I’m on the fence about astrobiology, but I’ll go out on a limb and say the solar system is not a connected ecosystem “like Earth”.
Any actions we take that ruin parts of the quite likely ecosystem that lives on Mars will have zero effect on Earth’s.
I think a more likely threat is that we may bring something nasty back with a human mission to Mars, as unlike a sample return, you can’t keep the crew and every part of the returning spacecraft in a NASA biocontainment lab indefinitely.
Any actions we take that ruin parts of the quite likely ecosystem that lives on Mars will have zero effect on Earth’s.
I think a more likely threat is that we may bring something nasty back with a human mission to Mars, as unlike a sample return, you can’t keep the crew and every part of the returning spacecraft in a NASA biocontainment lab indefinitely.