Birth of the Solar System

More than 10 billion years since the universe exploded into existence, the solar system--including the [blank_start]Sun[blank_end], the eight planets, their moons, and various [blank_start]asteroids[blank_end]-- forms from interstellar [blank_start]gas[blank_end] and [blank_start]dust[blank_end].

ESCAPING GASES BUILD EARTH'S ATMOSPHERE [blank_start]Gases[blank_end] released from [blank_start]magma[blank_end] inside earth collected to form an atmosphere probably composed of [blank_start]nitrogen[blank_end], [blank_start]carbon dioxide[blank_end], [blank_start]water vapor,[blank_end] [blank_start]methane[blank_end], and ammonia, with little or no free [blank_start]oxygen[blank_end]

EARTH'S CORE FORMS [blank_start]Iron[blank_end] sinks deep inside Earth and form its [blank_start]core[blank_end]. Lighter elements like [blank_start]silica[blank_end], [blank_start]magnesium[blank_end], and [blank_start]aluminum[blank_end] rise to the surface, and later harden to form the [blank_start]outer crust[blank_end]

GREAT OCEANS FORM Water [blank_start]condenses[blank_end] in the cooling atmosphere, and heavy [blank_start]rains[blank_end] pour down on the planet. After several [blank_start]hundred million[blank_end] years of falling rain, great [blank_start]oceans[blank_end] form.

SMALL CONTINENTS FORM As several chunks of land blocks collide, the earliest [blank_start]continents[blank_end] form.

CONTINENTS BEGIN SHIFTING Driven by [blank_start]heat[blank_end] deep within the Earth, the continents' underlying [blank_start]plates[blank_end] begin to [blank_start]shift[blank_end].

BANDED IRON FORMATIONS [blank_start]Banded iron[blank_end] formations are sedimentary rocks deposited on the ocean floor over hundred of [blank_start]millions[blank_end] of years. Their alternating rust-gray colored bands are evidence that [blank_start]oxygen[blank_end] is being produced in the oceans by [blank_start]photosynthetic[blank_end] bacteria. HINT...to get the last blank click on the blue circle directly below called "bacteria" on land and read it.

1,800 mya: Oldest eukaryote fossils Though eukaryotes are thought to have evolved much earlier, the oldest definitive fossils date to [blank_start]1,800 mya[blank_end]. Acritarchs are probably single-celled [blank_start]algae[blank_end] in a resting stage of their life cycle that have grown tough outer coats. They are spherical in shape and float at or near the surface of the [blank_start]water[blank_end]. 1,200 mya: True algae The oldest [blank_start]multicellular[blank_end] algae fossil dates to 1,200 mya. At this time, biological diversity increases greatly, with algae, [blank_start]fungi[blank_end], [blank_start]animals[blank_end], and [blank_start]plants[blank_end] diverging from their common eukaryote ancestors. 900 mya: Soft-bodied animals The oldest fossil evidence of multicellular animals, or metazoans, are burrows that suggest smooth, [blank_start]wormlike[blank_end] creatures live [blank_start]900 mya[blank_end] or more. Found in rocks in China, Canada, India, and elsewhere, the imprints of these [blank_start]soft-bodied[blank_end] creatures reveal little else besides their basic shapes.

RODINIA SUPERCONTINENT FORMS The late-Precambrian supercontinent [blank_start]Rodinia[blank_end] forms when smaller continents [blank_start]converge[blank_end].

Major glaciation period begins(Snowball Earth) The first of [blank_start]four[blank_end] global [blank_start]ice[blank_end] ages sets in. While cold-loving microbes evolve and thrive in the frigid waters and snow, repeated [blank_start]glaciation[blank_end] factors heavily into future [blank_start]extinctions[blank_end] of other organisms.

BREAKUP OF RODINIA SUPERCONTINENT As [blank_start]tectonic plates[blank_end] shift, the Rodinia supercontinent begins to [blank_start]break[blank_end] apart into the smaller [blank_start]continents[blank_end].

OZONE LAYERS IN PLACE With the exception of bacteria, life on Earth has been restricted to the [blank_start]oceans,[blank_end] where the risk of exposure to [blank_start]ultraviolet radiation[blank_end] is greatly reduced. But now, atmospheric [blank_start]oxygen[blank_end] levels are high enough to form a protective [blank_start]ozone[blank_end] layer over the planet. This shield is a critical factor in the emergence and survival of [blank_start]complex[blank_end] life on [blank_start]land[blank_end]