The hunting of the humpback whale to extinction in the 21st century, which leaves no one on 23rd-century Earth able to answer an alien probe.
The extinction of the humpback whale is the piece of history on which Star Trek IV: The Voyage Home turns. When a vast probe arrives at Earth in 2286 and its signal begins to vaporise the oceans, Spock works out that the transmission matches the song of humpback whales.
The species had been hunted until none were left, at some point in the 21st century, so nothing on Earth can reply. Kirk's crew take their captured Klingon Bird-of-Prey back to 1986 to find living humpbacks.
In San Francisco, the biologist Gillian Taylor tells Kirk how few of the animals remain even then. The crew bring the pair George and Gracie, and Taylor herself, forward to the 23rd century, where the whales answer the probe and the species is given a second beginning.