Chapter 12, Section 3 And parts of 12.4 Origins of Life Chapter 12, Section 3 And parts of 12.4

Early Theories Spontaneous generation = the idea that living things could come from nonliving things Three experiments disproved this theory: Francesco Redi (1665) Lazzaro Spallanzani (1767) Louis Pasteur (1862)

Redi’s Experiment People thought maggots came from meat Redi showed that maggots came from flies laying eggs on the meat

Spallanzani’s Experiment People still thought that microorganisms could spontaneously generate Spallanzani boiled two flasks of broth, then left one open and sealed one Bacteria grew in the open flask The sealed flask remained sterile People convinced that spontaneous generation exists said that boiling the broth killed a “vital principle” in the air

Pasteur’s Experiment Disproved spontaneous generation once and for all Microorganisms only grew in the flask when the swan neck was broken The swan neck prevented particles in the air from entering the broth Animation

Biogenesis – Life from Life A possible sequence: Inorganic molecules form and make small organic molecules Small organics join to form macromolecules / polymers Origin of RNA / DNA to make inheritance possible Packaging within membranes

Related Vocabulary Inorganic – any substance that doesn’t contain both carbon (C) and hydrogen (H) Organic – any substance that contains both C and H; usually comes from something that is, or once was, living Polymer – substance made up of many repeating subunits (monomers) Macromolecule – large molecules; biological examples include carbohydrates, lipids, proteins, and nucleic acids

Early Earth For the first 700 million years, Earth was most likely very hot and in a molten state Over time, the materials making up Earth separated into Earth’s layers (crust, mantle, core) Gases released from Earth’s interior formed an atmosphere Early Life on Earth – 4:37

Oparin and Haldane – 1920s Theory for how life may have developed on early Earth; based on assumptions that: There was little or no oxygen present The atmosphere was mainly formed from volcanic vapors – methane, ammonia, hydrogen, water vapor Felt it would be possible for inorganic molecules to be converted to organic forms using energy from the sun and lightning At the time, no effective way to test this

Miller and Urey – 1950s Tested Oparin-Haldane hypothesis Simulated atmosphere containing gases Oparin & Haldane thought were present Exposed gases to electric shocks to simulate lightning Produced small organic compounds – mainly amino acids Animation

Follow-up to Miller/Urey Based on the gases emitted from volcanoes today, scientists think the atmosphere would have been different from what Oparin & Haldane proposed More carbon monoxide (CO), carbon dioxide (CO2), nitrogen gas (N2) and water vapor Similar experiments using this “updated” representation of the atmosphere produced smaller amounts of amino acids

Meteorite Hypothesis Analysis of meteorites indicate that organic molecules can be found in space This suggests the following possibilities: Amino acids may have been present when Earth formed Organic molecules may have arrived on Earth through meteorite / asteroid impact

Formation of the First Cells Once organic molecules / compounds are formed, how did they get packaged into cells? Iron-sulfide bubbles hypothesis Lipid membrane hypothesis

Iron-Sulfide Bubbles Iron sulfide rising from deep sea vents combines with cool ocean water to form chimney-like structures with many compartments Biological molecules may have combined inside these compartments, which acted as membranes With the right combination of ingredients, the first organic cell membranes may have formed

Lipid-membrane Hypothesis Lipids spontaneously form membrane-enclosed spheres called liposomes Liposomes could act as membranes around a variety of organic molecules, separating them from the environment

The First Genetic Material It has been hypothesized that RNA was the genetic material for the earliest life forms Cech & Altman (1980s) discovered that RNA can: Catalyze reactions Copy itself

The First Eukaryotes Fossil evidence indicates that the first living things were prokaryotes (bacteria) First appeared ~3.5 BYA Eukaryotes – cells with a nucleus and other organelles – don’t appear in the fossil record until approx. 1.5 BYA How did the first eukaryotic cells develop?

Endosymbiosis Suggested by Lynn Margulis (1970s) Idea that mitochondria and chloroplasts used to be simple prokaryotic cells that were engulfed by larger prokaryotes Animation

Endosymbiosis, cont’d What evidence supports endosymbiosis? Both mitochondria and chloroplasts: Have their own DNA Have their own ribosomes Can copy themselves Are about the same size as prokaryotes Have DNA in the shape of a circle, like bacterial / prokaryotic DNA