Since life began, more than 3.5 billion years ago,
evolution has produced an (14)---- variety of living
things. Some (15)---- as fossils that tell us about how
they lived. They show that animal life underwent an
explosive burst of evolution about 545 billion years
ago. Some of these animals had strange body forms
that have not been seen since. Furthermore, the first
plants moved from water (16)---- land about 440
million years ago, and by about 325 million years
ago, the first flying insects had taken to air. (17)----,
there have also been setbacks. (18)---- at least five
occasions, mass extinctions have wiped out huge
number of species.