section 12.3 key concept the origin of life on earth remains a … · 2018-09-09 · the origin of...
TRANSCRIPT
Section 12.3
KEY CONCEPT
The origin of life on Earth remains a puzzle.
Earth was very different billions of years ago.
Most scientists agree on two key points:
1. Earth is billions of years old.
2. The conditions of the early planet and its atmosphere were
very different from those of today.
Earth was very different billions of years ago.
The solar system was formed by a condensing nebula.
A nebula is a cloud of gas and dust in space.
Earth was very different billions of years ago.
The sun is suggested to be 4.6 billion years old.
The planets were formed over millions of years.
Early Earth was hot and violent for
the first 700 million years.
Early Earth contained compounds
such as ammonia (NH3), water vapor,
Methane (CH4) and carbon dioxide
(CO2), but oxygen gas did not exist
until about 2 billion years ago.
Several sets of hypotheses propose how life began on
Earth.
There are two organic molecule hypotheses.
1. Miller-Urey experiment
electrodes
heat source amino acids
water
“atmosphere”
“ocean”
An experiment designed by
Stanley Miller and Harold Urey
in 1953 showed that organic
compounds could be made by
heating and passing an
electrical current, to simulate
lightning, through a mixture of
gases.
They produced many organic
compounds, such as amino
acids and nucleotides.
Several sets of hypotheses propose how life began on
Earth.
There are two organic molecule hypotheses.
2. Meteorite hypothesis
After analyzing a meteorite that
fell in Australia in 1969, it was
revealed that organic molecules
can be found in space.
The meteorite contained more
than 90 amino acids.
This evidence suggests that
organic molecules may have
arrived on Earth through
meteorite or asteroid impacts.
Section 12.4
KEY CONCEPT
Single-celled organisms existed 3.8 billion years ago.
Microbes have changed the physical and chemical
composition of Earth.
• The oldest known fossils are a group of marine
cyanobacteria.
– prokaryotic cells
– carry out photosynthesis
• Changed Earth by:
– added oxygen to
atmosphere
– deposited minerals
• Fossil stromatolites provide evidence of early colonies of
life.
– stromatolites are domed, rocky structures made of layers
of cyanobacteria and sediment.
Microbes have changed the physical and chemical
composition of Earth.
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.
1 2 3
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.
Section 12.5
KEY CONCEPT
Multicellular life evolved in distinct phases.
Life moved onto land during the Paleozoic Era.
• Multicellular organisms first
appeared during the
Paleozoic era.
• The era began 544 million
years ago and ended 248
million years ago.
• The Cambrian explosion led
to a huge diversity of animal
species.
• Life moved onto land in the middle of the Paleozoic era.
Life moved onto land during the Paleozoic Era.
Reptiles radiated during the Mesozoic era.
• The Mesozoic era is known as the Age of Reptiles.
• It began 248 million years ago and ended 65 million years
ago.
• Dinosaurs, birds, flowering plants, and first mammals
appeared.
Mammals radiated during the Cenozoic era.
• The Cenozoic era began 65 million years ago and continues today.
• Placental mammals and monotremes evolved and diversified.
• Anatomically modern humans appeared late in the era.