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The Endosymbiotic Theory
The Tree of Life and its Main Branches
At the highest level, life can be divided into three main groups, called Domains: the
Archaea, the Eubacteria, and the Eukarya.!
Characteristic
Size
Metabolism
Bacteria
small (0.2 - 10mm)
all types of energy
metabolism: some
chemo-autotrophs, many
fermenters, diverse
photo- autotrophs, some
respirers
single loop
no nucleus
none
Archaea
small (0.2 - 10mm)
all types of energy
metabolism: many
heterotrophs and
chemo-autotrophs
Membrane
lipids
Cell walls
ester links
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Stiff
Stiff
Reproduction
Asexual - fission
Asexual - fission
Body plan
Habitats
unicellular/colonial
anaerobic and aerobic
habitats; very tolerant
unicellular/colonial
anaerobic and
aerobic habitats;
extremely tolerant
of hot or acidic or
salty habitats
DNA structure
Organelles?
!
single loop
no nucleus
none
Eukarya
large (10 – 100mm)
more limited types
of metabolism:
fermentation,
oxygenic-producing
photo-autotroph and
aerobic respiration
chromosomes in
nucleus
Mitochondria,
plastids, flagella
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Stiff (plants); flexile
(animals )
Asexual - mitosis
Sexual - meiosis
chiefly multicellular
most require oxygen
and are intolerant of
nasty environments
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The Universal Tree of Life
This phylogenetic tree shows the relationships of the major groups (Kingdoms) of organisms.! These
Kingdoms are arranged within the three Domains of life, Archaea, Eukarya, and Eubacteria.! The tree is
constructed from comparative analysis of the sequence of nitrogen-bearing bases on RNA molecules from
organisms in each group.! Longer branches indicate greater genetic distance between organisms. (modified
from J.I. Lunine 1999. Evolution of a Habitable World. Cambridge Univ. Press)
The most primitive eukaryote - Giardia:no mitochondria or plastids, doesn't tolerate O2.
More derived eukaryotes - a diversity of unicellular eukaryotes or Protists: all have
mitochondria (e.g., amoebas, ciliates), some have plastids too (e.g., euglenids)
Big Bang of Eukarya - Multicellularity arose many times within the Eukarya:! green
algae, red algae, multicellular plants, multicellular animals, slime molds, and fungi: All
these groups have mitochondria, plastids independently appeared in many different
groups.
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How did eukaryotes originate?
The evolution of the compartmentalized nature of eukaryotic cells may have resulted
from two processes
Specialization of plasma membrane invaginations.
Invaginations and subsequent specializations may have given rise to the nuclear
envelope, ER, Golgi apparatus, and other components of the endomembrane system
Endosymbiotic associations of prokaryotes may have resulted in the appearance of some
organelles.
Mitochondria, chloroplasts, and some other organelles evolved from prokaryotes living
within other prokaryotic cells
The Endosymbiotic Hypothesis
The theory focuses mainly on the origins of chloroplasts and mitochondria
It has been argued that mitochondria and plastids (and perhaps flagella) were once freeliving bacteria that took up residence inside the cell of another organism, probably an
Archaean.
More specifically, chloroplasts are believed to have descended from endosymbiotic
photosynthesizng prokaryotes, such as cyanobacteria, living in larger cells
Mitochondria are postulated to be descendents of prokaryotic areobic heterotrophs.
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What observations support this hypothesis?
They are of the appropriate size to be descendents of eubacteria
They have inner membranes containing several enzymes and transport systems similar to
those of prokaryotic plasma membranes
The organelles are separated from the cytoplasm by complex membranes.
Organelles have their own DNA (a simple loop); they reproduce by simple fission.
Comparisons of the genetic contents of this DNA reveal that the organelles are more
closely related to particular types of bacteria than they are to the nucleus of the cell in
which they reside. Mitochondria are closely related to purple bacteria; chloroplasts are
related to cyanobacteria.
Chloroplasts have ribosomes more similar to prokaryotic ribosomes (with regards to size,
biochemical characters, etc) than to eukaryotic ribosomes
Mitochondrial ribosomes vary, but are also more similar to prokaryotic ribosomes
The RNA of chloroplasts is more similar in basic sequence to RNA from certain
photosynthetic eubacteria than to rRNA in eukaryotic cytoplasm
Chlorplast rRNA is transcribed from genes in the chloropast while eukaryotic rRNA is
transcribed from nuclear DNA
Mitochondrial rRNA also has a base sequence which supports the eubacterial origin
Environmental Implications?
Because they have bigger cells, and because nearly all do aerobic respiration, Eukarya
require higher levels of O2 to survive.
Many Archaea and Eubacteria can’t tolerate O2.
The rise in oxygen may have driven them into oxygen-poor zones where eukaryotes can’t
go.
What does the fossil record say about this transition?
!
Sterols at 2.7 Bya. Sterol, which are present in the cell walls of the Eukarya, may just
indicate that the "host" cell that was eventually invaded by Purple Bacteria and
Cyanobacteria had evolved. They do not provide evidence that the key symbiotic events
had taken place at 2.7 Bya.
A better way to spot Eukarya in the fossil record is to look for big cells.
Bacteria and Archaea typically form small cells.! If a cell is bigger than 60 microns, it
was almost certainly was a eukaryote.
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~ 2.0 Bya: The Gunflint Chert
These rocks contain the first abundant and diverse record of unicellular organisms.!
The organisms are small, and all have morphologies very similar to living cyanobacteria.
Why have cyanbacteria failed to evolve for 2.0 billion years?
Asexual/Clonal reproduction - No sexual mixing of the genome. Genetic novelties can
only arise by mutation. This limits the new genetic combinations upon which natural
selection can work.
2.1 - 1.7 Bya: The first large fossil: Grypania
It is not yet clear what Grypania is, but it is so large, it almost certainly is a eukaryote.
Either large and unicellular with many nuclei or multicellular
1.7 to 1.2 Bya: non-diverse communities of unicellular Eukarya
Fossils are much more common in rocks of this age.!
Many contain unicellular organisms that are bigger than 60 microns, so we are probably
dealing with eukaryotes
1.2 - 0.7 Bya:! Unicellular Eukarya diversify
Acritarchs: a group of unicellular, eukaryotic fossils common at this time.
Based on biochemical and morphologic evidence, it is thought that they are the resting
cysts of protozoans that are common in the ocean today.
Why the increase in diversity at 1.2 Bya?
Invention of Sexual Reproduction
Sexually reproducing organisms get copies of genes from each parent, then shuffle the
genes from each parent before making gametes.!
This allows for the perpetual generation of new genetic combinations.!
Natural selection is presented with new genetic combinations every generation.!
Much higher levels of variation allows much more rapid evolutionary change.