In the quest to know how life came to a planet, we have a strange behavior on the only life populated planet we know about: The cambrian explosion.

Without it, life is limited to algae and plant or mushroom for 4000 millions years. Then 500 millions years ago all modern animals came to existence. That's the cambrian explosion.

If the cambrian event is a very very rare occurrence, it might be a good candidate to explain why the universe is so empty. It might be a bottleneck event or a great filter preventing the universe to be populated by intelligent civilization. Something special that happened to Earth, and only here.

If it is a real thing!

If it's just an artifact: a period where skeleton became usual -and could be more easily fossilized- , or when fossilization became more easy because of propriety of soil or something... Then... We're more likely to be doomed: