Chapter 26Early Earth and the Origin of Life
Life on Earth originated between 3.5 and 4.0 billion years ago
•The Earth formed about 4.5 billion years ago
•No clear fossils have been found in the oldest surviving Earth rocks, from 3.8 billion years ago.
•The oldest fossils that have been uncovered were embedded in rocks from western Australia that are 3.5 billion years ago.
Prokaryotes dominated evolutionary history from 3.5 to 2.0 billion years ago
•The earliest organisms were prokaryotes.
•Relatively early, prokaryotes diverged into two main evolutionary branches, the bacteria and the archaea.
Oxygen began accumulating in the atmosphere about 2.7 billion years ago
•Photosynthesis probably evolved very early in prokaryotic history.
•Cyanobacteria, photosynthetic organisms that split water and produce O2 as a byproduct, evolved over 2.7 billion years ago.
•This early oxygen initially reacted with dissolved iron to form the precipitate iron oxide. This can be seen today in banded iron formations.
Eukaryotic life began by 2.1 billion years ago
•The first clear evidence of a eukaryote appeared about 2.1 billion years ago.
•This is the same time as the oxygen revolution that changed the Earth’s environment so dramatically.
•Chloroplasts and mitochondrion evolved
Multicellular eukaryotes evolved by 1.2 billion years ago
•A great range of eukaryotic unicellular forms evolved into the diversity of present-day “protists.”
Animal diversity exploded during the early Cambrian period
•Most of the major groups of animals make their first fossil appearances during the relatively short span of the Cambrian period’s first 20 million years.
Plants, fungi, and animals colonized the land about 500 million years ago
•Most orders of modern mammals, including primates, appeared 50-60 million years ago.
•Humans diverged from other primates only 5 million years ago
The Origin of Life
•Spontaneous generation- life arises from nonliving matter
•In 1862, Louis Pasteur conducted broth experiments that rejected the idea of spontaneous generation even for microbes.
•Biogenesis- all life today arises only by the reproduction of preexisting life.
One hypothesis is that chemical and physical processes in Earth’s primordial environment eventually produced simple cells.
1) theabiotic synthesis of small organic molecules
2) joining these small molecules into polymers
3) origin of self-replicating molecule
4) packaging of these molecules into “protobionts”
1. Abiotic synthesis of organic molecules
1920’s, A.I. Oparinand J.B.S. Haldane
- Conditions on the early Earth favored the synthesis of organic compounds from inorganic precursors.
- The reducing environment in the early atmosphere would have promoted the joining of simple molecules to form more complex ones.
- The energy required could be provided by lightning and the intense UV radiation that penetrated the primitive atmosphere.
1953, Stanley Miller and Harold Urey
- The atmosphere model consisted of H2O, H2, CH4, and NH3.
- It produced a variety of amino acids and other organic molecules.
2. Laboratory simulations of early-Earth conditions have produced organic polymers
•Monomers should link to form polymers without enzymes and other cellular equipment.
•Researchers have produced polymers, including polypeptides, after dripping solutions of monomers onto hot sand, clay, or rock.
3. RNA may have been the first genetic material
•Many researchers have proposed that the first hereditary material was RNA, not DNA.
•RNA can also function as an enzyme
•Short polymers of ribonucleotides can be synthesized abiotically in the laboratory.
•1980’s Thomas Cech- discovered that RNA molecules are important catalysts in cells.
•Ribozymes- RNA catalysts that remove introns from RNA.
•Ribozymes also help catalyze the synthesis of new RNA polymers.
4A. Protobionts can form by self-assembly
•Protobionts- maintain an internal chemical environment from their surroundings and may show some properties associated with life, metabolism, and excitability.
•Liposomes- droplets of abiotically produced organic compounds that form from lipids. Forms a molecular bilayer like the lipid bilayer of a membrane.
•If enzymes are added, they are incorporated into the droplets.
•The protobionts are then able to absorbsubstrates fromtheir surroundingsand release theproducts of thereactions catalyzedby the enzymes.
4B. Natural section could refine protobionts containing hereditary information
•Once primitive RNA genes and their polypeptide products were packaged within a membrane, the protobionts could have evolved as units.
•The most successful protobionts would grow and split, distributing copies of their genes to offspring.Later, DNA would replace RNA for storing genetic information.
Debates about the origin of life abounds
•Laboratory simulations cannot prove that these kinds of chemical processes actually created life on the primitive Earth. They describe steps that could have happened.
•Among the debates is whether organic monomers on early Earth were synthesized in deep-sea vents or reached Earth on comets and meteorites.
The five-kingdom system reflected increased knowledge of life’s diversity
•1969, R.H Whittaker, A five-kingdom system: Monera, Protista, Plantae, Fungi, and Animalia.
Arranging the diversity of life into the highest taxa is a work in progress
•Three-domain system: Bacteria, Archaea, and Eukarya, as superkingdoms.
Chapter 27 Prokaryotes and the Origins of Metabolic Diversity
They’re (almost) everywhere! An overview of prokaryotic life
•Prokaryotes were the earliest organisms on Earth and evolved alone for 1.5 billion years.
•Live in diverse environments.
•Most are benign or beneficial but others cause disease.
•Diverse in structure and in metabolism.
•About 5,000 species are known, but actual diversity may range from about 400,000 to 4 million species.
Bacteria and archaea are the two main branches of prokaryote evolution
The Structure, Function, and Reproduction of Prokaryotes
•Most prokaryotes are unicellular.
•The most common shapes among prokaryotes are spheres (cocci), rods (bacilli), and helices.
Nearly all prokaryotes have a cell wall external to the plasma membrane
•Most bacterial cell walls contain peptidoglycan, a polymer of modified sugars cross-linked by short polypeptides.
Gram stain- used identify bacteria based on differences in their cell walls.
•Gram-positive bacteria have simpler cell walls, with large amounts of peptidoglycans.
•Gram-negative bacteria have more complex cell walls and less peptidoglycan.
•Generally more threatening. The outer membrane protects against host defenses. More resistant to antibiotics.
•Many prokaryotes secrete a sticky protective layer, the capsule, outside the cell wall.
•Pili- an appendage used to adhere to surfaces or other prokaryotes. Also used for conjugation.
Many prokaryotes are motile
•Flagella
•Taxis- movement toward or away from a stimulus (chemicals, light, magnetic fields).
The cellular and genomic organization of prokaryotes is fundamentally different from that of eukaryotes
•Prokaryotic cells lack a nucleus enclosed by membrane. They have smaller, simpler genomes than eukaryotes and have small rings of DNA, plasmids, that consist of only a few genes.
Populations of prokaryotes grow and adapt rapidly
•Prokaryotes reproduce only asexually via binary fission, synthesizing DNA almost continuously.
•Transformation- a cell can absorb and integrate fragments of DNA from their environment.
•Conjugation- one cell directly transfers genes to another cell.
•Transduction- viruses transfer genes between prokaryotes.
Endospore- forms when a cell replicates its chromosome and surrounds one chromosome with a durable wall.
Can survive lack of nutrients and water, extreme heat or cold, and most poisons.
May be dormant for centuries or more.
When the environment becomes more hospitable, the endospore absorbs water and resumes growth.
Nutrition and Metabolic Diversity
Prokaryotes can be grouped into four categories according to how they obtain energy and carbon
•Species that use light energy are phototrophs.
•Species that obtain energy from chemicals in their environment are chemotrophs.
•Organisms that need only CO2 as a carbon source are autotrophs.
•Organisms that require at least one organic nutrient as a carbon source are heterotrophs.
•Photoautotrophs- photosynthetic organisms that harness light energy to drive the synthesis of organic compounds from carbon dioxide.
•Ex: cyanobacteria.
•Chemoautotrophs need only CO2 as a carbon source, but they obtain energy by oxidizing inorganic substances, rather than light.
•These substances include hydrogen sulfide (H2S), ammonia (NH3), and ferrous ions (Fe2+) among others.
•Photoheterotrophs use light to generate ATP but obtain their carbon in organic form.
•Chemoheterotrophs must consume organic molecules for both energy and carbon.
Prokaryotes are responsible for the key steps in the cycling of nitrogen through ecosystems.
•Some convert ammonium (NH4+) to nitrite (NO2-).
•Others convert nitrite or nitrate (NO3-) to N2 (gas).
•Nitrogen fixation-convert N2 to NH4+, making atmospheric nitrogen available to other organisms for incorporation into organic molecules.
Obligate aerobes- require O2 for cellular respiration.
Facultative anerobes- will use O2 if present but can also grow by fermentation in an anaerobic environment.
Obligate anaerobes- are poisoned by O2 and use either fermentation or anaerobic respiration.
Photosynthesis evolved early in prokaryotic life
•The very first prokaryotes were heterotrophs, but photosynthesis capabilities soon followed.
A Survey of Prokaryotic Diversity
Molecular systematics is leading to phylogenetic classification of prokaryotes
•Carl Woese clustered prokarotes into taxonomic groups based on comparisons of nucleic acid sequences- small-subunit ribosomal RNA (SSU-rRNA)
Researchers are identifying a great diversity of archaea in extreme environments and in the oceans
•Archaea are extremophiles, “lovers” of extreme environments.
Methanogens obtain energy by using CO2 to oxidize H2 replacing methane as a waste.
•They live in swamps and marshes where other microbes have consumed all the oxygen.
•Methanogens are important decomposers in sewage treatment.
•Other methanogens live in the anaerobic guts of herbivorous animals, playing an important role in their nutrition.
Extreme halophiles live in such saline places as the Great Salt Lake and the Dead Sea.
Extreme thermophiles thrive in hot environments, 60oC-80oC, hot sulfur springs or at 105oC water near deep-sea hydrothermal vents
Most known prokarotes are bacteria
Proteobacteria, Chlamydias, Spirochetes, Gram-Positive Bacteria, Cyanobacteria
The Ecological Impact of Prokaryotes
Prokaryotes are indispensable links in the recycling of chemical elements in ecosystems
•Decomposers
•Metabolize inorganic molecules containing elements such as iron, sulfur, nitrogen, and hydrogen
•Restore oxygen to the atmosphere
•Fix nitrogen
Many prokaryotes are symbiotic
•Commensalism, one symbiont receives benefits while the other is not harmed or helped by the relationship.
•Parasitism, one symbiont, the parasite, benefits at the expense of the host.
•Mutualism, both symbionts benefit.
Pathogenic prokaryotes cause many human diseases
•Some pathogens are opportunistic.These are normal residents of the host, but only cause illness when the host’s defenses are weakened.
Exotoxins are proteins secreted by prokaryotes.
C. botulinum, V. cholerae, E. coli
Endotoxins are components of the outer membranes of some gram-negative bacteria.
Salmonella
Humans use prokaryotes in research and technology
•The application of organisms to remove pollutants from air, water, and soil is bioremediation.
•Decomposers treat human sewage.
•Soil bacteria have been developed to decompose petroleum products at the site of oil spills or to decompose pesticides.
•The chemical industry produces acetone, butanol, and other products from bacteria.
•The pharmaceutical industry cultures bacteria to produce vitamins and antibiotics.
•The food industry used bacteria to convert milk to yogurt and various kinds of cheese.
Chapter 28 The Origins of Eukaryotic Diversity
Introduction to the Protists
•The first eukaryotes were unicellular.
•Eukaryotic fossils date back 2.1 billion years
•For about 2 billion years, eukaryotes consisted of mostly microscopic organisms- “protists.”
Systematists have split protists into many kingdoms
•Systematists have split the former kingdom Protistainto as many as 20 separate kingdoms.
Protists are the most diverse of all eukaryotes
•Most of the 60,000 known protists are unicellular, but some are colonial and others multicellular.
•Protists are the most nutritionally diverse of all eukaryotes.
•Aerobic w/ mitochondria, photoautotrophs w/ chloroplasts, heterotrophs, mixotrophs- combining photosynthesis and heterotrophic nutrition.
•Euglena, mixotrophic, can use chloroplasts to undergo photosynthesis if light is available or live as a heterotroph by absorbing organic nutrients from the environment.
The Origin and Early Diversification of Eukaryotes
•Unique cellular structures and processes:
•Membrane-enclosed nucleus, Endomembrane system, Mitochondria, Chloroplasts, Cytoskeleton, 9 + 2 flagella, Multiple chromosomes of linear DNA with organizing proteins, Life cycles with mitosis, meiosis, and sex.
Endomembranes contributed to larger, more complex cells
•The endomembrane system of eukaryotes (nuclear envelope, endoplasmic reticulum, Golgi apparatus, and related structures) may have evolved from infoldings of plasma membrane.
Mitochondria and plastids evolved from endosymbiotic bacteria
•The theory of serial endosymbiosis proposes that mitochondria and chloroplasts were formerly small prokaryotes living within larger cells.
•These ancestors probably entered the host cells as undigested prey or internal parasites.
•This evolved into a mutually beneficial symbiosis.
The eukaryotic cell is a chimera of prokaryotic ancestors
•mitochondria from one bacteria
•plastids from another
•nuclear genome from the host cell
Secondary endosymbiosis increased the diversity of algae
•The chloroplasts of plants and green algae have two membranes.
•The plastids of others have three or four membranes.
•These include the plastids of Euglena (with three membranes) that are most closely related to heterotrophic species.
•Those algal groups with more than two membranes were acquired by secondary endosymbiosis.
•Each endosymbiotic event adds a membrane derived from the vacuole membrane of the host cell that engulfed the endosymbiont.
Research on the relationships between the three domains is changing ideas about the deepest branching in the tree of life
•All three domains seem to have genomes that are chimeric mixes of DNA that was transferred across the boundaries of the domains.
•In this new model, the three domains arose from an ancestral community of primitive cells that swapped DNA promiscuously.
The origin of eukaryotes catalyzed a second great wave of diversification
•The development of clades among the diverse groups of eukaryotes is based on comparisons of cell structure, life cycles, and molecules.
A Sample of Protistan Diversity
Diplomonadida and Parabasala
The diplomonads have multiple flagella, two separate nuclei, a simple cytoskeleton, and no mitochondria or plastids.
•Giardialamblia, a parasite that infects the human intestine.
The parabasalids include trichomonads.
•Trichomonasvaginalis, inhabits the vagina of human females.
Euglenozoa
The euglenozoa includes both photosynthetic and heterotrophic flagellates
•The euglenoids (Euglenozoa) are characterized by an anterior pocket from which one or two flagella emerge.
•The kinetoplastids (Kinoplastida) have a single large mitochondrion associated with a unique organelle, the kinetoplast.
•The kinetoplast houses extranuclear DNA.
•Trypanosoma causes African sleeping sickness.
Alveolata
The alveolata are unicellular protists with subcellular cavities (alveoli)
•Flagellated protists (dinoflagellates), parasites (apicomplexans), and ciliated protists (the ciliates).
•Alveoli- small membrane-bound cavities, under the cell surface.
•Their function is not known, but they may help stabilize the cell surface and regulate water and ion content.
•Dinoflagellates are abundant components of the phytoplankton that are suspended near the water surface.
•Dinoflagellates and other phytoplankton form the foundation of most marine and many freshwater food chains.
•Dinoflagellate blooms, characterized by explosive population growth, cause red tides in coastal waters.
•Pfiesteriapiscicida, is carnivorous. This organism produces a toxin that stuns fish.
•Some dinoflagellates are bioluminescent.
•Apicomplexans are parasites of animals and some cause serious human diseases.
•Tiny infectious cells (sporozoites) penetrate host cells and tissues.
•Intricate life cycles with both sexual and asexual stages and often require two or more different host species for completion.
•Plasmodium, the parasite that causes malaria, spends part of its life in mosquitoes and part in humans.
•Ciliophora (ciliates), a diverse protist group, is named for their use of cilia to move and feed.
•Paramecium, cilia along the oral groove draw in food that is engulfed by phagocytosis.
•Ciliates have two types of nuclei, a large macronucleus and usually several tiny micronuclei.
•Conjugation- micronuclei are exchanged.
Stramenopila
Heterotrophic and photosynthetic protists.
Numerous fine, hairlike projections on the flagella.
In most stramenopile groups, the only flagellated stage is motile reproductive cells.
The heterotrophic stramenopiles, the oomycotes, include water molds, white rusts, and downy mildews.
Water molds are important decomposers, mainly in fresh water.
White rusts and downy mildews are parasites of terrestrial plants.
The photosynthetic stramenopiles are known collectively as the heterokont algae.
•The heterokont algae include diatoms, golden algae, and brown algae.
•Diatoms (Bacillariophyta) have unique glasslike walls composed of hydrated silica embedded in an organic matrix.
•Golden algae (Chrysophyta), named for the yellow and brown carotene and xanthophyll pigments, are typically biflagellated.
•Brown algae (Phaeophyta) are the largest and most complex algae.Most brown algae are multicellular.
Structural and biochemical adaptations help seaweeds survive and reproduce at the ocean’s margins
•Brown, red, and green algae inhabit the intertidal and subtidal zones of coastal waters.
•Thallus or body of the seaweed: rootlikeholdfast and a stemlikestipe, which supports leaflike photosynthetic blades.
•Giant brown algae, known as kelps, form forests in deeper water.