Life Out There…

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

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

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

Life on Earth Carbon immense flexibility in creating covalent chemical bonds with non-metallic elements (N, O, H) CO 2 & H 2 O 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

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 SiO 2 (analog to CO 2 ) non-soluble in ranges where water is liquid

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

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

Alternative biochemistries Nitrogen & Phosphorus NO 2 atmosphere NH 3 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

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

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

Alternative biochemistries other non-water solvents methanol hydrogen sulfide or hydrogen chloride low abundances hydrocarbon mixtures methane/ethane on Titan lack polarity

Life on Other Worlds The Nature of Life The Origin of Life Communication with Distant Civilizations

Life on Other Worlds The Nature of Life The Physical Basis of Life Information Storage and Duplication Modifying the Information

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

Life on Other Worlds Communication with Distant Civilizations Travel Between the Stars Radio Communication How Many Inhabited Worlds?

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

Life on Earth Physical Basis of Life The Tobacco Mosaic Virus contains a single strand of RNA, about 0.1 mm long

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.

Life on Earth Information Storage & Duplication All information guiding all processes of life stored in long spiral molecules of DNA (deoxyribonucleic dcid)

Life on Earth Information Storage & Duplication basic building blocks are four amino acids Adenine Cytosine Guanine Thymine

Life on Earth Information Storage & Duplication Information is encoded in the order in which those amino acids are integrated in the DNA molecule

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)

Life on Earth RNA ribonucleic acid several roles in translating information from DNA into protein products forms vital portions of ribosomes

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

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

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

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 years ago

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

Life on Earth Duplication & Division Process must be sufficiently accurate, but also capable of occasional minor mistakes to allow for evolution

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

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

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

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)

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

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

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

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

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

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

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

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?

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?

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?

Life on Earth

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!

Life on Other Worlds Requirements liquid water atmosphere moderate temperatures time

Life on Other Worlds Requirements liquid water for chemical reactions and as transport medium

Life on Other Worlds Requirements atmosphere avoid rapid vaporization of water gasses needed for organic compounds

Life on Other Worlds Requirements moderate temperatures keep water liquid avoid disintegration of organic compounds activate complex chemical reactions

Life on Other Worlds Requirements time evolution from simple organic compounds into higher life forms several billion years

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

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

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

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

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

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

Life on Other Worlds Aricebo message dedication of Arecibo Radio Observatory blocks of 1679 pulses were emitted 23x73

Life on Other Worlds Aricebo message

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

Life on Other Worlds Search for Extraterrestrial Life (SETI) Only certain wavelength ranges are suitable for this search

Life on Other Worlds Drake Equation factors to consider when calculating number of technologically advanced civilizations

Life on Other Worlds Drake Equation N c = N* · f p · n LZ · f L · f l · F S

Fermi Paradox 1950 Enrico Fermi apparent contradiction between high estimates of the probability of existence and lack of evidence

Fermi Paradox lack of evidence probes spacecraft radio transmissions

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.

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.

Fermi Paradox Resolutions…

Fermi Paradox They do not exist… OK, that was easy

Fermi Paradox They do not exist… …and never did …because an inhospitable universe destroys complex intelligent life  ice ages  impacts  volcanism  supernovae

Fermi Paradox They do not exist… …and never did …because it is in the nature of intelligent life to destroy others

Fermi Paradox They do not exist… …and never did …because God created humans alone  but not in a science class…

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

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

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

Fermi Paradox They do exist… …and they are here unobserved hiding their presence we refuse to see or misunderstand the evidence

Fermi Paradox They do exist… …and they are here unobserved hiding their presence we refuse to see or misunderstand the evidence

Fermi Paradox They do exist… …and they are here unobserved hiding their presence we refuse to see or misunderstand the evidence