Astrobiology Flashcards

6.2 The Origin of Life

• The central question is: How did life begin?
• Several hypotheses exist:
  • Miller-Urey Experiment: Demonstrated that organic building blocks can form easily and naturally, although there is still uncertainty about early Earth conditions.
  • Life could have been brought from space.
  • Deep-sea vents could have been the origin.
  • Clay and other minerals may have acted as catalysts to build RNA strands.
  • Lipid pre-cells may have formed.
  • RNA self-replication could have occurred, allowing natural selection and improving replication until life emerges.

Basic Units of Life

• Living cells are the basic units of life on Earth.
• A cell membrane separates organic material from inorganic material.

Defining Life

Molecular Components of Cells

• Major molecular components include:
  • Carbohydrates
  • Lipids
  • Proteins: Enzymes are catalysts that speed up biochemical reactions.
  • Nucleic acids: DNA and RNA
  • Amino acids: Build proteins
• Handedness (or Chirality):
  • Life on Earth uses left-handed proteins and mostly right-handed carbohydrates.

Major Groupings of Life on Earth

• Eukaryotes: Cells with nuclei.
• Prokaryotes: Cells without a nucleus.
• Super-kingdoms (or Domains): Classification based on chemical properties (biochemistry):
  • Bacteria
  • Archaea
  • Eukarya
• Tree of life represents common ancestry.

Could Life Have Migrated to Earth?

• Migration from Venus or Mars is a possibility.
• Impact blasts from one world to another could have transferred life.
• Meteorites from Mars could carry life.
• There is a chance of survival while traveling through space.
• Migration from other star systems is highly unlikely.

3. The Universal Context of Life

3.1 The Universe of Life

Major Lessons from Modern Astronomy

• The universe is old and vast.
• Elements of life are widespread; chemicals of life are found everywhere, such as in nebula clouds.
• The same physical laws operate on Earth and in the rest of the universe.

3.2 The Structure, Scale, and History of the Universe

Structure of the Universe

• Cosmic Address structure:
  • Planets, moons, asteroids, comets.
  • Milky Way Galaxy: Contains over 100 billion stars.
  • Galaxies: Contains hundreds of millions to trillions of stars and nebulae.
  • Local Group: Milky Way is part of a group of more than 70 galaxies.
  • Superclusters: Regions where galaxies and galaxy clusters are grouped; our local supercluster is Laniakea.

Scale of the Solar System

• Planets, moons, asteroids, comets, and Kuiper Belt Objects are distributed in the solar system.
• Astronomical Unit (AU): Average distance between Earth and Sun.

Distances to Stars

• Measured in Lightyears: The distance light travels in one year, Approximately 1 \text{ lightyear} = 10^{13} \text{km}.
• Alpha Centauri: Closest star system to the Sun at 4.4 lightyears.
• The speed of light is fixed at 300,000,000 \text{ m/s} or 300,000 \text{ km/s}.

Scale of the Galaxy

• Over 100 billion stars; it would take about 3000 years to count them.
• Milky Way galaxy diameter: Approximately 100,000 \text{ lightyears}.
• Center of our Milky Way: Approximately 27,000 \text{ lightyears} from Earth/Sun.
• Orion Nebula: Located in Orion Constellation, approximately 1350 \text{ lightyears} from Earth/Sun.
• The Milky Way galaxy is a barred spiral galaxy with a flattened disk and central bulge; Dark Matter is believed to surround it in a massive halo.

Content of the Universe

*Ordinary (Baryonic) Matter (Makes up ~5% of the Universe)

• Made of three quarks. Quark Flavors: (Up, Down, Charm, Strange, Top, and Bottom)
  *Up, Charm, and Top quarks have +2/3 charge
  *Down, Strange, and Bottom quarks have -1/3 charge
  *Protons made of uud (three quarks): total charge is +1
  *Neutron made of udd (three quarks): total charge is 0
• Ordinary (Baryonic) Matter (Makes up ~5% of the Universe)
  • Baryonic Matter - made of three quarks.
  • Up, Charm, and Top quarks have +2/3 charge
  • Down, Strange, and Bottom quarks have -1/3 charge
  • Protons made of uud (three quarks): total charge is +1
  • Neutron made of udd (three quarks): total charge is 0
• Ordinary Matter interacts with light (electromagnetic radiation)
• Dark Matter (Makes up ~25% of the Universe)
  • does not interact with light (electromagnetic radiation).
  • observed with gravitational effects (like gravitational lensing & incorrect angular velocities of spinning galaxies).
  • believed to be key role in formation of galaxies
• Dark Energy (Makes up ~70% of the Universe)
  • Light / type of pure energy, but not electromagnetic energy.
  • believed to be cause of expansion of the universe (analogous to how radiation pressure / light pressure from nuclear fusion balancing the size of a star)
  • complete mystery to scientist, so far.

History of the Universe

• Big Bang and Expanding Universe
  • Expanding universe, not an explosion
  • Early universe much smaller and hotter
  • time of the Quark-Gluon Plasma (made up of u,d,s,c,t,b quarks and gluons)
  • As universe expanded, it cooled & protons and neutrons formed from quark-gluon plasma.
  • Protons slowed down (as temperature cooled) and electrons bound to protons to form Hydrogen atoms
  • Once atoms formed - photons (particles) of light could be produced - light can still be seen today as the Cosmic Microwave Background.
  • Cosmic Microwave Background (CMB) - remnants of the Big Bang - “glow” of radiation that can be seen in every direction of the universe - even in darkest/coldest regions of universe.
    *Early Universe - lacked heavier elements to make like Milky Way Galaxy - likely 12 -13 billion years old
• Stellar Lives and Galactic Recycling
  • Gravity drives collapse of clouds of gas and dust to form stars and planets (nebular hypothesis)
  • Life cycle of stars - cloud of gas compresses due to gravity and center more dense and hot enough to generate energy/light by nuclear fusion
  • Nuclear fusion - lighter elements (starts with hydrogen) hot/fast enough to fuse/combine together to form heavier nuclei/elements.
    *As giant cloud of gas heats up and spins - it spins faster and faster - flattening into disk of gas and dust.
    *Planets form on outer edges of the spinning nebular disk & star in center is formed.
    *Star formed - light/energy released due to nuclear fusion in star’s core.
    *At least 4 hydrogen required to form helium in core of a star.
    *Matter is converted into Energy - Recalls Einstein’s equation: E = mc^2 (where m is mass, E is energy, and c is the speed of light).
    *Specific predictions - relative abundance of heavier elements in massive stars at different ages - Carbon and oxygen atoms should be more abundant than Nitrogen atoms, and Neon atoms should be more abundant than Fluorine atoms - Theory is consistent with observations.
• We Are Star Stuff
  • Stars manufacture heavier elements toward end of their life cycle
  • Above a critical mass, massive stars explode through supernova releasing heavier elements
  • Recycling process to make new stars and continue the cycle
  • Relative abundances of each element can be studied to determine age of stars
• Implications For Life in the Universe
  *Expect many star systems to be made of similar chemical composition as our solar system - therefore conclude many star systems have the chemical composition to make and support life as we know it.

Scale of Time

• Universe 14 billion years old
• Condense into Cosmic Calendar to fit in one year
• Midnight January 1st - start of Big Bang
• Present day would be 11:59 pm December 31st.
• First humans would have appeared on December 31st at around 9pm.
  *Each month would represent a little more that 1 billion years.
  *Earth would form around September 3rd.
  Life would have had many chances to potentially star life, before Earth even formed.

How big is the universe?

*The universe is thought to be made up of sum total of all matter and energy.

• Could universe be infinite?
  *Limited with what we can observe, based on the age of your universe and limit light can travel in that time frame.

observable Universe

• Looking back in time when looking at the more distant objects from earth
• Andromeda galaxy is 2.5 million lightyears way - means light we are receiving from Andromeda left 2.5 million years ago or we are seeing what it looked like 2.5 million years ago.

Worlds Beyond Imagination

*10^22 stars in the observable universe - equal to number of grains of sand in all the beaches of the world.

• Determined from Hubble Space Telescope - Hubble Extreme Deep Field

Fine-Tuned Universe

• Fine-tuned for life - anthropic principle - conditions were just right for our universe to form
  *Expansion of universe not too fast (to not allow for galaxies to form) or not too slow then gravity would pull all matter back into itself (collapsing the universe).
  *Ratio and strengths of the four fundamental forces balance just right to coexist with the laws of quantum physics.

3.3 A Universe of Matter and Energy

Building Blocks of Matter

• Atoms - Protons, Neutrons, Electrons
• Electron energy levels
• Atoms and molecules - absorbing and emitting light
• Chemical elements - different number of protons
• Isotopes, Ions, Molecules
• Stars of Matter - Solid, Liquid, Gas

Energy

• Making matter move
• Different forms of energy
  • Kinetic energy - energy of motion
  • Potential energy - stored energy
  • Radiative energy - energy of light
• Conservation of energy

Light

*Electromagnetic wave
*Particle vs. wave nature of light photons - pieces of light
*Wavelength, frequency, energy of a wave
*Speed of a wave and speed of light (c = \lambda f)
Frequency and wavelength inversely related
Visible spectrum - ROYGBIV
Radio waves, microwaves, infrared (IR), visible light, ultraviolet, X-rays, gamma rays
Spectroscopy - finger-prints of the atoms and molecules
Information light carries from object it came from
Thermal energy (blackbody radiation) - peak wavelength tells astronomers temperature of the star (or blackbody)
Intensity of light - brightness / number of photons
Doppler effect - red shift object moving away for observer / blue shift object moving toward observer

1. The Possibility of Life Beyond Earth

1.1 What are we searching for?

• Difficulty clearly defining life
• Still no evidence of life beyond Earth
  *Search for life similar to life on Earth
  *Single and multicellular life
• Is it reasonable to imagine life beyond Earth?
• Examining possibility of life beyond Earth using modern science

1.2 The Scientific Context of the Search

How does astronomy help us understand the possibilities for extraterrestrial life?

• Same physical laws operate on Earth as in the universe
  Universality of Physics, Chemistry, and Biology

How does planetary science help us understand the possibilities for extraterrestrial life?

Expect planets common around other stars
Extrasolar planets
Habitable worlds

How does biology help us understand the possibilities for extraterrestrial life?

Three lines of evidence suggesting life might be common on habitable worlds:

1. Life quickly arising quickly on Earth
2. Meteorites and interstellar clouds observed with organic molecules common through galaxy
3. Some life on Earth surviving extreme conditions

1.3 Places to Search

Where should we search for life in the universe?
Where else can we search in our solar system
Direct searches on other planets and moons:

• Mars
  *Jupiter’s moons:
  *Ganymede, Callisto, and Europa
  *Saturn’s moons:
  *Titan and Enceladus
  Telescopic searches for life on other stars
  Could aliens be searching for us?
  If life common, then intelligent life could be as well
  Possibility of other civilizations conducting their own searches
  SETI (Search for Extraterrestrial Intelligence)

1.4 The New Science of Astrobiology

How do we study the possibility of life beyond Earth?

Astrobiology Three major areas:

• Study conditions conducive for origin and ongoing existence of life
• Finding habitable conditions within our own solar system and other stars
• Find actual occurrences of life beyond Earth
  *Develop connections between living organisms and places they live

2.1 The Ancient Debate About Life Beyond Earth

How did attempts to understand the sky start us on the road to science?

The Sky Observed By Many Civilizations
*Ancient Chinese
*Babylonians (near Iraq)
*Mayans (Central America)
Early Greek Science
*Thales (624 - 546 B.C.)
*Anaximander (610 - 547 B.C.)
*Pythagoras (560 - 480 B.C.) - mathematical perfection
*Plato (428 - 348 B.C.)
*Aristotle (384 - 322 B.C.) - tutor to Alexander the Great (Egypt - Library of Alexandria)
*Eratosthenes (around 240 B.C.) - Circumference of the Earth
Geometry Models - conceptual representations to explain and predict observed phenomena
Led to Development of Modern Science

The Geocentric Model

Celestial Sphere - Anaximander
Geocentric Model (supporting arguments)
Ptolemy (A.D. 100 - 170)
Epicycles - circle within circles
The Mystery of Planetary Motion
Planet - “wanderer” - generally move eastern relative to fixed stars
Visible with naked eye: Mercury, Venus, Mars, Jupiter, Saturday
Seven Days in a Week

Explanation for Apparent Retrograde Motion

Ptolemaic Model - geocentric with epicycles (1500 years) - Almagest
Heavenly perfection
Still no evidence of life beyond Earth

An Alternate Model

Aristarchus (310 - 230 B.C.)
Sun-Centered Model w/ Sun Much Larger Than Earth
Arguments Against - inconsistent with observation of stellar positions
Stars lie a different distances rather than all on the same spheres
Stellar Parallax
Stars too far away to be detectable w/ naked eye
The Roots of Modern Science
Greeks reject idea of Sun-centered Model (Heliocentric Model)
Modern Science: Reason vs. aesthetics, Models Consistent with Observations
Nicholas Copernicus (1473 - 1543)
Re-visits Aristarchus’ Model
Led to development of Modern Science

Why did the Greeks argue about the possibility of life beyond Earth?

Atomists (Democritus - 460 - 370 B.C.)
World form by infinite number of indivisible atoms - implies existence of other worlds
More associated with Atheism
Aristotelians
all earth must fall to center of universe
geocentric universe
Earth is unique
Heavens fundamentally different from Earth
No Other Worlds
Four Elements:
Earth, Water, Air, Fire
Fifth Element: Aether (or Ether) or Quintessence
Thomas Aquinas (1225 - 1274) - Connection to Christian Theology

2.2 The Copernican Revolution

Europe in Dark Ages
Baghdad, Iraq - Intellectual Center of World (Middle Ages)
Byzantine Empire (Eastern Roman Empire) - Islam sought knowledge
Better understand wisdom of Allah
Greek works translated into Islam
Mathematics & astronomy
Algebra developed
Capital Constantinople (Modern Day Istanbul, Turkey)
Falls (1453)
Scholars Leave East & go West (Europe)
Start of European Renaissance

How did the Copernican revolution further the develop the science?

Copernican Revolution
Nicholas Copernicus (1473 - 1543)
Book published after his death: “Concerning the Revolutions of the Heavenly Spheres”
Heliocentric Model of Universe
Debated for 50 years / Not any better at prediction than Ptolemaic Model
Ideas and Debates - formed foundations of modern science
Moved Earth away from center of universe & replaced it with the Sun
Kept concept of perfectly circular orbits
People did not want to replace 2000 year old (Ptolemaic Model) - that produced similar inaccuracies
Old Data not revised for 200 years

Tycho - A New Standard in Observational Data

Lack of Data - could not validate Ptolemaic Model vs. Copernican Model
Telescope Not Yet Invented
Tycho Brahe (1546 - 1601) - From Dutch Aristocratic Family
Built largest naked-eye observatory - giant protractors
30 years of observations (w/in 1 arc minute - less than thickness of thin part of a fingernail)
Most accurate data at the time
Kepler - A Successful Model of Planetary Motion
Johannes Kepler (1571 - 1630) - German Astronomer & Mathematician
Used Tycho Brahe’s data to validate Ptolemaic Model vs. Copernican Model
Deeply Religious
Understanding the Geometry of the Heavens
Initially used circular orbits to model Tycho’ data
Mars data did not match Kepler’s predictions by 8 arc minutes (with circular orbits)
Did not ignore the discrepancy or blame the data
Abandoned perfectly circular orbits
Used special shape - called an ellipse (type of oval)

Kepler’s Laws of Planetary Motion

Kepler’s 1st Law of Planetary Motion
Each planet orbits the sun in an ellipse with sun at one foci
Terms to Know:
Perihelion, aphelion, semi-major axis, center-to-focus distance, eccentricity
Eccentricity of 1 (straight line) & Eccentricity of 0 (perfect circle)
Calculation of Eccentricity: e = \frac{\text{center-to-focus distance}}{\text{semi-major axis}}
Kepler’s 2nd Law of Planetary Motion
A planet moves faster in part of its orbit nearer the Sun and slower when farther from the Sun
Sweeping our equal areas (A) in equal times (t): A/t = \text{constant}
Kepler’s 3rd Law of Planetary Motion
Planets farther from the Sun orbit at slower average speeds than planets closer to the sun
Obeying the mathematical relationship:
p^2 = a^3
p is the orbital period of the planet (time to complete one orbit)
a is the semi-major axis (or average distance for the sun)
Kepler’s Laws of Planetary Motion - far more accurate than Ptolemaic Model.
Describes the motion but does not explain why the planets behave that way.

Galileo - Answering the Remaining Objections

Still strong objections to the heliocentric model regardless of Kepler’s Success
Three Basic Objections:

1. Earth cannot move because objects would be left behind as Earth moved
   Like feeling a continuous wind on Earth
2. Non-circular orbits contradicted idea of heavens being perfect
   Realm of heavens:
   Sun, Moon, Planets, and Stars
   Must be perfect and unchanging
3. No detection of Stellar Parallax
   Should be observed if Earth revolved around the Sun
   And the fixed stars were not that far away

Galileo Galilei (1564 - 1642) - Italian Mathematical and Astronomer Answers to Three Objections:

1. Overturned Aristotelian view of physics (heavier objects fall to the ground faster than lighter objects)
   Create experiments with rolling balls down an ramp (or incline)
   Argued moving objects would remain in motion unless a force acted to stop it
   Also known as Newton’s 1st Law of Motion (or the Law of Inertia)
   Therefore, all objects on Earth share Earth’s motion
   And stays with the Earth rather than falling behind (or feeling a continuous wind)
2. Heavenly Perfection
   Tycho Brahe
   Observes new star (supernova explosion)
   Proved comets lie beyond the moon (not earthly objects, but belonged as heavenly objects)
   Heavens could undergo changes
   Galileo
   Built telescope (late 1609) - more powerful than naked-eye
   Observed sunspots on sun (imperfections)
   Moon had mountains & valleys
   Noticed shadows near moon’s Terminator (line separating illuminated and dark sides of moon)
   Kepler
   Elliptical orbits and not perfect circles
3. Absence of Observable Stellar Parallax
   Tycho’s data (naked eye observations) - precision not enough to observe
   Needed to prove distances were much, much farther away
   Galileo’s telescope
   Resolved individual stars of the Milky Way
   Number of stars in Milky Way were distant stars
   Farther than Tycho anticipated
   Fours moons (companion stars) orbiting Jupiter, and not the Earth (Earth Not Center of Everything)
   Phases of Venus:
   Galileo observes phases just like the moon:
   New Moons, Crescents, Quarter Moons, Gibbous, and Full Moons
   Ptolemaic Model only allows for Crescents and New Moons
   Heliocentric Model allows for all phases as observed by Galileo
   Conclusive evidence supporting the heliocentric model
   Newton - The Revolution Concludes

Kepler’s Laws Described Motion of the Planets

Elliptic orbits w/ varying speeds
Accepted - But did not know why
Issac Newton (1642 - 1727) - English Physicist
Invented Calculus
Published Book (1687): “Principia”
Precise mathematical description of motion
Explained and discovered many fundamental physics principles
Newton’s Three Laws of Motion:

• 1st Law - Law of Inertia (Galileo)
• 2nd Law - Net Unbalanced Force on an Object Causes Acceleration of that Object
• 3rd Law - Action-Reaction Law - Forces Exist in Pairs

Kepler’s Laws describe only motion of planets around the sun - Newton’s Laws explain motion of all objects

Newton’s Law of Universal Gravitation

explains why Kepler’s Laws Describing motion of planets works.
Created new model for inner workings of the universe
Motion governed by clear laws & force of gravity
Explained motion of objects on Earth and the Heavens
Combined Earth and Heavens as a Universe (One-verse)
Destroyed Geocentric Model of the Universe

Looking Back at Revolutionary Science:
Copernicus - De Revolutionibus (1543)
Newton - Principia (1687)
Revolution ~150 years changed human history
Radical Change in Human Perspective:
Earth shifted its central role to just another planet
Changed perspective how to acquire knowledge
No longer tolerating inaccuracies in predictions
Models of nature reproducing observations
Importance to ask why nature works (not just how)
Past Thinking:
Focussed on cultural aesthetics
Acceptance even w/o evidence
After Newton:
Guessing no longer good enough
Hard evidence / Consistent w/ observations
Rigorous mathematics
Convincing colleagues through clear logic and debate
Closer to understanding nature of universe

How did the Copernican revolution alter the ancient debate on extraterrestrial life?

Recall Ancient Greek Debate:
Atomists (many worlds) vs. Aristotelians (one unique world / central everything falls toward center)
Copernican Revolution
Aristotle wrong / Heliocentric view of universe
Heavens are just other worlds, not just dots in the sky
Does not necessarily mean Atomists are right
Galileo - speculated moon covered with land and water like Earth
Kepler
Suggested moon had an atmosphere & inhabited by intelligent beings
Science fiction story - called Somnium (The Dream”)
Giordano Bruno (Dominican Friar & Philosopher)
Convinced of Extraterrestrial Life
Battles with Authorities
Burned at the Stake
William Herschel (1738 - 1822) w/ sister Caroline
Discovered Uranus
Assumed all planets were inhabited
Percival Lowell (1855 - 1916)
Discovered Pluto (PL may be his initials)
Believed saw canals on Mars
Popularized idea Martians might want to invade Earth
Possible to argue endlessly, as long as no evidence to support your arguments

2.4 The Fact and Theory of Gravity

Important to life in universe

If no gravity - life would float away
Planets could not have formed
Life couldn’t form

How does the fact of gravity differ from the theory of gravity?

Fact of Gravity:
Objects do fall to the ground
Planets really do orbit the Sun
Theory of Gravity:
Used to explain why gravity acts as it does
Many Theories Improved Over Time:
Aristotle’s Theory of Gravity (claimed heavier objects fell to ground faster that lighter objects)
Galileo’s Theory of Gravity (all objects fall at same rate - if no air resistance)

Newton’s Law of Universal Gravitation

Cause of apple falling from tree - same cause as moon orbiting earth (recall orbital cannon thought experiment)
Earth and Heavens governed by a single principle - a universe
Newton’s Law of Universal Gravitation

1. Every mass attracts every other mass through force of gravity
2. Strength of gravitational force is directly proportional to product of masses
   FG proportional to m1 and m2
3. Strength of gravitational force between two objects increases or decreases as the square of the distances between them
   FG proportional to 1/d^2 (Inverse Square Law)
   All three statements combined:
   FG = \frac{G m1 m2}{d^2}
   G = 6.67 \times 10^{-11} \frac{Nm^2}{kg^2}
   Explained Galileo’s observations of falling objects
   Explained Kepler’s Law of Planetary Motion
   Edmond Halley (British Astronomer)
   Predicted when a specific comet would return to earth
   Returns every 76 years (Halley’s comet) using Newton’s Law
   Urbain Leverrier (French Astronomer)
   1846 - strange behavior of Uranus’ orbit
   Hypothesized an 8th planet - Neptune
   Johann Galle (German - Berlin Observatory)
   Observes Neptune w/in 1 degree of where Leverrier predicted (using Newton’s Law)
   Applies to Not just Sun and Planets, also applies to:

Orbits of extrasolar planets around their respective stars
Stars around the Milky Way Galaxy
Galaxies orbiting each other
Moons orbiting planets
Artificial satellites orbiting the Earth, etc,…

Problems with Newton’s Law of Universal Gravitation

Discrepancy with observations of Mercury’s orbit (only observed with Mercury)
Leverrier hypothesized another planet closer to the sun - he called Vulcan (never found)
Newton never could explain spooky "action at a distance”
Distant masses never in contact w/ each other - how is a force then applied?

Einstein’s General Theory of Relativity

Einstein’s Solution:
Action at a Distance - assumed all objects reside in something called spacetime
4-Dimensional Spacetime (x, y, z, ct) - “Fabric”
where x, y, and z are space dimensionsAnd t is a time dimension, but ct can also become a spatial dimension.
Recall c is the speed of light (~3.00 \times 10^8 \text{ m/s}) & nothing can travel faster than c.
2-D analogy:
Bowling ball on a mattress - bowling ball causes indentation on the mattress
Called a Gravitational Well - an object moving near the indentation will be deflected or trapped inside (like a planet orbiting the sun)
Action at a Distance - gravity arises from the way mass affects the structure of the spacetime “fabric”
Gravity is “curvature of spacetime”
Newton’s Law of Universal Gravitation still works in general
But not with extreme conditions - when gravity is very strong (like being too close to the sun).
Otherwise, can still just use Newton’s Laws

In the future, a better theory than Einstein’s Theory of General Relativity may be discovered - people are currently working to do this
Need to find out what happens with
*high density blackholes
*infinitely small spaces (quantum mechanics)

Organic Building Blocks

• Likely abundant due to at least three other sources of organic molecules.

RNA World

• RNA is simpler than DNA.
• RNA can hold hereditary information (e.g., many viruses).
• RNA can also catalyze chemical reactions (“ribozymes”) and could self-replicate.
• Experiments show RNA strands can form naturally through clay catalysis.
• Lipid “pre-cells” also form naturally under similar conditions.
• Suggests that simple self-replicated strands could evolve through a chemical analog to natural selection within naturally forming "pre-cells".
  *Providing a plausible scenario for origin of life:
  Organic molecules were abundant in at least some environments on the early Earth.
  Clay minerals catalyzed the production of RNA and the membranes that trapped RNA and other organic molecules in pre-cells.
  Pre-cells acted as microscopic factories in which RNA evolved through molecular natural selection; some of this RNA became capable of self-replication.
  RNA world
  RNA and proteins both evolved through molecular natural selection, eventually leading to the true living cells (capable of full self-replication) of "RNA world."
  DNA-based life
  Biological evolution in RNA world eventually led to the first DNA molecules. The advantages of DNA allowed it to replace RNA as the hereditary molecule of life on Earth.

Another Possibility: Migration from Elsewhere (panspermia)

• Some life can survive some periods of time in space.
• Impacts blast material from one world to others—e.g., Martian meteorites.
• Venus, Earth, and Mars have been “sneezing” on each other for billions of years.
  *If we find life on Mars, does it show evidence of common origin with Earth life???