Lecture 18
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All living things evolved from a common ancestor
Origin of life on Earth: Lecture outline No. 18

 The origin of life on earth

 The last common ancestor.

When the earth formed some 4.6 billion years ago, it was lifeless and inhospitable to living organisms. One billion years later it was already teeming with prokaryotic life forms, ancestors to all present living things. What would these early progenitors of life be like? If we make the reasonable assumption that the last common ancestor of all presently living organisms must have had those characteristics which are now shared by the organisms which constitute the five living kingdoms, then a listing of the common characteristics of living species also describes the minimum characteristics of the last common ancestor. Harold Horowitz compiled the following list in his book, "Beginnings of Cellular Life" (Yale University Press, 1992)

All life is cellular.
All living things are from 50 to over 90% water, the source of protons, hydrogen and oxygen in photosynthesis and the solvent of biomolecules.
The major elements of covalently bound biomolecules are carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur.
There is a universal set of small molecules: (i.e. sugars, amino acids, nucleotides, fatty acids, phospholipids, vitamins and coenzymes.)
The principle macromolecules are proteins, lipids, carbohydrates and nucleic acids.
There is a universal type of membrane structure (i.e. the lipid bilayer).
The flow of energy in living things involves formation and hydrolysis of phosphate bonds, usually ATP.
The metabolic reactions of any living species is a subset of a universal network of intermediary metabolism (i.e. glycolysis; the Krebs cycle, the electron transport chain)
Every replicating cell has a genome made of DNA that stores the genetic information of the cell which is read out in sequences of RNA and translated into protein.
All growing cells have ribosomes, which are the sites of protein synthesis.
All living things translate information from nucleotide language through specific activating enzymes and transfer RNAs.
All replicating biological systems give rise to altered phenotype due to mutated genotypes.
Reactions that proceed at appreciable rates in all living cells are catalyzed by enzymes.

How did they get there? What mechanism(s) could produce such a complex organism from inanimate matter? Darwin offered two answers, one public and the other private. In the final chapter of the "Origin of Species", he wrote "the Creator... originally breathed life.... into a few forms or one. ... From so simple a beginning endless forms most beautiful and most wonderful have been, and are being evolved". In private correspondence he suggested life could have arisen through chemistry "in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, etc. present".

Leslie Orgel of the Salk Institute has said that "For most of the 20th century, origin-of-life research has been trying to flesh out Darwin's private hypothesis - to elucidate how, without divine intervention, spontaneous interaction of relatively simple molecules dissolved in lakes and oceans of the prebiotic world could have yielded life's last common ancestor." (Scientific American, October 1994, p. 77).

Biologists and paleontologists have defined five basic questions that need to be answered when discussing the origin of life.

(1) Where did the raw materials for life come from?
(2) How did monomers develop?
(3) How did polymers develop?
(4) How did an isolated cell form?
(5) How did reproduction begin?

The following are only tentative and speculative answers to these questions

1) Where did the raw materials come from?

The early earth is presumed to have provided all of the elements and chemicals needed for life to begin.

2) How did monomers form?

The Miller-Urey experiments in the late 40’s and early 50’s showed that organic molecules could be formed by inorganic processes under primitive earth conditions. By discharging electric sparks in a large flask containing boiling water, methane, hydrogen and ammonia, conditions presumed to be similar to those of the early earth, they produced amino acids and other organic molecules experimentally. Using variations of their technique, most of the major building blocks of life have been produced: amino acids, sugars, nucleic acid bases and lipids.

Another source of amino acids and other organic molecule is meteorites. The amino acid content of the Murchison meteorite, for example, is surprisingly similar to that formed in the Miller-Urey experiments.

Both by earth-formed and meteorite-delivered processes, the early ocean could have become the thin "organic soup" proposed independently many years earlier by Alexander Oparin and J. B. S. Haldane as the starting place for life. The first "organisms" presumably consumed these molecules both as building blocks and as sources of energy. Upon the exhaustion of these early molecules, other strategies had to be develop such as photosynthesis. The first forms of photosynthesis was probably non-oxygenic using inorganic molecules as a source of electrons to reduce carbon dioxide, however, when these sources were exhausted, oxygen generating photosynthesis was developed using water as the electron source. The generation of oxygen had a most dramatic effect on future evolution.

3) How did polymers develop?

Various suggestions about this process exist. Polymerization on clays or the evaporation of amino acid containing water near volcanic vents. Sidney Fox has demonstrated such polymerizations experimentally. Such reactions could have led to the polymerization of amino acids and nucleotides. Others believe that polymerizations occurred in cold environments where the polymers would be more stable.

4) How did an isolated cell form?

Harold Morowitz has proposed that the formation of closed, membrane vesicles was an early event in cellular evolution. Lipid molecules spontaneously form membrane vesicles or liposomes. ("Beginnings of Cellular Life", 1992, Yale University Press). Consider the following properties of membrane vesicles, which are also the properties of cells.

1) They maintain separate stable phases in an aqueous environment.
2) They maintain different chemical compositions between intra- and extra-cellular compartments.
3) They maintain substantial transbilayer electrical voltages, pH differences, and oxidation potentials (necessary for chemiosmotic processes).
4) They form spontaneously from abiotically formed amphipathic lipid molecules

"What is impressive in simply listing the properties of vesicles is how many cellular features are already present in these simple systems. Strong reasons for assuming the importance of vesicles in biogenesis are their spontaneous formation and the continuity they make with contemporary cells in so many ways".

5) How did reproduction begin?

By what series of chemical reactions did the complex structure and the interdependent system of nucleic acids and proteins come about? Carl Woese, Francis Crick and Leslie Orgel propose that RNA came first. It could self-replicate and possibly serve as enzymes for protein synthesis. In 1983 Thomas Cech and, independently, Sidney Altman discovered ribozymes, that is, the ability of RNA to catalyze its own modifications without the use of protein enzymes. They received the Nobel Prize and the RNA world is now with us. Eventually the RNA it would be replaced by DNA and protein enzymes to take over information storage and enzymatic functions, respectively.

Ribozymes exist and have been modified to carry out some of the important reactions of RNA replication such as stringing up nucleotides and oligonucleotides using ATP. Derived ribozymes can also be made to cleave chemical bonds including peptides. In translation on ribosomes it is probably the rRNA, not the protein, that forms the peptide bonds. Furthermore, ATP and all coenzymes are ribonucleotides which some consider are relics of the original RNA World. Thus there is reason to believe that there was an original RNA world which invented protein synthesis and only later was supplanted by DNA.

The extraterrestrial origin of life?

 Svante Arrhenius in 1908 proposed the "panspermia theory" - that life originated on Earth with the arrival of spores that had drifted through space from some other planetary or solar system. Among those who favor this hypothesis, Francis Crick argues that the overwhelming biochemical and molecular evidence suggests that the last common ancestor was already on earth 3.5 to 3.6 billion years ago when the history of life began on earth.

Is the Panspermia idea a viable one? The possibility that life once existed on Mars made much news last year. The evidence is questionable but still a possibility. Is it likely that microbial life came to earth from Mars or some more distant extraterrestrial source? "Deinococcus radiodurans, a bacterium highly resistant to radiation, would be a good vector for panspermia, said Dr. [Kenneth W.] Minton [of the Uniformed Services University of Health Sciences in Bethesda, MD]. While drifting through interstellar space for many thousands of years, it might acquire a shell of interstellar crud that could protect it [from the intense heat generated] when it entered some planet’s atmosphere space".

Final Caveat

 Almost all of this section is highly conjectural.  Read it with this in mind.