What makes it clear?
Firstly, the earth consists of many beings, including plants as well as animals and insects. Some of these are quite complex and many scientists think they are self-aware just like we are or in a similar way (simians, dolphins, whales, elephants....).
Then, given the vastness of the universe and the great number of planets, it is very likely mathematically that other planets exist/ed with sentient life forms on them, so the universe is "for" them as well. And if you allow for the possibility that there are lifeforms, sentient and self-aware, that follow completely different rules than we do (say, energy-based life forms, silicon-based life forms etc etc), it is even more likely that the beauty and complexity of life is infinite. And if you allow even more for the thought that modern scientists have brought up, that there are 11+ dimensions of which we know only 4 and that there are also many universes - well?
Restricting this universe and your thinking to "humans only" is, in my eyes, very flat-earth like
.