and Genetics Evolution IB 201
How Did Life Begin?
The origin of life begins with chemistry, which ultimately led to self-replicating molecules. Not much of these earliest stages of life on earth are likely to be preserved in the fossil record. Until very recently, there was no record of life before 550 Ma, which was a big dilemma for Darwin's views of evolution because all life was predicted to have evolved from even simpler forms than those represented in the fossils available in his day. Even today, as they discover more fossils, pushing back in time the age of many phyla, there are very few fossils to help scientists recover the origin of the first single-celled organisms. The difficulties of estimating the origin of life: vast age of the origin of life-- ~ 4 Billion years ago no direct physical record of events, no rocks dating from earliest formation of Earth, so we are left with only indirect evidence. Answers to three major questions should help scientists reconstruct the earliest biological events. 1. What was the Earth's environment like when life arose? 2. a) How is life defined, and b) how did life arise from inorganic forms? 3. What were the characteristics of the very first cells? We will consider the first two of these in this lecture, and pick up the third question in lecture 5 (Tree of Life). 1. Early Environment Using radioisotopic dating of meteorites gives Earth's age as about 4.5 billion years. Initially, the Earth was too hot for life. It took several hundred million years to cool enough for an outer crust to form. Then as water vapor was released from the Earth's interior it cooled, condensed and formed the planet's oceans. 2a. What exactly is the definition of life? How to define basic life? ability to store and transmit information having a genotype ability to express the information--having a phenotype
ability to evolve (to change over time) Non-living systems do not have these fundamental capabilities.
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2b. The RNA World: Ribozymes: small RNA molecules (single-stranded) that can catalyze a reaction, similar to protein enzymes. They contain both genotypic and phenotypic properties: -- its genotype is the nucleotide sequence of the ribozyme molecule, which has the ability to store and transmit information. -- its phenotype is its single-stranded 3-dimensional structure with an active site for catalyzing a chemical rxn to: form and break phosphoester bonds in RNA or DNA; a process required for self-replication (as with nucleic acids) Example: Tetrahymena thermophila bacterium containing a ribozyme intron (catalytic RNA) Is RNA old enough to be the precursor of complex life? Evidence: Discovery of ribozymes--catalytic RNA RNA is in all cell machinery-- i.e., ribosomes contain RNA that appears to be catalytic in protein synthesis Ribonucleoside triphosphates (ATP and GTP) are required for energy-transfer in all cells
Can RNA evolve? RNA molecules can store heritable information through complementary base pairing --i.e. replicate. Example: RNA virus, HIV, uses reverse transcriptase to copy itself into DNA RNA can evolve by natural selection in a test tube--examples: 1. bacterial RNA virus Q 2. evolution catalytic of ability of ribozyme Tetrahymena But where did RNA come from? Precursors need to be simple inorganic compounds Need source of energy to make larger molecules from simple precursors Building blocks need to self-assemble into polymers of RNA and polypeptides Large molecules need protection from harsh environment--How occur in early Earth? 1. Panspermia Hypothesis: Life originated in or outside the solar system and arrived on Earth 2. Carbonaceous Chondrites (organic carbon containing meteorites): Murchison Meteorite contained traces of 18 amino acids--most of which have been made in laboratory experiments First such experiment was that of Stanley Miller, as grad student at Univ. of Chicago, 1953. Suggests that early-Earth conditions simulated in Miller's experiments were present on the asteroid that generated the meteorite (a small planet-like body)--and gave rise to the same kinds of monomeric compounds. 3. All living systems are basically composed of: CHON (Carbon, Hydrogen, Oxygen and Nitrogen) plus (SP: Sulphur, Phosphorous) 99% of all living systems These simple elements combine with one another to form the simple sturdy molecules of CH4, NH3, H2, H20, C02, NH2 2
All of life's basic building-block molecules are composed of CHON (SP), including monomers of sugars, amino acids, DNA and RNA purines and pyrimidines polymers of proteins, carbohydrates and nucleic acids are formed by linking together of these monomers These compounds have been found in the Hydrogen-rich clouds around Jupiter and Saturn, one of Saturn's moons (Titan) and in various comets (e.g., Hale-Bopp), even beyond the Solar System. Organic compounds are widespread in the Cosmos--in the dust clouds of interstellar space. Oparin-Haldane Model (first half 20th century): Describes the hypothetical steps leading to the first self-replicating organisms on Earth An inorganic soup of simple inorganic comp...