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Chapter 25
Early Earth and The
Origin of Life
Essential Knowledge
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1.A.4 – Biological evolution is supported by
scientific evidence from many disciplines,
including mathematics
1.B.1 – Organisms share many conserved
core processes and features that evolved and
are widely distributed among organisms
today
1.C.1 – Speciation and extinction have
occurred throughout the Earth’s history
Essential Knowledge
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1.D.1 – There are several hypotheses about
the natural origin of life on Earth, each with
supporting scientific evidence
1.D.2 – Scientific evidence from many
different disciplines supports models of the
origin of life
4.B.3 – Interactions between and within
populations influence patterns of species
distribution and abundance
Early History of Life
Early History of Life
Solar system = 12 billion years
ago (bya)
 Earth = 4.6 bya
 Life = 3.5 bya
 Prokaryotes dominated Earth =
3.5 to 2.0 bya
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– Stromatolites = hold the first living
fossil
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Oxygen accumulation = 2.7 bya
– photosynthetic cyanobacteria =
created oxygen in the atmosphere
Eukaryotic life = 2.1 bya
 Multicellular eukaryotes = 1.2
bya
 Land colonization = 500 million
years ago (mya)
 Animal diversity with the
Cambrian explosion = 543 mya
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Geologic Time Scale
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Major Events in Evolution
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4.6 bya – formation of the Earth (Precambrian)
3.5 bya – prokaryotic cells
2.2 bya – eukaryotic cells
600 mya – soft-bodied invertebrates
500 mya – colonization of land plants (Paleozoic)
420 mya – jawless fish
375 mya – bony fish, amphibians, insects
325 mya – first seed plants, reptiles
220 mya – cone-bearing plants (Mesozoic)
175 mya – dinosaurs abundant
80 mya – angiosperms
60 mya – mammals, birds, pollinating insects
(Cenozoic)
♦ 30 mya – primate groups
♦ 2.5 mya – apelike ancestors
Plate Tectonics and Pangaea
The 4 Stage Origin of
Life Hypothesis:
1) Abiotic synthesis of organic monomers
2) Polymer formation
3) Molecule packaging (“protobionts”) –
membrane containing droplets
4) Origin of self-replicating molecules
Origin of Life
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Primitive Earth provided inorganic
precursors from which organic molecules
could have been synthesized
– This is due to the presence of available free
energy and the absence of a significant quantity
of oxygen in the atmosphere
Early Atmosphere
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The Precambrian atmosphere was
composed mainly of nitrogen and carbon
dioxide. Also had some methane and
ammonia
Volcanoes released water vapor, carbon
monoxide, and even more nitrogen and
carbon dioxide
But no free oxygen was present
Origin of Life
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This early atmosphere provided molecules
that served as monomers or building blocks
for the formation of more complex
molecules, including amino acids and
nucleotides
Origin of Life
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The joining of these monomers produced
polymers with the ability to replicate, store
and transfer information
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These complex reaction sets could have
occurred in solutions (organic soup model)
or as reactions on solid reactive surfaces
Organic Monomer/Polymer
Synthesis
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Oparin /Haldane hypothesis (1920s):
– primitive earth: volcanic vapors (reducing
atmosphere which means electron-adding) with
lightening & UV radiation
– This will enhance complex molecule formation
(no O2)
– Haldane coined the phrase “primitive soup”
because he suggested the oceans were a solution
of organic molecules from which life arose
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Miller/Urey experiment (1953):
– Water, hydrogen, methane,
ammonia, all 20 amino acids,
nitrogen bases, & ATP formed, but
not organic molecules
– Evidence suggests that that the
atmosphere was probably not
reducing or oxidizing (electronremoving)
• Possible that just areas around volcanic
openings were reducing in order to
create molecules
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Fox experiment (1959):
– Suggested that proteinoid formation (abiotic
polypeptide spheres) occurs from organic
monomers dripped on hot sand, clay or rock
– Hot, dry conditions are needed followed by being
dissolved in water
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Oparin also proposed that:
– coacervates (spherical droplet of assorted
macromolecules) and then protobionts (abiotic
aggregate of macromolecules surrounded by a
membrane) formed surrounded by a shell of H2O
molecules
Abiotic Genetic Replication
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The RNA World hypothesis proposes that
RNA could have been the earliest genetic
material (1986)
– First genetic material = self-replicating
RNA
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Abiotic production of
ribonucleotides
Ribozymes = short sequence
of RNA that act as a catalyst
(enzyme)
Formation of short
polypeptides occurred
RNA to DNA template?
First Organisms
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Prokaryotes were the first organisms on
Earth and cyanobacteria is the oldest known
fossil
Stromatolites are rock-like buildups of mats
of bacteria.
– Photosynthetic cyanobacteria began adding
oxygen to the atmosphere by taking in carbon
dioxide to produce food
– Today, cyanobacteria are still around and also
contribute to converting atmospheric nitrogen
into a form plants can use (nitrogen cycle)
Endosymbiotic Theory
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The theory proposes that mitochondria and
plastids (chloroplasts) were once free
living prokaryotes that were engulfed by
another prokaryotic cell and developed
into a symbiotic relationship within the
cell
– This is the connection between prokaryotic
cells and unicellular eukaryotic cells
Endosymbiotic Theory Evidence
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Both organelles have enzymes and transport
systems homologous to living prokaryotes
Both replicate by a splitting process similar
to prokaryotes
Both contain a single, circular DNA
molecule
Both have ribosomes that can translate their
DNA into proteins
Evidence
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Scientists determine information about the
origin of species using:
– Relative dating = order of rock strata
determines relative age
– Radiometric dating = decay of radioactive
isotopes determines the exact age
• Half-life
Relative Dating
Sedimentary Rock Are The
Richest Source of Fossils
Dinosaur Bones
Skull
Scorpion in Amber
Petrified Trees
Leaf
Fossilized Seashell
The Fossil Record
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The fossil record is far from being complete, it is
slanted in favor of species that existed for a long
time, were abundant and widespread, and had
shells or hard skeletons.
A substantial fraction of species that have lived
probably left no fossils, most fossils that formed
have been destroyed, and only a fraction of the
fossils have been discovered.
Radiometric Dating
Evidence
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Molecular and genetic evidence from extant
and extinct organisms indicates that all
organisms on Earth share a common
ancestral origin of life
– Molecular building blocks are common to all
life
– Common genetic code
Rise and Fall of Species
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Continental drift = movement of continents
have altered habitats and promotes speciation
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Adaptive radiation = periods of evolutionary
change in which groups of organisms begin to
fill different ecological niches
Rise and Fall of Species
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Mass extinctions = loss of large number of
species, which can drastically alter an
ecological community
Species extinction rates are rapid at times of
ecological stress
– Ex: 5 major extinctions through the geologic time
scale has drastically reduced the number of species
on Earth
– Ecology Ex: Human impact on ecosystems can
lead to species extinction rates increasing
Major Mass Extinctions
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Permian Period – 250 mya
• 90% of marine animal went extinct
• 8 out of 27 orders of Permian insects did not survive
• The extinction occurred in less than 5 million years
• Reasons
• Occurred about the same time the continents merged to form
Pangaea – Marine and terrestrial habitats disturbed
• Massive volcanic eruptions in what is now Siberia – increase
in atmospheric carbon dioxide = global warming
• Global warming = reduced temp. differences between the
poles and equator. Could lead to uneven mixing of the
oceans which decrease amount of dissolved oxygen
Major Mass Extinctions
Cretaceous Period – 65 mya
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Extinction of dinosaurs
Killed more than half of the marine species
Exterminated many families of terrestrial plants and
animals
Reasons
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The climate became cooler, and shallow seas receded
from continental lowlands
Large volcanic eruption in what is now India
“Impact Hypothesis” – a large comet (dirt and ice) or
small asteroid (rock and metal) collided with Earth
Changes in Body Form
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Evolutionary novelty = when structures had one
role originally, but have gradually acquired a
different role
– Feather in birds – first for thermoregulation, now for
flight
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Heterochrony = evolutionary change in the rate or
timing of developmental events
– Small changes in the embryo can have big impacts on
the adult form
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Homeotic genes = master regulatory genes that
determine location and organization of body parts
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Video: Human Evolution
Video: Abiogenesis
Phylogeny and Systematics
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Phylogeny – evolutionary history of a species or
group of related species
Systematics – study of biological diversity in an
environmental context (tracing phylogeny)
Taxonomy - science of naming, identifying, and
describing diverse forms of life
Geologic
Time
Scale
Taxonomy
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Taxonomy – Binomial;
based on a 2-part Latin
name; genus and species.
Ex. Pseudacris nigrita,
Homo sapiens
Hierarchical
Classification – way for
us to structure and view
of our world
Grizzly bear
Black bear
Giant panda
Red fox
KINGDOM Animalia
PHYLUM Chordata
CLASS Mammalia
ORDER Carnivora
FAMILY Ursidae
GENUS Ursus
SPECIES Ursus arctos
Abert
squirrel
Coral snake Sea star
Phylogenetic Systematics
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Phylogenetic Tree – reflect the hierarchical
classification of taxonomic groups
Cladogram – a “tree” constructed from a series of
dichotomies, or 2-way branch points that represent
divergence of an animal from a common ancestor;
the “deeper” the branch to greater the divergence
The sequence symbolizes historical chronology
Clades – each branch in a cladogram; ancestral
species and all of its decendents
Phylogenetic Systematics
Phylogenetic Systematics
Phylogenetic Systematics
How Do We Construct a Cladogram?
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Homology – likeness attributed to shared ancestry;
all forelimbs of mammals are homologous
Not all likeness qualifies as homology
Convergent Evolution – Species from different
evolutionary branches may come to resemble one
another if they have similar ecologocial roles and
natural selection has shaped analogous adaptation.
Similarity due to convergence is called analogy.
Example – the wings of a bird, bat, and bee.
Convergent Evolution &
Analogous Structures
Ocotillo of southwestern North America
Alluaudia of Madagascar
How Do We Construct a Cladogram?
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As a general rule – the greater the number of homologous
parts between two species, the more closely the species are
related.
The more complex two similar structures are, the less likely
it is they evolved independently
– Example – the human skull and chimpanzee skull match almost
perfectly bone for bone, the only difference
is theway they fuse together.
Most likely, the genes required to
build these skulls were inherited
from a common ancestor.
Identifying Shared Derived Characteristics
These characteristics allow us to identify the sequence in which
derived characters evolved during vertebrate phylogeny
Identifying Shared Derived Characteristics
Phylogeny Can Be Inferred
From Molecular Data
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Anatomical characteristics and homology alone
cannot account for all evolutionary relationships
Systematists compare genes (DNA) and gene
products (proteins) to determine evolutionary
relationships
Phylogeny Can Be Inferred
From Molecular Data
Appendages
Crab
Conical Shells
Barnacle
Limpet
Crustaceans
Crab
Gastropod
Barnacle
Molted
exoskeleton
Segmentation
TRADITIONAL
CLASSIFICATION
CLADOGRAM
Limpet
Phylogeny Can Be Inferred
From Molecular Data
Modern Systematics is Shaking Some
Phylogenetic Trees