ConclusionIn spite of Mars' similarity to Earth in size and closeness to the sun, the environment of Mars seems unfriendly toward life as we know it on Earth.
Mars is small, so there is not much gravity. For this reason, much of the atmosphere of Mars has drifted away. With little atmosphere, and no ozone layer, there is less protection from the ultraviolet radiation of the sun, which is very harmful to life.
With little atmosphere, there is a only a small buffer between the surface and space itself. This means that the temperature above the surface is cold. With little atmosphere, there is also little pressure, which sophisticated life forms such as humans need to keep blood from boiling. (Remember what happened to the characters of the movie "Total Recall" when they encountered the surface of Mars)!
The Viking lander, which carried the first experiments designed to detect life on Mars, did not find evidence of even the simplest organic molecule. The Mars Pathfinder came up with negative results when looking for signs of life on Mars. We know, however, that there are life forms on Earth which can survive in very harsh environments. It's possible that if life ever existed on Mars or if it will ever exist on Mars, it will look very different from life as we know it!