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Life Out There…
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Extraterrestrial Life Can/does life exist out there?
astrobiology/xenobiology no evidence accepted by scientific
community most scientists hold that, if extraterrestrial
life exists, it likely occurred independently in many places
(minority) panspermia hypothesis, where life originated in few points of origin and spread
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Life on Earth Water
neutral pH can dissolve positive metallic ions & negative non-metallic ions with equal ability
medium for interactions organic molecules can be hydrophobic
(repelled) or hydrophilic (soluble) enables formation of water-enclosing membranes
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Life on Earth Water
solid ice is less dense than liquid water ice floats oceans don’t freeze
latent heat of vaporization moderates climate
cools tropics warms poles helps maintain thermodynamic equilibrium
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Life on Earth Carbon
immense flexibility in creating covalent chemical bonds with non-metallic elements (N, O, H)
CO2 & H2O together enable storage of solar energy in sugars
oxidation of glucose fuels all other biochemical reactions
able to form organic acids & amine bases permits building DNA, ATP, other stuff
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Alternative biochemistries Silicon
chemically similar to carbon large mass & size
formation of double & triple covalent bonds difficult
long-chain silicone molecules are more unstable than carbon counterparts
SiO2 (analog to CO2) non-soluble in ranges where water is liquid
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Alternative biochemistries Silicon
cosmic abundances of carbon & silicon roughly 10:1
molecules identified in interstellar medium 84 carbon-based 8 silicon-based
terrestrial planets are exceptionally Si-rich, C-poor
yet carbon is used on Earth some suggestion early life forms may have
been Si-based
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Alternative biochemistries Nitrogen & Phosphorus
Phosphorus can build long chain molecules like carbon
fairly reactive nitrogen can allow more stable covalent
bonds Nitrogen is nearly inert and energetically
expensive to “fix” due to triple bond
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Alternative biochemistries Nitrogen & Phosphorus
NO2 atmosphere NH3 atmosphere
lots of debate – several aspects of this biology would be energy deficient
nitrogen & phosphorus are unlikely to be formed in useful quantities & ratios
Carbon likely wins again
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Alternative biochemistries Chlorine
much less abundant than oxygen typically becomes bound in salts & other
inert compounds Sulfur
high reactivity problems of phosphorus & silanes
but there are strains of sulfur-reducing bacteria
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Alternative biochemistries ammonia as solvent
hydrogen bonds weaker low heat of vaporization
surface tension three times smaller reduces ability to concentrate non-polar
molecules through hydrophobic effects seems unlikely to hold prebiotic molecules to
allow emergence of self-replicating system Highly combustible in an oxidizing
environment
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Alternative biochemistries other non-water solvents
methanol hydrogen sulfide or hydrogen chloride
low abundances hydrocarbon mixtures
methane/ethane on Titan lack polarity
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Life on Other Worlds The Nature of Life The Origin of Life Communication with Distant
Civilizations
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Life on Other Worlds The Nature of Life
The Physical Basis of Life Information Storage and Duplication Modifying the Information
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Life on Other Worlds The Origin of Life
The Origin of Life on Earth Geologic Time Life in Our Solar System Life in Other Planetary Systems
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Life on Other Worlds Communication with Distant
Civilizations Travel Between the Stars Radio Communication How Many Inhabited Worlds?
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Life on Earth Physical Basis of Life
All life forms on Earth, from viruses to complex mammals (including humans) are based on carbon chemistry
Carbon-based DNA and RNA molecule strands are the basic carriers of genetic information in all life forms on Earth
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Life on Earth Physical Basis of Life
The Tobacco Mosaic Virus contains a single strand of RNA, about 0.1 mm long
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Life on Earth Physical Basis of Life
The Tobacco Mosaic Virus (TMV)
first virus discovered single strand of RNA, ~0.1
mm long
This complex mammal contains about 30 AU of DNA.
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Life on Earth Information Storage & Duplication
All information guiding all processes of life stored in long spiral molecules of DNA (deoxyribonucleic dcid)
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Life on Earth Information Storage
& Duplication basic building blocks
are four amino acids Adenine Cytosine Guanine Thymine
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Life on Earth Information Storage
& Duplication Information is
encoded in the order in which those amino acids are integrated in the DNA molecule
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Life on Earth RNA
ribonucleic acid several roles in
translating information from DNA into protein products
messenger between DNA and the protein synthesis complexes (ribosomes)
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Life on Earth RNA
ribonucleic acid several roles in
translating information from DNA into protein products
forms vital portions of ribosomes
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Life on Earth RNA
ribonucleic acid several roles in
translating information from DNA into protein products
acts as essential carrier molecule for amino acids used in protein synthesis
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Life on Earth Processes of Life in Cells
Information stored in the DNA in the nucleus is copied over to RNA (ribonucleic acid) strands, which act as messengers to govern the chemical processes in the cell
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Life on Earth Human DNA & descent
mitochondrial DNA Mitochondrial Eve (mt-mrca)
matrilineal most recent common ancestor (MRCA) for all living humans
mitochondrial DNA (mtDNA) passed down from mothers to offspring for over 100,000 years
now found in all living humans: every mtDNA in every living person is derived from hers
believed to have lived ~140,000 years ago in what is now Ethiopia, Kenya or Tanzania
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Life on Earth Human DNA & descent
Y-chromosomal Adam (Y-mrca) patrilineal human most recent common
ancestor (mrca) from whom all Y chromosomes in living men are
descended geneticist Spencer Wells has concluded all
humans alive today are descended from single African man who lived ~60,000 - 90,000 years ago
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Life on Earth Duplication & Division
In the course of cell division, DNA strands in nucleus (chromosomes) duplicate by splitting double-helix strand and replacing open bonds with corresponding amino acids
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Life on Earth Duplication & Division
Process must be sufficiently accurate, but also capable of occasional minor mistakes to allow for evolution
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Life on Earth Origin of Life on Earth
Life develops into more complex forms through gradual evolution, spanning many thousands of generations
Life began in the sea as single-celled creatures
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Life on Earth Origin of Life on Earth
Those as well as early multi-celled creatures had no hard parts to leave fossils
Earliest, microscopic fossils date back ~ 4 billion years
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Life on Earth Origin of Life on Earth
~½ billion years ago, in the Cambrian Period, the diversity and complexity of life on Earth dramatically increased
“Cambrian Explosion” trilobites from Cambrian period all known fossils from Cambrian
period sea creatures no traces of life on land until ~400
million years ago
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Life on Earth Miller Experiment
(1952) simulating conditions on
Earth when life began ~4 billion years ago
water (oceans) primitive atmosphere
gases (hydrogen, ammonia, methane)
energy from electric discharges (lightning)
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Life on Earth Miller Experiment
(1952) simulating conditions
on Earth when life began ~4 billion years ago
produced some of the fundamental building blocks of life: amino acids, fatty acids, and urea
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Life on Earth Miller Experiment (1952)
shows that basic building blocks of life form naturally
Amino acids and other organic compounds naturally tend to link up to form more complex structures
early oceans on Earth were probably filled with a rich mixture of organic compounds: the “Primordial Soup”
Chemical evolution leads to the formation and survival of the most stable of the more complex compounds
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Life on Earth Miller Experiment (1952)
recent studies – amino acid composition of products of “old” areas in “old” genes
defined as those found to be common to organisms from several widely separated species
assumed to share only the last universal ancestor (LUA) of all extant species
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Life on Earth Miller Experiment (1952)
recent studies – amino acid composition of products of “old” areas in “old” genes
found that the products of these areas are enriched in those amino acids that are also most readily produced in the Miller-Urey experiment
suggests that original genetic code based on smaller number of amino acids – only those available in prebiotic nature – than the current one
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Life on Earth Extraterrestrial Origin of
Life on Earth Alternative theory: most
primitive living entities transported to Earth in meteorites or comets
some meteorites do show traces of amino acids
theory of extraterrestrial origin of life is currently untestable
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Life on Earth Age of Life on Earth
oldest fossils known are stromatolites built up layer by layer from single-celled
creatures, similar to bacteria ~3.5 billion years ago
During the Cambrian period (~500 million years ago), life became complex
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Life on Earth Age of Life on Earth
In geologic terms, higher life forms have evolved only very recently
mammals & humans in particular
humans have existed only ~3 million yrs
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Life on Other Worlds Evolution of Life
Could life originate on another world if conditions were suitable?
Will life always evolve toward intelligence? How common are suitable conditions for
the beginning of life?
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Life on Other Worlds Evolution of Life
Could life originate on another world if conditions were suitable?
Miller experiment etc. indicate: probably yes
Will life always evolve toward intelligence? How common are suitable conditions for
the beginning of life?
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Life on Other Worlds Evolution of Life
Could life originate on another world if conditions were suitable?
Miller experiment etc. indicate: probably yes
Will life always evolve toward intelligence? If intelligence favors one species over
another: probably yes How common are suitable conditions for
the beginning of life?
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Life on Earth
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Life on Other Worlds Evolution of Life
How common are suitable conditions for the beginning of life?
investigate conditions on other planets and statistics of stars in our Milky Way
Astronomy!
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Life on Other Worlds Requirements
liquid water atmosphere moderate temperatures time
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Life on Other Worlds Requirements
liquid water for chemical reactions and as transport
medium
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Life on Other Worlds Requirements
atmosphere avoid rapid vaporization of water gasses needed for organic compounds
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Life on Other Worlds Requirements
moderate temperatures keep water liquid avoid disintegration of organic compounds activate complex chemical reactions
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Life on Other Worlds Requirements
time evolution from simple organic compounds
into higher life forms several billion years
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Life on Other Worlds Life in the Solar System
most planets & their moons are unlikely to have ever provided suitable conditions for life
best candidate: Mars Mars rovers Spirit and Opportunity found
evidence for past water on Mars possibly some evidence of past life on Mars,
but questionable
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Life on Other Worlds Life in the Solar System
Meteorite ALH84001,0 probably originated on
Mars claimed traces of
microscopic fossils may well be regular mineral formations in the rock
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Life on Other Worlds Requirements outside Solar System
planetary systems are probably common stable orbit around the star
consider only single stars time for evolution
consider only F5 or less massive stars Moderate temperatures
life zone around the star
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Life on Other Worlds Requirements outside Solar System
planetary systems are probably common stable orbit around the star time for evolution Moderate temperatures
life zone around the star
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Life on Other Worlds Communication with distant civilizations
direct space travel to other stars not feasible large distances/long travel times
viable alternative: radio communication still long answer times due to light-travel time messages can be arranged in blocks of certain
length that is a product of two prime numbers only two ways to arrange them in a rectangle
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Life on Other Worlds Communication with distant
civilizations viable alternative: radio communication
messages can be arranged in blocks of certain length that is a product of two prime numbers
only two ways to arrange them in a rectangle
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Life on Other Worlds Aricebo message
dedication of Arecibo Radio Observatory
blocks of 1679 pulses were emitted
23x73
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Life on Other Worlds Aricebo message
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Life on Other Worlds Search for Extraterrestrial Life (SETI)
In addition to sending messages to possible extraterrestrial civilizations, there are also programs to listen for intelligent messages from space: SETI
Only certain wavelength ranges are suitable for this search
SETI program is highly controversial because of the uncertain prospects of positive results
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Life on Other Worlds Search for Extraterrestrial Life (SETI)
Only certain wavelength ranges are suitable for this search
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Life on Other Worlds Drake Equation
factors to consider when calculating number of technologically advanced civilizations
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Life on Other Worlds Drake Equation
Nc = N* · fp · nLZ · fL · fl · FS
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Fermi Paradox 1950 Enrico Fermi
apparent contradiction between high estimates of the probability of existence and lack of evidence
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Fermi Paradox lack of evidence
probes spacecraft radio transmissions
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Fermi Paradox
The size and age of the universe suggest that many technologically advanced extraterrestrial civilizations ought to
exist. However, this belief seems logically inconsistent with the lack of
observational evidence to support it.
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Fermi Paradox
Either the initial assumption is incorrect and technologically advanced intelligent life is
much rarer than believed, current observations are incomplete and human
beings have not detected other civilizations yet, or search methodologies are flawed and
incorrect indicators are being sought.
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Fermi Paradox Resolutions…
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Fermi Paradox They do not exist…
OK, that was easy
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Fermi Paradox They do not exist…
…and never did …because an inhospitable universe
destroys complex intelligent life ice ages impacts volcanism supernovae
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Fermi Paradox They do not exist…
…and never did …because it is in the nature of intelligent
life to destroy others
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Fermi Paradox They do not exist…
…and never did …because God created humans alone
but not in a science class…
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Fermi Paradox They do exist…
…but communication is impossible due to problems of scale
they’re too far away space time
too expensive to physically spread they haven’t gotten back to us yet
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Fermi Paradox They do exist…
…but communication is impossible due for technical reasons
we are listening improperly period of radio signals may be brief tend to experience a technological
singularity
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Fermi Paradox They do exist…
…and they choose not to communicate Earth is purposely isolated (zoo hypothesis) they are too alien they are not interested they do not believe in life elsewhere
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Fermi Paradox They do exist…
…and they are here unobserved hiding their presence we refuse to see or misunderstand the
evidence
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Fermi Paradox They do exist…
…and they are here unobserved hiding their presence we refuse to see or misunderstand the
evidence
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Fermi Paradox They do exist…
…and they are here unobserved
hiding their presence we refuse to see or
misunderstand the evidence
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