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.