XIV. –THE ORIGIN OF LIFE
A. – Biogenesis
1. Biogenesis – the current model, which states that all living things come from other living things.
2. Spontaneous Generation – states that living things arise from nonliving things.
3.
Redi’s
Experiment a. Francesco Redi questioned the commonly held belief that flies were generated spontaneously from rotting meat. b. Redi devised an experiment to test his hypothesis that meat kept away from flies would not get maggots.
1. Experimental consisted of net-covered jars that contained meat.
2. Control consisted of uncovered jars containing meat. c. Result maggots swarmed only over meat in open jars. d. Conclusion flies come only from eggs laid by other flies.
4. Spallanzani’s Experiment a. Lazzaro Spallanzani tested the hypothesis of spontaneous generation of microorganisms. b. Spallanzani reasoned that boiling broth in a flask would kill any microorganisms.
1. Experimental consisted of sealed flasks full of boiled clear, fresh broth.
2. Control consisted of unsealed flasks full of boiled clear, fresh broth. c. Result the broth in the sealed flasks remained clear, while that in the open flasks became cloudy due to contamination with microorganisms. d. Conclusion the broth became contaminated only when microorganisms entered with the air.
5. Pasteur’s Experiment a. Louis Pasteur devised an experiment in which a flask with a curved but open neck prevented microorganisms from entering. b. Broth boiled in the flasks became contaminated by microorganisms only when the necks were removed. c. Conclusion the contamination was due to microorganisms in the air.
B. Earth’s History
1. The Formation Of Earth a. The solar system formed from a mass of gas and dust at the center of a giant molecular cloud.
49 b. Most of this material fell inward, forming a stellar core that

would eventually achieve hydrogen fusion and become the Sun. c. The remainder of the material circled as a protoplanetary disk.
The terrestrial planets formed as solid debris accumulated, and the gas planets formed from large rocky cores that had enough gravitational attraction to pull in gas from the surrounding disk. d. The estimated age of Earth is 4.5 billion years . e. Radioactive Dating – methods of establishing the age of materials. f. Isotopes – atoms of the same element that differ in the number of neutrons they contain.
1. The mass number of an isotope is the total number of protons and neutrons .
2. Isotopes are designated by their chemical name followed by their mass number, ex. carbon-12 . g. Radioactive Decay – the release of particles or radiant energy from unstable nuclei. ( through beta decay )
1. Radioactive Isotopes – isotopes that are radioactive and are undergoing decay.
2. Half-life (represented by λ ) – the length of time it takes for ½ of an isotope to decay.
3. The age of a material can be determined by measuring the amount of a radioactive isotope it contains. h. The age of Earth was estimated by using a method based on the decay of uranium and thorium in rock crystals.
2. The First Organic Compounds a. Alexander Oparin (1894-1980) suggested that the atmosphere of the primitive Earth was composed of:
1. Ammonia ( NH
3
)
2. Hydrogen Gas ( H
2
)
3. Water Vapor ( H
2
O )
4. Methane ( CH
4
) b. These chemicals may have rearranged under the searing heat to form organic molecules. c. Lightning and UV radiation may have catalyzed the formation of molecules essential to life. d. Stanley L. Miller and
Harold C. Urey built an apparatus to test Oparin’s hypothesis.
1. Their apparatus included a chamber containing the gases Oparin assumed were present in the young atmosphere.
2. As the gases circulated in the chamber, electric sparks drove chemical reactions.
3. Result the production of a variety of organic compounds, including all amino acids.

50 e. Organic compounds are present in the atmospheres of most of the planets and in interstellar space.
3. From Molecules To Cell-Like Structures a. Cell-like structures have formed from solutions of simple organic molecules.
1. Microspheres – spheres composed of protein molecules organized as a membrane.
2. Coascervates – collections of droplets that are composed of molecules of different types, such as amino acids and sugars.
C. The First Life-Forms
1. The Origin Of Heredity a. The t shape of tRNA is dictated by hydrogen bonds between particular nucleotides. b. This observation led to speculation that RNA molecules may behave like enzymes.
2. The Roles of RNA a. Ribozyme ( portmanteau of ribosome and enzyme ) – a type of
RNA that can act as an enzyme. b. Life may have started with self replicating molecules of RNA.
3. The First Prokaryotes a. The first cells were probably anaerobic heterotroph prokaryotes, since there was little oxygen. b. The next generation was probably autotroph prokaryotes. c. Archaebacteria – living organisms that may be similar to the first organisms. They are unicellular organisms that thrive in seemingly inhospitable places. d. Photosynthetic life appeared about 3 billion years ago. e. Cyanobacteria – a group of photosynthetic unicellular prokaryotes possibly related to the first photosynthetic life. f. It took about one billion years for the O
2 in the atmosphere to reach current levels. g. O
2 can be split into 2 O by high energy light. Each reacts with
O
2 to form O
3
, or ozone. Ozone is toxic but is responsible for absorbing UV in the upper atmosphere.
4. The First Eukaryotes a. About 2.0-1.5 billion years ago, a small aerobic prokaryote began to live and reproduce inside larger, anaerobic prokaryotes.
1. It is thought that it gave rise to mitochondria .
2. A relative of photosynthetic cyanobacteria gave rise to the chloroplast . b. Endosymbiosis – a successful, mutually beneficial relationship.