Here is a link to the latest from Reasons to Believe (reasons.org).
Some scientists have maintained that the water habitable zone is wide because even if a planet is farther from its star, more carbon dioxide can keep the planet warm enough for ice to melt. This new study shows that above very strict limits, CO2 becomes toxic to oxygen-breathing animals. The water habitable zone just got much, much, smaller. There is much more. Treat yourself. Here it is:
The “fine-tuning of the universe” for complex life gets finer and finer the more we learn about nature. The bottom line: it was very, very hard to create a universe in which even one earthlike planet could exist. It would not happen by accident in a trillion, trillion, trillion years.
 Water habitable zone: The range of distances from its star within which a planet must be located in order for the temperature to be mild enough for liquid water to exist on the surface of the planet.