Humans are the basic species in the Twilight series, from which vampires, shape-shifters, and werewolves originate. Lacking supernatural enhancements, they are considered prey by most vampires, who need their blood for sustenance.
Human beings have roamed Earth for as early as time itself. In the past, everyone was aware of the existence of vampires and werewolves. However, after the Volturi established the laws that prohibited vampires from exposing themselves to humans, the knowledge slowly faded to fiction and myths. Humans who discover the existence of vampires are considered a threat, and a Volturi-enforced law has been implemented dictating that said humans either be killed or changed into vampires, causing Jane, Alec, Demetri and Felix to be sent to Forks in Eclipse. It is also illegal to create immortal children, that is to turn human children into vampires, because they always lack concentration and self-control, and are therefore un-trainable and dangerous.
A human who is to be changed into a vampire needs just to be bitten once to allow the venom into their bloodstream. If blood is consumed by the vampire during this time, it is highly unlikely that he will resist the temptation to kill the human before completion of the transformation. The pain accompanying the transformation is described as the worst possible by all those transformed into vampires, and in Breaking Dawn, it is described as a burning feeling by Bella. A human, however, can also be changed into a werewolf by being bitten by one, but it remains unknown whether the conversion is painless.
Despite lacking the physical qualities that could protect them from vampires, certain humans can also be supernaturally gifted. For example, one could learn or adapt to a particularly useful skill that gives him or her an advantage in a needed situation, such as sensing and evading danger, or being sensitive to the minds of others. If a gifted human converts into a vampire, their gift will become enhanced also, making it more powerful than its human version.
According to Aro, the human race has advanced so far in modern technology that they now possess weapons that could potentially destroy vampires, and maintaining the secret of their existence is the one thing that protects them from destruction.