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Chapter 25
The History of Life on Earth
Antarctica many
millions of years ago
Antarctica now…
WOW!!
• Past organisms were very different from today’s.
• The fossil record shows macroevolutionary
changes over large time scales including
– The origin of photosynthesis
– The emergence of terrestrial vertebrates
– Long-term impacts of mass extinctions
Prebiotic Chemical Evolution & the Origin of Life
-
Hypothesis: First cells originated by
chemical evolution
non living materials became organized into
molecules; molecules were able to replicate &
metabolize.
-
possible because atmosphere was really different; no
O2, volcanoes, UV, lightning, etc.
Four Main Stages of Cell Emergence:
1. small organic molecules are made
abiotically
2. monomers  polymers (macromolecules)
3. protocells (droplets of aggregated
molecules)
4. Origin of self replicating molecules/
beginning to heredity
Stage 1: Synthesis of Organic
Compounds on Early Earth
• Earth formed about 4.6 bya
• Earth’s early atmosphere likely contained water vapor
and chemicals released by volcanic eruptions (nitrogen,
nitrogen oxides, carbon dioxide, methane, ammonia,
hydrogen, hydrogen sulfide)
TED Talk: The Line
Between Life and Nonlife
• A. I. Oparin & J. B. S.
Haldane hypothesized
that the early atmosphere
was a reducing
environment (no oxygen)
• Stanley Miller and Harold
Urey conducted lab
experiments that showed
that the abiotic synthesis
of organic molecules in a
reducing atmosphere is
possible
= Primeval Soup Hypothesis
OR…
Organic compounds
were created near
hydrothermal vents
OR…
They rained down
from outer
space
Video: Hydrothermal Vent
Stage 2: Abiotic Synthesis of
Macromolecules
• What came first, the amino
acid or the enzyme?
– How would macromolecules
form without
enzymes/dehydration
synthesis?
• Dilute solutions containing
monomers dripped onto hot
sand, clay, or rock vaporizes
water
– “Proteinoids” (proteins
formed abiotically) were
made this way
• Maybe waves splashed
monomers onto hot lava?
Stage 3:
Protocells
• Replication & metabolism
are key properties of life
• Protocellss are aggregates
of abiotically produced
molecules surrounded by a
membrane or membranelike structure
• Exhibit
– simple reproduction
– metabolism
– maintain an internal chemical
environment
Protocells can behave similarly to a cell
(osmotic swelling, membrane potential like nerve cell)
20 µm
Glucose-phosphate
Glucose-phosphate
Phosphatase
Starch
Phosphate
(a) Simple
reproduction by
liposomes (aggregates
of lipids)
Amylase
Maltose
Maltose
(b) Simple metabolism
Possible to contain
enzyme within; catalyze
RXNs, give off product
Stage 4: Self-Replicating RNA and
the Dawn of Natural Selection
• RNA = probably the first
genetic material, then DNA
• Ribozymes can make
complementary copies of
short stretches of their own
sequence or other short
pieces of RNA
• Base sequences provide
blueprints for amino acid
sequence (polypeptides)
• Early protocells with self-replicating, catalytic RNA would
have been more effective at using resources (“fitness”) &
would have increased in # due to natural selection.
• RNA could have provided template for DNA (more stable,
better at replicating)
The
stage
has now
been
set for
life!
Fig. 25-7
Humans
Colonization
of land
Animals
Origin of solar
system and
Earth
4
1
Proterozoic
2
Archaean
3
Multicellular
eukaryotes
Single-celled
eukaryotes
Atmospheric
oxygen
Prokaryotes
Fig. 25-4
Present
Rhomaleosaurus victor,
a plesiosaur
Dimetrodon
Casts of
ammonites
Hallucigenia
Coccosteus cuspidatus
Dickinsonia
costata
Stromatolites
Tappania, a
unicellular
eukaryote
Fossilized
stromatolite
Table 25-1
Table 25-1a
Table 25-1b
Animation: The Geologic Record
Fig 25-UN2
1
4
2
3
Prokaryotes
The First Single-Celled
Organisms
• Oldest known fossils are
stromatolites
– rock-like structures
composed of many layers
of bacteria and sediment
– Dated 3.5 billion years ago
• Prokaryotes were Earth’s
sole inhabitants from 3.5 to
about 2.1 billion years ago
Fig. 25-4i
Stromatolites
3.5 BYA
Fossilized stromatolite
Fig 25-UN3
1
4
2
3
Atmospheric
oxygen
Photosynthesis & the
Oxygen Revolution
• By about 2.7 bya, O2 began accumulating in the atmosphere
rusting iron-rich terrestrial rocks
– O2 produced by oxygenic photosynthesis reacted with dissolved
iron and precipitated out to form banded iron formations
• “Oxygen revolution” = rapid increase in O2 around 2.2 bya
– Posed a challenge for life; some microbes hid out in anaerobic
environments
– Provided opportunity to gain energy from light
– Allowed organisms to exploit new ecosystems as old ones died,
opening up new niches
• Source of O2 was likely bacteria similar to modern
cyanobacteria
– Later rapid increase attributed to evolution of eukaryotes
Fig. 25-8
Fig 25-UN4
1
4
2
Singlecelled
eukaryotes
3
The First Eukaryotes
Plasma membrane
Cytoplasm
Ancestral
prokaryote
Endoplasmic reticulum
DNA
• Oldest fossils of eukaryotes go
back 2.1 bya
• Endosymbiosis
Nucleus
Nuclear envelope
Aerobic
heterotrophic
prokaryote
Photosynthetic
prokaryote
Mitochondrion
Ancestral
heterotrophic
eukaryote
Mitochondrion
Plastid
– mitochondria & plastids
(chloroplasts & related
organelles) were formerly
small prokaryotes living within
larger host cells
– At first, undigested prey or
internal parasites?
– 2 became interdependent;
host + endosymbionts became
a single organism
Ancestral photosynthetic
eukaryote
Evidence supporting
endosymbiosis:
– Similarities in inner
membrane structures
and functions between
chloroplasts/mitochondr
ia and prokaryotes
– Organelle division is
similar to prokaryotes
– Organelles transcribe &
translate their own DNA
– Organelle ribosomes are
more similar to
prokaryotic ribosomes
than eukaryotic
ribosomes
Fig. 25-4h
1.5 BYA
Tappania, a unicellular eukaryote
The Origin of Multicellularity
• eukaryotic cells allowed
for a greater range of
unicellular forms
• Once multicellularity
evolved then… algae,
plants, fungi, and animals
• Ancestor appeared rougly
1.5 bya, though oldest
fossil is algae dated to 1.2
bya
Multicellular
eukaryotes
1
4
2
3
Ediacaran biota (Proterozoic Eon)
– large & more diverse soft-bodied organisms that lived
from 565 to 535 mya after snowball Earth
– Thaw opened up niches that allowed for speciation
Fig. 25-4g
565 MYA
Dickinsonia costata
2.5 cm
Fig 25-UN6
Animals
1
4
2
3
The Cambrian Explosion
• sudden appearance of fossils resembling
modern phyla in the Cambrian period
(Phanerozoic Eon, 535 to 525 mya)
• first evidence of predator-prey
interactions; claws, hard-shells, spikes, etc.
Burgess Shale
Fig. 25-4f
525 MYA
Hallucigenia
Fig. 25-4e
Coccosteus cuspidatus
400 MYA
Early
Paleozoic
era
(Cambrian
period)
542
Late
Proterozoic
eon
Sponges
500
Arthropods
Molluscs
Annelids
Brachiopods
Chordates
Echinoderms
Cnidarians
Millions of years ago
Fig. 25-10
Fig. 25-11
(a) Two-cell stage
150 µm
(b) Later stage
200 µm
Fig 25-UN7
Colonization of land
1
4
2
3
The Colonization of Land
• Fungi, plants, and animals began to
move to land 500 mya
• Plants & fungi 420 mya: adaptations to
reproduce on land
• Arthropods & tetrapods are the most
widespread and diverse land animals
– Tetrapods evolved from lobe-finned
fishes around 365 million years ago
– Amphibians, reptiles, then birds and
mammals
Fig 25-UN8
1.2 bya:
First multicellular eukaryotes
2.1 bya:
First eukaryotes (single-celled)
535–525 mya:
Cambrian explosion
(great increase
in diversity of
animal forms)
3.5 billion years ago (bya):
First prokaryotes (single-celled)
Millions of years ago (mya)
500 mya:
Colonization
of land by
fungi, plants
and animals
Major Influences on Life on Earth
• Continental Drift: 3 occasions of formation, then
separation of supercontinents; next one will occur
in roughly 250 million years.
– Collision and separation of oceanic and terrestrial plates shape
mountains, cause earthquakes
– Pangaea (250 mya) caused drastic changes in habitats = evolution!
• Mass extinctions: 5 major ones in Earth’s history
– Opens up niches for future species
– Usually takes 5-10 million years to return diversity to its preextinction levels
• Adaptive Radiation: Periods of evolutionary change in
which groups of organisms form many new species whose
adaptations allow them to fill different niches (with little
competition)
Adaptive Radiation
– Occur after mass extinctions
• Rise of mammals after Cretaceous extinction
– Colonized regions (i.e. new islands)
• Hawaiian Islands
How can evolutionary novelties/major
changes in form come about?
• Evolutionary developmental biology, or evodevo, is the study of the evolution of
developmental processes in multicellular
organisms
• Genomic information shows that minor
differences in gene sequence or regulation
can result in major differences in form
…think fruit flies with legs instead of antennae
•allometric growth
Newborn
2
Evo-devo
5
Age (years)
15
Adult
(a) Differential growth rates in a human
Chimpanzee fetus
Chimpanzee adult
• Changes in rate and timing
(regulation) of developmental
genes is called heterochrony
– Accelerated growth in bone
structures (finger bones to wings in
bats) or slowed growth (reduction in
leg bones in whale ancestors)
– Paedomorphosis: fast development
of reproductive system compared to
other development; leads to
maintenance of juvenile features
though sexually mature (phenotypic
variation)
Gill
s
Human fetus
Human adult
(b) Comparison of chimpanzee and human skull growth
More Evo-devo
Fig. 21-17
• Changes in spatial pattern of
developmental genes (homeotic
genes = master regulatory genes)
– determine where, when, and
how body segments develop
– Small changes in regulatory
sequences of certain genes
lead to major changes in body
form
Adult
fruit fly
Fruit fly embryo
(10 hours)
Fly
chromosome
Mouse
chromosomes
Mouse embryo
(12 days)
Adult mouse
Hox genes of the fruit fly and
mouse show the same linear
sequence on the chromosomes
• Homeobox/Hox genes code
for transcription factors
that turn on developmental
genes in embryos
The expression of 2 Hox genes
in snakes suppresses the
development of legs…the same
genes are expressed in
chickens in the area between
their limbs
Hypothetical vertebrate
ancestor (invertebrate)
with a single Hox cluster
First Hox
duplication
Hypothetical early
vertebrates (jawless)
with two Hox clusters
Second Hox
duplication
Vertebrates (with jaws)
with four Hox clusters
• Change in location of two Hox genes in
Crustaceans led to the conversion of
swimming appendage to feeding appendage
• Duplications of Hox genes in vertebrates
may have influenced the evolution of
vertebrates from invertebrates
Even more Evo-devo
Fig. 25-22
• Changes in genes and
where they are expressed
– Differing patterns of Hox
gene expression = variation
in segmentation
– Suppression of leg
formation in insects vs.
crustaceans
– Change in expression, not
gene, can cause differences
in form Genital
Fig. 21-18
Thorax
Thorax
segments
Abdomen
Hox gene 6
Hox gene 7
Hox gene 8
Ubx
About 400 mya
insect
crustaceans
Artemia
Drosophila
Ubx gene expressed in
Abdomen – supressing leg formation
Ubx gene expressed
In main trunk – doesn’t supress legs
Abdomen
Brine shrimp Artemia in comparison to
grasshopper Hox expression
Fig. 25-23
RESULTS
Test of Hypothesis A:
Differences in the coding
sequence of the Pitx1 gene?
Result:
No
Test of Hypothesis B:
Differences in the regulation
of expression of Pitx1 ?
Result:
Yes
Marine stickleback embryo
Close-up
of mouth
Close-up of ventral surface
The 283 amino acids of the Pitx1 protein
are identical.
Pitx1 is expressed in the ventral spine
and mouth regions of developing marine
sticklebacks but only in the mouth region
of developing lake stickbacks.
Lake stickleback embryo
Evolutionary “Novelties” are actually just new forms arising by
slight modifications of existing forms
Fig. 25-24
Pigmented
cells
Pigmented cells
(photoreceptors)
Epithelium
Nerve fibers
(a) Patch of pigmented cells
Fluid-filled cavity
Epithelium
Optic
nerve
Nerve fibers
(b) Eyecup
Cellular
mass
(lens)
Pigmented
layer (retina)
(c) Pinhole camera-type eye
Cornea
Optic nerve
(d) Eye with primitive lens
Cornea
Lens
Retina
Optic nerve
Eye Evolution Video
(e) Complex camera-type eye