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CHAPTER 26
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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
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• 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
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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
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• 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
• 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
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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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• 6-kingdom system
– 4 eukaryotic kingdoms
•
•
•
•
Plantae
Each fundamentally different
Fungi
Animalia
Protista – did not fit into 3 other kingdoms
– 2 prokaryotic kingdoms
• Archaea
• Bacteria
12
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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
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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
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Archaea
• Shared characteristics
– Cell walls lack peptidoglycan (found in bacteria)
– Membrane lipids are different from all other
organisms
• 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
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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
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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
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• Land plants arose from an ancestral green alga only once during
evolution
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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
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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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