There is no proof the 1976 Viking landers missed finding life on Mars. There is, however, an intriguing new theory about how they could have.
Geology professor Dirk Schulze-Makuch has published a paper suggesting that the VIking experiments could actually have killed life isntead of finding it. By adding water to Martian soil and then warming it - a logical way to look for Earth-type microbes - Schulze-Makuch suggests the experiments may have drowned and/or baked life forms with significantly different chemistry.
In the cells of Earth-based life forms, the basic internal liquid is some variant of salt water. On the cold, dry Red Planet, a mix of water and hydrogen peroxide may have evolved instead. Astrobiologist Mitch Sogin, a member of a National Research Council committee on alien life, said, "I'm open to the possibility that it could be the case." Future probes carrying more sophisticated experiments may settle the question.