I'm not 100% sure but I do think it is very possibly that there use to be some kind of life living on Mars. Whether it just be bacterial or animal, I think that Mars was once a very habitable place for life.
Life itself have been proven to form in the most extreme conditions. Even in the most coldest places of Antarctica to even molten lava they have found life.
Why not Mars?

Some scientists say Mars was once like Earth.

I don't know much about it, since I haven't had very many opportunities to study Space very much, but there probably could have been.

@rainandbomba
It could be true. Mars could be once like Earth. But again, it's the "Solar Wind & Magnetic Field" theory people! It's still very reliable, though. Hope Curiosity would do its job well.