The Lizard is the alter-ego Dr. Curt Conners, of a human scientist whose fields of expertise encompass both herpetology and gene therapy.
Conners, whose right arm had been amputated, hoped to find a way to regenerate the lost or damaged limbs of disabled people. To this end, he studied the regenerative nature of reptiles and amphibians. He developed a radically advanced form of therapy which could reproduce these regenerative ablities in humans and began to use this therapy in experiments. Unfortunately, Conners was contaminated by his own serum and physically transformed into a bipedal, sentient lizard.
As "The Lizard" Conners became a threat to humanity, with an insane determination to wipe out the human race and replace its population with reptiles. The Lizard is extremely strong, can scale walls much like Spider-Man, has a powerful tail, and can communicate and command reptiles with limited telepathy. The Lizard is, however, not as intelligent as the human Conners, and like other reptiles is sensitive to cold.
Eventually defeated by Spider-Man and returned to his benevolent human self, Conners still transforms back into the evil Lizard to plague the web-slinger every now and then.