Download CHAPTER 4

Chapter 15
The Evolution of Microbial Life
Biology and Society:
Can Life Be Created in the Lab?
• How did life first arise on Earth?
• To gain insight, scientists have
– Synthesized the entire genome of Mycoplasma genitalium, a species of
bacteria found naturally in the human urinary tract
– Transplanted the complete genome of one species of Mycoplasma bacteria
into another
• A research group led by Craig Venter hopes to
– Create an artificial genome
– Transplant it into a genome-free host cell
• An artificial organism that could be completely controlled might
– Clean up toxic wastes
– Generate biofuels
– Be unable to survive outside rigidly controlled conditions
MAJOR EPISODES IN THE HISTORY OF LIFE
• Earth was formed about 4.6 billion years ago.
• Prokaryotes
– Evolved by 3.5 billion years ago
– Began oxygen production about 2.7 billion years ago
– Lived alone for almost 2 billion years
– Continue in great abundance today
• Single-celled eukaryotes first evolved about 2.1 billion years ago.
• Multicellular eukaryotes first evolved at least 1.2 billion years ago.
• All the major phyla of animals evolved by the end of the Cambrian explosion,
which began about 540 million years ago and lasted about 10 million years.
• Plants and fungi
– First colonized land about 500 million years
– Were followed by amphibians that evolved from fish
THE ORIGIN OF LIFE
• We may never know for sure how life on Earth began.
Resolving the Biogenesis Paradox
• All life today arises by the reproduction of preexisting life, or biogenesis.
• If this is true, how could the first organisms arise?
• From the time of the ancient Greeks until well into the 19th century, it was
commonly believed that life regularly arises from nonliving matter, an idea called
spontaneous generation.
• Today, most biologists think it is possible that life on early Earth produced simple
cells by chemical and physical processes.
A Four-Stage Hypothesis for the Origin of Life
• According to one hypothesis, the first organisms were products of chemical
evolution in four stages.
Stage 1: Abiotic Synthesis of Organic Monomers
• The first stage in the origin of life has been the most extensively studied by
scientists in the laboratory.
• Experiment: An apparatus was built to mimic the early Earth atmosphere and
included
• Hydrogen gas (H2), methane (CH4), ammonia (NH3), and water vapor
(H2O)
• Sparks, discharged into the chamber to mimic the prevalent lightning of
the early Earth
• A condenser to cool the atmosphere, causing water and dissolved
compounds to “rain” into the miniature “sea”
Chapter 15
The Evolution of Microbial Life
•
Results: After the apparatus had run for a week, an abundance of organic
molecules essential for life had collected in the “sea,” including amino acids, the
monomers of proteins.
• Since Miller and Urey’s experiments, laboratory analogues of the primeval Earth
have produced
• All 20 amino acids
• Several sugars
Stage 2: Abiotic Synthesis of Polymers
• Researchers have brought about the polymerization of monomers to form polymers,
such as proteins and nucleic acids, by dripping solutions of organic monomers onto
– Hot sand
– Clay
– Rock
Stage 3: Formation of Pre-Cells
• A key step in the origin of life was the isolation of a collection of abiotically
created molecules within a membrane.
• Laboratory experiments demonstrate that pre-cells could have formed
spontaneously from abiotically produced organic compounds.
• Such pre-cells produced in the laboratory display some lifelike properties. They:
• Have a selectively permeable surface
• Can grow by absorbing molecules from their surroundings
• Swell or shrink when placed in solutions of different salt concentrations
Stage 4: Origin of Self-Replicating Molecules
• Life is defined partly by the process of inheritance, which is based on selfreplicating molecules.
• One hypothesis is that the first genes were short strands of RNA that replicated
themselves without the assistance of proteins, perhaps using RNAs that can act as
enzymes, called ribozymes
From Chemical Evolution to Darwinian Evolution
• Over millions of years
– Natural selection favored the most efficient pre-cells
– The first prokaryotic cells evolved
• Prokaryotes lived and evolved all alone on Earth for 2 billion years before
eukaryotes evolved.
• Prokaryotes
– Are found wherever there is life
– Far outnumber eukaryotes
– Can cause disease
– Can be beneficial
• Compared to eukaryotes, prokaryotes are
– Much more abundant
– Typically much smaller
• Prokaryotes
– Are ecologically significant, recycling carbon and other vital chemical
elements back and forth between organic matter, the soil, and atmosphere
– Cause about half of all human diseases
– Are more typically benign or beneficial
The Structure and Function of Prokaryotes
• Prokaryotic cells
– Lack true nuclei
– Lack other membrane-enclosed organelles
– Have cell walls exterior to their plasma membranes
Chapter 15
•
The Evolution of Microbial Life
Procaryotic Forms
• Prokaryotes come in several shapes:
– Spherical (cocci)
– Rod-shaped (bacilli)
– Spiral
• Additionally they are clasified as
– Strep form generally strait chains
– Staph (greek for bunch of grapes) form bunches
• Most prokaryotes are
– Unicellular
– Very small
• Some prokaryotes
– Form true colonies
– Show specialization of cells
– Are very large
• About half of all prokaryotes are mobile, using flagella.
• Many have one or more flagella that propel the cells away from unfavorable places
or toward more favorable places, such as nutrient-rich locales.
• Procaryotic Reproduction
• Most prokaryotes can reproduce by binary fission and at very high rates if
conditions are favorable.
• Some prokaryotes
– Form endospores, thick-coated, protective cells that are produced within
the cells when they are exposed to unfavorable conditions
– Can survive very harsh conditions for extended periods, even centuries
Procaryotic Nutrition
• Prokaryotes exhibit four major modes of nutrition.
– Phototrophs obtain energy from light.
– Chemotrophs obtain energy from environmental chemicals.
– Species that obtain carbon from carbon dioxide (CO2) are autotrophs.
– Species that obtain carbon from at least one organic nutrient—the sugar
glucose, for instance—are called heterotrophs.
• We can group all organisms according to the four major modes of nutrition if we
combine the
– Energy source (phototroph versus chemotroph) and
– Carbon source (autotroph versus heterotroph)
The Two Main Branches of Prokaryotic Evolution: Bacteria and Archaea
• By comparing diverse prokaryotes at the molecular level, biologists have identified
two major branches of prokaryotic evolution:
– Bacteria
– Archaea (more closely related to eukaryotes)
• Some archaea are “extremophiles.”
– Halophiles thrive in salty environments.
– Thermophiles inhabit very hot water.
– Methanogens inhabit the bottoms of lakes and swamps and aid digestion
in cattle and deer.
Bacteria That Cause Disease
• Bacteria and other organisms that cause disease are called pathogens.
• Most pathogenic bacteria produce poisons.
– Exotoxins are poisonous proteins secreted by bacterial cells.
– Endotoxins are not cell secretions but instead chemical components of the
outer membrane of certain bacteria.
Chapter 15
The Evolution of Microbial Life
PROTISTS
•
Protists
– Are eukaryotic
– Evolved from prokaryotic ancestors
– Are ancestral to all other eukaryotes, which are
– Plants
– Fungi
– Animals
The Origin of Eukaryotic Cells
• Eukaryotic cells evolved by
– The infolding of the plasma membrane of a prokaryotic cell to form the
endomembrane system and
– Endosymbiosis, one species living inside another host species, in which
free-living bacteria came to reside inside a host cell, producing
mitochondria and chloroplasts
The Diversity of Protists
• Protists can be
– Unicellular
– Multicellular
• More than any other group, protists vary in
– Structure
– Function
• Protists are not one distinct group but instead represent all the eukaryotes that are
not plants, animals, or fungi.
• The classification of protists remains a work in progress.
• The four major categories of protists, grouped by lifestyle, are
– Protozoans
– Slime molds
– Unicellular algae
– Seaweeds
Protozoans
• Protists that live primarily by ingesting food are called protozoans.
• Protozoans with flagella are called flagellates and are typically free-living, but
sometimes are nasty parasites.
• Amoebas are characterized by
• Great flexibility in their body shape
• The absence of permanent organelles for locomotion
• Most species move and feed by means of pseudopodia (singular,
pseudopodium), temporary extensions of the cell.
• Ciliates
• Are mostly free-living (nonparasitic), such as the freshwater ciliate
Paramecium
• Use structures called cilia to move and feed
• Plasmodial slime molds
• Can be large
• Are decomposers on forest floors
• Are named for the feeding stage in their life cycle, an amoeboid mass
called a plasmodium
• Cellular slime molds have an interesting and complex life cycle that changes
between a
• Feeding stage of solitary amoeboid cells
• Sluglike colony that moves and functions as a single unit
• Stalklike reproductive structure
Chapter 15
The Evolution of Microbial Life
Unicellular and Colonial Algae
• Algae are
– Photosynthetic protists
– Found in plankton, the communities of mostly microscopic organisms
that drift or swim weakly in aquatic environments
• Unicellular algae include
– Diatoms, which have glassy cell walls containing silica
– Dinoflagellates, with two beating flagella and external plates made of
cellulose
• Green algae are
– Unicellular
– Sometimes flagellated, such as Chlamydomonas
– Colonial, sometimes forming a hollow ball of flagellated cells, as seen in
Volvox
Seaweeds
• Seaweeds
– Are only similar to plants because of convergent evolution
– Are large, multicellular marine algae
– Grow on or near rocky shores
– Are often edible
• Seaweeds are classified into three different groups, based partly on the types of
pigments present in their chloroplasts:
– Green algae
– Red algae
– Brown algae (including kelp)