Answer:
An ice age caused Earth’s oceans to cool, which killed many of the unicellular organisms. When the ice age ended and Earth warmed, the continents broke apart. This event allowed the shallow seas to cover the continents. The warming of Earth, accompanied by the seas covering the land, created the proper conditions for multicellular organisms to form.
Explanation:
This is the exact answer on Edmentum so I advise that you do not copy and paste this answer.
