2.3: Eukaryotic Evolution and Diversity
pg. 67
For about 1.5 billion years Prokaryotes were on the only living
organism on Earth.
3.5 to 2 billion years ago Prokaryotes thrive in many different
environments.
2 billion years ago Eukaryotes evolved, which led to an increase in
diversity of life on Earth.
Eukaryotic organisms, are more complex, have far more genes,
greater cellular diversity, (size, shape, mobility, and specialized
functions).
How did eukaryotic organisms develop?
Endosymbiosis
Endosymbiosis: theory that explains how eukaryotic cells evolved
from the symbiotic relationship between two or more prokaryotic
cells.
One cell engulfs a different type of cell. The engulf cell survives
and becomes an internal part of the engulfing cell.
Prokaryotes do not have membrane bond organelles, yet
eukaryotes have membrane bond organelles. These organelles may
have been ancestors of once free-living prokaryotes.
There are two organelles that support this theory. The Chloroplast
is an organelle found in photosynthetic eukaryotes, to convert
sunlight energy into chemical potential energy, glucose. The other
organelle is the Mitochondria, an organelle responsible for cellular
respiration and the breakdown of glucose into a chemical potential
energy known as ATP. ATP is the energy source which allows
cells to do work, metabolism.
Figure 2.14 A likely event in the evolution of the eukaryotic cells was the endosymbiotic
engulfing of smaller cells that became part of the larger one.
Chloroplasts and Mitochondria
Based on this theory, the chloroplast and mitochondria were once
smaller prokaryotic cells. At separate times these prokaryotes were
engulfed by larger prokaryotes, leading to the evolution of a
Eukaryotic cell.
These cells were not digested, when engulfed, but continued to
perform their function while inside the larger cell.
The engulfed cell is called an endosymbiont, and the engulfing cell
is called a host cell. The host cell benefited greatly from this
arrangement.
Evidence to support this theory:
a) The membranes of the chloroplast and mitochondria are
similar to that of the prokaryotic cell.
b) Ribosomes found inside these organelles to produce proteins
are similar to prokaryotes
c) These organelles reproduce by binary fission.
d) Each of the organelles have their own DNA, a circular
chromosome, and gene content similar to prokaryotes.
Chloroplast genetic sequence is matches closely to the modern
cyanobacteria, which photosynthesis.
Figure 2.15 Mitochondria have their own genetic material in a bacteria-like
chromosome. Like bacteria, they also divide by binary fission.
Multi-cellularity
Although endosymbiosis may have led to the complexity of a
Eukaryotic cell, it did not account for the multi-cellularity.
Cells living prokaryotic aggregation carry out the same function,
without specialization. These groups are just a collection of cloned
individuals.
When eukaryotic cells appear on Earth, it was of the unicellular
variety.
1.2 to 1.5 billion years ago, it is estimated that the first multicellular organisms evolved. Fossil evidences of the Red Algae
support the evolution of simple multi-cellular organism. Larger
more complex eukaryotes developed 550 million years ago.
Multi-cellular Organisms, first lived in colonies of single cells. As
reproduction occurred, the genetic information of some cells led to
specialization, where some cells took on specific roles to support
the colony. Specialization saw some cells become responsible for
absorbing nutrients, and responding to environmental stimuli.
Life Cycles and Reproduction
Eukaryotes have greater reproduction diversity than prokaryotes.
Prokaryotes have asexual reproduction, binary fission.
Eukaryotes also have asexual reproduction, complex binary fission,
budding, cloning, and fragmentation.
Multi-cellular organisms also reproduce by sexual reproduction.
Reproduction occurs between two different individual organisms,
one donating an egg and the other sperm cells which are haploid.
Fertilization occurs either internally or externally. After
fertilization the new cell is diploid.
The process of Meiosis is responsible for producing sex cells,
gametes.
Multi-cellular organisms grow in size, by a process known as
Mitosis.
Figure 2.17 Life cycles can asexual or sexual.
Learning Check: questions 13 – 18, pg. 69
Section 2.3 Review, questions 1 – 13, pg. 71