Transcript Slide 1

Dawn of Eukarya
Chapter 28
The Steps in the Development
of a Eukaryote Cell
 The evolution of eukaryotic cells
included the following components:
The origin of a flexible cell surface
The origin of a cytoskeleton
The origin of a nuclear envelope
The appearance of digestive vesicles
The endosymbiotic acquisition of certain
Revision of the cell – first
 The first step toward the eukaryotic
condition may have been the loss of
the cell wall by an ancestral
prokaryotic cell.
Steps in the formation of a
Eukaryote cell
 A surface that is flexible enough to allow for
infolding lets the cell exchange materials
with its environment rapidly enough to
sustain a larger volume and more rapid
 A flexible surface also allows endocytosis.
 An infolded plasma membrane attached to a
chromosome within an ancestral prokaryote
may have led to the formation of the nuclear
The development of a
eukaryote cell
 The early steps in the evolution of the
eukaryotic cell likely included three
The formation of ribosome-studded
internal membranes, some of which
surrounded the DNA
The appearance of a cytoskeleton
The evolution of digestive vesicles
Cytoskeleton and size and
 A cytoskeleton allowed the now much larger
cell to manage changes in its shape,
distribute daughter chromosomes, and move
materials from one part of the cell to
 The origin of the cytoskeleton is a mystery;
the genes that encode it are found in neither
bacteria nor archaea.
 A controversial hypothesis suggests that
these genes may have originated in a longextinct fourth domain of life that
transferred them laterally to an ancestor of
the early eukaryotes.
Intermediate cell
 From an intermediate kind of cell, the
next advance was likely to have been a
motile phagocyte.
 The first true eukaryotic cell
possessed a cytoskeleton and a nuclear
envelope; it also may have had an
associated endoplasmic reticulum and
Golgi apparatus and perhaps one or
more flagella
The Effects of Oxygen
 During the early stages of eukaryotic evolution, the
O2 levels in the atmosphere were increasing as a
result of the photosynthetic activities of the
 Most living things were unable to tolerate this new
aerobic, oxidizing environment, but some prokaryotes
and ancient phagocytes were able to survive.
 One hypothesis suggests that the key to the survival
of the early phagocytes was the ingestion of a
prokaryote that became symbiotic and evolved into
the peroxisomes of today.
Blending of the two Domains?
 It is clear that the eukaryotic genome
is a mixture of genes with two distinct
 Recently, it has been suggested that
the Eukarya may have arisen from the
mutualistic fusion of a Gram-negative
bacterium and an archaean.
Giardia – A link?
Figure 28.1 Three Protists
Polyphyletic Origins
 Most protists are aquatic, occupying a
variety of environments including marine and
fresh waters, the body fluids of other
organisms, and soil water.
 Most are unicellular, but some are
multicellular, and a few are very large.
 Some protists are heterotrophs, some are
autotrophs, and some switch between these
two modes of nutrition.
 The terms protozoan and algae actually lump
together many phylogenetically distant
protist groups.
Cell Surfaces
Cell surfaces
Contractile vacuoles
Food vacuoles
Apicomplexans - Malaria
Advanced ciliates
Sex – Conjugative exchange
Red Tides- Dinoflagellates
Reproductive strategies
Slime molds