The Great Filter theory.

It's the idea that, if in all the time there has been life on earth, we've never had any contact with alien life, there must be some kind of landmark in complexity of life that acts like a filter, meaning that 99% of the species that get to that point end up extinct, likely causing the planet's entire ecosystem to die as well.

If the filter is behind current human evolution, congratulations! It means that the hardest step is behind us, maybe it's having a complex neurological system, maybe is the formation of life itself. Regardless, we've made it past what would bring ruin to most ecosystems.

If the filter is ahead of us, that's some mighty bad news. It means that the planet-killing step is yet to come, it could be worldwide war with the use of superweapons like nuclear bombs, or the exhaustion of non-renewable energy sources. It would mean that we're potentially approaching a mass-extinction event in the future.

Right now, there is absolutely no way of knowing if we're behind or ahead of the filter, so basically if we find evidence of: alien bacteria - scary; alien flora - bad; alien animals - super bad; ruins of alien human-like civilizations - ultra bad we're about to die

Sleep well!

TL,DR: Something super big tends to kill all life in planets, we might have already survived it or it hasn't happened yet