Transcript chapter19
Chapter 19 Life’s Origin and Early Evolution Albia Dugger • Miami Dade College 19.1 Looking for Life • Astrobiology is the study of life’s origins and distribution – astrobiologists study Earth’s extreme habitats to determine the range of conditions living things can tolerate • Life on Earth is protected by the ozone layer, which serves as a natural sunscreen, preventing most UV radiation from reaching the planet’s surface • Life can adapt to nearly any environment with sources of carbon and energy – including extreme temperatures, pH, salinity, or pressure Lessons from Chile’s Atacama Desert 19.2 The Early Earth • Knowledge of modern chemistry and physics are the basis for scientific hypotheses about early events in Earth’s history Origin of the Universe and Our Solar System • Big bang theory • The universe began in an instant, 13-5 billion years ago • All existing matter and energy suddenly appeared and exploded outward from a single point • The universe is still expanding • Earth formed from dust and debris orbiting the sun, about 4.6 billion years ago Formation of the Earth Conditions on the Early Earth • Earth’s early atmosphere came from gas released by volcanoes, and was low in oxygen • Rain washed minerals and salts out of rocks to form early seas ANIMATION: Origin of organelles To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE Early Earth Take-Home Message: What were conditions like on the early Earth? • Earth’s early atmosphere had little or no oxygen • Meteorites pummeled the planet’s surface, and volcanic activity was more common than it is today 19.3 Formation of Organic Monomers • All living things are made from the same organic subunits: amino acids, fatty acids, nucleotides, and simple sugars • Small organic molecules that serve as building blocks of life can be formed by nonliving mechanisms Possible Sources of Life’s First Building Blocks 1. Stanley Miller showed that amino acids form in conditions that simulate lightning in the atmosphere of early Earth 2. Wächtershäuser and Huber synthesized amino acids in a simulated hydrothermal vent environment 3. Amino acids, sugars, and nucleotide bases may have formed in interstellar clouds and been carried to Earth on meteorites electrodes to vacuum pump CH4 NH3 H2O H2 spark discharge gases water out condenser water in water droplets boiling water water containing organic compounds liquid water in trap Figure 19-4 p311 A Hydrothermal Vent on the Seafloor Take-Home Message: What was the source of organic molecules to build the first life? • Small organic molecules that serve as the building blocks for living things can be formed by nonliving mechanisms. • For example, amino acids form in reaction chambers that simulate conditions on the early Earth, and are present in some meteorites ANIMATED FIGURE: Miller's reaction chamber experiment To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE 19.4 From Polymers to Protocells • We will never know for sure how the first cells came to be, but we can investigate the possible steps on the road to life Properties of Cells • All living cells carry out metabolic reactions, are enclosed within a plasma membrane, and can replicate themselves • Cells have a genome of DNA that enzymes transcribe into RNA, and ribosomes that translate RNA into proteins • Studies support the hypothesis that cells arose from a stepwise process that began with inorganic materials inorganic molecules …self-assemble on Earth and in space organic monomers …self-assemble in aquatic environments on Earth organic polymers …interact in early metabolism …self-assemble as vesicles …become the first genome protocells in an RNA world …are subject to selection that favors a DNA genome DNA-based cells Figure 19-6 p312 Origin of Metabolism • Before cells, nonbiological process that concentrate organic subunits might increase the chance of polymer formation • Concentration of molecules on clay particles in tidal flats may have caused organic subunits to bond as polymers • The iron–sulfur world hypothesis holds that the first metabolic reactions began on the surface of rocks around hydrothermal vents Origin of the Genome • An RNA-based system of inheritance may have preceded DNA-based systems • RNA world hypothesis • RNA may have stored genetic information and functioned like an enzyme in protein synthesis • RNAs that function as enzymes (ribozymes) are common in living cells today Origin of the Plasma Membrane • The cell’s plasma membrane allows organic molecules to concentrate and undergo reactions • Membranous sacs (protocells) containing interacting organic molecules may have formed prior to the earliest life forms • In experiments, small organic molecules can react with minerals and seawater to form vesicles with a bilayer membrane Laboratory-Produced Protocells Testing a Hypothesis Take-Home Message: What do experiments reveal about steps that led to the first cells? • All living cells carry out metabolic reactions, are enclosed within a plasma membrane, and can replicate themselves • Metabolic reactions may have begun when molecules became concentrated on clay particles or in tiny rock chambers near hydrothermal vents • RNA can serve as an enzyme, as well as a genome. An RNA world may have preceded evolution of DNA-based genomes • Vesicle-like structures with outer membranes form spontaneously when certain organic molecules are mixed with water 19.5 Life’s Early Evolution • Fossils and molecular comparisons among modern organisms inform us about the early history of life Origin of Bacteria and Archaea • Life that arose 3-4 billion years ago was probably anaerobic and used dissolved carbon dioxide as a carbon source • Early fossil cells are similar in size and structure to modern archaea and bacteria • The first photosynthetic cells were bacteria that used the cyclic pathway (does not produce O2) Fossil Prokaryotic Cells The Proterozoic Era • The oxygen-producing, non-cyclic pathway of photosynthesis first evolved in cyanobacteria • In the Proterozoic era Layers of photosynthetic bacteria formed large dome-shaped, layered called stromatolites • Oxygen accumulation in air and seas halted spontaneous formation of molecules of life, formed a protective ozone layer, and spurred evolution of organisms using aerobic respiration Stromatolites The Rise of Eukaryotes • The earliest evidence of eukaryotes is lipids in 2.7-billionyear-old rocks – the lipids are biomarkers for eukaryotes • A red alga that lived 1.2 billion years ago is the oldest species known to reproduce sexually, a trait unique to eukaryotes • Multicellularity and cellular differentiation allowed evolution of larger bodies with specialized parts • Spongelike animals evolved about 870 million year ago; animals with more complex bodies existed about 570 mya Fossils of Some Early Eukaryotes What was early life like and how did it change Earth? Take-Home Message: • Life arose by 3–4 billion years ago; it was probably anaerobic and did not have a nucleus • An early divergence separated ancestors of modern bacteria from the lineage that lead to archaea and eukaryotic cells • The first photosynthetic cells were bacteria that used the cyclic pathway; later, the oxygen-producing, noncyclic pathway evolved in cyanobacteria • Oxygen accumulation in air and seas halted spontaneous formation of the molecules of life, formed a protective ozone layer, and favored organisms that carried out the highly efficient pathway of aerobic respiration 19.6 How Did Eukaryotic Traits Evolve? • Eukaryotic cells have a composite ancestry, with different components derived from different lineages • Archaea-like nuclear genes govern genetic processes (DNA replication, transcription, translation) • Bacteria-like nuclear genes govern metabolism and membrane formation Origins of Internal Membranes • In eukaryotes, DNA resides in a nucleus bordered by a nuclear envelope – a double membrane with pores that control the flow of material into and out of the nucleus • A few modern bacteria also have internal membraneenclosed compartments • The nucleus and endomembrane system probably evolved from infoldings of plasma membrane infolding of plasma membrane ER nuclear envelope 19-10a p1910 Bacteria with Internal Membranes Evolution of Mitochondria and Chloroplasts • The endosymbiont hypothesis holds that mitochondria and chloroplasts descended from bacteria that were prey or parasites of early eukaryotic cells • Mitochondria are genetically similar to aerobic bacteria called rickettsias; chloroplasts are similar to photosynthetic bacteria called cyanobacteria Rickettsia prowazekii Evidence of Endosymbiosis • Endosymbiosis can occur when a bacterium infects a eukaryotic cell • Eventually, host and symbiont become incapable of living independently • Example: The photosynthetic organelles of glaucophytes are dependent on their host – they can’t survive on their own photosynthetic organelle that resembles a cyanobacterium Figure 19-12a p317 photosynthetic organelle that resembles a cyanobacterium Figure 19-12b p317 Take-Home Message: How might eukaryotic organelles have evolved? • A nucleus and other organelles are defining features of eukaryotic cells • The nucleus and ER may have arisen through modification of infoldings of the plasma membrane • Mitochondria and chloroplasts most likely descended from bacteria 19.7 Time Line for Life’s Origin and Evolution Table 19-1 p320 ANIMATION: Evolutionary tree of life To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE ANIMATED FIGURE: Milestones in the history of life To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE