Lizards such as pet iguanas can produce venom that should be studied for its potential pharmaceutical value, say scientists who studied the evolution of snakes and lizards.

Textbooks say advanced snakes and two species of lizard, the gila monster and the beaded lizard, have venom delivery systems.

Pain and swelling from lizard bites are commonly attributed to bacteria infections. However, the new findings suggest the damage is from venom itself.

Two more groups of lizards, varanids such as the Komodo dragon, and iguanians, also produce venom toxin and have venom glands, according to a team of researchers led by Bryan Fry of the University of Melbourne in Australia.

Fry consulted on three cases of humans bitten by Komodo dragons, spotted tree monitors and lace monitors. Komodo dragons are the world's largest lizard, weighing up to 160 kilograms.

The lizards don't deliver enough venom to kill a person, but can paralyze small animals such as birds or mice, the team reports in the Nov. 16 online issue of the journal, Nature.

The rapid appearance of symptoms in humans included:

• Swelling within minutes.
• Dizziness.
• Localized disruption of blood clotting.
• Shooting pain on the affected finger up to the elbow, with some symptoms lasting for hours.

Researchers isolated nine venom toxins from both lizard and snake tissues.

"These molecules represent a tremendous hitherto unexplored resource not only for understanding reptile evolution but also for use in drug design and development," the study's authors concluded.

A DNA analysis suggests venomous lizards and snakes shared a common ancestor, which would mean the two groups are more closely related than first thought.

A single evolutionary origin of venom delivery may explain the ecological success of "advanced" snakes, with 2,500 out of 3,000 snake species being venomous, the researchers said.

They call for future research to explore the relationship between the relative toxicity of a venom and the complexity of the gland where it's secreted.