Taking this one step further, it is also possible that life has been exchanged between our two worlds. Large meteor collisions in the past have moved parts of Mars to Earth, and possibly also Earth to Mars. And some small life can exist clung to rocks in space (we know of such creatures here on Earth)! So perhaps, Mars was once wet and warm and full of life. Perhaps it suffered a large impact of some kind billions of years ago, and perhaps that impact flung Mars rocks with little tiny Martians on them towards Earth. In fact, this could have happened thousands of times.

It's possible, anyway. You know, while we're speculating.