Viking 1 & 2 returned positive results in the seventies but these have been played down or hand waved. I think there is good evidence of microbial life in the soil or underground. We should be wary of bringing Martian microbes back to Earth, because they may find our environment too hospitable and end up invasive species.
> We should be wary of bringing Martian microbes back to Earth, because they may find our environment too hospitable and end up invasive species.
They could even already have invaded in the past, and we descended of them[1]. Earth as Mars ancient lifeboat[2].
[1] https://www.bbc.com/news/science-environment-23872765
[2] https://badspacecomics.com/apostles-of-mercy
Yes, that is perfectly possible. There could be some transfer, but there is a scenario where Martian microbes breed on Earth and have no natural controls. An Australia type scenario on a planetary scale.