219.1 Looking for LifeAstrobiology is the study of life’s origins and distribution – astrobiologists study Earth’s extreme habitats to determine the range of conditions living things can tolerateLife on Earth is protected by the ozone layer, which serves as a natural sunscreen, preventing most UV radiation from reaching the planet’s surfaceLife can adapt to nearly any environment with sources of carbon and energy – including extreme temperatures, pH, salinity, or pressure
3Lessons from Chile’s Atacama Desert Figure 19.1 The Mars-like landscape of Chile’s Atacama Desert. Scientist Jay Quade, visible in the distance at the right, was a member of a team that found bacteria living beneath this arid desert soil.
419.2 The Early EarthKnowledge of modern chemistry and physics are the basis for scientific hypotheses about early events in Earth’s history
5Origin of the Universe and Our Solar System Big bang theoryThe universe began in an instant, 13-5 billion years agoAll existing matter and energy suddenly appeared and exploded outward from a single pointThe universe is still expandingEarth formed from dust and debris orbiting the sun, about 4.6 billion years ago
6Formation of the EarthFigure 19.2 Artist’s depiction of our sun surrounded by a cloud of dust, debris, and gases. Earth and other planets formed from the material in this cloud.
7Conditions on the Early Earth Earth’s early atmosphere came from gas released by volcanoes, and was low in oxygenRain washed minerals and salts out of rocks to form early seas
8ANIMATION: Origin of organelles To play movie you must be in Slide Show ModePC Users: Please wait for content to load, then click to playMac Users: CLICK HERE
9Early EarthFigure 19.3 An artist’s depiction of early Earth.
10Take-Home Message: What were conditions like on the early Earth? Earth’s early atmosphere had little or no oxygenMeteorites pummeled the planet’s surface, and volcanic activity was more common than it is today
1119.3 Formation of Organic Monomers All living things are made from the same organic subunits: amino acids, fatty acids, nucleotides, and simple sugarsSmall organic molecules that serve as building blocks of life can be formed by nonliving mechanisms
12Possible Sources of Life’s First Building Blocks Stanley Miller showed that amino acids form in conditions that simulate lightning in the atmosphere of early EarthWächtershäuser and Huber synthesized amino acids in a simulated hydrothermal vent environmentAmino acids, sugars, and nucleotide bases may have formed in interstellar clouds and been carried to Earth on meteorites
14A Hydrothermal Vent on the Seafloor Figure 19.5 A hydrothermal vent on the seafloor. Mineral-rich water heated by geothermal energy streams out, into cold ocean water. As the water cools, dissolved minerals come out of solution and form a chimney-like structure around the vent.
15Take-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
16ANIMATED FIGURE: Miller's reaction chamber experiment To play movie you must be in Slide Show ModePC Users: Please wait for content to load, then click to playMac Users: CLICK HERE
1719.4 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
18Properties of CellsAll living cells carry out metabolic reactions, are enclosed within a plasma membrane, and can replicate themselvesCells have a genome of DNA that enzymes transcribe into RNA, and ribosomes that translate RNA into proteinsStudies support the hypothesis that cells arose from a stepwise process that began with inorganic materials
19…self-assemble on Earth and in space inorganic molecules…self-assemble on Earth and in spaceorganic monomers…self-assemble in aquatic environments on Earthorganic polymers…interact in early metabolism…self-assemble as vesicles…become the first genomeprotocells in an RNA world…are subject to selection that favors a DNA genomeDNA-based cellsFigure 19-6 p31219
20Origin of MetabolismBefore cells, nonbiological process that concentrate organic subunits might increase the chance of polymer formationConcentration of molecules on clay particles in tidal flats may have caused organic subunits to bond as polymersThe iron–sulfur world hypothesis holds that the first metabolic reactions began on the surface of rocks around hydrothermal vents
21Origin of the GenomeAn RNA-based system of inheritance may have preceded DNA-based systemsRNA world hypothesisRNA may have stored genetic information and functioned like an enzyme in protein synthesisRNAs that function as enzymes (ribozymes) are common in living cells today
22Origin of the Plasma Membrane The cell’s plasma membrane allows organic molecules to concentrate and undergo reactionsMembranous sacs (protocells) containing interacting organic molecules may have formed prior to the earliest life formsIn experiments, small organic molecules can react with minerals and seawater to form vesicles with a bilayer membrane
23Laboratory-Produced Protocells A Illustration of a laboratory-producedprotocell with a bilayer membrane of fattyacids and strands of RNA inside.B Laboratory-formed protocell consistingof RNA-coated clay (red) surrounded byfatty acids and alcohols.
24Testing a HypothesisC Field-testing a hypothesis about protocell formation.David Deamer pours a mix of small organic moleculesand phosphates into a hot acidic pool in Russia.
25Take-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 themselvesMetabolic reactions may have begun when molecules became concentrated on clay particles or in tiny rock chambers near hydrothermal ventsRNA can serve as an enzyme, as well as a genome. An RNA world may have preceded evolution of DNA-based genomesVesicle-like structures with outer membranes form spontaneously when certain organic molecules are mixed with water
2619.5 Life’s Early Evolution Fossils and molecular comparisons among modern organisms inform us about the early history of life
27Origin of Bacteria and Archaea Life that arose 3-4 billion years ago was probably anaerobic and used dissolved carbon dioxide as a carbon sourceEarly fossil cells are similar in size and structure to modern archaea and bacteriaThe first photosynthetic cells were bacteria that used the cyclic pathway (does not produce O2)
28Fossil Prokaryotic Cells A Possible cells from 3.5 billion years ago.B Two types of cyanobacteria from 850 million years ago.
29The Proterozoic EraThe oxygen-producing, non-cyclic pathway of photosynthesis first evolved in cyanobacteriaIn the Proterozoic era Layers of photosynthetic bacteria formed large dome-shaped, layered called stromatolitesOxygen 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
30StromatolitesC Artist’s depiction of stromatolites. The surface of each stromatolite consists of amat of living photosynthetic bacteria. Beneath it are layers of earlier generations ofbacteria that trapped sediment and precipitated minerals. The inset photo showsthe layered structure of a fossil stromatolite.
31The Rise of EukaryotesThe earliest evidence of eukaryotes is lipids in 2.7-billion-year-old rocks – the lipids are biomarkers for eukaryotesA red alga that lived 1.2 billion years ago is the oldest species known to reproduce sexually, a trait unique to eukaryotesMulticellularity and cellular differentiation allowed evolution of larger bodies with specialized partsSpongelike animals evolved about 870 million year ago; animals with more complex bodies existed about 570 mya
32Fossils of Some Early Eukaryotes A Grypania spiralis; 2.1 billion years agoB Tawuia; 1.6 billion years agoC Bangiomorpha pubescens; 1.2 billion years ago
33Take-Home Message: What was early life like and how did it change Earth? Life arose by 3–4 billion years ago; it was probably anaerobic and did not have a nucleusAn early divergence separated ancestors of modern bacteria from the lineage that lead to archaea and eukaryotic cellsThe first photosynthetic cells were bacteria that used the cyclic pathway; later, the oxygen-producing, noncyclic pathway evolved in cyanobacteriaOxygen 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
3419.6 How Did Eukaryotic Traits Evolve? Eukaryotic cells have a composite ancestry, with different components derived from different lineagesArchaea-like nuclear genes govern genetic processes (DNA replication, transcription, translation)Bacteria-like nuclear genes govern metabolism and membrane formation
35Origins 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 nucleusA few modern bacteria also have internal membrane-enclosed compartmentsThe nucleus and endomembrane system probably evolved from infoldings of plasma membrane
36infolding of plasma membrane ERFigure Evolution of internal membranes.A Infoldings of the plasma membrane could have evolved intothe eukaryotic nuclear envelope and endomembrane system.nuclear envelope19-10a p191036
37Bacteria with Internal Membranes B Bacterium (Nitrosococcus oceani)with highly folded internal membranesvisible across its midline.C Bacterium (Gemmata obscuriglobus)that has DNA enclosed by a two-layermembrane (indicated by the arrow).
38Evolution of Mitochondria and Chloroplasts The endosymbiont hypothesis holds that mitochondria and chloroplasts descended from bacteria that were prey or parasites of early eukaryotic cellsMitochondria are genetically similar to aerobic bacteria called rickettsias; chloroplasts are similar to photosynthetic bacteria called cyanobacteria
40Evidence of Endosymbiosis Endosymbiosis can occur when a bacterium infects a eukaryotic cellEventually, host and symbiont become incapable of living independentlyExample: The photosynthetic organelles of glaucophytes are dependent on their host – they can’t survive on their own
41photosynthetic organelle that resembles a cyanobacterium Figure Cyanophora paradoxa, a glaucophyte protist with large photosynthetic organelles that make the bacterial protein peptidoglycan.Figure 19-12a p317
42photosynthetic organelle that resembles a cyanobacterium Figure Cyanophora paradoxa, a glaucophyte protist with large photosynthetic organelles that make the bacterial protein peptidoglycan.Figure 19-12b p317
43Take-Home Message: How might eukaryotic organelles have evolved? A nucleus and other organelles are defining features of eukaryotic cellsThe nucleus and ER may have arisen through modification of infoldings of the plasma membraneMitochondria and chloroplasts most likely descended from bacteria