Despite the bleak-looking ending, is humanity's doom really assured? As noted on the main page, Earth is a new environment for Calvin, one that it might not be able to adapt to, thus it and its spawn might die out War of the Worlds style. On top of that, none of the crew members aside from Rory actively tried to fight it - and with an impromptu weapon, to boot - so it's up in the air whether or not Calvin can shrug off bullets or chemicals. A confirmed weakness is low temperature, so there's a chance that good old General Winter might prove to be a major obstacle in Calvin's would-be conquest. In the end, with all these factors involved, Calvin's chances of actually wiping out humanity aren't astronomically high.
Also on earth, it wouldn't be facing six humans in a relatively tiny space. It would be up against a few million military members and the entire world.
The fate of the humanity is not focus of the movie. Tragedy happened either way - even if humanity survives and get rid of Calvin (lets be honest, even Calvin can't survive nuclear blast, too high energy to sustain its form without being reduced to singular molecules), the Earth's ecosystem will be damaged. And the movie concentrates on crew's carelessness and human mistakes being the main reason for that.
Also the space is the problem. It now has an entire planet to hide on so even if it never becomes the dominant lifeform, Calvin-type lifeforms will be a constant threat given their seemingly ability to reproduce From a Single Cell.
