Humans are a presumably extinct species that once populated the world 12,000 years ago, before the start of the Mollusc Era. It is stated that the rising sea levels caused the extinction which in turn were the result of a massive apocalyptic war combined with an upsurge in volcanic activity. This cataclysm killed most of the population and the rest attempted escape underground or into space. While none are known to have survived to the present day, many remaining artifacts from the humans are still present and discovered by the current inhabitants, such as Inklings. Human fossils and bones have been found and are exhibited in museums, and reverse-engineering human designs has seemingly developed most technology.
Currently, the only specific human being that is known is "the Professor", who created the Telephone, an AI designed to pass down humanity's knowledge to the next intelligent life, but which reprogrammed itself to recreate life to its specifications after deeming Inkling and Octarian society unworthy of its knowledge. The professor was also Judd's original owner,[1] and cryogenically sealed Judd in a machine to preserve him for the future. The machine is also capable of creating a clone of Judd when reactivated, as the human feared that the cat would be lonely.
Prior to their extinction, Humans built large networks of underground dome cities to escape the rising sea levels. These domes were seemingly abandoned by the time following The Great Turf War. The largest of these was a facility called Alterna, built in the crater of a massive volcano. Humans decided that their civilisation should be lead by scientists placed their survival in their inventions. However after years of living in Alterna, the younger generation of scientists longed to see the surface again. Despite warnings from their elders about the risks, they built a rocketship in an attempt to escape Alterna; however the rocket overloaded the facility and so caused the destruction of the inhabitants of Alterna.
Humanity had also built a spaceship called Ark Polaris, attempting to fly to space in hopes of finding another habitable planet. Many species of Earth were taken on board the vessel too, and were presumably tested on. This plan failed however, as the Ark Polaris was hit by debris at the edge of the Solar System, and was left to float aimlessly in space for ten thousand years, before inevitably crashing into Earth. The only known survivor of the crash was Bear #3. The remains of the space ship can be seen in the Ruins of Ark Polaris.
Return of the Mammalians reveals that humans were indirectly responsible for the genesis of the sentient life that exists in the Mollusc Era, as the humans of Alterna had developed a form of liquid crystal based on squid bodily fluids that reacted to thoughts and emotions; following the destruction of Alterna the crystals entered the underground seas, infusing the marine life with humanity's lingering emotions and drastically accelerating their evolution.
The Art of Splatoon expands on Splatoon's background story, revealing that the extinction of humans was caused by a nuclear missile detonating on Antarctica during one of a series of wars, which melted its ice cap and worsened the already rising sea levels.
In the same art book, it is also implied that humans created the Octarian domes found in Octo Valley as an unsuccessful shelter during the flooding.
Return of the Mammalians confirms this and further clarifies that due to numerous wars and climate change, the surface had become inhospitable. So while there existed remaining land, the climate of the surface wasn't suitable for humans during this period.
The NILS Statue resembles a giant human being, more specifically the mythological Greek god Hermes.
English (both American and European) and Latin American Spanish localizations of the Inkopolis News dialogue of Shellendorf Institute reference humans, with the English localizations in particular also referencing a popular real-life Miiverse post that reads "i wish squids were real".[2]
Human writing remains in the ruins of human artifacts. The Ruins of Ark Polaris features English text such as "Active Bear Area".
Alterna is implied to have human writings on their facilities, as Marie comments she can't read the text in Alterna. However the text seems to be made of a fictional language than an existing human language.
