1. Essentials of the Living World Evolution of Microbial Life
Second Edition
George B. Johnson
Jonathan B. Losos
Chapter 17
Evolution of Microbial Life
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. 17.1 How Cells Arose
No one knows for sure where the first organisms (thought to be like today’s bacteria) came from
There are, in principle, at least three possibilities for the origin of life on earth
extraterrestrial origin
special creation
evolution

3. 17.1 How Cells Arose The earth formed 4.5 billion years ago
The first life originated around 2.5 billion years ago
Figure 17.1 A clock of biological time.

4. 17.1 How Cells Arose
When life formed, the earth’s atmosphere contained little or no oxygen but contained lots of hydrogen-rich gases, such as hydrogen sulfide (SH2), ammonia (NH3), and methane (CH4)
Electrons of these gases would have been frequently pushed to higher energy levels by photons from the sun or by electrical energy in lightning

5. 17.1 How Cells Arose
Stanley Miller and Harold Urey reconstructed the oxygen-free atmosphere of the early earth in their laboratory
They subjected it to the lighting and UV radiation that it would have experienced then
They found that many of the building blocks of organisms formed spontaneously
They concluded that life may have evolved in a “primordial soup” of biological molecules formed in the early earth’s oceans

6. 17.1 How Cells Arose
Critics of the Miller-Urey experiment say that, because there was no oxygen in the early atmosphere, there would have been no protection from ozone against UV
The UV radiation would have destroyed ammonia and methane in the atmosphere, without which, the building blocks cannot be synthesized

7. 17.2 How Cells Arose
An alternative to the Miller-Urey explanation for the origin of the building blocks of life is the bubble model
this model asserts that the key chemical processes generating the building blocks takes place within bubbles on the ocean’s surface
the bubbles were produced by wind, wave action, the impact of rain drops, and volcanic action
chemical reactions would proceed fast inside the bubbles where polar reactants would be concentrated
the bubbles would also provide protection from UV radiation

8. Figure 17.2 A chemical process involving bubbles may have preceded the origin of life

9. 17.1 How Cells Arose
Most scientists assume the first cells aggregated spontaneously
When organic molecules are present in water, they tend to cluster together in suspensions called microspheres
these microspheres have many cell-like properties
Little is known about how the first cells developed from microspheres

10. 17.2 The Simplest Organisms
Prokaryotes are the simplest and most abundant organisms on earth
they are small, simply organized, single cells that lack a nucleus
the prokaryotic cell’s plasma membrane is encased within a cell wall
in bacteria, the cell wall is made of peptidoglycan
in archaea, the cell wall lacks peptidoglycan and is made of either protein and/or polysaccharides

11. 17.2 The Simplest Organisms
In many bacteria, called gram-negative bacteria, the cell wall has an outer layer of lipopolysaccharide covering a thinner layer of peptidoglycan
If this membrane is absent, then the bacterium is called gram-positive because its peptidoglycan layer is much thicker

12. Figure 17.3 The structure of bacterial cell walls

13. 17.2 The Simplest Organisms
Many bacteria have a gelatinous layer beyond the membrane and the cell wall
this layer is called the capsule
Additional features of some bacteria include
flagella are long strands of protein used in swimming
pili are shorter strands that act as docking cables
endospores are thick-walled enclosures of DNA and a small bit of cytoplasm that are extremely resistant to environmental stress

14. 17.2 The Simplest Organisms
All prokaryotes can reproduce via binary fission
after replicating DNA, the plasma membrane and cell wall grow inward and eventually divide the cell
Some bacteria can exchange genetic information via plasmids passed from one cell to another
this process is called conjugation and the transfer a special pilus making contact with another cell
the connection is called a conjugation bridge

15. Figure 17.4 Bacterial conjugation

16. Figure 17.5 Contact by a pilus

17. 17.3 Comparing Prokaryotes to Eukaryotes
Prokaryotes have evolved many more ways than eukaryotes to acquire the carbon atoms and energy necessary for growth and reproduction
photoautotrophs perform photosynthesis
chemoautotrophs obtain energy by oxidizing inorganic substances
photoheterotrophs are purple non-sulfur bacteria that use light as an energy source but obtain carbon from organic molecules produced by other organisms
chemoheterotrophs obtain both energy and carbon from organic molecules produced by other organisms

18. Table 17.1 Prokaryotes Compared to Eukaryotes

19. 17.4 Viruses Infect Organisms
Viruses are parasitic chemicals, segments of DNA (or sometimes RNA) wrapped in a protein coat
they cannot reproduce on their own
they occur in all organisms
bacterial viruses are called bacteriophages
many viruses form an additional layer around their DNA core; this is called a capsid
in turn, the capsid may be encased by a membranelike envelope, rich in proteins, lipids, and glycoproteins

20. Figure 17.6 The structure of bacterial, plant, and animal viruses

21. 17.5 The Origin of Eukaryotic Cells
Eukaryotic cells possess an internal structure called a nucleus
The endosymbiotic theory suggests that at a critical stage in the evolution of eukaryotic cells, energy-metabolizing bacteria came to reside symbiotically within larger early eukaryotic cells

22. Figure 17.7 The theory of endosymbiosis

23. 17.6 General Biology of Protists
The only unifying thing about protists is that they are not fungi, plants, or animals
otherwise, they are extremely variable eukaryotes
protists have varied types of cell surfaces
all have a cell membrane but many have cell walls or glass shells
movement in protists is accomplished by diverse mechanisms
cilia, flagella, pseudopods, or gliding mechanisms

24. 17.6 General Biology of Protists
Some protists can survive harsh environmental conditions by forming cysts
cysts are dormant forms of cells with a resistant outer covering in which cell metabolism is more or less completely shut down

25. 17.6 General Biology of Protists
Protists employ every form of nutritional acquisition except chemoautrophy
phototrophs are photosynthetic autotrophs
among heterotrophic forms,
phagotrophs ingest visible particles of food
the ingested food is put into intracellular vesicles called food vacuoles that are then broken down by lysosomes
osmotrophs ingest food in soluble form

26. 17.6 General Biology of Protists
Protists typically reproduce asexually, most reproducing sexually only in times of stress
fission and budding are common forms of asexual reproduction
sexual reproduction occurs among some protists, especially algae and sporozoans

27. 17.6 General Biology of Protists
A single cell has limits but protists have evolved ways to go beyond a single cell
colonial organism is a collection of cells that are permanently associated but in which little or no integration of cell activities occurs
aggregation is a more transient collection of cells that come together for a period of time and then separate
this is common in cellular slime molds when aggregations called slugs form to move to a new feeding location
multicellular organism involves cells in contact with each other and coordinated in their activities
three phyla of protists have evolved examples of mutlicellularity
brown, green, and red algae

28. Unicellular versus Colonial Protist
Figure 17.8 A unicellular protist
Figure 17.9 A colonial protist

29. 17.7 Kinds of Protists
The protists are the most diverse of the four kingdoms in the domain Eukarya
the phyla of protists are only distantly related to each other
there are 15 distinct phyla groups into five general groups based on some major shared characters

30. Figure 17.10 The major protist groups

31. 17.8 A Fungus Is Not a Plant
Fungi lack chlorophyll and resemble plants only because of their general appearance and lack of mobility
But fungi differ from plants in significant ways in that fungi
are heterotrophs
have filamentous bodies
have nonmotile sperm
have cell walls made of chitin
have nuclear mitosis

32. 17.8 A Fungus Is Not a Plant
Fungi exist mainly in the form of slender filaments called hyphae (singular, hypha)
different hyphae then associate with each other to form much larger structures
a mass of hyphae is called a mycelium (plural, mycelia)
fungal cells are able to exhibit a high degree of communication within a mycelium
cytoplasm is able to cross between adjacent hyphal cells by a process called cytoplasmic streaming
multiple nuclei can be connected through the shared cytoplasm

33. Figure Mushrooms

34. 17.8 A Fungus Is Not a Plant
Fungi reproduce both asexually and sexually
in many cases, fungi of two different mating types produce haploid gametes
the gametes do not fuse in fertilization but instead coexist in a common cytoplasm; this is called being dikaryotic
a heterokaryon describes two nuclei from two different mating types
a homokaryon describes two nuclei from the same mating type

35. 17.8 A Fungus Is Not a Plant
When reproductive structures form in fungi, they are separated from the other parts of the mycelium
There are three kinds of reproductive structures
gametangia produce gametes
sporangia produce haploid spores that can be dispersed
conidophores produce asexual spores that can be produced quickly to colonize rapidly a new food source

36. Figure 17.12 Many fungi produce spores

37. 17.8 A Fungus Is Not a Plant
All fungi are heterotrophs and externally digest food by secreting enzymes into their surroundings and then absorbing the nutrients back into the fungus
some fungi are predatory, such as the oyster fungus

38. Figure 17.13 The oyster mushroom

39. 17.9 Kinds of Fungi
There are nearly 74K species described species spread across four phyla
most of the phyla are distinguished based on their mode of sexual reproduction
imperfect fungi are outside of the four phyla because sexual reproduction in this group has not been observed
Together with bacteria, fungi are the principal decomposers in the biosphere

40. Table Fungi

41. 17.9 Kinds of Fungi
Two kinds of mutualistic associations between fungi and autotrophic organisms are ecologically important
mycorrhizae are fungal/plant associations
these interactions expedite the plant’s absorption of essential nutrients, such as P, in the roots
lichens are fungal/algal or fungal/cyanobacterial associations
these can grow in harsh habitats, such as bare rock
In each of these associations, a photosynthetic organism fixes atmospheric CO2 and makes organic material available to fungi

42. Inquiry & Analysis
What is the impact of exposure to chytrids upon development of the frog-killing disease?
Graph of Effects of Exposure of Frogs to Chytrids