Guy Threepwood
Mighty Pirate
There are complex life forms at the bottom of the ocean, though: an environment that wouldn't be much different than what probably exists on Europa. It is due to those same complex life forms that we know that life can survive at extreme pressures. That being said, another way to regulate climate other than having a large moon would be to have a very thick atmosphere like what Venus has. The pressures at the sea floor are greater than on Venus' surface, too. So we know complex life could survive on a cooler, wetter version of Venus.
The biggest issue, to me at least, is not knowing the minimum requirements for abiogenesis. Even if life can survive in exotic conditions, we don't know if it could actually develop there or not. We need to do more research first.
I take that point, though 'complex' is a little subjective, I don't think extremophile tube worms clinging to an ocean vent are going to be joining the space race anytime soon?
So too with Venus, the Earth's complex, rich, diverse climate, seasons, environs etc are uniquely tuned to develop complex, rich, diverse life in turn. Yet also with an improbably unique stabilizing satellite limiting the swings- not to mention the role tides may have had in developing land animals etc..
in short simple conditions = simple life, which again we can observe on Earth- remaining practically unchanged for millions of years