If we accidentally spread microorganisms to other planets in our solar system, could they evolve into complex life?

Question

Is the following story plausible: a spacecraft transports microorganisms from Earth to another planet or moon in our solar system. Those microorganisms survive, and eventually, evolve into complex sentient life. Humans don't affect these planets again in this scenario.

I know that it's plausible for microorganisms to survive a journey on a spacecraft and survive on another planet. That's why there is the planetary protection principle and why NASA sterilizes spacecrafts. But is there any planet or moon in our solar system that has or will have appropriate conditions for complex sentient life to evolve?

Answer 1

It seems plausible with some conditions

The location of the body determines its golden age in terms of Earth life. Earth doesn't have a billion years left (maybe not a hundred...). Mars might be a good candidate from 1.5-2.5 billion years from now. (As others point out, the organisms need to survive in the meanwhile, but there are some earthly organisms that seem like they might be able to make a go of existing sheltered subsurface environments on Mars, and only a proper experiment could prove otherwise!)

The length of time the life needs to evolve depends on how much it has to work with. If you send another planet a coronavirus it will never have life; if you send it one bacterium with 3 million base pairs it will take a long while to work something out - though with the machinery of protein synthesis, it is billions of years ahead of where Earth life started. If you send it a well balanced care package of tardigrades, lichens, and extremophiles (such as Deinococcus radiodurans) they should be able to arrange something faster.

How fast? Nobody knows. The Fermi paradox is that we don't see life all over the sky, and one reason might be that it doesn't get past some "evolutionary bottleneck". What that bottleneck is, nobody knows. Body plans of major animal phyla have remained rather steady in some cases, over half a billion years; but then again, sometimes they don't (human vs. sea squirt). The bottleneck might be a combination of ecological niche and evolution preserving a status quo within it, which might depend on what happens to the planet later. (Nobody knows if any dinosaurs were on the brink of becoming intelligent per Jurassic Park, or whether they ever would have)

There are little guarantees in worldbuilding. Even God seems rather disappointed with the results.

Answer 2

Panspermia is a theory postulating exactly that.

However you are asking about complex life, and that poses additional constrains on the type of planet. For that one needs sufficient energy supplied at the start of the food chain, to allow such chain to grow longer and with more branches.

That practically singles out Earth as the only place: Mars has not enough atmosphere and water for hosting life, Venus is too hot and Europa with its oceans is too far away from the Sun to plausibly have anything more complex than unicellular beings or simple multicellular creatures like jellyfishes and worms.

Answer 3

Implausible

Certainly no other planet or moon in the solar system has the necessary conditions to support the kind of complex life present on Earth. We could not even walk outside without spacesuits.

Of course you did not specify you wanted Earth-like sentient life. The difficulty with asking about other stuff is we have no examples.

All we know is that Early Earth was suitable to evolve life. As for the others, they are much colder or hotter or they have no water on them.

We don't even know how suitable Early Earth was to create life in the first place. It might have been inevitable. It might have been a minuscule probability. The Anthropic principle you see.

Perhaps other planets in the solar system have the potential to evolve a completely different form of life. But we cannot even begin to guess what such life would look like.