Doctor Curtis Connors was a Floridian scientist who lost his right-arm while serving for the US military. Curt was a family-man who was obsessed with getting back his lost limb resulting in him beginning to study how to splice human DNA with that of a reptile as some reptiles could regenerate their limbs.
Eventually Curt learned how to properly splice the genetics but in addition to regrowing his arm, the results transformed Curt into a giant lizard/human hybrid. Taking on the moniker of "The Lizard", Curt lost control and became a super-villain who used his powers and ability to communicate with reptiles to try and turn other humans into creatures like him. Lizard became an enemy of Spider-Man who eventually cured the doctor with Cut becoming a recurring ally to him despite sometimes loosing control and falling back into villainy.