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)

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

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

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

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

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


The Tobacco Mosaic Virus contains a single strand of RNA, about 0.1 mm long


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



forms vital portions of ribosomes

Life on Earth RNA



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 - 90,000 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


Those as well as early multi-celled creatures had no hard parts to leave fossils

Earliest, microscopic fossils date back ~ 4 billion years


~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


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


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?



Miller experiment etc. indicate: probably yes

Will life always evolve toward intelligence? How common are suitable conditions for




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


Life on Earth


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


liquid water for chemical reactions and as transport

medium


atmosphere avoid rapid vaporization of water gasses needed for organic compounds


moderate temperatures keep water liquid avoid disintegration of organic compounds activate complex chemical reactions


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

Nc = N* · fp · nLZ · fL · fl · FS

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


…and never did …because an inhospitable universe

destroys complex intelligent life ice ages impacts volcanism supernovae


…and never did …because it is in the nature of intelligent

life to destroy others


…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


…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


…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


…and they are here unobserved hiding their presence we refuse to see or misunderstand the

evidence


…and they are here unobserved

hiding their presence we refuse to see or

misunderstand the evidence

Life Out There…

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