The fossil record. There is little evidence of very complex animal life before about 541 million years ago, which is when the Cambrian explosion begun.
Before "modern" life evolved in the pre-cambrian era, there was the Ediacaran life forms that were complex multicellular life, but died out millions of years before the Cambrian explosion.
There's quite a few fossils showing single-celled organisms from 3+ billion years ago. I imagine if more complex life existed the fossils could've survived
