In the movies, first contact with aliens unfolds with great drama. Often, extraterrestrials arrive with laser guns blazing; in more benevolent scenarios, they show up in the form of hooded Vulcans, glowing-fingered empaths or black-box artifacts buried on the moon. Regardless, the main message is clear: We are not alone in the universe.

In reality, the discovery of alien life will come with a lot of ambiguity and skepticism. How can I be so certain? Because it has already happened. Twice. 

In 1976, the twin Viking 1 and Viking 2 landers touched down on Mars and initiated the most ambitious effort ever undertaken to seek life on another world. The robots scooped up soil and ran four experiments. In one of them, a sample was incubated in a nutrient broth. It immediately emitted a belch of a carbon-containing gas (most likely carbon dioxide), and for a staggering moment it looked like Viking had found an alien biosignature — chemical proof of biology in action. 

Then came the gloomy realization that the chemicals of the Martian surface could produce the same effect without the aid of microbes. The other Viking experiments came up empty; NASA’s official verdict was “no evidence of life at either landing site.”