Good question! We still don’t know whether there ever was, or still is, life on Mars. However, it is an important scientific question that many people are interested in. I predict that if a scientific paper reported solid evidence of life on Mars it would be the most popular article in history! So, why do we think that Mars could possibly harbor life (now or in the past)? The key is… water. Liquid water is necessary for life (as we understand it). We have already discovered ice on Mars, but was it ever liquid? NASA’s Mars rovers mission has identified strong evidence that there was once flowing liquid water on the surface of the planet. This evidence is mostly from certain structures in sedimentary rocks (like sandstones) on Mars that only form from flowing water. This evidence doesn’t necessarily point towards the presence of life, but it means that the most important ingredient for life was once present on the surface of Mars. I recommend checking out the NASA website on the Mars rover mission for more information and really cool pictures

Mars is an extreme environment, but life exists in extreme environments on earth, from underwater volcanoes to polar deserts in Antarctica (for example, the “Dry Valleys”). Many researchers look to these extreme environments on earth to gather information on how life might exist on other planets.

Nobody knows. We haven't found any yet, which means that it can't be very common or prevalent or we would have found it (we've looked). However, that doesn't mean that it isn't there and just rare enough that we've yet to come across it. The vast majority of the Martian surface has not been explored except by photographs taken from space, and you can't see bacteria from space.