Human

Humans are an extinct species of primate that once populated the world over 12,000 years before the present day.

History
The history of humanity as it exists in the Splatoon series is heavily implied to have mirrored that of real life humanity until a mass extinction event took place approximately 12,000 years ago. As a result of global warming, the Earth's sea levels had begun to rise. This was exacerbated by a series of large-scale wars that lead to Antarctica being destroyed with a nuclear missile. The rising sea levels combined with an upsurge in volcanic activity lead to the Earth becoming uninhabitable. This cataclysm killed most of the human population, while those that survived attempted to 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 are a common discovery, many of which are exhibited in museums, and reverse-engineering human designs has seemingly developed most technology.

Currently, the only notable human character is "the Professor", who created the Telephone, an AI designed to pass down humanity's knowledge to the next intelligent life, 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, who cryogenically sealed him in a machine to preserve him for the future. The machine is also capable of creating a clone of Judd when reactivated, as the Professor feared that their cat would be lonely.

Shelters
Prior to their complete extinction, the humans that survived the mass-extinction event built large networks of underground dome cities to escape the rising sea levels, where the majority of them perished. The largest of these domes was a facility called Alterna, built in the crater of a massive volcano. These humans decided that their civilization should be lead by scientists who placed their survival in their inventions. After years of living in Alterna, the younger generation of humans longed to see the surface again. Despite warnings from their elders about the risks, they new scientists 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. Both Alterna and the remaining domes were seemingly abandoned in the time preceding the Great Turf War. After losing the war, the Octarians repurposed an unspecified amount of human domes to form their own society separate from the Inklings.

Humanity had also built a spaceship called Ark Polaris, a vessel created in hopes of finding another habitable planet. Many species of Earth were taken on board the vessel, some of which had been experimented upon. This plan failed however, as the Ark Polaris was hit by debris at the edge of the solar system, damaging its navigational system, which lead the ship's crew to redirect its trajectory back towards Earth. Midway, the ship ran out of fuel was left to float aimlessly in space for ten thousand years before eventually being pulled back into Earth's orbit and crash landing. The only survivor of the crash was Bear #3. The remains of the space ship can be seen in the Ruins of Ark Polaris.

Technology
The humans of Splatoon were more technologically advanced than the humans of today. Among their technological breakthroughs were the successful creation of an Artificial Intelligence, effective cryosleep machines, cloning machines, preservational spaceships designed to seek out other inhabitable planets long-term, as well as advanced animal experimentation that lead at least one mammalian subject to stay conscious as he dreamt.

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 cephalopodic 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.

Trivia

 * 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".
 * Pearl mentions that she has dug up human bones before, to which Marina replies that they are found everywhere.
 * 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.
 * SashiMori's music incorporates vocals that are implied to be surviving recordings of humans.