Chapter 33: Origin of Life
I. How did life begin?
A. How could living cells have evolved from
non-living inorganic molecules?
1. Panspermia: Maybe simple organic molecule came to earth on meteorites or were brought here by Klingons.
2. Spontaneous generation
3. Abiogenesis: the abiotic synthesis of small organic molecules (monomers).
a. Oparin and Haldane (1920’s): postulated what the conditions of the early earth would have had to been like for organic molecules to form. This required evidence for what the early atmosphere was like. Where would you look for such evidence?
1. Miller-Urey experiments: Miller was able to show, using the ‘spark-discharge apparatus’, that by simulating the Oparin/Haldane postulates they could form examples of all the major types of organic molecules
2. assembling these monomers into polymers – macromolecules
a. Fox: produced proteinoids (polypeptides produced by abiotic means) by heating up dry mixtures of amino acids and plopping them in water. The amino acids spontaneously assembled into the proteinoids.
b. a possible role for clay: a major component of clay, aluminosilicates, provides a charged surface that facilitates the binding of metal ions and organic molecules creating an environment promoting chemical reactions. Polypeptides can be produced using such catalytic surfaces.
c. deep sea hydrothermal vents: some thermoacidophiles, a type of Archaebacteria, flourish around hydrothermal vents. Many of the inorganic and organic precursors to more complex molecules, thought to have been abiotically produced, are found around these vents, for example acetyl coenzyme A.
d. Catalysts: metal ions and their sulfides. These inorganic ions and their salts commonly participate in chemical reactions as catalysts either alone or as cofactors.
e. Experiments show that molecules like formaldehyde are formed readily under abiotic conditions. Formaldehyde readily reacts with other simple molecules to form porphyrin rings, which are important structures in chlorophyll, cytochromes, nucleotides and some amino acids.
3. Once organic molecules are present the question becomes how could cells have formed? Some organic molecules readily organize into cell-like structures sometimes referred to as protobionts. Protobionts are the scientific postulate for the precursors to living cells. Types of protobionts include:
a. Microspheres: Fox demonstrate thet protobionts can form from protenoids in water.
b. Liposomes: protobionts that form from phospholipids in water.
c. Coacervates: Oparin was able to produce membrane-like structures by mixing proteins and carbohydrates in water. Since then mixtures of macromolecules (of all types) have been used to contribute to the structure of and form protobionts.
4. Origin of heredity
a. RNA could have been the first genetic material, since it can encode information and catalyze reactions. Eventually became the intermediary in the central dogma, DNAàRNAàproteins.
b. RNA is capable of autocatalytic synthesis and replication
c. ribozymes: RNA catalysts or enzymes.
III. Who evolved first?
A. Cenancestor: last common ancestor to all life
B. Earliest cell fossils are prokaryotic.
1. Stromatalites: fossils of cyanobacteria-like cells approx. 3.6byo.
2. However, most biologists believe the first living cell was a chemoheterotroph.
1. endosymbiosis hypothesis
2. endomembrane hypothesis