Earth in Sixth Major Extinction

                     As most kids probably know, dinosaurs came to a sudden end about 65 million years ago.  Did you know that there have been four other super-big, or mass, extinctions of life on Earth?
                     Most scientists believe we are in the middle of a sixth mass extinction.  But this time, the extinction isn't being caused by an asteroid or volcanoes.  Its causes can be traced to us.
                     To find out more about this event,  the newspaper talked with Michael Brett-Surman from the Smithsonian National Museum of Natural History.
What is a mass extinction?
                     When a species, or type of life, becomes extinct, it means there are no more members of that species alive.
                      Extinctions are normal.  Usually, there is a steady rate of extinctions during every million years.  Life-forms naturally appear and disappear over time.
                      In a mass extinction, at least one-fourth of all plants and animals on the planet might be wiped out very quickly, much faster than normal.  Huge numbers of species die, and no new species appear in that time.
                      Scientists are seeing this happen now on Earth.
Climate change
                      Each time there has been a mass extinction, it was because something caused the climate to change.  Many types of life could not adapt, or change, quickly enough, and they died.
                      During the current mass extinction, humans will be able to adapt, but our crops and animals might not.  Life as we know it will keep changing.
The ages of Earth
                      Each time there is a mass extinction, a new age begins on Earth.  As older species die out, other species suddenly have no competition.  They begin to fill in the Earth's habitats.
                      Over millions of years, newer types of life develop.  Diversity increases again.  The makeup of the planet changes yet again.
                      Let's explore Earth's earlier mass extinctions.
End of the Ordovician
                     The Ordovician (or-doh-VIH-shun) Period ended about 445 million years ago.  Most life lived in the oceans at this time.  Experts believe more than 50 percent of life was wiped out at the end of this age.
                     One reason for the extinctions might have been a drop in sea level.  Huge glaciers might have formed, locking up much of the planet's water.  This would have of the planet's water.  This would have caused ses levels to drop.  Life in shallower waters might not have been able to adapt quickly enough to survive.