in Australia.
Besideschangesinlifeandtheatmosphere, otherchangeshavealsohappenedsincetheEarth
was first formed. Early volcanic eruptions on Earth released large amounts of water vapor
into the atmosphere. The water vapor slowlycondensedand returned to Earth’s surface
in rainfall. This formed the oceans. Water began to cycle on Earth, and events like rainfall
and storms next began to change the Earth’s surface through weathering and erosion. The
Earth’s Fresh Waterchapter gives more detail on how water cycles on Earth.
The continents were in very different locations than they are now. Scientists do not know
how Earth’s land looked exactly after the planet’s first formation. They do know that North
America and Greenland formed one giant landmass called Laurentia about 1.8 billion years
ago. By about 1 billion years ago, Antarctica may have been close to the equator, even
though it now sits at Earth’s South Pole. Today, Earth’s continents continue to slowly shift
around the globe.
Lesson Summary
- The Earth formed more than 4 billion years ago along with the other planets in our
solar system. - The early Earth had no ozone layer and was probably very hot. The early Earth also
had no free oxygen. - Without an oxygen atmosphere very few things could live on the early Earth. Anaer-
obic bacteria were probably the first living things on Earth. - The early Earth had no oceans and was frequently hit with meteorites and asteroids.
There were also frequent volcanic eruptions. Volcanic eruptions released water vapor
that eventually cooled to form the oceans. - Theatmosphereslowlybecamemoreoxygen-richassolarradiationsplitwatermolecules
and cyanobacteria began the process of photosynthesis. Eventually the atmosphere be-
came like it is today and rich in oxygen. - The first complex organisms on Earth first developed about 2 billion years ago.
Review Questions
- Describe how the different layers of the Earth vary by density. When did the materials
that make the Earth separate out by density? - Explain two reasons why having an oxygen-rich atmosphere is important for life on
Earth. - Scientists believe that Earth’s ozone layer is shrinking because of human activities and
air pollution. What affect might this have on Earth’s life forms? - Describe the role of cyanobacteria in changing Earth’s early atmosphere.
- List three ways the Earth was different today from when it was first formed.