[Razengan]

> You could travel 10,000 light years but you will find them only on Earth.

(sorry for the Reddit'ism, but:) This. That sentence is subtly and unexpectedly profound.

Imagine having developed interstellar or even intergalactic travel, and after having colonized multiple solar systems, we find that life isn't so rare after all, but the specific lifeforms on Earth were.

What if we find that each life-bearing planet has its own unique creatures, not to be found anywhere else in the infinity of the Universe, and we may never again see the creatures we wiped out here on Earth?