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Endosymbiotic Theory
Universal Tree of Life
Or…...
• According to this tree the
earliest eukaryotic cells
were Archaezoa that are
amitochondriate organisms
that were adapted to an
anaerobic environment
such as the extant
Diplomonads (Giardia),
Parabasalia (trichomonads)
and the Microsporidia.
Mitochondria then were
acquired at a later stage
from a bacterial
endosymbiont belonging to
the group of the alphaproteobacteria.
Key Differences among the Major domains of Life
Single-celled Eukaryotes
Giardia
euglenids
Multi-cellular Eukaryotes
Green algae (Ulva)
Red algae
Multi-cellular Eukaryotes cont.
Multi-cellular plants (ferns)
Multi-cellular animals
Multi-cellular Eukaryotes cont.
Slime molds
Fungi
Endosymbiotic
Theory
The evolution of the
compartmentalized nature of
eukaryotic cells may have
resulted from two processes:
 Specialization of plasma
membrane invaginations
 Endosymbiotic associations
of prokaryotes may have
resulted in the appearance of
some organelles
Endosymbiotic Theory cont.
• The theory was developed extensively by Lynn Margulis
• It focuses mainly on the origins of chloroplasts and mitochondria
• 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.
• Perhaps they gained entry as parasites or undigested prey of larger
prokaryotes.
Then the association progressed from parasitism or predation to mutualism.
Observations supporting the hypothesis
 The organelles 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.
 The organelles have their own DNA and they reproduce by simple
fission.
 DNA of the organelles are more closely related to particular types of
bacteria than they are to the nucleus of the cell in which they reside.
 Chloroplasts have ribosomes more similar to prokaryotic ribosomes
(with regards to size, biochemical characters, etc) than to eukaryotic
ribosomes
• Molecular systematic evidence: RNA of chloroplasts is more similar
in basic sequence to RNA from certain photosynthetic eubacteria than
to rRNA in eukaryotic cytoplasm
Environmental Implications?
• Because they have bigger cells, and because nearly all do aerobic
respiration, Eukarya require higher levels of O2 to survive.
• Mitochondria - can do aerobic respiration when O2 is 2% of
modern.
• Chloroplasts - can do photosynthesis when O2 is 10% of
modern.
• 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.
~ 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..
• Cyanobacteria are very tolerant. They are relatively immune to
selection driven by environmental change.
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.
Grypania spp.
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.
• The number of different types of cells is low, however.
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?
Evolution 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.
• Much higher levels of variation allows much more rapid
evolutionary change.