Life’s Origin and Early Evolution Chapter 18 Biology Concepts and Applications, Eight Edition, by Starr, Evers, Starr. Brooks/Cole, Cengage Learning 2011.

18.2 Beginnings: The Big Bang Earth formed approximately 4.6 billion years ago

Big Bang Theory Big Bang Theory Model describing the formation of the universe as a nearly instant distribution of matter through space The universe began 13-15 billion years ago (13.7BYA) The Event: all existing matter and energy appeared and exploded outward from a single point

Big Bang Theory The Event: Single elements of hydrogen and helium formed Over millions of years, gravity drew the gases together and formed giant stars and eventually galaxies Asteroids collided and merged together The heavier the asteroid (pre-planetary rock)  the more gravitational pull they exerted and the more material they gathered 5 billion years ago  the sun was formed 4.6 billion years ago  Earth and planets in our solar system formed

Conditions on Early Earth First Air Contained  water vapor, carbon dioxide, and gaseous hydrogen and nitrogen LITTLE OR NO OXYGEN Later, life began in salty runoff pooled in early seas

18.3 Stanley Miller’s Experiment Proposed that reactions in Earth’s early atmosphere could have produced building blocks for the first life – Chemical Hypothesis Experiment Procedure: Placed water and gases (methane, ammonia, and hydrogen gas) into a reaction chamber Observation: As the mix circulated, sparks from electrodes simulated lightning Result: Within weeks, a variety of amino acids and other small molecules formed

18.3 Stanley Miller’s Experiment

Reactions at Hydrothermal Vents Hydrothermal vent – variation on the chemical hypothesis Rocky, underwater opening where mineral-rich water heated by geothermal energy steams out Reactions at vents can produce organic building blocks Experiment: Hot water + carbon monoxide + potassium cyanide + metal ions (like those near vents)  Result: amino acid formation

Extraterrestrial Hypothesis Organic compounds were brought to earth by comets, meteorites and other materials from space Evidence: (from the video Origins) Amino acids found in meteorites that was found in Australia Capsule experiment (amino acids can become peptides under conditions of high temp and pressure Space dust and comets have amino acids

Key Concepts: ABIOTIC SYNTHESIS OF ORGANIC COMPOUNDS When Earth first formed about 4 billion years ago, conditions were too harsh to support life Over time, its crust cooled, seas formed, and organic compounds of the sort now found in living cells may have formed spontaneously or arrived in meteorites

18.4 From Polymers to Cells All cells have a plasma membrane, genome of DNA that is transcribed into RNA and protein All cells replicated and pass on copies of genetic material to descendants Self-replicating genetic systems require proteins (including enzymes) and nucleic acids

Origin of Metabolism Proteins and nucleic acids may self-assemble when certain conditions are met Clay-template hypothesis Clay (- charge) attach + charge molecules Low tide, evaporation concentrated the subunits Concentration and energy from the sun caused the molecules to bond together as polymers Hydrothermal vent hypothesis High pressure, high temperature environment Iron sulfide in rocks donated electrons to dissolved carbon monoxide  start reactions that lead to the formation of larger organic compounds

Protocells: Origins of the Plasma Membrane? Laboratory produced protocell Lipid bilayer encloses RNA Cell grows by adding fatty acids and nucleotides Mechanical force causes protocell division

Origin of the Plasma Membrane Protocell Membranous sac that contains lipid-enclosed collections of interaction molecules It is able to take up materials and replicate itself Hypothesized to have formed prior to the earliest life forms

Proposed sequence for the evolution of cells living cells membrane-bound proto-cells Self-replicating system enclosed in a selectively permeable, protective lipid sphere enzymes and other proteins DNA RNA Proposed sequence for the evolution of cells formation of protein-RNA systems, evolution of DNA formation of lipid spheres spontaneous formation of lipids, carbohydrates, amino acids, proteins, nucleotides under abiotic conditions Fig. 18.7, p.295

Origins of Self-Replicating Genetic Systems Hypothesis: RNA world RNA stores genetic information, but breaks apart easily and mutates often Ribozymes: Catalytic RNAs Switch from RNA to DNA Makes the genome more stable Defense against viruses that attack RNA-based cells

18.5 Life’s Early Evolution 3.8 billion years ago, oxygen levels in atmosphere and seas were low Early prokaryotic cells probably were anaerobic Stromatolites Dome-shaped structures composed of layers of bacterial cells and sediment Cyanobacteria and photosynthetic bacteria grow here

Stromatolites

The Oxygen Atmosphere Cyanobacteria evolved photosynthesis and released waste gas oxygen Changed Earth’s atmosphere Free oxygen levels increased and carbon dioxide levels decreased Increased oxygen favored aerobic respiration Key innovation in evolution of eukaryotic cells

The Oxygen Environment Consequences of Life Oxygen interferes with self-assembly of complex organic compounds. Life no longer could arise from nonliving materials Oxygen put organisms that thrived in aerobic conditions at an advantage. Aerobic respiration allowed the evolution of multicelled eukaryotes Some oxygen molecules broke apart and then recombined as ozone (O3). The ozone layer reduced the amount of solar UV radiation. Without the ozone life could not have moved onto land

Key Concepts: ORIGIN AND EARLY EVOLUTION OF CELLS Laboratory experiments and advanced computer simulations support the hypothesis that forerunners of living cells arose through known physical and chemical processes, such as tendency of lipids to assemble into membrane-like structures when mixed with water

Key Concepts: ORIGIN AND EARLY EVOLUTION OF CELLS (cont.) First cells probably were anaerobic prokaryotes Some gave rise to bacteria, others to archaeans and to ancestors of eukaryotic cells Photosynthetic bacteria started releasing free oxygen into the atmosphere Oxygen accumulated over time and became a global selection pressure

18.4 Where Did Organelles Come From? Eukaryotic internal membranes may have evolved through infoldings of cell membrane

infolding of plasma membrane DNA infolding of plasma membrane Fig. 18.10a, p.298

Fig. 18.10b, p.298

Endosymbiosis Endosymbiosis  One cell lives and reproduces inside another Host and guest cells come to depend upon one another for essential metabolic processes Mitochondria and chloroplasts may have evolved by endosymbiosis

Key Concepts: HOW THE FIRST EUKARYOTIC CELLS EVOLVED A nucleus, ER, and other membrane-enclosed organelles are among the defining features of eukaryotic cells Some organelles may have evolved from infoldings of the plasma membrane Mitochondria and chloroplasts probably are descendants of bacterial cells that became modified after taking up residence in host cells

Key Concepts: VISUAL PREVIEW OF THE HISTORY OF LIFE Key events in life’s origin and early evolution can be correlated with the geologic time scale A time line for milestones in the history of life offers insight into shared connections among all organisms