Has life existed on Mars before?

We have no idea that it ever existed. Claims that fossil bacteria were discovered in 1996 in a meteorite recovered from Antarctica have been severely tested and the scientific jury is still out. The best we can say is that there is probably a lot of water and ice under the surface and in the polar ice caps, but until we actually go to mare and poke around on the surface to find fossils of live bacterial colonies, we really can't prove that life has ever been on Mars.