic
100 MYA
150 MYA
Diversification of flowering plants
First flowering plants, birds,
marsupial mammals
Gondwana begins
to break apart;
interior less arid.
Gondwana
Jurassic
200 MYA
First dinosaurs
Triassic
First gymnosperms
Pangea intact.
Interior of Pangea
arid. Climate
very warm.
250 MYA
Permian
300 MYA
First reptiles
Carboniferous
Paleozoic
Phanerozoic
Cretaceous
350 MYA
First amphibians
Devonian
400 MYA
Bony fish, tetrapods, seed plants,
and insects appear
Supercontinent of
Laurentia to the
north and
Gondwana to the
south. Climate mild.
Laurentia
Laurentia
Gondwana
Silurian
Early vascular plants diversify
450 MYA
Ordovician
Cambrian
500 MYA
Invertebrates dominate
First land plants
Cambrian explosion; increase in diversity
12
Eras
Eons
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Periods
500 MYA
Late
Proterozoic
1000 MYA
Appearance of animals and plants
First multicellular organisms
Supercontinent of
Gondwana forms.
Oceans cover
much of North
America. Climate
not well known.
Middle
Early
1500 MYA
Oldest definite fossils of eukaryotes
2000 MYA
Appearance of oxygen in atmosphere
Most of Earth
is covered in
ocean and ice.
2500 MYA
Archaean
Late
3000 MYA
Middle
3500 MYA
Early
Oldest fossils of prokaryotes
Molten-hot surface of Earth becomes somewhat cooler
Hadean
Precambrian
Cyanobacteria
4000 MYA
Oldest rocks
4500 MYA
Formation of Earth
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Classification of Organisms
• More than 2000 years ago, Aristotle
divided living things into animals and
plants
• Later, basic units were called genera
– Felis (cats) and Equus (horses)
• In the 1750s, Carolus Linnaeus instituted
the use of two-part names, or binomials
– Apis mellifera the European honeybee
– Genus name capitalized, all in italics
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• Taxonomy is the
science of
classifying living
things
– A classification
level is called a
taxon
• Scientific names
avoid the
confusion caused
by common names
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The Linnaean Hierarchy
• Taxa are based on shared characteristics
– Domain → → → Species
• Early taxonomists not aware of distinction
between derived and ancestral traits
– Many hierarchies now being re-examined
• Categories at the different levels may include
many, a few, or only one taxon
• Limitations
– Many higher ranks are not monophyletic
– Linnaean ranks not equivalent in any meaningful way
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Grouping Organisms
Carl Woese proposed a 6-kingdom system
Prokaryotes
Eukaryotes
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• 6-kingdom system
– 4 eukaryotic kingdoms
•
•
•
•
Plantae
Each fundamentally different
Fungi
Each probably monophyletic
Animalia
Protista – did not fit into 3 other kingdoms
– Probably paraphyletic
– 2 prokaryotic kingdoms
• Archaea
• Bacteria
20
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Domain
Bacteria
(Bacteria)
Domain
Archaea
(Archaebacteria)
Domain
Eukarya
(Eukaryotes)
Common Ancestor
a.
• 3 domain system
– Domain Archaea
– Domain Bacteria
– Domain Eukarya
– Each of these domains forms a clade
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• Tree based on rRNA analysis
• Archaea and Eukarya are more closely related to each
other than to bacteria
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Bacteria
• Most abundant organisms on Earth
• Key roles in biosphere
– Extract nitrogen from the air, and recycle
carbon and sulfur
– Perform much of the world’s photosynthesis
• Responsible for many forms of disease
• Highly diverse
• Most taxonomists recognize 12–15
different groups
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Archaea
• Shared characteristics
– Cell walls lack peptidoglycan (found in bacteria)
– Membrane lipids are different from all other
organisms
– Distinct rRNA sequences
• Divided into three general categories
– Methanogens
– Extremophiles
– Nonextreme archaea
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• Methanogens
– Use H2 to reduce CO2 to CH4
– Strict anaerobes that live in swamps and guts
• Extremophiles
– Thermophiles – High temperatures (60–80ºC)
– Halophiles – High salt
– Acidophiles – Low pH (pH = 0.7)
• Nonextreme archaea
– Grow in same environments as bacteria
– Nanoarchaeum equitens – Smallest cellular genome
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Eukarya
• Prokaryotes ruled the earth for at least one
billion years
• Eukaryotes appeared about 2.5 BYA
• Their structure and function allowed
multicellular life to evolve
• Eukaryotes have a complex cell
organization
– Extensive endomembrane system divides the
cell into functional compartments
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• Mitochondria and
chloroplasts most likely
gained entry by
endosymbiosis
• Mitochondria were
derived from purple
nonsulfur bacteria
• Chloroplasts from
cyanobacteria
28
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Archaebacteria
Animalia
Fungi
Protista
Plantae
Bacteria
Brown
algae
Red Green
algae algae
Photosynthetic
protists
Nonphotosynthetic
protists
Chloroplasts
Mitochondria
Ancestral
eukaryotic cell
Halophiles
Thermophiles
Methanogens
Purple
bacteria
Photosynthetic
bacteria
Other
bacteria
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Key Eukaryotic Characteristics
• Compartmentalization
– Allows for increased subcellular specialization
– Nuclear membrane allows for additional levels
of control of transcription and translation
• Multicellularity
– Allows for differentiation of cells into tissues
• Sexual reproduction
– Allows for greater genetic diversity
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Viruses
• Are literally “parasitic” chemicals
– DNA or RNA wrapped in protein
• Cannot reproduce on their own
• Not considered alive – cannot be placed in
a kingdom
• Viewed as detached fragments of a
genome
• Tobacco mosaic virus (TMV) first
discovered in 1933
33
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Vaccinia virus
(cowpox)
Herpes simplex
virus
Rhinovirus
(common
cold)
Influenza
virus
T4 bacteriophage
HIV-1
(AIDS)
Tobacco mosaic
virus (TMV)
Adenovirus
(respiratory
virus)
Poliovirus
(polio)
100 nm
Ebola virus
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Making Sense of the Protists
• Represents tension between traditional
classification and use of evolutionary
relationships
• Eukaryotes diverged rapidly as atmosphere shift
from anaerobic to aerobic
– May never be able to sort out relationships during this
time
• Protist is a catchall for eukaryotes that are not
plant, fungus, or animal
35
• 6 main branches of protists are current working hypothesis
– At least 60 protists do not fit into these groups
• New kingdom called Viridiplantae would include all green
36
algae and land plants
Origin of Plants
• Land plants arose from an ancestral green
alga only once during evolution
• Green alga consist of 2 monophyletic
groups
– Chlorophyta
– Streptophyta
• Composed of seven clades, including land plants
– Kingdom Viridiplantae would include
Chlorophyta and Streptophyta
37
• Mesostigma represent the earliest Streptophyte branch
• Charales is the sister clade to land plants
38
– Split 420 MYA
Some land plants
show evidence of
horizontal gene
transfer
Amborella has some
mitochondrial
genes from moss
Close contact with
epiphytes increases
the probability of
HGT
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Sorting Out the Animals
• Origins of segmentation
– Used in the past to group arthropods and
annelids close together
– rRNA sequences now suggest that these two
groups are distantly related
– Segmentation likely evolved independently in
these two groups, as well as in chordates
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• Division based on embryonic development
– Protostomes develop the mouth before the
anus in embryonic development
• Annelids and arthropods among others
– Deuterostomes develop the anus first
• Chordates including humans
• Protostomes divided further into
– Lophotrochozoans
• Flatworms, mollusks, and annelids
– Ecdysozoans
• Roundworms and arthropods
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• Segmentation is regulated by the Hox gene family
– Hox ancestral genes already present in ancestor to all
groups
– Members were co-opted at least three times
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Within the arthropods, insects have traditionally
been separated from the crustaceans
– Uniramous vs. biramous appendages
However, molecular
data is questioning
this classification
– Distal-less, a
Hox gene, initiates
development of
both types of
appendages
43
The Mammalian Family Tree
• Over 90% of mammals are eutherians or
placental mammals
– Now divided into four major groups
• First major split occurred 100 MYA when Africa
split from South America
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• Origin of whales and hippos debated for
200 years
– Whales thought to be relatives of pigs based
on skull and teeth
– DNA sequences reveal a close relationship
between whales and hippos
– Some adaptations to aquatic origins had a common
origin
– Recent fossil finds confirm the artiodactyl origin
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• Understanding evolutionary relationships
among organisms accomplishes these
things
– Provides an orderly and logical way to name
organisms
– Allows researchers to ask important questions
about physiology, behavior, and development
using information already known about a related
species
– Provides insights in understanding the history of
major features and functions
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