"Worlds Enough, and Time" is a short story by Harry Turtledove, first published as the "Probability Zero" feature in Analog, Jan-Feb, 2008, and reprinted in Reincarnations. The story is two-pages long, with a "twist" ending.
A family travels in their starship to a planet in a very formative stage. There are oceans filled with plants and primitive animals. The land, however, is barren.
The father of the family, while normally enjoying such worlds, is disappointed. His wife is not, as she expected a poor world. When they've decided they've seen enough, they prepare to leave. However, theirchildren come out of the ship, and announce that the environmental unit of their aquarium has died, and they have forgotten the spare. After some pleading, the father agrees to let his children empty the aquarium to give the animals a chance. This is against the rules of their home planet, so they agree to keep quiet.
The last paragraph of the story features a professor named Jack Conway giving a lecture to his class about the explosion of metazoan body plans at the beginning of the Cambrian, showing several examples of prehistoric fauna. Conway tells the class that life on Earth was almost nonexistent before a certain point, but abundant and diverse species appeared in the fossil record almost overnight. No one really knows why, says Conway, but there are many interesting theories...