The Origin of Life
April 11, 2001
"The earth hath bubbles, as the water has, and these are of them."
- William Shakespeare, 1564-1616 [from Macbeth I, iii]
The cell theory states that all cells come from other cells - but, if that is the case, how did the first living cell come about?
* To qualify as a living cell, it would have to grow and reproduce (at the very minimum).
* Since this cell was the first living thing, it had to come from non-living materials....but HOW?
A current hypothesis on the origin of life:
I. Early Earth - the primitive Earth of 4 billion years ago did not contain life.
The early earth, a mass of hot, glowing atoms, began to solidify 4 BYA.
II. Life definitely existed ~3.5 - 3.8 billion years ago.
III. So what do scientists propose happened in between???? How might the first cells have originated?
Hypothesis: chemical evolution or the chemosynthetic theory: life developed from non-living materials eventually, by the process of natural selection, over hundreds of millions of years, became able to self-replicate and metabolize. This hypothesis presumes that at least 4 steps happened to bring about this chemical evolution
But - Is Chemical Evolution a scientific hypothesis - can it be tested? Do we have any scientific evidence that these processes can happen???? Yes, to all of the above.
Note: Laboratory experiments cannot prove that
life was created in this way on the primitive earth, only that the key steps
could have happened.
1. Abiotic synthesis of amino acids and nucleotides - could it happen? Yes.
The Miller-Haldane-Urey experiment (Nobel Prize) - recreated the atmosphere of Early Earth. (Figure 8-11)
7 simple molecules in --------->> Urea, 6 amino acids, lipids, carbohydrates, and two nucleic acid bases (adenine and cytosine) OUT!
These experiments indicate that these simple molecules - amino acids and nucleotides - could have been formed under the atmospheric conditions of Early Earth.
2. Joining of monomers into polymers - could it happen? Yes.
Remember those macromoleules - polymers of smaller building blocks?!? Today, formation of polymers from monomers in cells is speeded up greatly by the presence of enzymes that join monomers into polymers. The enzymes help form or break the chemical bonds between molecules to put small building blocks together, usually removing a molecule of water in the process. .
However, enzymes are only one way to speed chemical reactions up. Heat is another way to join monomers into polymers. Scientists have shown that when organic monomers (like amino acids) are heated and splashed onto hot sand or rocks, the heat vaporizes the water and links the monomers into polymers - which scientists call 'proteinoids'. [Image]
This suggests that waves or rain may have splashed amino acid monomers onto hot rocks or fresh lava, and rinsed polymers - proteinoids - back into the sea....
Neither oxidation (there was no O2) nor decay (there were no bacteria) would have destroyed these molecules, and they would have accumulated in the early oceans for hundreds of millions of years, eventually producing a thick, warm, organic soup or primordial soup containing a variety of organic molecules.
3. The self-assembly of molecules into droplets, or 'Protobionts' - could it happen? Yes.
4. The ability to replicate - could it happen? Yes.
Livin' in an RNA World: The RNA World hypothesis: The first hereditary molecule on Earth were RNA molecules that served as both a genome as well as the enzymes to copy itself.
In 1980, Tom Cech at U Colorado Boulder made a revolutionary and unexpected finding about RNA
A. RNA can function as an enzyme in cells - called a ribozyme. RNA has been shown to remove its own introns as well as synthesize new RNA (mRNA, rRNA, and tRNA). There are over 500 different ribozymes known today.
B. RNA can make a copies of itself in a test tube. If RNA in a test tube is supplied with monomers (ribonucleotides A, C, U and G), sequences 5-10 nucleotides long can be copied from the template according to base-pairing rules. If zinc is added as a catalyst, sequences up to 40 nt long are copied with less than 1% error.
In 1989, Tom Cech won the Nobel Prize in Chemistry for his discovery of Ribozymes.:
RNA, being capable of self-replication and catalytic, fits one criteria needed life = replication
All it would take is one protobiont with the ability to replicate, and an inefficient replication process that would generate inevitable copying errors (mutations) to its descendants to produce a diverse population of living cells....
Once replication (and its inevitable mistakes, or mutations) was possible, so was evolution (change over time).
It is hypothesized that only after a differential reproductive success was seen in cells that stored their genetic 'blueprint" as the more stable molecule DNA did RNA taking on the intermediate role in the translation of genetic material into physical characteristics.
More clues to the origin of life and the RNA World hypothesis
IV. A few more "big questions" about the origin of life...
Where did life begin?
When do we first call it "life"?
Does anyone know for sure how life actually began?
Could there be life on other planets? (From: Biology, by Peter Raven)
As long as a functional genetic system capable of replication, mutation, and adaptation came about, life could theoretically evolve in different ways on other planets.
"Beam me up Scotty!"
V. Timeline of Life on Earth (handout)
|Objectives for Chapter 8:
Describe the Miller-Urey experiment, the results, and the
implications for the origin of life.