The father

During the Cambrian period, a father took his family to visit planet Earth. Earth was in its formative stages. Its oceans supported primitive plant and animal life. While the father enjoyed such planets, its lack of life on dry land disappointed him. He and his wife were preparing to leave when his children informed him that the environmental unit of their aquarium had died, and that they'd forgotten the spare, he agreed that his daughter let the animals in the aquarium into the ocean, a violation of their civilization's rules.

The result was an explosion of animal life on planet Earth.