Lecture 11 -- The Origins of Life
Tuesday, June 18
How do scientists know how old a surface is (whether it's been reworked by volcanism or tectonism)?
We think that the number of objects flying around the solar system has been decreasing exponentially since the solar system began
We know that parts of the moon are very old, because it has collected impacts for a long time without wiping them out
We know the surface of Venus is very young, because it doesn't have very many
DRAW surface, collecting craters every million years
More small ones than big ones, because there are more smaller rocks than big ones
DRAW binned size frequency distribution
Where the line falls tells you how old the surface is
The tilt of the line tells you that small craters may have been wiped out
So what planetary scientists do when they first see a planetary surface is to figure out how long ago a planet has been geologically active. Count up all the craters, and put them in bins, and then make a plot of the size-frequency distribution
They compare the height of the line with the line made by the craters counted around where Apollo sampled rocks -- ones we know the age of
So you see fewer craters on the maria -- how much younger are they?
Since we are immersed in and part of a biosphere
Therefore, we should recognize that we have some biases and perspectives about what life, essentially hard-wired in
Analogy -- Earth centered Universe, our cosmology reflected our biases
Our view of life may be reflected by biases inherent in human consciousness
What distinguished life from a rock?
Commonly agreed -- replicating molecules
But rocks replicate -- crystal structure remains intact after cleavage
Then how are they different?
In life, replicating molecules contain information, and that information is passed on with replication
The information in contained in the sequence of nucleotide bases making up DNA -- the genes
The information tells how to build the organism from the available materials
The expression of the genes in the form and function of the organism is its phenotype
The genetic information is subject to changes and errors as copies are made of copies, of copies, etc.
Random changes in the genetic information cause random changes in the organism's phenotype
This changes may cause the organism to be either less or more competitive with other replicating organism, in a given environment
If the organism becomes more competitive, that random genetic change, or mutation, survives
Organisms with that advantage proliferate
Natural selection -- Darwinian Evolution
So life is: a self sustaining chemical system capable of undergoing Darwinian evolution
We implicitly recognize life -- it is in our genes
But with this definition, is life always recognizable?
There is only one kind of life on the Earth -- the DNA and protein based life form that is common to all known terrestrial biology
All the molecules in our body are built of carbon chains, with a variety of other elements attached.
We need the chains to give us structural strength, and to give organic molecules the complex shapes required for specific chemical tasks
But what about life made from chains of different elements, Si, S, Ge (they also form chains)
What about life that uses a completely different information carrying system, genes made from sound waves?
Earth is 4.5 By old, from the radioactive decay of elements
Radioactive dating -- Start with 1 m atoms of 235U
Half-Life -- 700 my -- 500,000 atoms of U
If rock formed with pure 235U grains at the start of the solar system, it would have 2% of its U235 atoms left, or 20,000
Can date any rock we find on the surface
Oldest rocks are found in old sediments of the centers of the continents
They date to about 4 By
Fossils are the main source of information about past life
Animals die and get contained in the sediments, hard parts preserved with the sediments
Almost all our fossil record is from the Cambrian, 600 my or less
But there are some fossils from very long ago
Photosynthetic bacteria, called cyanobacteria -- 3.5 By old
Cyanobacteria are also called blue-green algae
Stromatolites -- layered deposits of calcium carbonates and sand, created by ancient cyanobacteria
Also, cyanobacteria form the blue-green scum that grows in a water trough
They use the sun for photosynthesis: They build their carbon chains from CO2 in the atmosphere, and use the energy of sunlight to do it
DNA based life, works just like us
Even older evidence -- isotope record of carbon
Life alters the isotopes of carbon -- creates a geochemical signature -- 3.8 by
But we know from the bombardment record of the Moon that the Earth-Moon system had heavy bombardment until about 3.8 By ago
Impact frustration of life -- impacts would have heated huge areas, cut off the sun, melt things.
But life must have started very quickly after the number of impacts began to die down
The information in DNA is carried by a code of 4 letters, (molecules), G, T, A, C
Nucleotides -- guanine, thymine, adenine and cytosine
This is the alphabet -- Sections of the code mean -- 'make this protein'
Proteins are the building blocks of all life
They also act as catalysts, controlling and speeding up reactions -- enzymes
RNA is a kind of molecule like DNA (helix), but has a code with A, C, G,U Uracil
RNA copies portions of the genetic information from the DNA -- messenger RNA -- transcription
Takes it to a ribosome -- work bench that takes the RNA-transcribed information, and assembles proteins
Proteins have a 20 letter alphabet -- the amino acids
Process of taking the messenger RNA code and translating it into the 20 letter amino acid alphabet is translation
Millions of different proteins, all have a unique function
Their shapes define their function
Complex chemical reactions happen by the fitting of one molecule into another
Proteins control all reactions, including the replication of DNA
Proteins codes determine how the nucleotides are sequenced
Fundamental problem -- DNA is needed to make proteins, but proteins are needed to make DNA!
Must have been some vastly simpler scheme for carrying and replicating information from early organisms
RNA world -- RNA instead of DNA as prime information carrier
Thomas Cech, CU, demonstrated that RNA was capable of enzymatic activity
That is it could work as genes and as proteins
But RNA is very, very complex, and couldn't just come about by random chemical reaction
Maybe there was an even simpler scheme -- clays act as catalysts for organic reactions, they have crystal structure, can be replicated, mutations
Pictures of filamentary clays that grow and replicate in the spaces of rock
simple information systems -- RNA -- DNA -- genetic takeover
CO2 and H2O atmosphere
Lots of volcanism, heat from core formation and radioactive decay
Wet and hot
Chemicals mix in the ocean
Water is a solvent, so it mobilized all kinds of atoms and gets them in contact with each other
Thermal energy -- molecules are vibrating and rotating, and bumping into each other, so that they can eventually fit together and react
Both these things are very important
Most origin of life research demands an aqueous environment, so that molecules are free to interact and reactions can occur
Organic molecules dissolved in sea (remember that atmosphere was CO2)
Urey Miller experiment
Took primordial Earth atmosphere (CH4,NH3, H2, CO2, H2O) and zapped with lightening
Found many complex organic molecules, including amino acids
Gas mixture was not quite right, but still happens with CO2 + H2O, CH4
So lightening makes complex organics, they dissolve and react in sea
At tide pools, water evaporates, and the organic molecules get more concentrated--lots of reactions
Information molecules resulted.
Process called 'chemical evolution'
Darwinism takes off, and there you go
Maybe some hitches -- could be times of genetic takeover
VIDEO: THE THIRD PLANET, MIRACLE PLANET SERIES (First Half)