Chapter 19
Life’s Origin and
Early Evolution
Albia Dugger • Miami Dade College
19.1 Looking for Life
• Astrobiology is the study of life’s origins and distribution –
astrobiologists study Earth’s extreme habitats to determine
the range of conditions living things can tolerate
• Life on Earth is protected by the ozone layer, which serves
as a natural sunscreen, preventing most UV radiation from
reaching the planet’s surface
• Life can adapt to nearly any environment with sources of
carbon and energy – including extreme temperatures, pH,
salinity, or pressure
Lessons from Chile’s Atacama Desert
19.2 The Early Earth
• Knowledge of modern chemistry and physics are the basis for
scientific hypotheses about early events in Earth’s history
Origin of the Universe
and Our Solar System
• Big bang theory
• The universe began in an instant, 13-5 billion years ago
• All existing matter and energy suddenly appeared and
exploded outward from a single point
• The universe is still expanding
• Earth formed from dust and debris orbiting the sun, about 4.6
billion years ago
Formation of the Earth
Conditions on the Early Earth
• Earth’s early atmosphere came from gas released by
volcanoes, and was low in oxygen
• Rain washed minerals and salts out of rocks to form early
seas
ANIMATION: Origin of organelles
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Early Earth
Take-Home Message: What were conditions
like on the early Earth?
• Earth’s early atmosphere had little or no oxygen
• Meteorites pummeled the planet’s surface, and volcanic
activity was more common than it is today
19.3 Formation of Organic Monomers
• All living things are made from the same organic subunits:
amino acids, fatty acids, nucleotides, and simple sugars
• Small organic molecules that serve as building blocks of life
can be formed by nonliving mechanisms
Possible Sources of
Life’s First Building Blocks
1. Stanley Miller showed that amino acids form in conditions that
simulate lightning in the atmosphere of early Earth
2. Wächtershäuser and Huber synthesized amino acids in a
simulated hydrothermal vent environment
3. Amino acids, sugars, and nucleotide bases may have formed
in interstellar clouds and been carried to Earth on meteorites
electrodes
to
vacuum
pump
CH4
NH3
H2O
H2
spark
discharge
gases
water out
condenser
water in
water droplets
boiling water
water containing
organic compounds
liquid water in trap
Figure 19-4 p311
A Hydrothermal Vent on the Seafloor
Take-Home Message: What was the source of
organic molecules to build the first life?
• Small organic molecules that serve as the building blocks for
living things can be formed by nonliving mechanisms.
• For example, amino acids form in reaction chambers that
simulate conditions on the early Earth, and are present in
some meteorites
ANIMATED FIGURE: Miller's reaction
chamber experiment
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19.4 From Polymers to Protocells
• We will never know for sure how the first cells came to be, but
we can investigate the possible steps on the road to life
Properties of Cells
• All living cells carry out metabolic reactions, are enclosed
within a plasma membrane, and can replicate themselves
• Cells have a genome of DNA that enzymes transcribe into
RNA, and ribosomes that translate RNA into proteins
• Studies support the hypothesis that cells arose from a
stepwise process that began with inorganic materials
inorganic molecules
…self-assemble on Earth
and in space
organic monomers
…self-assemble in aquatic
environments on Earth
organic polymers
…interact in early metabolism
…self-assemble as vesicles
…become the first genome
protocells in an RNA world
…are subject to selection
that favors a DNA genome
DNA-based cells
Figure 19-6 p312
Origin of Metabolism
• Before cells, nonbiological process that concentrate organic
subunits might increase the chance of polymer formation
• Concentration of molecules on clay particles in tidal flats may
have caused organic subunits to bond as polymers
• The iron–sulfur world hypothesis holds that the first
metabolic reactions began on the surface of rocks around
hydrothermal vents
Origin of the Genome
• An RNA-based system of inheritance may have preceded
DNA-based systems
• RNA world hypothesis
• RNA may have stored genetic information and functioned
like an enzyme in protein synthesis
• RNAs that function as enzymes (ribozymes) are common in
living cells today
Origin of the Plasma Membrane
• The cell’s plasma membrane allows organic molecules to
concentrate and undergo reactions
• Membranous sacs (protocells) containing interacting organic
molecules may have formed prior to the earliest life forms
• In experiments, small organic molecules can react with
minerals and seawater to form vesicles with a bilayer
membrane
Laboratory-Produced Protocells
Testing a Hypothesis
Take-Home Message:
What do experiments
reveal about steps that led to the first cells?
• All living cells carry out metabolic reactions, are enclosed
within a plasma membrane, and can replicate themselves
• Metabolic reactions may have begun when molecules
became concentrated on clay particles or in tiny rock
chambers near hydrothermal vents
• RNA can serve as an enzyme, as well as a genome. An RNA
world may have preceded evolution of DNA-based genomes
• Vesicle-like structures with outer membranes form
spontaneously when certain organic molecules are mixed with
water
19.5 Life’s Early Evolution
• Fossils and molecular comparisons among modern
organisms inform us about the early history of life
Origin of Bacteria and Archaea
• Life that arose 3-4 billion years ago was probably anaerobic
and used dissolved carbon dioxide as a carbon source
• Early fossil cells are similar in size and structure to modern
archaea and bacteria
• The first photosynthetic cells were bacteria that used the
cyclic pathway (does not produce O2)
Fossil Prokaryotic Cells
The Proterozoic Era
• The oxygen-producing, non-cyclic pathway of photosynthesis
first evolved in cyanobacteria
• In the Proterozoic era Layers of photosynthetic bacteria
formed large dome-shaped, layered called stromatolites
• Oxygen accumulation in air and seas halted spontaneous
formation of molecules of life, formed a protective ozone
layer, and spurred evolution of organisms using aerobic
respiration
Stromatolites
The Rise of Eukaryotes
• The earliest evidence of eukaryotes is lipids in 2.7-billionyear-old rocks – the lipids are biomarkers for eukaryotes
• A red alga that lived 1.2 billion years ago is the oldest species
known to reproduce sexually, a trait unique to eukaryotes
• Multicellularity and cellular differentiation allowed evolution of
larger bodies with specialized parts
• Spongelike animals evolved about 870 million year ago;
animals with more complex bodies existed about 570 mya
Fossils of Some Early Eukaryotes
What was early life like
and how did it change Earth?
Take-Home Message:
• Life arose by 3–4 billion years ago; it was probably anaerobic
and did not have a nucleus
• An early divergence separated ancestors of modern bacteria
from the lineage that lead to archaea and eukaryotic cells
• The first photosynthetic cells were bacteria that used the
cyclic pathway; later, the oxygen-producing, noncyclic
pathway evolved in cyanobacteria
• Oxygen accumulation in air and seas halted spontaneous
formation of the molecules of life, formed a protective ozone
layer, and favored organisms that carried out the highly
efficient pathway of aerobic respiration
19.6 How Did Eukaryotic Traits Evolve?
• Eukaryotic cells have a composite ancestry, with different
components derived from different lineages
• Archaea-like nuclear genes govern genetic processes (DNA
replication, transcription, translation)
• Bacteria-like nuclear genes govern metabolism and
membrane formation
Origins of Internal Membranes
• In eukaryotes, DNA resides in a nucleus bordered by a
nuclear envelope – a double membrane with pores that
control the flow of material into and out of the nucleus
• A few modern bacteria also have internal membraneenclosed compartments
• The nucleus and endomembrane system probably evolved
from infoldings of plasma membrane
infolding of plasma membrane
ER
nuclear envelope
19-10a p1910
Bacteria with Internal Membranes
Evolution of Mitochondria and Chloroplasts
• The endosymbiont hypothesis holds that mitochondria and
chloroplasts descended from bacteria that were prey or
parasites of early eukaryotic cells
• Mitochondria are genetically similar to aerobic bacteria called
rickettsias; chloroplasts are similar to photosynthetic bacteria
called cyanobacteria
Rickettsia prowazekii
Evidence of Endosymbiosis
• Endosymbiosis can occur when a bacterium infects a
eukaryotic cell
• Eventually, host and symbiont become incapable of living
independently
• Example: The photosynthetic organelles of glaucophytes are
dependent on their host – they can’t survive on their own
photosynthetic organelle that
resembles a cyanobacterium
Figure 19-12a p317
photosynthetic organelle that
resembles a cyanobacterium
Figure 19-12b p317
Take-Home Message: How
might eukaryotic
organelles have evolved?
• A nucleus and other organelles are defining features of
eukaryotic cells
• The nucleus and ER may have arisen through modification of
infoldings of the plasma membrane
• Mitochondria and chloroplasts most likely descended from
bacteria
19.7 Time Line for
Life’s Origin and Evolution
Table 19-1 p320
ANIMATION: Evolutionary tree of life
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ANIMATED FIGURE: Milestones in the
history of life
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