Grade 11 University Biology – Unit 1 Diversity of Life
Eukaryotic Evolution and Diversity
Section 2.3 (Pages 67-71)
The fossil record indicates that simple unicellular organisms – prokaryotes – were present on Earth as
early as 3.5 billion years ago (BYA).
Recall
Prokaryotes
 Small and simple
 Lacking many internal structures such as membrane-bound organelles
 Mostly bacteria
 Have an outer layer called the cell membrane
 Inside is a watery fluid (… cytoplasm…) which is about 70% water plus 30% proteins
(…enzymes…) and smaller molecules like amino acids, glucose molecules and ATP.
 Contains simple genetic DNA
 May occur as colonials or aggregate forms
 Some can photosynthesis, others extract energy from the environment directly
 Divide cells by binary fission
 Reproduce by conjugation
Via prokaryote life, Earth conditions changed (e.g.,
photosynthetic prokaryotes gave off oxygen that slowly
altered Earth’s atmosphere).
About 2 BYA, Eukaryotes appeared. Eukaryotes are more
complex and larger. They also contain membrane-bound
organelles such as mitochondria and Golgi bodies that
perform specific tasks inside the cell. However, the first
Eurkaryotes were simple and unicellular protists (…all
processes of life are completed within one cell, although protists can occur in a wide variety of structural,
locomotive and nutritional patterns).
About 1.5 BYA, multicellular Eukaryotes formed.
It is believed that protists are the ancestral organisms for all multicellular life (i.e., plants, animals, fungi).
 Fungi and animals appeared 545 million years ago (MYA) from a common colonial protistan
ancestor similar to a flagellated eukaryote. Simple invertebrates (e.g., sponges) appeared first,
while more complex forms (e.g., fish, amphibians, reptiles, birds, mammals) appeared later.
 Plants appeared 450 MYA from a common protistan ancestor similar to green algae
Thus, the theoretical order of evolution is -Simple prokaryotes
Simple unicellular eukaryotes
Multicellular eukaryotes
Question
How is it possible that a simple prokaryote with no internal membrane-bound organelles can become a
more complex eukaryote with multiple membrane-bound organelles with specific functions?
 Working in small groups and using the materials, develop your ideas (hypothesis) to explain the
development
 Present your ideas to the class
Endosymbiosis
 Theory that explains how eukaryotic ells evolved from the symbiotic relationship between two or
more prokaryotic cells
 Symbiosis means “living together”
 In endosymbiosis, one cell engulfs a different type of cell. However, the engulfed cell survives
and becomes part of the internal functioning of the engulfing cell.
 The prevailing endosymbiotic theory suggests free-living prokaryotes that photosynthesized and
produced energy were engulfed by larger cells. Intact in the larger cell, they became the
chloroplasts and mitochondria, respectively. The engulfed cell (…now organelle…) is the
ENDOSYMBIONT and the engulfing cell is a HOST CELL.
 The proof for the Endosymbiosis theory includes (1) some organelles reproduce inside
the cell via binary fission (see Figure 2.15 on Page 68), (2) the membranes of the
chloroplasts and mitochondria are very similar to the membranes of the prokaryotes, (3)
photosynthesis began in prokaryotes, and (4) ribosomes that assemble proteins are
similar structurally to prokaryote ribosomes.
Multicellularity
 Large, complex multicellular eukaryotes first developed 550 MYA
 These complex organisms arose from colonies created by masses of individual cells. Cells within
the masses specialized in function. What would be the evolutionary advantage of cell
specialization?
Life Cycles and Reproduction
 Cell division in eukaryotes is diverse. Asexual mitosis still occurs, although many eukaryotes
employ MULTIPLE FISSION in which multiple copies of a cell are made at one time.
 To produce a new individual, the process is sexual reproduction. Two individuals make eggs and
sperm, called GAMETES. Gametes are haploid cells that contain only one set of chromosomes
(…other cells have two sets of chromosomes…these cells are diploid). When the sperm and egg
cells fuse, they form a diploid ZYGOTE. It contains two set on chromosomes. One set is from
the male and one from the female. This process is called MEIOSIS. See Figure 2.17 on Page 70
to contrast asexual and sexual life cycles.
Questions
 Page 69, Questions 13-17
 Page 71, Questions 1, 4-6, 8-11