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. Warfare combined with rising sea levels and 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 dominant species. Human fossils and bones are a common discovery, many of which are exhibited in museums.

Currently, the only notable human character is "the Professor", who created Commander Tartar, 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 prioritized the Alternans' 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, the new scientists built a rocketship in an attempt to escape Alterna. The rocket was unable to launch and overloaded the facility, causing the destruction of the shelter and its inhabitants alike. For an unspecified amount of time the remains of Alterna deteriorated to the state of a wasteland. 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. Alterna was later rebuilt to its current state by Mr Grizz.

Humanity had also built a spaceship called the Ark Polaris, a vessel created to find another habitable planet to replace Earth. Many species of Earth were taken on board the vessel and kept in cryogenic stasis, some of which had been experimented upon. This plan failed however, as the Polaris was hit by debris at the edge of the solar system, damaging its navigational system. This lead the ship's crew to redirect its trajectory back towards Earth, though there was not enough remaining fuel to attempt landing. The ship floated aimlessly in space for 10,000 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 stage.

Technology
The humans of Splatoon were more technologically advanced than the humans of today. Among their technological breakthroughs were the successful creation of advanced artificial intelligence, effective cryosleep machines, cloning machines, preservational spaceships designed to seek out other inhabitable planets in the 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 passive desire to see the surface, drastically accelerating their evolution.

Trivia

 * The NILS Statue resembles a giant human being, more specifically the mythological Greek god Hermes.
 * Inkopolis News dialogue of Shellendorf Institute reference the fact the museum displays human bones, though they are exclusive to certain localizations.
 * Marina says "I wish humans were real", referencing a popular real-life Miiverse post that reads "i wish squids were real".
 * 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.
 * Throughout Alterna various English-speaking voices can be heard in what seems to be a series of rocket launch announcements emanating from the Alterna Space Center. Totaling at seven clips, these recordings play at random, with each consecutive unlocking of a Site bringing a new clip into the "rotation".
 * The clip "unlocked" last is glitchier than the rest and contains several emergency alarm sirens. This suggests it is the final rocket launch announcement recording conducted by the Alternan humans before Alterna's collapse, which is mentioned in Alterna Log 004.
 * SashiMori's music incorporates vocals that are implied to be surviving recordings of humans.
 * In a concept manga seen in the Art of Splatoon, the sea levels' sudden and dramatic rise was due to the launch of a nuclear missile towards Antarctica.