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CAMPBELL BIOLOGY IN FOCUS Urry • Cain • Wasserman • Minorsky • Jackson • Reece 24 Early Life and the Diversification of Prokaryotes Lecture Presentations by Kathleen Fitzpatrick and Nicole Tunbridge © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Figure 24.1 Overview: The First Cells • Earth formed 4.6 billion years ago • The oldest fossil organisms are prokaryotes dating back to 3.5 billion years ago • Prokaryotes are single-celled organisms in the domains Bacteria and Archaea © 2014 Pearson Education, Inc. Concept 24.1: How did the first cells form? • Chemical and physical processes on early Earth may have produced very simple cells through a sequence of stages 1. Abiotic synthesis of small organic molecules 2. Joining of these small molecules into macromolecules 3. Packaging of molecules into protocells, membranebound droplets that maintain a consistent internal chemistry 4. Origin of self-replicating molecules © 2014 Pearson Education, Inc. Synthesis of Organic Compounds on Early Earth • Earth’s early atmosphere likely contained water vapor and chemicals released by volcanic eruptions (nitrogen, nitrogen oxides, carbon dioxide, methane, ammonia, and hydrogen) • No free oxygen • As Earth cooled, water vapor condensed into oceans, and most of the hydrogen escaped into space © 2014 Pearson Education, Inc. Chemical Hypothesis Theory • In the 1920s, A. I. Oparin and J. B. S. Haldane hypothesized that the early atmosphere was a reducing environment • In 1953, Stanley Miller and Harold Urey conducted lab experiments that showed that the abiotic synthesis of organic molecules in a reducing atmosphere is possible © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Chemical Hypothesis Theory • Miller-Urey-type experiments demonstrate that organic molecules could have formed with various possible atmospheres • Instead of forming in the atmosphere, the first organic compounds may have been synthesized near volcanoes or deep-sea vents • Organic molecules have also been found in meteorites © 2014 Pearson Education, Inc. Other theories Extraterrestrial Theory • Organic compounds from space • Murchison meteorite contained amino acids • Organic compounds in comets etc. • Org. in space © 2014 Pearson Education, Inc. Clay Theory • Small organic molecules polymerize when they are concentrated on hot sand, clay, or rock • RNA monomers have been produced spontaneously from simple molecules Formation of Protocells • Spontaneous vesicle formation with hydrophobic lipids enclosing hydrophilic organics Can be accelerated with clay • Adding clay can increase the rate of vesicle formation • Vesicles exhibit simple reproduction and metabolism and maintain an internal chemical environment © 2014 Pearson Education, Inc. Relative turbidity, an index of vesicle number Figure 24.4 0.4 Precursor molecules plus montmorillonite clay 0.2 Precursor molecules only 0 0 40 20 Time (minutes) 60 Vesicle boundary 1 m (a) Self-assembly 20 m (b) Reproduction (c) Absorption of RNA Protocells • Replication and metabolism are key properties of life and may have appeared together • Protocells may have been fluid-filled vesicles with a membrane-like structure • In water, lipids and other organic molecules can spontaneously form vesicles with a lipid bilayer • RNA first genetic material and catalytic molecule • For example, ribozymes can make complementary copies of short stretches of RNA © 2014 Pearson Education, Inc. • Natural selection has produced self-replicating RNA molecules • RNA molecules that were more stable or replicated more quickly would have left the most descendant RNA molecules • The early genetic material might have formed an “RNA world” © 2014 Pearson Education, Inc. • Vesicles with RNA capable of replication would have been protocells • RNA could have provided the template for DNA, a more stable genetic material © 2014 Pearson Education, Inc. First life form - Prokaryote • Prokaryotes are the most abundant organisms on Earth • There are more in a handful of fertile soil than the number of people who have ever lived • Prokaryotes thrive almost everywhere, including places too acidic, salty, cold, or hot for most other organisms • Some prokaryotes colonize the bodies of other organisms © 2014 Pearson Education, Inc. Fossil Evidence of Early Life • Many of the oldest fossils are stromatolites, layered rocks that formed from the activities of prokaryotes up to 3.5 billion years ago • Ancient fossils of individual prokaryotic cells have also been discovered • For example, fossilized prokaryotic cells have been found in 3.4-billion-year-old rocks from Australia • Heterotroph hypothesis – first cell was anaerobe and heterotroph. © 2014 Pearson Education, Inc. 5 cm 30 m Figure 24.5 1.1-billion-year-old fossilized stromatolite 3-billion-year-old fossil of a cluster of nonphotosynthetic prokaryote cells 10 m Stromatolites Nonphotosynthetic bacteria 1.5-billion-year-old fossil of a cyanobacterium Possible earliest appearance in fossil record 4 Cyanobacteria 3 2 Time (billions of years ago) 1 0 Sequence of eventS in evolution • Chlorophyll evolved – autotrophic prokaryote • The cyanobacteria that form stromatolites were the main photosynthetic organisms for over a billion years • Began the release of oxygen into Earth’s atmosphere • Surviving prokaryote lineages either avoided or adapted to the newly aerobic environment. • Aerobic prokaryotes evolved © 2014 Pearson Education, Inc. Concept 24.2: Diverse structural and metabolic adaptations have evolved in prokaryotes • Most prokaryotes are unicellular, although some species form colonies • Most prokaryotic cells have diameters of 0.5–5 µm, much smaller than the 10–100 µm diameter of many eukaryotic cells • Prokaryotic cells have a variety of shapes • The three most common shapes are spheres (cocci), rods (bacilli), and spirals © 2014 Pearson Education, Inc. 1 m (a) Spherical (b) Rod-shaped 3 m 1 m Figure 24.6 (c) Spiral Cell-Surface Structures • A key feature of nearly all prokaryotic cells is their cell wall, which maintains cell shape, protects the cell, and prevents it from bursting in a hypotonic environment • Eukaryote cell walls are made of cellulose or chitin • Bacterial cell walls contain peptidoglycan, a network of modified sugars cross-linked by polypeptides © 2014 Pearson Education, Inc. Figure 24.11 1 m 0.2 m Respiratory membrane Thylakoid membranes (a) Aerobic prokaryote (b) Photosynthetic prokaryote Nutritional and Metabolic Adaptations © 2014 Pearson Education, Inc. The Role of Oxygen in Metabolism • Prokaryotic metabolism varies with respect to O2 • Obligate aerobes require O2 for cellular respiration • Obligate anaerobes are poisoned by O2 and use fermentation or anaerobic respiration, in which substances other than O2 act as electron acceptors • Facultative anaerobes can survive with or without O2 © 2014 Pearson Education, Inc. Concept 24.3: Rapid reproduction, mutation, and genetic recombination promote genetic diversity in prokaryotes • Prokaryotes have considerable genetic variation • Three factors contribute to this genetic diversity • Rapid reproduction • Mutation • Genetic recombination © 2014 Pearson Education, Inc. Figure 24.18 Euryarchaeotes Crenarchaeotes UNIVERSAL ANCESTOR Nanoarchaeotes Domain Archaea Korarchaeotes Domain Eukarya Eukaryotes Proteobacteria Spirochetes Cyanobacteria Gram-positive bacteria Domain Bacteria Chlamydias Archaea • Archaea share certain traits with bacteria and other traits with eukaryotes © 2014 Pearson Education, Inc. Figure 24.UN02 Eukarya Archaea Bacteria Table 24.2a Table 24.2b Concept 25.1: Eukaryotes arose by endosymbiosis more than 1.8 billion years ago Early eukaryotes were unicellular Eukaryotic cells have organelles and are structurally more complex than prokaryotic cells A well-developed cytoskeleton enables eukaryotic cells to have asymmetrical forms and to change shape © 2014 Pearson Education, Inc. The Fossil Record of Early Eukaryotes Chemical evidence for the presence of eukaryotes dates back to 2.7 billion years ago The earliest fossils of eukaryotic cells are 1.8 billion years old © 2014 Pearson Education, Inc. Origin of Mitochondria and Plastids Endosymbiont theory proposes that mitochondria and plastids were formerly small prokaryotes that began living within larger cells An endosymbiont is a cell that lives within a host cell Prokaryote ancestors to mitochondria and plastids probably entered the host cell as undigested prey or internal parasites © 2014 Pearson Education, Inc. Endosymbiont theory The relationship between endosymbiont and host cells was mutually beneficial Anaerobic host cells benefited from endosymbionts’ ability to take advantage of an increasingly aerobic world Heterotrophic host cells benefited from the nutrients produced by photosynthetic endosymbionts In the process of becoming more interdependent, the host and endosymbionts would have become a single organism © 2014 Pearson Education, Inc. Figure 25.3 Cytoplasm DNA Ancestral prokaryote Plasma membrane Endoplasmic reticulum Engulfing of aerobic bacterium Engulfing of photosynthetic bacterium Nucleus Nuclear envelope Mitochondrion Mitochondrion Ancestral heterotrophic eukaryote Plastid Ancestral photosynthetic eukaryote Endosymbiont theory Key evidence supporting an endosymbiotic origin of mitochondria and plastids: ◦ Inner membranes are similar to plasma membranes of prokaryotes ◦ Division is similar in these organelles and some prokaryotes ◦ DNA structure is similar to that of prokaryotes ◦ These organelles transcribe and translate their own DNA ◦ Their ribosomes are more similar to prokaryotic than eukaryotic ribosomes © 2014 Pearson Education, Inc. Endosymbiont theory DNA sequence analysis indicates that mitochondria arose from an alpha proteobacterium Eukaryotic mitochondria descended from a single common ancestor Plastids arose from an engulfed cyanobacterium Some photosynthetic protists may have been engulfed to become endosymbionts themselves © 2014 Pearson Education, Inc.