Post on 28-Jun-2020
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Chapter 3
What the rocks say:
Part 3
How do early organisms fit in the
tree of life?
Earliest fossils:
potentially 3.45 myo;
abundant by ~2.6
mya, corresponding
to rise in oxygen
Earliest fossils:
~3.5 bya
Earliest fossils:
~1.8 bya
How do early organisms fit in the tree
of life?
Earliest fossils:
potentially 3.45 myo;
abundant by ~2.6
mya, corresponding
to rise in oxygen
Earliest fossils:
~3.5 bya
Earliest fossils:
~1.8 bya
Endosymbiotic TheoryLynn Margulis 1967
⚫ More than 4 billion years ago, it is argued, primitive cells
had one of at least three modes of nutrition,
⚫ Phagocytosis - Invaginate food particles
⚫ Photosynthesis
⚫ Absorb free floating organic nutrients in their
environment.
⚫ Theory first published by L. Margulis, and is now
supported by detailed experimental testing. There are
also living examples of intermediates
⚫ Eukaryotic cells containing cyanobacteria
Endosymbiotic Theory
⚫ The theory argues that cells able to respire aerobically
were incorporated into cells that invaginate their food.
⚫ The resultant symbiosis had the capacity to carry out
aerobic respiration.
⚫ Maintenance of the symbiosis required the two cell types
to reproduce at about the same rate, and that one not
digest the other.
Endosymbiotic Theory
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Endosymbiotic Theory
⚫ Plant (and algal) cells arose when photosynthetic
prokaryotic cells were incorporated into one line of the
aerobic cell.
⚫ The result is that the three – way symbiosis could now fix
its own carbon using light energy, and carry out aerobic
respiration.
⚫ View animation of eukaryotic evolution (170 Kb).
Endosymbiotic Theory
Endosymbiotic Theory
⚫ Eukaryotic cells originated more than 2.7 billion
years ago.
⚫ The mitochondria (and plastids) of extant eukaryotic
cells are remarkably similar suggesting aerobic
bacteria (or cyanobacteria) were incorporated on only
one occasion each.
⚫ We might assume that the evolution of the Eukaryotic
cell was a most unusual event because an amazing
array of other cellular characteristics has evolved over
the same period.
Endosymbiotic Theory
⚫ Evidence in support of theory:
⚫ Mitochondria and chloroplasts have their own DNA
and can reproduce
⚫ Mt and Chl have ribosomes and can synthesize
proteins
⚫ Presence of double membrane shows that they were
engulfed by the host cell.
⚫ Thylakoids of chloroplast have same structure as the
membranes in cyanobacteria that carry the pigment
molecules
⚫ Chl a and Chl b in cyanobacteria and in chloroplast
Evidence in support of theory:
⚫ Both mitochondria and chloroplasts can arise only
from preexisting mitochondria and chloroplasts.
⚫ They cannot be formed in a cell that lacks them
because nuclear genes encode only some of the
proteins of which they are made.
Evidence in support of theory:
⚫ Both mitochondria and chloroplasts
have their own genome, and it
resembles that of bacteria not that of
the nuclear genome.
⚫ Both genomes consist of a single
circular molecule of DNA.
⚫ There are no histones associated
with the DNA.
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Evidence in support of theory:
⚫ Both mitochondria and chloroplasts have their
own protein-synthesizing machinery, and it more
closely resembles that of bacteria than that
found in the cytoplasm of eukaryotes.
⚫ The first amino acid of their transcripts is always fMet
as it is in bacteria (not methionine [Met] that is the first
amino acid in eukaryotic proteins).
⚫ A number of antibiotics (e.g., streptomycin) that act
by blocking protein synthesis in bacteria also block
protein synthesis within mitochondria and
chloroplasts. They do not interfere with protein
synthesis in the cytoplasm of the eukaryotes.
Evidence in support of theory:
⚫ Conversely, inhibitors (e.g., diphtheria toxin) of
protein synthesis by eukaryotic ribosomes do not —
sensibly enough — have any effect on bacterial
protein synthesis nor on protein synthesis within
mitochondria and chloroplasts.
⚫ The antibiotic rifampicin, which inhibits the RNA
polymerase of bacteria, also inhibits the RNA
polymerase within mitochondria. It has no such effect
on the RNA polymerase within the eukaryotic nucleus.
Evid
en
ce
in
su
pp
ort
of
theo
ry
Origin of multicellularity a major
transition in history of life
⚫ Evolved independently in different lineages
⚫ Extant organisms provide clues about origin of
multicellularity
Advantages of
being multicellular
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Origin of multicellularity
⚫ Oldest known possible multicellular eukaryote
is Grypania spiralis, a coiled, ribbon-like
fossil two millimeters wide and over ten
centimeters long.
⚫ It looks very much like a coiled multicellular
alga and has been described from banded
iron formations in Michigan 2.1 billion years
old.
Grypania spiralis
Oldest known Eukaryote 2000
million years old Grypania spiralis,
coiled thin films of carbon from the
Negaunee Iron-Formation in
Michigan, USA, provide the first
evidence for eukaryotic life on
Earth (Image courtesy of Dr.
Runnegar, UCLA).
Pleodorina (a larger colony of Chlamydomonas-like cells)
Gonium (a colony of Chlamydomonas-like cells)
Pleodorina (a larger colony of Chlamydomonas-like cells)
Volvox, a true multicellular organism
Origin of
multicellularity
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Evolution of Multicellularity
⚫ Arose separately in plants, fungi and animals
⚫ At least 25 times in total
⚫ Once for metazoa
⚫ Multiple times (with secondary losses) for plants, fungi
and eubacteria
⚫ Still happening today
⚫ Probably evolved because of division of labor
between different cell types with different
functions
Evolution of Multicellularity
⚫ Tissues and organs and development of
organ systems
⚫ Required cell signaling mechanisms, cell
adhesion, cell-cell communication and
coordination and programmed cell death
⚫ Genes may have multiple binding sites for
different transcription factors, and contribute to
diverse, complex developmental pathways.
Advantages Of Multicellularity
⚫ Size-Related Advantages
⚫ Ecological
⚫ Escape predators: selective agents were phagotrophic
organisms that consumed unicellular prey.
⚫ Once multicellular predators and suspension feeders
evolved, the size race was on between consumers
and their victims predator-prey “coevolutionary arms
race”
Advantages Of Multicellularity
⚫ Ecological
⚫ In the absence of phagotrophs increased size may
have been favored to
▪ Give a competitive advantage in benthic forms,
perhaps via overgrowth
▪ Provide storage reserves when nutrients are limiting
▪ Expand feeding opportunities [e.G., Group-feeding
myxomycetes and myxobacteria that produce
extracellular digestive enzymes]
▪ Generate an internal environment protected by an
external layer of cells
▪ Allow novel metabolic opportunities
▪ Enhance motility for dispersal or foraging
Advantages Of Multicellularity
⚫ Functional Specialization and Division of Labor
⚫ Simultaneously partition complementary tasks among
different cells.
▪ When development occurs clonally, the benefits of
division of labor are shared primarily among close
relatives, fueling the evolution of division of labor
⚫ Metabolic cooperation
▪ Some key metabolic processes cannot concurrently take
place within a cell.
▪ Ex. Nitrogen fixation and photosynthesis in cyanobacteria –
Evolution of heterocysts
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⚫ Metabolic cooperation
⚫ Motility-mitosis trade-offs - loss
of mitotic activity in the somatic
cells of large volvocaceans
Advantages Of Multicellularity
Eukaryotic multicelluar life
⚫ Earliest fossils of algae date
to 1.6 bya
⚫ Red algae: 1.2 bya
⚫ Green algae: 750 mya
Red algae fossil; 1.2 bya
Evolution of Multicellularity
⚫ Multicellular eukaryotes may have existed 1
billion years ago but a major burst of
metazoan diversification occurred about
600 – 700 Mya, at a time of dramatic
increases in atmospheric and oceanic
oxygen.
Evolution of Multicellularity
⚫ Oldest fossils of multicellular animals – 575
myo
⚫ Ediacaran Fauna of late Paleozoic and early
Cambrian
⚫ Mostly soft bodied, no skeletons
⚫ Flat and crept or stood on the sea floor
⚫ Some may have been stem groups for the
Cnidaria and Bilateria
⚫ No mouthparts, or locomotory appendages
⚫ Little evidence that they were subject to predation
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Evolution of Multicellularity
⚫ One group of flagellates, the
choanoflagellates, is thought to
have given rise to the simplest
animals, the sponges.
Evolution of Multicellularity
⚫ Bacteria influence choanoflagellate colony
formation
⚫ It is plausible that bacteria also helped to
stimulate multicellularity in the ancestors of
animals.
pinacoderm
mesohyle with amoebocytes
choanoderm
Cell Layers Choanocytes
Amoebocytes in the mesohyl
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Choanocyte or Collar cell
Note similarity to group of flagellated protozoans called the Choanoflagellates!
nucleus
collar
flagellum
microvillus
Search for early animals can be
controversial
Animal embryos or animal relatives?
Doushantuo Cysts
from Chinese
Lagerstätte
Complex Single-celled Life
⚫ Acritarchs⚫ Acid-resistant, organic-
walled, micro-plankton
⚫ Common Proterozoic
eukaryote fossils
⚫ Represent encysted "resting
stage" of organism
⚫ Resting cysts of algae
⚫ Egg cases of small
metazoans
⚫ Others are cysts of unknown
eukaryotes
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The dawn of animals
⚫ Early animal life resemble sponges⚫ Oldest fossils 650 myo
⚫ Biomarkers also demonstrate existence of sponges during this time
⚫ Cholesterol-like molecule similar to one made by one group of sponges today
BIOMARKERS
⚫ Read pages 60-61 on biomarkers
Ediacaran fauna
⚫ Diverse and unique animals dominated the oceans from 575 – 535 mya⚫ Many hard to place taxonomically
Evolution of Ediacaran fauna
Precambrian Animals650 to 544 mya
Ediacaran Fauna of Australia and
Newfoundland, CA
Soft Bodied Invertebrates
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Life in the Late Proterozoic
⚫ In the latest part of the Proterozoic (~ 600 mya), multi-cellular,
complex life is recorded in the fossil record.
⚫ Oldest fossils within Kingdom Animalia are Vendian age 650 to 544
mya, and are found at nearly 30 locations around the world, and are most distinctive.
⚫ Fossils are preserved as thin impressions on bedding surfaces of fine
to medium-grained sedimentary rocks.
⚫ Organisms were very thin, lacked any minerallized hard parts or well
developed organs or organ systems, and had a quilt-like outer
surface.
⚫ Uncertainty about what groups of animals these fossils might represent, and, if they were ancestral to the animals that appeared in
the late Cambrian.
Avalon Peninsula Newfoundland, CA
Ediacaran Biota
⚫ Appeared immediately after the prolonged period of
global glaciations towards the end of the
Precambrian
⚫ Extraordinary organisms:
⚫ Discs
⚫ Fronds
⚫ Segmented morphologies
⚫ Locations
⚫ Flinders Range (Ediacaran Hills), South Australia
⚫ Avalon Zone of Newfoundland
⚫ Nama Group in Namibia
•Jellyfish
•Worm Tubes
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Cyclomedusa a jellyfish Charnia masoni - no known
living descendants
Dickinsonia costata – early relative of earthworms?
f09_27_pg19
f09_28_pg20
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Trace fossils of primitive
worms
Spriggina
Parvancorina
Dickinsonia
Charnia
Cyclomedusa
Cloudinia
Precambrian Ocean – Ediacaran Fauna
620 to ~543 mya
Cambrian
Cambrian Fauna
Early Cambrian: 542 – 511 mya
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Burgess Shale fossils
⚫ These fossils merit special interest for
several reasons:
⚫ They were buried in an underwater avalanche of fine
mud that preserved exceptionally fine details of the
structure of their soft parts. Only hard parts are
preserved in most other Cambrian deposits, obviously
limiting information within the geologic record.
⚫ They represent an early snapshot of the complexity of
evolving life systems. The Burgess Shale fossils as a
group have already developed into a variety of sizes
and shapes from the much simpler, pre-Cambrian life
forms.
Burgess Shale fossils
⚫ Many of them appear to be early ancestors of higher
forms; from algae to the chordates (a major group of
animals that includes human primates). Others appear
unrelated to any living forms and their later
disappearance presents an intriguing mystery.
Currently existing lineages recognizable
during the early Cambrian
Early Cambrian: 542 – 511 mya
Anomalocaris canadensis
Marella
Hallucigenea
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Jellyfish?Shrimp?
Sponge?
Parts of Anomalocaris were at first thought to be three
separate animals.
Anomalocaris
canadensis
A proto-arthropod
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Wiwaxia corrugata
Wiwaxia corrugata
Marrella splendens
Hallucigenia
Chordates emerged during early
CambrianKey Concepts
⚫ Only a fraction of Ediacaran fauna share
traits with existing lineages
⚫ Almost all extinct within 40 million years
⚫ Most existing lineages are found in the fossil
record during the Cambrian period
⚫ Includes our own lineage, the chordates