Transcript 12.3 Origin of Life

12.3 Origin of Life
KEY CONCEPT
The origin of life on Earth remains a puzzle.
12.3 Origin of Life
Earth was very different billions of years ago.
There have been many hypotheses of Earth’s origins.
• The most widely accepted hypothesis of Earth’s origins is
the nebula hypothesis.
the solar system formed
by a condensing nebula (a
cloud of gas and dust in
space)
12.3 Origin of Life
Ancient Earth
• Atmosphere contents:
– Ammonia, H2O vapor,
Methane, H2, CO2
• Climate:
– Extreme heat due to
meteor impacts &
volcanic activity
• Eventually Cooling:
Water vapor
condensed (oceans
formed)
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12.3 Origin of Life
Several sets of hypotheses propose how life began on
Earth.
There are two organic molecule hypotheses.
– Miller-Urey experiment
organic compounds could be
made by passing an electrical
current (to simulate lightning),
through a closed system that held
a mixture of gases (to simulate
the atmosphere)
electrodes
“atmosphere”
water
“ocean”
heat source
– meteorite hypothesis
 amino acids may have arrived on Earth
through meteorite or asteroid impacts.
amino acids
12.3 Origin of Life
There are different hypotheses of early cell structure.
– iron-sulfide bubbles hypothesis
– lipid
membrane
biological
moleculeshypothesis
combined
in spheres, or
 Lipid
compartments
of could
liposomes,
chimney like structures
form around a variety
on the ocean floor.
organic molecules,
Theof
compartments
as early
actedacting
as the first
cell cell
membranes.
membranes.
12.3 Origin of Life
RNA as Early Genetic Material
• A hypothesis proposes that RNA was the first genetic material.
– Ribozymes are RNA
molecules that catalyze
their own replication.
– DNA needs enzymes to
replicate itself.
12.3 Origin of Life
Eukaryotic cells may have evolved through
endosymbiosis.
• Endosymbiosis is a relationship in which one organism lives
within the body of another.
• Mitochondria and chloroplasts may have developed through
endosymbiosis.
12.3 Origin of Life
The evolution of sexual reproduction led to increased
diversity.
• Genetic variation is an advantage of sexual reproduction.
• Sexual reproduction may have led to the evolution of
multicellular life.
12.3 Origin of Life
How do we know how old some of these fossils are?
• Dating fossils:
1. Relative Dating:
– compares placement of
fossils in layers of rock
– Estimates the time
period that the
organism lived
12.3 Origin of Life
2.Radiometric dating:
– uses decay of unstable isotopes.
– Isotopes are atoms of an element that differ in their
number of neutrons.
neutrons
– Isotopes are
unstable in their nuclei,protrons
therefore they decay
– The rate of decay of any isotope is known as a half
life (number of years it takes for half of the isotopes
to decay).
– 14C has a half life of approximately 5700 years, and 14N
is a decay product.
– Scientists look at the ratio of 14C and 14N to tell us
how old something is.
12.3 Origin of Life