Genetics and 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 1 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 of catalytic 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 compounds Hydrogen cyanide methane gave rise to monomeric building blocks amino acids and nucleotides rxns took place in water building blocks assembled into biological polymers proteins and nucleic acids The reactions took place in water of the early Earth biological polymers assembled into self replicating cells feeding off the simpler organic building blocks Criticisms of the hypothesis 1 Did liquid water actually exist on Earth when life first formed 2 Earth s atmosphere appears not to have been as Hydrogen rich as originally thought thus reactions were much less efficient Controversial whether RNA was the first stage in the evolution of a replicating molecular world Possibly other polymers were self replicating earlier including polymers of inorganic substances such as clay Protective Enough Environment 3 4 billion years ago Apatite crystals produced by living organisms 1 3 billion years ago Evidence from rock formations on Akilia Island Greenland History of many large meteoritic impacts hitting Earth billions of years ago One major one 4 5 3 8 billion years ago probably generated enough heat to vaporize the entire ocean We are left with many exciting and unanswered questions about origins References Text F H pp 615 636 3