Humans

Humans (人間, Ningen) are the dominant race of living beings on Earth.

Overview
Humans are a race of sapient beings that inhabit the world of One-Punch Man. They are the dominant and most abundant species on the planet.

A human's size varies according to ethnicity and age. Humans possess skin color which ranges in variety from dark brown to light hues. Certain body parts, mostly facial structures of humans, vary depending on ethnicity and age, and genetics.

A running gag in the series is that a lot of humans are unable to accept Saitama's immense strength despite witnessing him in battle. For example, they call him a fraud after he defeats the Sea King despite him defeating the monster in one punch, as well as blowing the rain away. Another time is when Saitama battles Suiryu in the Super Fight Tournament, the security staff didn't want to intervene when Suiryu was fighting Saitama, yet they were willing to try to pin Saitama down and capture him, not even thinking of how Saitama was unfazed by Suiryu's brutal attacks and defeated him in the first place. Furthermore, during the hero costume competition, most of the audience states that the monster Saitama effortlessly kills is fragile or even self-destruct not noting the fact that many heroes even A-class cannot defeat it. All these factors indicate that the majority of humans suffer from Sunnydale Syndrome.

Furthermore, many humans care more about the reputation and appearance of a person without considering their true power. Only a handful of people like Bang, Satoru, Sonic, Fubuki, King and Suiryu are perceptive enough to determine the true power of a person. Despite the situation, it is shown that some people or heroes are willing to accept Saitama's tremendous power, either through witnessing or losing to him in combat. For example, Sweet Mask who used to only care about appearances was willing to praise Saitama after seeing him defeat Garou. The police chief also calls Saitama a hero after he brings back the people's hope in the police force and Sneck admits that Saitama is stronger than him after he defeated him in a single blow.

History
Many years ago, a number of nations existed on Earth. They fought for land and resources through countless world wars, and the human population suffered as a result. In order to preserve their posterity, the nations unified and established a single world government and a universal language. This period was identified as the first era of change.

Despite the end of the warring period, the environment suffered its harmful effects. An increase in environmental toxicity, rapid climate change, and a rise in sea levels gave birth to a multitude of harmful lifeforms, which forced humans to migrate to the center of the supercontinent to reestablish their civilization. This period was identified as the second era of change.

According to Sweet Mask, the world is currently in its third era of change, characterized by an increase in occurrences of Mysterious Beings with high intelligence.

Humans appeared in the world at an unknown period in time. They were the dominant species in the world 15 years ago when a race of creatures labeled Mysterious Beings appeared and started wreaking havoc and killing humans. At first, they could be easily dealt with. However, Mysterious Beings grew in strength to the point where regular weapons were useless against them. In order to organize individuals that could combat these threats, Agoni founded the Hero Association.

Abilities & Powers
Humanity's main power lies in its intelligence and numbers. They vastly outnumber the Mysterious Beings and possess advanced technology which allows them to multiply across the planet. However, most of the weapons they use prove to be useless against stronger Mysterious Beings.

Some humans acquire superhuman abilities through experimentation, talent, or hard work. Other humans are born with supernatural powers, such as Psychokinesis or Plant Manipulation. Many use their powers for hero work. Some humans are born with genius intellect, like Dr. Genus.

Heroes from the Hero Associaton are humanity's first line of defense against Mysterious Beings. Humans also defend themselves from Mysterious Beings using weaponry.

Humans can transform into monsters through a desire to turn into something else. They may also ingest monster cells to become monsters.

Trivia

 * It's hinted by Dr. Genus that Saitama's powers were granted by breaking his human limits.