This race of lizard creatures developed on a warm and marshy planet thousands of years ago. Many millennia ago these creatures were little more than just amphibious frog like creatures. As time passed and they evolved, they moved from the swampy areas of their planets to the more rugged and warmer mountainous areas.
As they were moving into these warmer, rocky, areas they very closely resembled the iguanas of Earth. Several thousand years ago a meteor struck the planet. This meteor apparently contained many unstable radioactive elements that were released into the atmosphere of the Lizard's homeworld.
These radioactive elements caused and amazing mutation in the genetic code of the Lizards . The mutation caused this race to grow and change. Their cranial capacity enlarged and their brains grew. They started to stand upright and developed an opposable thumb. After several hundred years of unstable genetic code their DNA finally stabilized once again and they now appear very humanoid.
Though they had become very humanoid in manner and appearance they still retained much of their amphibian and reptilian ancestry. Although they walk upright like a human their bodies are covered with a tough scaly hide. They average a height of seven feet. They are also enormously strong and durable.
Even as their bodies grew faster and larger, their intelligence grew at a proportional rate. They became very intelligent and developed all the facets of government and science that they needed to form a strong and advanced society. They even developed the technology they needed to explore space and to protect their world(s). The one problem that this race has yet to overcome is a horrible lisp that seems to plague all of their kind.