30EARLY EARTH FIRST LIFE EXPLOSION OF LIFE
TIMELINE OF LIFE ON EARTH
3.5 billion years ago:
first living cells.4.5 billion years
ago: Earth forms
into a red-hot ball
of liquid rock.4.2 billion years
ago: Earth
develops a crust,
and oceans form.416 million years ago:
first land animals appear.500 million years ago:
Earth’s atmosphere
becomes breathable.630 million years
ago: complex
(multicellular)
animals evolve.Our unique world
Among all the planets in the solar system, Earth
is the only one known to support life. It is a
perfect distance from the Sun, has a breathable
atmosphere, and is bathed in life-giving water.
EARTH’S STRUCTURE
Earth may look like a solid ball of rock, but
if you slice through it you can see it is made
up of different layers. At the center is a hot
metallic core. It is surrounded by the stony
mantle, which is like sticky
caramel. On top is a
thin crust that
forms the
continents
and ocean
floor.EARTH’S CORE
The core is a mixture of iron and
nickel mixed with lighter elements.
The pressure at the center is so high
that the inner core remains solid.EARTH
The outer core is made
of molten metal.The inner mantle has
a temperature of about
5,400ºF (3,000ºC.The outer mantle is
fused to the crust.The atmosphere
gives us our weather,
provides us with
oxygen, and protects
us from harmful
solar energy.The crust shapes
the continents
and seafloor.The temperature of the
inner core is about
10,800ºF (6,000ºC).540 million years ago: the
Cambrian explosion—sudden
appearance of many new species
with teeth, feet, intestines,
spines, and hard shells.