A Short Look at Earth History
Formation of Sun
• Formation of Universe: 13.7 billion years
• Formation of Galaxy: 11 billion Years
• Formation of Solar System: 4.6 billion
years
• Sun is probably a third generation star
• Probably takes 10-100 million years for
planets to form
Formation of Planets
• Planets made of same material as Sun, minus
elements that remain mostly in gases
• Inner Rocky Planets: iron and magnesium
silicates
• Outer gas giants and moons: water ice
• If a protoplanet gets big enough, it can hold
everything (Jupiter, Saturn)
• Very far from sun: methane, ammonia, nitrogen
ice
Our Solar System May Not Be Typical
• Over 400 extrasolar planets known
• Barely can detect Jupiter-size planets,
don't yet have technology to see small
planets
• Many have very eccentric orbits
• Some have gas giants very close in to sun
("hot Jupiters")
Formation of Earth
• Planets formed by accretion of smaller objects =
impact
• Very tiny objects hold together by atomic forces
• Objects kilometers across hold together by gravity
• How do objects the size of a refrigerator hold
together?
• As planets get bigger, gravity gets stronger, impacts
get more violent
• Big impacts throw out ejecta, trap heat
• Magma ocean
• Formation of core early in earth history as iron sinks
Formation of Moon
• It's very hard to account for the Moon:
– Very big compared to its parent planet
– Orbits nearly in plane of earth's orbit, not over
equator.
• Co-creation with Earth?
• Fission?
• Capture?
Collision
• A collision takes no finesse at all
• Can explain why moon orbits in earth's orbital
plane
• Can explain why moon's composition differs
from earth
• Models of solar system evolution suggest that
last stage is mega-collisions
• Impact would have melted most of earth and
moon
• Earth would have been incandescent for about
10,000 years.
Unstable Early Earth
• May have been several moon-forming events
• 1000-km impactors can melt crust
• 100-km impactors create temporary atmosphere of
vaporized rock, vaporize oceans
• Life not possible until large impacts cease
• To have life on Earth, we need Jupiter?
– Sweeps up debris and reduces impacts
– Stabilizes orbits of other planets
• To have life on Earth, we need Moon?
– Stabilizes changes in earth's axis tilt
Conditions on Early Earth
• Oldest existing earth materials: 4.1 billion years
old
• Oldest rocks: 3.9 billion years old
• Oxygen-poor atmosphere (present oxygen is
created by life)
• Faint Early Sun: perhaps 30 per cent less bright
• Evidence for liquid water from very early on
• Atmosphere and sun must have evolved in
tandem
• Carbonate-Silicate Cycle: life not essential but
liquid water is.
Life
• What Is It?
• "Life is what dies when you stomp on it“
--Dave Barry
• A self-replicating chemical system
• Feedback (Homeostasis)
How Did Life Originate?
• Certainly not as complex as anything now
alive
• Lots of candidates for first self-replicators
• Role of minerals as catalysts and
templates?
• Simplest organisms are extremophiles
• At least since 3 billion years, probably
much earlier
Tolerance Ranges
Major Events in the History of
Life
• Oxygen levels in atmosphere
– Plants release waste oxygen
– Eventually organisms developed a way to utilize
oxygen (6CO2 + 6H2O = C6H12O6 + 6O2)
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Sex: Who Needs It?
We are a team: Mitochondria
Snowball Earth: what survived and how?
Cambrian "Explosion"
Mass Extinctions
Causes ?
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Climate Change
Disease
Mountain-building
Sea Level Change
Competing Organisms
Over-specialization
Volcanism
Meteor Impact
Plate Tectonics
• Very early earth may have had thin, unstable
crust
• Large areas of continental crust existed by 2.5
billion years
• Plate tectonics since at least two billion years
• Wilson Cycle and Supercontinents
– Supercontinents
– Dispersal
– Reassembly
– Rodinia 800 million years
– Pangaea 250 million years
Humans
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Lumpers vs. Splitters
Australopithecus > 2 m.y.
Homo erectus 1-2 m.y.
Homo ergaster 1-2 m.y.
Homo habilis < 1 m.y.
Homo neanderthalensis 500,000-30,000
yr.
• Homo sapiens<100,000 years